Showing 1–50 of 252 results for author: Kumar, S

An ultracompact dilution refrigerator for fast quantum device characterization

Authors: Clment Geffroy, Dorian Nicolas, Eric Eyraud, Shelender Kumar, Supriya Mandal, Julien Jarreau, Laura Kowalski, Laurent Del-Rey, Didier Dufeu, Nicolas Roch, Wolfgang Wernsdorfer, Quentin Ficheux, Matias Urdampilleta

Abstract: Rapid thermal cycling is a central bottleneck in the development of superconducting quantum devices: conventional dilution refrigerators require cooldowns of a day or more and substantial cryogenic infrastructure, which throttles the fabricate-measure-redesign loop. We present an ultracompact dilution refrigerator (3 kg in mass and 100 mm in diameter) that completes a full cooldown-warm-up cycle t… ▽ More Rapid thermal cycling is a central bottleneck in the development of superconducting quantum devices: conventional dilution refrigerators require cooldowns of a day or more and substantial cryogenic infrastructure, which throttles the fabricate-measure-redesign loop. We present an ultracompact dilution refrigerator (3 kg in mass and 100 mm in diameter) that completes a full cooldown-warm-up cycle to a base temperature of 70 mK in 1.2 hours when unloaded, and in 2.1 hours when fully equipped with the microwave wiring required for qubit measurements, while delivering 20 microW of cooling power at 100 mK. We validate the platform through a complete characterization of a two-fluxonium device: we extract the full circuit Hamiltonian by two-tone spectroscopy, measure energy-relaxation and coherence times, and benchmark single-qubit control. Although the relaxation time is limited by the base temperature of the system, we reach a single-qubit gate fidelity of up to 99%, at the coherence limit set by our operating temperature. These results demonstrate that compact, fast-cycling dilution refrigeration can support state-of-the-art quantum-device characterization without sacrificing measurement quality, offering a practical route to high-throughput quantum-hardware development. △ Less

Submitted 19 August, 2026; originally announced August 2026.

Comments: 8 pages, 5 figures, two appendixes

High-Harmonicity Planar Penning Traps for Single-Electron Qubits

Authors: Kedar Mal, A. N. Agnihotri, Sugam Kumar, Wolfgang Quint, Manuel Vogel

Abstract: We present a detailed account of the design choices required for planar Penning traps that feature a highly harmonic confining potential. High harmonicity is indispensable for a number of applications, particularly for confinement of single electrons as qubits in quantum-information processing. The present work extends previous studies [D. Goldmann and G. Gabrielse, Phys. Rev. A 81, 052335 (2010)]… ▽ More We present a detailed account of the design choices required for planar Penning traps that feature a highly harmonic confining potential. High harmonicity is indispensable for a number of applications, particularly for confinement of single electrons as qubits in quantum-information processing. The present work extends previous studies [D. Goldmann and G. Gabrielse, Phys. Rev. A 81, 052335 (2010)] by a fully analytic treatment of finite electrode gaps and their relevance for small traps on the millimetre size scale and below, when relative gap sizes are non-negligible. We derive the overall trap potential with a particular focus on different models of finite-gap potentials and show how to find the optimum trap geometry and electrode voltages to minimize anharmonicities. The analytic calculations are compared with detailed finite-element simulations. △ Less

Submitted 18 August, 2026; originally announced August 2026.

Iterative linear quadratic regulator on SU(N) for multi-qubit gate synthesis

Authors: Dirk Heimann, Felix Wiebe, Elie Mounzer, Shivesh Kumar

Abstract: In quantum optimal control theory, gradient-based trajectory optimization techniques have proven versatile in designing multi-qubit quantum gates. Furthermore, incorporating the underlying Lie-group structure can accelerate the optimization process. In this work, we adapt the Lie-group formulation of the iterative linear quadratic regulator (iLQR) to the special unitary group SU(N) and apply it to… ▽ More In quantum optimal control theory, gradient-based trajectory optimization techniques have proven versatile in designing multi-qubit quantum gates. Furthermore, incorporating the underlying Lie-group structure can accelerate the optimization process. In this work, we adapt the Lie-group formulation of the iterative linear quadratic regulator (iLQR) to the special unitary group SU(N) and apply it to quantum gate synthesis, systematically comparing it against the standard Euclidean iLQR formulation across multiple two- to five-qubit gates. We find that in the idealized, unconstrained setting, where all Lie-algebra basis elements are available as drive Hamiltonian terms, the Lie-group formulation converges faster than the Euclidean iLQR formulation. If drive terms are constrained to 2-local Hamiltonian terms, the Lie-group variant converges faster in early optimization iterations, but exhibits greater sensitivity to initialization and a stronger tendency towards local minima. These results demonstrate that incorporating Lie-group geometry into iLQR substantially improves convergence and highlight important next steps for improvements in constrained control settings. △ Less

Submitted 4 August, 2026; originally announced August 2026.

Comments: 21 pages, 11 figures

Quantum Remote Implementation of Hybrid Operations on Hyperstates Using Hyperentangled States

Authors: Satish Kumar, Anirban Pathak

Abstract: Quantum remote control, also known as quantum remote implementation of an operator (QRIO), enables the remote manipulation of an arbitrary quantum state by implementing a desired quantum operation at a distant location. Significant progress has recently been made in developing QRIO protocols and their variants. Most existing schemes employ hyperentangled states where entanglement is shared across… ▽ More Quantum remote control, also known as quantum remote implementation of an operator (QRIO), enables the remote manipulation of an arbitrary quantum state by implementing a desired quantum operation at a distant location. Significant progress has recently been made in developing QRIO protocols and their variants. Most existing schemes employ hyperentangled states where entanglement is shared across multiple degrees of freedom (DOFs). However, these protocols typically exploit only one degree of freedom at a time. In this work, we propose a QRIO protocol that simultaneously utilizes the polarization and spatial DOFs of a two-qubit hyperentangled state to remotely implement an arbitrary hybrid operator on an unknown single-photon two-qubit hyperstate. The shared hyperentangled resource is realized using the polarization and spatial modes of photons, while the protocol is constructed using linear optical elements and cross-Kerr nonlinear interactions to facilitate effective photon-photon coupling. Furthermore, the effects of measurement errors arising from finite coherent state distinguishability and coherent state dissipation are analyzed and the corresponding success probability of the protocol is evaluated. The results demonstrate that an appropriate choice of the cross-Kerr phase shift and coherent state amplitude significantly enhances the protocol performance, making the proposed scheme a promising candidate for hybrid quantum communication and distributed quantum information processing. △ Less

Submitted 16 July, 2026; originally announced July 2026.

Comments: 8 pages, 3 figures

Plaquette: A hardware-aware design platform for fault-tolerant quantum computers

Authors: Raul Conchello Vendrell, Carlos Díaz López, Ish Dhand, Kshitij Kapoor, Davide Laureti, Marcello Massaro, Pranjal Nayak, Ivan Ogloblin, Martin B. Plenio, Shreya Prasanna Kumar, Matteo Santandrea, Varun Seshadri, Antal Száva, Trevor Vincent, Raphael Weber

Abstract: Hardware teams building fault-tolerant quantum computers (FTQCs) must decide which imperfections to suppress, and that decision requires the logical performance of the architecture under the device's actual noise. Hardware noise often departs from the stochastic Pauli models used by scalable stabilizer simulators: superconducting transmons leak out of the computational subspace, neutral atoms scat… ▽ More Hardware teams building fault-tolerant quantum computers (FTQCs) must decide which imperfections to suppress, and that decision requires the logical performance of the architecture under the device's actual noise. Hardware noise often departs from the stochastic Pauli models used by scalable stabilizer simulators: superconducting transmons leak out of the computational subspace, neutral atoms scatter through intermediate states, trapped ions heat as their motional modes absorb phonons, and miscalibrated controls over-rotate coherently. We present Plaquette, a theoretical framework and software suite that computes the logical performance of fault-tolerant architectures directly from the physics of such imperfections. In Plaquette, a hardware error model is specified once, as Kraus operators, Hamiltonian-Lindblad dynamics, or an experimentally reconstructed quantum channel, and is compiled automatically into the exact or approximate representation required by each of four sampler classes: stabilizer sampling for Pauli noise, the new XPauli sampler for leakage and environment sectors, near-Clifford samplers for coherent errors, and full-state simulation for exact reference calculations. We validate the XPauli and near-Clifford samplers against full-state simulation, which they can match within statistical uncertainty while Pauli twirling can fall short depending on the error model. We demonstrate the framework on three error models: leakage in superconducting qubits, intermediate-state scattering in neutral atoms, and heating in trapped ions. The size of the discrepancy between Plaquette and Clifford-only simulations varies with platform and noise process, so reliable thresholds, error budgets, and overhead estimates require the most accurate simulation available. Plaquette provides a direct path from the open-system physics of a device to the logical performance of the FTQC built on it. △ Less

Submitted 9 July, 2026; originally announced July 2026.

Phase-correlation-free quantum key distribution source operating at gigahertz rates

Authors: Shashank Kumar, Alessandro Marcomini, Loïc Millet, Towsif Taher, Aurélien Cavalié, Raphael Houlmann, David Cabrerizo, Gianluca Boso, Marcos Curty, Rob Thew, Boris Korzh

Abstract: Phase randomization is essential for the security of practical decoy-state quantum key distribution (QKD) systems. Commonly, implementations rely on laser sources which are either actively phase-randomized, or gain-switched. However, at high repetition rates these show correlations, which can ultimately compromise security and performance. We present a 1.25 GHz phase-randomized QKD source based on… ▽ More Phase randomization is essential for the security of practical decoy-state quantum key distribution (QKD) systems. Commonly, implementations rely on laser sources which are either actively phase-randomized, or gain-switched. However, at high repetition rates these show correlations, which can ultimately compromise security and performance. We present a 1.25 GHz phase-randomized QKD source based on a super-luminescent light emitting diode (SLED) operating in the C-band as a compact and cost-effective alternative. The source generates $\sim100$ ps optical pulses with $400$ ps pulse-to-pulse separation, compatible with high-speed time-bin encoding. Interferometric measurements demonstrate $>99\%$ visibility between adjacent time bins, confirming strong first-order coherence within the same quantum signals, while the spontaneous-emission-driven nature of the SLED ensures intrinsic global phase randomization between adjacent signals. This work establishes a scalable SLED-based platform for high-speed prepare-and-measure QKD systems. △ Less

Submitted 3 June, 2026; originally announced June 2026.

Comments: 11 pages (7 pages main text and 4 pages Supplementary), 10 figures (5 in main text and 5 in supplementary), Poster accepted in Qcrypt 2026

OAM-Induced Lattice Rotation Reveals a Fractional Optimum in Fault-Tolerant GKP Quantum Sensing

Authors: Simanshu Kumar, Nandan S Bisht

Abstract: Photon loss and dephasing rapidly degrade the sensitivity of quantum sensors, yet systematic methods for designing error-correcting codes whose geometry is simultaneously adapted to the sensing task and the noise channel do not exist. Here we establish that orbital-angular-momentum (OAM) encoding and Gottesman-Kitaev-Preskill (GKP) lattice geometry are structurally coupled: an OAM mode of topologi… ▽ More Photon loss and dephasing rapidly degrade the sensitivity of quantum sensors, yet systematic methods for designing error-correcting codes whose geometry is simultaneously adapted to the sensing task and the noise channel do not exist. Here we establish that orbital-angular-momentum (OAM) encoding and Gottesman-Kitaev-Preskill (GKP) lattice geometry are structurally coupled: an OAM mode of topological charge $\ell$ induces a phase-space rotation $θ_\ell=\ellπ/\ell_{\max}$, corresponding to a family of twisted GKP stabilizer lattices. Using an end-to-end differentiable Strawberry Fields--TensorFlow circuit, we jointly optimise $\ell$, the lattice aspect ratio $r$, and the finite-energy envelope $ε$ to maximise quantum Fisher information subject to $P_{\rm err}\leq10^{-3}$. The optimum occurs at the fractional charge $\ell=1.5$ ($θ=67.5^\circ$), implementable with a half-integer spiral phase plate, which reduces $P_{\rm err}$ by $23.9\times$ relative to the square-lattice baseline while leaving $\mathcal{F}_Q$ unchanged to within $0.2\%$. This surpasses the best integer value ($\ell=2$, $15.7\times$) and arises from an exact $180^\circ$ periodicity of the $P_{\rm err}(θ)$ landscape, confirmed analytically and numerically. We derive a transcendental balance equation for the optimal angle $θ^*(η,γ,r)$ and prove that it decreases with both $γ$ and $η$. A Shannon-inspired metrological capacity $\mathcal{C}=\mathcal{F}_Q\cdot(-\ln P_{\rm err})$, maximised at $\ell=1.5$ with a $41\%$ gain over the square lattice, quantifies the joint sensitivity--fault-tolerance resource. These results establish a geometric design principle for noise-adaptive quantum sensors and a fully open-source differentiable template extensible to other bosonic code families. △ Less

Submitted 14 May, 2026; v1 submitted 13 May, 2026; originally announced May 2026.

Comments: 21 pages (18 main + 3 appendix), 16 figures, 14 tables, and 42 references. Code available at https://github.com/simanshukumar369/oam-gkp-quantum-metrology (Zenodo: doi:10.5281/zenodo.20099263)

MSC Class: 81P40 81P68 81P70 78A60 65K10 ACM Class: J.2

Memory-assisted multimode microwave-to-optical transduction

Authors: Ujjwal Gautam, Nasser Gohari Kamel, Sourabh Kumar, Daniel Oblak

Abstract: Microwave-to-optical quantum transducers will enable coherent interconnection between distant superconducting quantum devices. Ongoing explorations with several platforms have shown promising results at single-photon levels. However, in all these demonstrations, elimination of noise due to the concurrence of the weak transduced signal with intense pump pulses remains a challenge, requiring high su… ▽ More Microwave-to-optical quantum transducers will enable coherent interconnection between distant superconducting quantum devices. Ongoing explorations with several platforms have shown promising results at single-photon levels. However, in all these demonstrations, elimination of noise due to the concurrence of the weak transduced signal with intense pump pulses remains a challenge, requiring high suppression filtering setups. A memory-assisted transducer, on the other hand, offers a versatile approach that not only mitigates the noise but also enables the on-demand retrieval of the transduced signal. Here, we integrate a quantum memory protocol with transduction in a three-level atomic system to demonstrate on-demand retrieval of transduced signals. Due to the zero-first-order Zeeman transitions at zero magnetic fields, providing long optical and spin coherence times, and GHz range hyperfine splitting, we use a low-doping concentration $^{171}{\rm Yb}^{3+}$:${\rm Y}_2{\rm SiO}_5$ crystal at 30\,mK temperature. We achieve on-demand transduction assisted by memory with $0.4\ (\text{and }0.3)$ noise photons in the detection window at a storage duration of $460\ (\text{and }620) \, μ\textrm{s}$. To demonstrate the coherent nature of the protocol, we show interference patterns resulting from transduced signals due to varying phase or frequency of the input microwave pulses. Further, multimode transduction capacity is demonstrated, utilizing the spin and optical inhomogeneous broadening. The on-demand capability of the protocol allows synchronizing qubits in a quantum repeater protocol, while multimode capacity increases the entanglement generation rate. To the best of our knowledge, this is the first demonstration of an on-demand microwave-to-optical transducer assisted by memory. △ Less

Submitted 2 May, 2026; originally announced May 2026.

Comments: 22 pages, 19 figures

Quantum Gatekeeper: Multi-Factor Context-Bound Image Steganography with VQC Based Key Derivation on Quantum Hardware

Authors: Sahil Tomar, Sandeep Kumar

Abstract: This paper presents Quantum Gatekeeper, a context-bound image steganography framework where successful payload recovery depends on both cryptographic decryption and the reconstruction of a precise extraction path. The system integrates lossless least significant bit (LSB) embedding with a deterministic variational quantum circuit (VQC)-derived gate key, multi-factor contextual binding, and authent… ▽ More This paper presents Quantum Gatekeeper, a context-bound image steganography framework where successful payload recovery depends on both cryptographic decryption and the reconstruction of a precise extraction path. The system integrates lossless least significant bit (LSB) embedding with a deterministic variational quantum circuit (VQC)-derived gate key, multi-factor contextual binding, and authenticated encryption. Payload extraction is contingent upon four requisite factors: a password, a shared secret, a user-supplied context string, and a reference image signature. Any deviation in these factors causes the system to read from an incorrect pixel sequence or fail authentication, resulting in silent rejection rather than partial disclosure. The proposed method derives a gatecontrolled extraction key from a seed-conditioned variational circuit, with parameters generated via cryptographic hash expansion and context-dependent image features. To ensure encode/decode consistency, the cryptographic key path is generated via exact statevector simulation; concurrently, IBM superconducting quantum hardware is utilized to evaluate the statistical behavior of the circuit family under physical noise. We introduce a dual-region image layout to resolve the nonce bootstrapping dependency, separating header recovery from payload recovery through independently derived keys. Experimental results confirm successful end-to-end message embedding and recovery on PNG images, demonstrating deterministic success under correct conditions and failure otherwise. The framework supports both text and image payloads; in the image-in-image configuration, a secret image is resized to a fixed resolution prior to embedding, enabling exact pixel-level recovery under correct contextual reconstruction. △ Less

Submitted 29 April, 2026; originally announced April 2026.

Co-Designing Error Mitigation and Error Detection for Logical Qubits

Authors: Rohan S. Kumar, Takahiro Tsunoda, Sophia H. Xue, Dantong Li, Robert J. Schoelkopf, Yongshan Ding

Abstract: Near-term quantum workloads demand error management, yet the two lightest-weight techniques, Quantum Error Detection (QED) and Probabilistic Error Cancellation (PEC), have complementary cost profiles whose joint architectural design space remains unexplored. QED encodes logical qubits and discards error-flagged runs, filtering noise with low qubit overhead but leaving residual errors; PEC can corr… ▽ More Near-term quantum workloads demand error management, yet the two lightest-weight techniques, Quantum Error Detection (QED) and Probabilistic Error Cancellation (PEC), have complementary cost profiles whose joint architectural design space remains unexplored. QED encodes logical qubits and discards error-flagged runs, filtering noise with low qubit overhead but leaving residual errors; PEC can correct these in software, but at exponential cost in noise strength. If QED efficiently reduces per-gate noise, PEC's cost savings can outweigh QED's discard overhead; realizing this, however, requires solving two system-level design challenges. First, the \textit{QED interval} -- how often detection cycles are inserted -- is a tunable architectural parameter governing the cost-accuracy tradeoff. We derive an efficiency condition and show that the canonical one-cycle-per-gate frequency does not achieve break-even in any code we evaluate, while optimized intervals on high-rate Iceberg codes do. Second, we discover that naive PEC+QED integration \textit{degrades} accuracy below the QED-only baseline. The root cause is a transient error profile in the first detection cycle that corrupts PEC's noise model. We develop \textit{steady-state extraction}, a co-designed characterization protocol that isolates steady-state error behavior, reducing estimation bias by up to $10.2\times$. On a $[[6,4,2]]$ Iceberg code running QAOA ($p{=}4$--$8$) with a fixed shot budget, PEC+QED achieves $2$--$11\times$ lower absolute error and up to $31\times$ lower MSE versus PEC on physical qubits, with per-interval savings compounding over interval depth. △ Less

Submitted 21 April, 2026; originally announced April 2026.

Comments: 15 pages, 9 figures, 2 tables

Unitary Quadratic Quantum Gravity in 4D

Authors: K. Sravan Kumar, João Marto

Abstract: In quadratic gravity, with a positive Weyl squared coefficient, the extra spin-2 sector is shown to correspond to a dual inverted harmonic oscillator, instead of a ghost. Using the Wightman spectrum condition, we prove that the associated Källén--Lehmann spectral density vanishes, reflecting the absence of a normalizable ground state and the spacelike nature of the propagator pole. This uniquely f… ▽ More In quadratic gravity, with a positive Weyl squared coefficient, the extra spin-2 sector is shown to correspond to a dual inverted harmonic oscillator, instead of a ghost. Using the Wightman spectrum condition, we prove that the associated Källén--Lehmann spectral density vanishes, reflecting the absence of a normalizable ground state and the spacelike nature of the propagator pole. This uniquely fixes the propagator to a principal value form as a theorem, not a prescription. The optical theorem is satisfied, the dual IHO spin-2 is not an asymptotic state, and gives only virtual contributions at all loop orders. As a result, unitarity is preserved consistently with renormalizability. △ Less

Submitted 7 July, 2026; v1 submitted 21 April, 2026; originally announced April 2026.

Comments: 9 pages, 2 figures. Minor typos corrected and extended discussion on unitarity and renormalizability added

Quantum-Enhanced Single-Parameter Phase Estimation with Adaptive NOON States

Authors: Simanshu Kumar, Nandan S Bisht

Abstract: Quantum metrology promises phase sensitivity surpassing the shot-noise limit by exploiting entanglement and photon-number correlations. NOON states-maximally path-entangled $N$-photon superpositions $(|N,0\rangle + |0,N\rangle)/\sqrt{2}$ -achieve the Heisenberg limit $1/N$ for single-parameter estimation, as demonstrated experimentally by Afek et al. (2010) using hybrid coherent-plus-squeezed ligh… ▽ More Quantum metrology promises phase sensitivity surpassing the shot-noise limit by exploiting entanglement and photon-number correlations. NOON states-maximally path-entangled $N$-photon superpositions $(|N,0\rangle + |0,N\rangle)/\sqrt{2}$ -achieve the Heisenberg limit $1/N$ for single-parameter estimation, as demonstrated experimentally by Afek et al. (2010) using hybrid coherent-plus-squeezed light up to $N=5$. We present an end-to-end differentiable quantum-optical framework-implemented in Strawberry Fields (Killoran et al., 2019) with a TensorFlow backend -that learns optimal circuit parameters by maximising the classical Fisher information (CFI) across all coincidence channels for $N=2,3,4,5$. Starting from proper numerical reproductions of the Afek et al. coincidence fringes, verified by FFT analysis and parity measurements, we apply gradient descent (Adam) to the eight trainable circuit parameters. Raw CFI improvements grow dramatically with photon number: $+153\%$ ($N=2$), $+834\%$ to $+956\%$ ($N=3$), $+829\%$ to $+1598\%$ ($N=4$), and $+1775\%$ ($N=5$), alongside post-selection rate improvements of $+153\%$ to $+3269\%$, and an $8\times$ to $133\times$ improvement in useful measurement events per pulse across $N=2$-$5$. A fundamental inter-channel trade-off is identified at $N=2$ but weakens at higher $N$ where the Afek initialisation is further from optimal. These results provide numerically rigorous benchmarks for adaptive single-parameter quantum sensing and demonstrate that the Afek working point is significantly suboptimal at $N\geq 3$. QFI calculations confirm that the optimised probe reaches $82\%$ of the Heisenberg limit at $N=2$ and improves from $36\%$ to $58\%$ at $N=5$, while useful measurement events per pulse improve by $8\times$ to $133\times$ across all $N$, making quantum-enhanced sensing at $N\geq 3$ experimentally practical. △ Less

Submitted 18 May, 2026; v1 submitted 14 April, 2026; originally announced April 2026.

Comments: v4: Corrected DOIs for refs 8, 14, 18, 21, 26; reordered bibliography to strict first-citation order; removed duplicate sentence from Acknowledgements; minor figure placement adjustment., added Zenodo DOI (10.5281/zenodo.20041907) and GitHub repository (https://github.com/simanshukumar369/noon-state-adaptive-metrology) to Data Availability

MSC Class: 81P45; 81P15; 78A60; 62F15 ACM Class: J.2

Spin qubit gates via phonon buses in electron nanowires

Authors: Dylan Lewis, Roopayan Ghosh, Sanjeev Kumar, Michael Pepper, Charles Smith, Karyn Le Hur, Sougato Bose

Abstract: Scalable architectures for quantum computing using semiconductor quantum dots require interactions between qubits beyond adjacent quantum dots. Here, we propose using nanowires of electrons to mediate the interaction between two quantum dots. Virtual phonons in the linear chain of electrons can mediate an interaction that gives rise to effective spin-spin coupling of the electrons in distant quant… ▽ More Scalable architectures for quantum computing using semiconductor quantum dots require interactions between qubits beyond adjacent quantum dots. Here, we propose using nanowires of electrons to mediate the interaction between two quantum dots. Virtual phonons in the linear chain of electrons can mediate an interaction that gives rise to effective spin-spin coupling of the electrons in distant quantum dots. We find coupling strengths of more than 30 MHz for experimentally realisable parameters in GaAs quantum dots. △ Less

Submitted 13 March, 2026; originally announced March 2026.

Comments: 6 + 9 pages, 5 figures

Integrated nanophotonic platform for on-chip quantum emitter interactions and entanglement

Authors: Yinhui Kan, Shailesh Kumar, Xujing Liu, Antonio I. Fernández-Domínguez, Sergey I. Bozhevolnyi

Abstract: Entanglement between solid-state quantum emitters (QEs) is a key resource for photonic quantum technologies. Achieving such entanglement requires strong and controllable long-range interactions between QEs. However, engineering such coupling remains challenging, particularly for on-chip distant solid-state QEs. Here, we introduce a forward-designed platform that enables ultracompact nanophotonic a… ▽ More Entanglement between solid-state quantum emitters (QEs) is a key resource for photonic quantum technologies. Achieving such entanglement requires strong and controllable long-range interactions between QEs. However, engineering such coupling remains challenging, particularly for on-chip distant solid-state QEs. Here, we introduce a forward-designed platform that enables ultracompact nanophotonic architectures to mediate enhanced long-range QE-QE interactions via engineered surface plasmon polariton interference. Using this strategy, we realize two distinct configurations: a phase-conjugated elliptic design for energy funneling, and a co-radiating hyperbolic design for its suppression. We experimentally demonstrate large enhancement and suppression of energy transfer rates compared to bare substrates. Furthermore, we predict transient entanglement between spatially separated QEs with concurrence peaking at 0.493, approaching the theoretical bound in the transient regime. Extending to the multi-QE case, we observe enhanced energy funneling and predict QE-QE entanglement in three-QE configurations. These results establish a compact and scalable framework for on-chip entanglement engineering in integrated quantum nanophotonic systems. △ Less

Submitted 27 February, 2026; originally announced February 2026.

Comments: 45 pages, 16 figures

Self-correction phase transition in the dissipative toric code

Authors: Sanjeev Kumar, Hendrik Weimer

Abstract: We analyze a time-continuous version of a cellular automaton decoder for the toric code in the form of a Lindblad master equation. In this setting, a self-correcting quantum memory becomes a thermodynamical phase of the steady state, which manifests itself through the steady state being topologically ordered. We compute the steady state phase diagram, finding a competition between the error correc… ▽ More We analyze a time-continuous version of a cellular automaton decoder for the toric code in the form of a Lindblad master equation. In this setting, a self-correcting quantum memory becomes a thermodynamical phase of the steady state, which manifests itself through the steady state being topologically ordered. We compute the steady state phase diagram, finding a competition between the error correction rate and the update rate for the classical field of the cellular automaton. Strikingly, we find that self-correction of errors is possible even in situations where conventional quantum error correction does not have a finite threshold. △ Less

Submitted 22 February, 2026; originally announced February 2026.

Comments: 6 pages, 5 figures

Phonon-enhanced strain sensitivity of quantum dots in two-dimensional semiconductors

Authors: Sumitra Shit, Yunus Waheed, Jithin Thoppil Surendran, Indrajeet Dhananjay Prasad, Kenji Watanabe, Takashi Taniguchi, Santosh Kumar

Abstract: Two-dimensional semiconductors have attracted considerable interest for integration into emerging quantum photonic networks. Strain engineering of monolayer transition-metal dichalcogenides (ML-TMDs) enables the tuning of light-matter interactions and associated optoelectronic properties, and generates new functionalities, including the formation of quantum dots (QDs). Here, we combine spatially r… ▽ More Two-dimensional semiconductors have attracted considerable interest for integration into emerging quantum photonic networks. Strain engineering of monolayer transition-metal dichalcogenides (ML-TMDs) enables the tuning of light-matter interactions and associated optoelectronic properties, and generates new functionalities, including the formation of quantum dots (QDs). Here, we combine spatially resolved micro-photoluminescence ($μ$-PL) spectroscopy from cryogenic (4$\text{-}$94 K) to room temperature with micro-Raman spectroscopy at room temperature to investigate the strain-dependent emission energies of thousands of individual QDs in ML-WS$_2$ and ML-WSe$_2$, integrated across multiple heterostructures and a piezoelectric device. Compared with delocalized excitons, QDs in both materials exhibit enhanced strain sensitivities of their emission energies $-$ approximately fourfold in WS$_2$ and twofold in WSe$_2$ $-$ leading to pronounced broadening of the ensemble emission linewidth. Temperature-dependent $μ$-PL spectroscopy combined with dynamic strain tuning experiments further reveal that the enhanced strain sensitivity of individual QDs originates from strengthened interactions with low-energy phonons induced by quantum confinement. Our results demonstrate a versatile strain-engineering approach with potential for spectral matching across solid-state, atomic, and hybrid quantum photonic networks, and provide new insights into phonon-QD interactions in two-dimensional semiconductors. △ Less

Submitted 19 February, 2026; originally announced February 2026.

Comments: 17 pages, 5 figures, methods, supporting information

Chip-integrated metasurface-enabled single-photon skyrmion sources

Authors: Xujing Liu, Yinhui Kan, Shailesh Kumar, Liudmilla F. Kulikova, Valery A. Davydov, Viatcheslav N. Agafonov, Sergey I. Bozhevolnyi

Abstract: Skyrmions, topologically stable field configurations, have recently emerged in classical optics as structured light for high-density data applications. Achieving controllable on-chip generation of single-photon skyrmions, while being highly desirable for quantum information technologies, remains challenging due to the nanoscale confinement of quantum emitters (QEs). Here we demonstrate a metasurfa… ▽ More Skyrmions, topologically stable field configurations, have recently emerged in classical optics as structured light for high-density data applications. Achieving controllable on-chip generation of single-photon skyrmions, while being highly desirable for quantum information technologies, remains challenging due to the nanoscale confinement of quantum emitters (QEs). Here we demonstrate a metasurface-integrated quantum emitter (metaQE) platform enabling room-temperature on-chip generation of single-photon skyrmions. Near-field coupling between QEs and propitiously designed surface arrays of meta-atoms mediates spin-orbit interaction, transforming nanoscale-localized dipole emission into free-propagating topologically structured photonic modes. By exploiting this approach for structuring quantum emission from different color centers in nanodiamonds, we realize diverse skyrmionic states, including high-order anti-skyrmions and skyrmionium, and thereby demonstrate its universality across QEs. Our work establishes a unified framework for on-chip structured quantum light sources, offering versatile control of high-dimensional topological states, such as skyrmions, and advancing scalable quantum photonic technologies. △ Less

Submitted 10 January, 2026; originally announced January 2026.

Comments: 24 pages, 11 figures

Hilbert space signatures of non-ergodic glassy dynamics

Authors: Aleksey Lunkin, Nicole S. Ticea, Shashwat Kumar, Connie Miao, Jaehong Choi, Mohammed Alghadeer, Ilya Drozdov, Dmitry Abanin, Amira Abbas, Rajeev Acharya, Laleh Beni, Georg Aigeldinger, Ross Alcaraz, Sayra Alcaraz, Markus Ansmann, Frank Arute, Kunal Arya, Walt Askew, Nikita Astrakhantsev, Juan Atalaya, Ryan Babbush, Brian Ballard, Joseph C. Bardin, Hector Bates, Andreas Bengtsson , et al. (270 additional authors not shown)

Abstract: Disorder in quantum many-body systems can drive transitions between ergodic and non-ergodic phases, yet the nature--and even the existence--of these transitions remains intensely debated. Using a two-dimensional array of superconducting qubits, we study an interacting spin model at finite temperature in a disordered landscape, tracking dynamics both in real space and in Hilbert space. Over a broad… ▽ More Disorder in quantum many-body systems can drive transitions between ergodic and non-ergodic phases, yet the nature--and even the existence--of these transitions remains intensely debated. Using a two-dimensional array of superconducting qubits, we study an interacting spin model at finite temperature in a disordered landscape, tracking dynamics both in real space and in Hilbert space. Over a broad disorder range, we observe an intermediate non-ergodic regime with glass-like characteristics: physical observables become broadly distributed and some, but not all, degrees of freedom are effectively frozen. The Hilbert-space return probability shows slow power-law decay, consistent with finite-temperature quantum glassiness. In the same regime, we detect the onset of a finite Edwards-Anderson order parameter and the disappearance of spin diffusion. By contrast, at lower disorder, spin transport persists with a nonzero diffusion coefficient. Our results show that there is a transition out of the ergodic phase in two-dimensional systems. △ Less

Submitted 15 April, 2026; v1 submitted 3 January, 2026; originally announced January 2026.

Constant Depth Digital-Analog Counterdiabatic Quantum Computing

Authors: Balaganchi A. Bhargava, Shubham Kumar, Anne-Maria Visuri, Paolo A. Erdman, Enrique Solano, Narendra N. Hegade

Abstract: We introduce a digital-analog quantum computing framework that enables counterdiabatic protocols to be implemented at constant circuit depth, allowing fast and resource-efficient quantum state preparation on current quantum hardware. Counterdiabatic protocols suppress diabatic excitations in finite-time adiabatic evolution, but their practical application is limited by the non-local structure of t… ▽ More We introduce a digital-analog quantum computing framework that enables counterdiabatic protocols to be implemented at constant circuit depth, allowing fast and resource-efficient quantum state preparation on current quantum hardware. Counterdiabatic protocols suppress diabatic excitations in finite-time adiabatic evolution, but their practical application is limited by the non-local structure of the required Hamiltonians and the resource overhead of fully digital implementations. Counterdiabatic terms can be expressed as truncated expansions of nested commutators of the adiabatic Hamiltonian and its parametric derivative. Here, we show how this algebraic structure can be efficiently realized in a digital-analog setting using commutator product formulas. Using native multi-qubit analog interactions augmented by local single-qubit rotations, this approach enables higher-order counterdiabatic protocols whose implementation requires a constant number of analog blocks for any fixed truncation order, independent of system size. We demonstrate the method for two-dimensional spin models and analyze the associated approximation errors. These results show that digital-analog quantum computing enables a qualitatively new resource scaling for counterdiabatic protocols and related quantum control primitives, with direct implications for quantum simulation, optimization, and algorithmic state preparation on current quantum devices. △ Less

Submitted 3 January, 2026; originally announced January 2026.

Operational entanglement of collective quantum modes at room temperature

Authors: Shalender Singh, Santosh Kumar

Abstract: Quantum entanglement is commonly assumed to be fragile at ambient temperature and over macroscopic distances, where thermal noise and dissipation are expected to rapidly suppress nonclassical correlations. Here we show that this intuition fails for collective quantum modes whose dynamics is governed by reduced open-system channels rather than by microscopic thermal equilibrium. For two spatially s… ▽ More Quantum entanglement is commonly assumed to be fragile at ambient temperature and over macroscopic distances, where thermal noise and dissipation are expected to rapidly suppress nonclassical correlations. Here we show that this intuition fails for collective quantum modes whose dynamics is governed by reduced open-system channels rather than by microscopic thermal equilibrium. For two spatially separated collective modes, we derive an exact entanglement boundary based on the positivity of the partial transpose, valid in the symmetric resonant limit. From this result we obtain an explicit minimum collective fluctuation amplitude, expressed entirely in measurable noise, bandwidth, dissipation, and distance-dependent coupling parameters, required to sustain steady-state entanglement at finite temperature. We further show that large collective occupation suppresses but does not eliminate quantum phase diffusion, so the steady state remains phase symmetric and does not collapse to a classical mean-field despite macroscopic signal amplitudes. Stochastic simulations of the reduced open-system dynamics, together with matched classical correlated-noise null models analyzed through an identical pipeline, confirm that entanglement witnesses are violated only in the quantum regime. Our results establish a minimal, platform-independent framework connecting collective-mode dynamics, noise injection, distance, and operational certification of macroscopic entanglement. △ Less

Submitted 27 December, 2025; originally announced December 2025.

Comments: 19 pages, 4 figures

Partial Collapse and Ensemble Invariance under Continuous Quantum Measurement

Authors: Shalender Singh, Santosh Kumar

Abstract: Wavefunction collapse is commonly associated with unavoidable physical disturbance of the measured system. Here we show that in driven-dissipative quantum systems, continuous measurement can induce strong trajectory-level collapse while leaving the ensemble-averaged steady state strictly invariant. We identify measurement-invariant steady states whose unconditional density matrix remains unchanged… ▽ More Wavefunction collapse is commonly associated with unavoidable physical disturbance of the measured system. Here we show that in driven-dissipative quantum systems, continuous measurement can induce strong trajectory-level collapse while leaving the ensemble-averaged steady state strictly invariant. We identify measurement-invariant steady states whose unconditional density matrix remains unchanged under continuous monitoring, despite pronounced measurement-induced localization in conditioned quantum trajectories. This separation between trajectory-level collapse and ensemble invariance defines a regime of partial collapse, in which measurement-induced localization is continuously counteracted by dissipative dynamics. We derive a necessary and sufficient condition for steady-state invariance under continuous measurement and identify Liouvillian symmetry as a concrete dynamical mechanism enforcing it. Our results clarify the distinction between conditional collapse and physical disturbance in open quantum systems and provide a framework for non-invasive continuous monitoring in driven-dissipative settings. △ Less

Submitted 10 January, 2026; v1 submitted 23 December, 2025; originally announced December 2025.

Comments: 24 pages, 2 figures

Bell-Inequality Violation for Continuous, Non-Projective Measurements

Authors: Shalender Singh, Santosh Kumar

Abstract: Many solid-state quantum platforms do not permit sharp, projective measurements but instead yield continuous voltage or field traces under weak, non-demolition readout. In such systems, standard Bell tests based on dichotomic projective measurements are not directly applicable, raising the question of how quantum nonlocality can be certified from continuous time-series data. Here we develop a gene… ▽ More Many solid-state quantum platforms do not permit sharp, projective measurements but instead yield continuous voltage or field traces under weak, non-demolition readout. In such systems, standard Bell tests based on dichotomic projective measurements are not directly applicable, raising the question of how quantum nonlocality can be certified from continuous time-series data. Here we develop a general theoretical framework showing that Bell-CHSH inequality violation can be extracted from continuous, non-projective measurements without assuming any specific collapse model or phase distribution. We show that sufficiently long continuous measurements of a single entangled pair sample its internal phase-probability structure, enabling effective dichotomic observables to be constructed through phase-sensitive projections and coarse-graining. The resulting Bell correlator is governed by two experimentally accessible resources: intrinsic single-qubit phase spread and nonlocal phase locking between qubits. We benchmark the resulting estimator against conventional projective-measurement CHSH tests implemented via quantum-circuit simulations using Qiskit, finding quantitative agreement in the Bell-violating regime without parameter fitting. Classical deterministic correlations cannot violate the CHSH bound, whereas quantum phase-locked systems recover the nonlinear angular dependence characteristic of entanglement. Our results provide a practical route to demonstrating Bell nonlocality in platforms where measurements are inherently continuous and weak. △ Less

Submitted 22 December, 2025; originally announced December 2025.

Comments: 25 pages, 1 figure

Journal ref: Sci. Rep. 16, 51406 (2026)

Observation of disorder-induced superfluidity

Authors: Nicole Ticea, Elias Portoles, Eliott Rosenberg, Alexander Schuckert, Aaron Szasz, Bryce Kobrin, Nicolas Pomata, Pranjal Praneel, Connie Miao, Shashwat Kumar, Ella Crane, Ilya Drozdov, Yuri Lensky, Sofia Gonzalez-Garcia, Thomas Kiely, Dmitry Abanin, Amira Abbas, Rajeev Acharya, Laleh Aghababaie Beni, Georg Aigeldinger, Ross Alcaraz, Sayra Alcaraz, Markus Ansmann, Frank Arute, Kunal Arya , et al. (277 additional authors not shown)

Abstract: The emergence of states with long-range correlations in a disordered landscape is rare, as disorder typically suppresses the particle mobility required for long-range coherence. But when more than two energy levels are available per site, disorder can induce resonances that locally enhance mobility. Here we explore phases arising from the interplay between disorder, kinetic energy, and interaction… ▽ More The emergence of states with long-range correlations in a disordered landscape is rare, as disorder typically suppresses the particle mobility required for long-range coherence. But when more than two energy levels are available per site, disorder can induce resonances that locally enhance mobility. Here we explore phases arising from the interplay between disorder, kinetic energy, and interactions on a superconducting processor with qutrit readout and control. Compressibility measurements distinguish an incompressible Mott insulator from surrounding compressible phases and reveal signatures of glassiness, reflected in non-ergodic behavior. Spatially-resolved two-point correlator measurements identify regions of the phase diagram with a non-vanishing condensate fraction. We also visualize the spectrum by measuring the dynamical structure factor. A linearly-dispersing phonon mode materializes in the superfluid, appearing even when disorder is introduced to the clean Mott insulator. Our results provide strong experimental evidence for disorder-induced superfluidity. △ Less

Submitted 3 February, 2026; v1 submitted 24 December, 2025; originally announced December 2025.

Comments: Supplement updated

Attosecond Control of Squeezed Light

Authors: Russell Zimmerman, Shashank Kumar, Shiva Kant Tiwari, Eric Liu, Francis Walz, Siddhant Pandey, George J. Economou II, Hadiseh Alaeian, Chen-Ting Liao, Valentin Walther, Niranjan Shivaram

Abstract: Squeezed light has revolutionized quantum metrology by enhancing interferometry for sensitive applications such as the detection of gravitational waves. Squeezed light has also played a pivotal role in quantum information science with numerous applications in quantum computing and communication. Previously, squeezed light has been primarily generated using nonlinear optical interactions, where con… ▽ More Squeezed light has revolutionized quantum metrology by enhancing interferometry for sensitive applications such as the detection of gravitational waves. Squeezed light has also played a pivotal role in quantum information science with numerous applications in quantum computing and communication. Previously, squeezed light has been primarily generated using nonlinear optical interactions, where control of the degree of squeezing was possible by tuning the nonlinearity of the generating medium using suitable material engineering. Here, we modulate the third-order nonlinear response in dielectrics with strong ultrafast laser fields to control the degree of squeezing on attosecond time scales. We demonstrate the ability to change the ultrafast squeezed light generated in the nonlinear process from amplitude-squeezed to phase-squeezed by controlling the strong-field-driven nonlinear response of the material through a sub-cycle phase delay between the input femtosecond laser pulses. The squeezing of quantum noise is measured using a frequency-resolved balanced homodyne detection scheme capable of extracting the field quadratures in different frequency modes simultaneously. Using this frequency-resolved measurement we extract the complete coherency matrix containing the quantum correlations between field quadratures across different frequency modes of the femtosecond squeezed light pulse. These results have major implications for the development of quantum light sources with unprecedented levels of control over quadrature squeezing, for applications in multimode quantum information processing, and for measuring transient quantum matter correlations via transduction to quantum field correlations in an ultrafast light-matter interaction. △ Less

Submitted 18 December, 2025; originally announced December 2025.

Comments: 13 pages, 6 figures in the main text. 3 pages and 3 figures in the supplementary material

Exchange Symmetry in Multiphoton Quantum Interference

Authors: Shreya Kumar, Alex E Jones, Daniel Bhatti, Stefanie Barz

Abstract: Photons are bosons, and yet, when prepared in specific entangled states, they can exhibit non-bosonic behaviour. While this phenomenon has so far been studied in two-photon systems, exchange symmetries and interference effects in multi-photon scenarios remain largely unexplored. In this work, we show that multi-photon states uncover a rich landscape of exchange symmetries. With three photons alrea… ▽ More Photons are bosons, and yet, when prepared in specific entangled states, they can exhibit non-bosonic behaviour. While this phenomenon has so far been studied in two-photon systems, exchange symmetries and interference effects in multi-photon scenarios remain largely unexplored. In this work, we show that multi-photon states uncover a rich landscape of exchange symmetries. With three photons already, multiple pairwise combinations are possible, where each pair of photons can exhibit either bosonic, fermionic, or anyonic exchange symmetry. This gives rise to mixed symmetry systems that are not possible to achieve with two photon alone. We experimentally investigate how these symmetry configurations manifest themselves in the observed interference of three photons. We show that multi-photon interference can be effectively turned on and off by tuning the symmetry of the constituent pairs. The possibility of accessing and tuning new quantum statistics in a scalable photonic platform not only deepens our understanding of quantum systems, but is also highly relevant for quantum technologies that rely on quantum interference. △ Less

Submitted 8 December, 2025; originally announced December 2025.

Comments: 14 pages, 5 figures

Quantum-Classical Separation in Bounded-Resource Tasks Arising from Measurement Contextuality

Authors: Shashwat Kumar, Eliott Rosenberg, Alejandro Grajales Dau, Rodrigo Cortinas, Dmitri Maslov, Richard Oliver, Adam Zalcman, Matthew Neeley, Alice Pagano, Aaron Szasz, Ilya Drozdov, Zlatko Minev, Craig Gidney, Noureldin Yosri, Stijn J. de Graaf, Aniket Maiti, Dmitry Abanin, Rajeev Acharya, Laleh Aghababaie Beni, Georg Aigeldinger, Ross Alcaraz, Sayra Alcaraz, Trond I. Andersen, Markus Ansmann, Frank Arute , et al. (258 additional authors not shown)

Abstract: The prevailing view is that quantum phenomena can be harnessed to tackle certain problems beyond the reach of classical approaches. Quantifying this capability as a quantum-classical separation and demonstrating it on current quantum processors has remained elusive. Using a superconducting qubit processor, we show that quantum contextuality enables certain tasks to be performed with success probab… ▽ More The prevailing view is that quantum phenomena can be harnessed to tackle certain problems beyond the reach of classical approaches. Quantifying this capability as a quantum-classical separation and demonstrating it on current quantum processors has remained elusive. Using a superconducting qubit processor, we show that quantum contextuality enables certain tasks to be performed with success probabilities beyond classical limits. With a few qubits, we illustrate quantum contextuality with the magic square game, as well as quantify it through a Kochen--Specker--Bell inequality violation. To examine many-body contextuality, we implement the N-player GHZ game and separately solve a 2D hidden linear function problem, exceeding classical success rate in both. Our work proposes novel ways to benchmark quantum processors using contextuality-based algorithms. △ Less

Submitted 1 December, 2025; originally announced December 2025.

Performance Analysis of One- and Two-way DV-QKD with MIMO FSO Communication Systems

Authors: Sushil Kumar, Soumya P. Dash, George C. Alexandropoulos

Abstract: This paper considers a multiple-input multiple-output (MIMO) wireless system wherein two legitimate users attempt to exchange secret keys over free-space optical (FSO) channels. Novel frameworks for the use of the one- and two-way discrete-variable quantum key distribution (DV-QKD) protocols, employing weak coherent pulses and decoy states, are presented. Focusing on the case where a photon-number… ▽ More This paper considers a multiple-input multiple-output (MIMO) wireless system wherein two legitimate users attempt to exchange secret keys over free-space optical (FSO) channels. Novel frameworks for the use of the one- and two-way discrete-variable quantum key distribution (DV-QKD) protocols, employing weak coherent pulses and decoy states, are presented. Focusing on the case where a photon-number-splitting attack is adopted by the eavesdropper and the legitimate multi-antenna receiver using threshold detection for the key extraction, novel expressions for the secret key rate and quantum bit error rate for both one- and two-way protocols are derived. The performance gain with larger MIMO configurations and the tradeoff between the performances with the one- and the two-way protocols with respect to the transmission distance of the legitimate FSO link are numerically assessed. △ Less

Submitted 7 November, 2025; originally announced November 2025.

Comments: 6 pages, 3 figures

Dynamical Phase Transitions Across Slow and Fast Regimes in a Two-Tone Driven Duffing Resonator

Authors: Soumya S. Kumar, Javier del Pino, Letizia Catalini, Alexander Eichler, Oded Zilberberg

Abstract: The response of nonlinear resonators to multifrequency driving reveals rich dynamics beyond conventional single-tone theory. We study a Duffing resonator under bichromatic excitation and identify a competition between the two drives, governed by their detuning and relative amplitudes. In the slow-beating regime, where the tones are closely spaced, the secondary drive acts as a modulation that indu… ▽ More The response of nonlinear resonators to multifrequency driving reveals rich dynamics beyond conventional single-tone theory. We study a Duffing resonator under bichromatic excitation and identify a competition between the two drives, governed by their detuning and relative amplitudes. In the slow-beating regime, where the tones are closely spaced, the secondary drive acts as a modulation that induces dynamical phase transitions between coexisting stationary states. We introduce the cycle-averaged amplitude as an order parameter and map the resulting phase diagram as a function of the drive detuning and amplitude ratio, capturing the pronounced asymmetry observed for blue versus red detuning in experiment. We devise a model to link the onset of these transitions to the resonance properties around the nonlinear stationary mode of the system. Our results provide a framework for controlling driven nonlinear systems, enabling state manipulation, and sensing in nanomechanical, optical, and superconducting circuit platforms. △ Less

Submitted 16 March, 2026; v1 submitted 3 November, 2025; originally announced November 2025.

Comments: 15 pages, 13 figures, 1 table

Digitized Counterdiabatic Quantum Sampling

Authors: Narendra N. Hegade, Nachiket L. Kortikar, Balaganchi A. Bhargava, Juan F. R. Hernández, Alejandro Gomez Cadavid, Pranav Chandarana, Sebastián V. Romero, Shubham Kumar, Anton Simen, Anne-Maria Visuri, Enrique Solano, Paolo A. Erdman

Abstract: We propose digitized counterdiabatic quantum sampling (DCQS), a hybrid quantum-classical algorithm for efficient sampling from energy-based models, such as low-temperature Boltzmann distributions. The method utilizes counterdiabatic protocols, which suppress non-adiabatic transitions, with an iterative bias-field procedure that progressively steers the sampling toward low-energy regions. We observ… ▽ More We propose digitized counterdiabatic quantum sampling (DCQS), a hybrid quantum-classical algorithm for efficient sampling from energy-based models, such as low-temperature Boltzmann distributions. The method utilizes counterdiabatic protocols, which suppress non-adiabatic transitions, with an iterative bias-field procedure that progressively steers the sampling toward low-energy regions. We observe that the samples obtained at each iteration correspond to approximate Boltzmann distributions at effective temperatures. By aggregating these samples and applying classical reweighting, the method reconstructs the Boltzmann distribution at a desired temperature. We define a scalable performance metric, based on the Kullback-Leibler divergence and the total variation distance, to quantify convergence toward the exact Boltzmann distribution. DCQS is validated on one-dimensional Ising models with random couplings up to 124 qubits, where exact results are available through transfer-matrix methods. We then apply it to a higher-order spin-glass Hamiltonian with 156 qubits executed on IBM quantum processors. We show that classical sampling algorithms, including Metropolis-Hastings and the state-of-the-art low-temperature technique parallel tempering, require up to three orders of magnitude more samples to match the quality of DCQS, corresponding to an approximately 2x runtime advantage. Boltzmann sampling underlies applications ranging from statistical physics to machine learning, yet classical algorithms exhibit exponentially slow convergence at low temperatures. Our results thus demonstrate a robust route toward scalable and efficient Boltzmann sampling on current quantum processors. △ Less

Submitted 30 October, 2025; originally announced October 2025.

Comments: 18 pages, 15 figures

Quantum Combinatorial Reasoning for Large Language Models

Authors: Carlos Flores-Garrigos, Gaurav Dev, Michael Falkenthal, Alejandro Gomez Cadavid, Anton Simen, Shubham Kumar, Enrique Solano, Narendra N. Hegade

Abstract: We design and implement a quantum combinatorial reasoning framework for large language models (QCR-LLM), integrating a real quantum computer in the hybrid workflow. QCR-LLM reformulates reasoning aggregation as a higher-order unconstrained binary optimization (HUBO) problem. In this sense, reasoning fragments are represented as binary variables and their interactions encode statistical relevance,… ▽ More We design and implement a quantum combinatorial reasoning framework for large language models (QCR-LLM), integrating a real quantum computer in the hybrid workflow. QCR-LLM reformulates reasoning aggregation as a higher-order unconstrained binary optimization (HUBO) problem. In this sense, reasoning fragments are represented as binary variables and their interactions encode statistical relevance, logical coherence, and semantic redundancy. We tackle the resulting high-order optimization problem both classically, via simulated annealing, and quantumly through the bias-field digitized counterdiabatic quantum optimizer (BF-DCQO) executed on IBM's superconducting digital quantum processors. Experiments on BIG-Bench Extra Hard (BBEH) benchmarks demonstrate that our QCR-LLM consistently improves reasoning accuracy across multiple LLM backbones, surpassing reasoning-native systems such as o3-high and DeepSeek R1 by up to $+9\,$pp. Despite requiring multiple reasoning samples per query, our QCR-LLM remains approximately five times more energy-efficient than o3-high, owing to the low per-token energy footprint of its GPT-4o backbone. These results constitute the first experimental evidence of quantum-assisted reasoning, showing that hybrid quantum-classical optimization can efficiently enhance reasoning coherence, interpretability, and sustainability in large-scale language models. We have opened the doors to the emergence of quantum intelligence, where harder prompts require quantum optimizers at quantum-advantage level. △ Less

Submitted 28 October, 2025; originally announced October 2025.

Strong-field Driven Sub-cycle Band Structure Modulation and Dephasing Control

Authors: Francis Walz, Shashank Kumar, Amirali Sharifi Olounabadi, Yuyan Zhong, Russell Zimmerman, Siddhant Pandey, Eric Liu, Liang Z. Tan, Niranjan Shivaram

Abstract: Over the past decade, ultrafast electron dynamics in the solid state have been extensively studied using various strong light-matter interaction techniques, such as high-harmonic generation. These studies lead to multiple interpretations of light-matter interaction in the strong-field regime, with exact mechanisms not yet fully understood. It is well known that strong-field interaction with a crys… ▽ More Over the past decade, ultrafast electron dynamics in the solid state have been extensively studied using various strong light-matter interaction techniques, such as high-harmonic generation. These studies lead to multiple interpretations of light-matter interaction in the strong-field regime, with exact mechanisms not yet fully understood. It is well known that strong-field interaction with a crystalline solid leads to significant modification of its band structure and, hence, its optical properties on ultrafast timescales. In this work, we present measurements of ultrafast electric-field observables in magnesium oxide using a non-resonant nonlinear optical interaction. Using field observables, we show that strong laser fields modulate the band structure on sub-cycle timescales, thereby altering the material's nonlinear optical response. We perform time-dependent perturbation theory calculations using a field-dependent dispersion relation and non-perturbative semiconductor Bloch equation calculations, both of which agree with experimental observations. Furthermore, we directly extract dephasing times from the real-time signal electric field envelope and show sub-cycle control of dephasing times. Our work offers a new perspective on strong-field-driven electron dynamics in solids through electric-field observables. The demonstrated attosecond modulation of the nonlinear response could have important implications for quantum light generation and quantum spectroscopy using nonlinear optical processes. △ Less

Submitted 10 February, 2026; v1 submitted 18 October, 2025; originally announced October 2025.

Comments: 18 pages, 8 figures. This new version has a modified title and contains new discussion on dephasing time measurement and control. Concept figure updated and added 3 new figures

Crystal-Field--Driven Magnetoelectricity in the Triangular Quantum Magnet CeMgAl$_{11}$O$_{19}$

Authors: Sonu Kumar, Gaël Bastien, Maxim Savinov, Petr Proschek, Adam Eliáš, Karol Załęski, Małgorzata Śliwińska-Bartkowiak, Ross H. Colman, Stanislav Kamba

Abstract: We report dielectric and magnetoelectric studies of single-crystalline \ce{CeMgAl11O19}, a Kramers triangular magnet embedded in a polarizable hexaaluminate lattice. In zero magnetic field, the permittivity $\varepsilon'(T)$ follows the Barrett law of a quantum paraelectric down to 25 K, below which a broad minimum develops near 3 K without evidence of static ferroelectric or magnetic order. Appli… ▽ More We report dielectric and magnetoelectric studies of single-crystalline \ce{CeMgAl11O19}, a Kramers triangular magnet embedded in a polarizable hexaaluminate lattice. In zero magnetic field, the permittivity $\varepsilon'(T)$ follows the Barrett law of a quantum paraelectric down to 25 K, below which a broad minimum develops near 3 K without evidence of static ferroelectric or magnetic order. Application of magnetic fields up to \SI{9}{\tesla} shifts this minimum to higher temperatures and broadens it, evidencing a tunable magnetoelectric response.The magnetoelectric coupling was characterized using results from magnetization measurements. The anomaly temperature $T^*$, extracted from the local minimum of $\varepsilon'(T)$, exhibits a linear dependence on the squared magnetization $M^2$, consistent with the biquadratic magnetoelectric coupling allowed in centrosymmetric systems. This magnetoelectric effect, mediated by spin-orbit-entangled Kramers doublets interacting with a frustrated antipolar liquid, establishes \ce{CeMgAl11O19} as a prototype for exploring quantum magnetoelectricity in frustrated systems. △ Less

Submitted 19 December, 2025; v1 submitted 9 October, 2025; originally announced October 2025.

Comments: v2-preprint (metadata update only; v1-preprint), Licence: CC BY 4.0

Ultra-narrow homogeneous linewidths of erbium-doped silica glass fibers at millikelvin temperatures: magnetic field and temperature dependence

Authors: Farhad Rasekh, Nasser Gohari Kamel, Mahdi Bornadel, Sourabh Kumar, Erhan Saglamyurek, Christoph Simon, Daniel Oblak

Abstract: Erbium-doped solids are promising candidates for fiber-based quantum networks due to their emission wavelength, which aligns with the telecom band over which optical fibers exhibit minimal loss. Among these, erbium-doped silica fiber (EDF) stands out for its availability, ease of use, and seamless integration with existing fiber-optic infrastructure. In this work, using the two-pulse photon-echo (… ▽ More Erbium-doped solids are promising candidates for fiber-based quantum networks due to their emission wavelength, which aligns with the telecom band over which optical fibers exhibit minimal loss. Among these, erbium-doped silica fiber (EDF) stands out for its availability, ease of use, and seamless integration with existing fiber-optic infrastructure. In this work, using the two-pulse photon-echo (2PPE) technique, we measured the homogeneous linewidth of the 4I13/2 <-> 4I15/2 optical transition under varying magnetic fields and temperatures. We observed an effective homogeneous linewidth of approximately 8 kHz at approximately 7mK at an optimal magnetic field of 0.09 T, representing over two orders of magnitude improvement compared to earlier reports measured at T = 700 mK. We also present a comprehensive model for the combined magnetic field and temperature dependency of the effective homogeneous linewidth. Additionally, we employed three-pulse photon-echo (3PPE) measurements to investigate spectral diffusion and decoherence processes, and conclude that Two-Level System (TLS) effects are significantly suppressed at sufficiently low temperatures, below approximately 100 mK. △ Less

Submitted 27 September, 2025; originally announced September 2025.

Integrated high-fidelity preparation and analysis of photonic two-qubit states for quantum network nodes

Authors: Jonas C. J. Zatsch, Tim Engling, Jeldrik Huster, Louis L. Hohmann, Shreya Kumar, Stefanie Barz

Abstract: The realisation of quantum networks requires local quantum information processing at the network nodes and highly efficient transmission of quantum information across the network. Integrated photonics, based on silicon-on-insulator, is a promising platform for quantum network nodes, as it supports low-loss propagation of telecom wavelength photons, making it compatible with existing optical fibre… ▽ More The realisation of quantum networks requires local quantum information processing at the network nodes and highly efficient transmission of quantum information across the network. Integrated photonics, based on silicon-on-insulator, is a promising platform for quantum network nodes, as it supports low-loss propagation of telecom wavelength photons, making it compatible with existing optical fibre networks. Here, we present a silicon-on-insulator integrated photonic chip, capable of bidirectional operation, enabling the preparation of arbitrary single- and two-qubit states, and performing full quantum state tomography on up to two qubits. Using our chip, we obtain preparation fidelities above 97% for on-chip prepared Bell states coupled into optical fibres. Furthermore, we demonstrate that we can distribute entanglement between network nodes by preparing a two-qubit cluster state on the first node and performing full quantum state tomography on the second node, achieving a fidelity of 90.0(16)%. This result proves that our approach allows the distribution of entanglement from one chip to another. The potential of bidirectional operation makes our circuit a versatile node in telecom quantum networks, both functioning as a sender and receiver unit, a key element for the deployment of fully photonic multi-purpose quantum networks. △ Less

Submitted 18 September, 2025; originally announced September 2025.

Comments: 7 pages, 7 figures

Broadband femtosecond lasers enable efficient two-photon excitation of the ultranarrow linewidth singlet 1s2s state in helium

Authors: Shashank Kumar, Justin D. Piel, Chris H. Greene, Niranjan Shivaram

Abstract: We propose a broadband, femtosecond two-photon excitation scheme for efficient population transfer to the ultra-narrow linewidth $1s2s\ ^1S_0$ metastable state in helium. Using $120$ nm vacuum ultraviolet (VUV) femtosecond laser pulses, we theoretically demonstrate that a direct two-photon excitation process can achieve a population transfer efficiency of $25-30\%$, even when photoionization losse… ▽ More We propose a broadband, femtosecond two-photon excitation scheme for efficient population transfer to the ultra-narrow linewidth $1s2s\ ^1S_0$ metastable state in helium. Using $120$ nm vacuum ultraviolet (VUV) femtosecond laser pulses, we theoretically demonstrate that a direct two-photon excitation process can achieve a population transfer efficiency of $25-30\%$, even when photoionization losses are included. The use of broadband pulses enables multiple excitation pathways to populate the excited state, in addition to compensating for significant AC Stark shifts occurring within the pulse duration. Furthermore, we introduce a two-color two-photon extreme ultraviolet-near infrared (XUV-IR) excitation scheme that will further reduce ionization losses and can achieve significantly higher transfer efficiencies of $\sim 70\%$. These results demonstrate that high excitation probability of ultra-narrow linewidth ($\sim 50$ Hz) excited states can be achieved with experimentally accessible femtosecond laser sources with a few THz bandwidth. △ Less

Submitted 16 September, 2025; originally announced September 2025.

Comments: 10 pages, 5 figures

A Quantum Bagging Algorithm with Unsupervised Base Learners for Label Corrupted Datasets

Authors: Neeshu Rathi, Sanjeev Kumar

Abstract: The development of noise-resilient quantum machine learning (QML) algorithms is critical in the noisy intermediate-scale quantum (NISQ) era. In this work, we propose a quantum bagging framework that uses QMeans clustering as the base learner to reduce prediction variance and enhance robustness to label noise. Unlike bagging frameworks built on supervised learners, our method leverages the unsuperv… ▽ More The development of noise-resilient quantum machine learning (QML) algorithms is critical in the noisy intermediate-scale quantum (NISQ) era. In this work, we propose a quantum bagging framework that uses QMeans clustering as the base learner to reduce prediction variance and enhance robustness to label noise. Unlike bagging frameworks built on supervised learners, our method leverages the unsupervised nature of QMeans, combined with quantum bootstrapping via QRAM-based sampling and bagging aggregation through majority voting. Through extensive simulations on both noisy classification and regression tasks, we demonstrate that the proposed quantum bagging algorithm performs comparably to its classical counterpart using KMeans while exhibiting greater resilience to label corruption than supervised bagging methods. This highlights the potential of unsupervised quantum bagging in learning from unreliable data. △ Less

Submitted 8 September, 2025; originally announced September 2025.

Suppression of measurement-induced state transitions in cosφ-coupling transmon readout

Authors: Cyril Mori, Francesca D Esposito, Alexandru Petrescu, Lucas Ruela, Shelender Kumar, Vishnu Narayanan Suresh, Wael Ardati, Dorian Nicolas, Giulio Cappelli, Arpit Ranadive, Gwenael Le Gal, Martina Esposito, Quentin Ficheux, Nicolas Roch, Olivier Buisson

Abstract: Drive-induced unwanted state transitions (DUST) are limiting both for microwave readout and parametric operations of superconducting qubits. Among them, measurement-induced state transitions (MIST) are due to intrinsic resonances described by the readout Hamiltonian. They were previously studied with a qubit linearly coupled to its readout mode, which constitutes the usual readout Hamiltonian. Sin… ▽ More Drive-induced unwanted state transitions (DUST) are limiting both for microwave readout and parametric operations of superconducting qubits. Among them, measurement-induced state transitions (MIST) are due to intrinsic resonances described by the readout Hamiltonian. They were previously studied with a qubit linearly coupled to its readout mode, which constitutes the usual readout Hamiltonian. Since MIST can appear even at moderate powers, they limit the readout SNR and the QND readout fidelity. In this work, we study the high-power readout regime in a different transmon readout scheme, implementing a nonlinear coupling called the cosφ-coupling. This coupling stems from a transmon molecule circuit and has symmetry properties that suppress nonparity-conserving MIST. We succeed in performing multi-state single-shot readout up to the fifth excited state of the transmon, which enables us to identify leakage pathways from the computational subspace. The measurements indicate that the system is free of MIST up to high powers, with more than 300 photons in the readout mode. The MIST can be controllably turned on by breaking the parity symmetry of the coupling using flux-tuning. These experimental results are corroborated by branch analysis and simulations of the classical chaotic dynamics, showing that the cosφ-coupling is very robust to readout photons compared to the usual transverse coupling. △ Less

Submitted 5 September, 2025; originally announced September 2025.

Comments: 20 pages; 11 Figures

Histogram Driven Amplitude Embedding for Qubit Efficient Quantum Image Compression

Authors: Sahil Tomar, Sandeep Kumar

Abstract: This work introduces a compact and hardware efficient method for compressing color images using near term quantum devices. The approach segments the image into fixed size blocks called bixels, and computes the total intensity within each block. A global histogram with B bins is then constructed from these block intensities, and the normalized square roots of the bin counts are encoded as amplitude… ▽ More This work introduces a compact and hardware efficient method for compressing color images using near term quantum devices. The approach segments the image into fixed size blocks called bixels, and computes the total intensity within each block. A global histogram with B bins is then constructed from these block intensities, and the normalized square roots of the bin counts are encoded as amplitudes into an n qubit quantum state. Amplitude embedding is performed using PennyLane and executed on real IBM Quantum hardware. The resulting state is measured to reconstruct the histogram, enabling approximate recovery of block intensities and full image reassembly. The method maintains a constant qubit requirement based solely on the number of histogram bins, independent of the resolution of the image. By adjusting B, users can control the trade off between fidelity and resource usage. Empirical results demonstrate high quality reconstructions using as few as 5 to 7 qubits, significantly outperforming conventional pixel level encodings in terms of qubit efficiency and validating the practical application of the method for current NISQ era quantum systems. △ Less

Submitted 5 September, 2025; originally announced September 2025.

Comments: 7 pages

Reinforcement Learning for Optimizing Large Qubit Array based Quantum Sensor Circuits

Authors: Laxmisha Ashok Attisara, Sathish Kumar

Abstract: As the number of qubits in a sensor increases, the complexity of designing and controlling the quantum circuits grows exponentially. Manually optimizing these circuits becomes infeasible. Optimizing entanglement distribution in large-scale quantum circuits is critical for enhancing the sensitivity and efficiency of quantum sensors [5], [6]. This paper presents an engineering integration of reinfor… ▽ More As the number of qubits in a sensor increases, the complexity of designing and controlling the quantum circuits grows exponentially. Manually optimizing these circuits becomes infeasible. Optimizing entanglement distribution in large-scale quantum circuits is critical for enhancing the sensitivity and efficiency of quantum sensors [5], [6]. This paper presents an engineering integration of reinforcement learning with tensor-network-based simulation (MPS) for scalable circuit optimization for optimizing quantum sensor circuits with up to 60 qubits. To enable efficient simulation and scalability, we adopt tensor network methods, specifically the Matrix Product State (MPS) representation, instead of traditional state vector or density matrix approaches. Our reinforcement learning agent learns to restructure circuits to maximize Quantum Fisher Information (QFI) and entanglement entropy while reducing gate counts and circuit depth. Experimental results show consistent improvements, with QFI values approaching 1, entanglement entropy in the 0.8-1.0 range, and up to 90% reduction in depth and gate count. These results highlight the potential of combining quantum machine learning and tensor networks to optimize complex quantum circuits under realistic constraints. △ Less

Submitted 28 August, 2025; originally announced August 2025.

Comments: 10 pages, 13 figures, 2 tables

MSC Class: 81P45; 68T07

Quantum Machine Learning for Optimizing Entanglement Distribution in Quantum Sensor Circuits

Authors: Laxmisha Ashok Attisara, Sathish Kumar

Abstract: In the rapidly evolving field of quantum computing, optimizing quantum circuits for specific tasks is crucial for enhancing performance and efficiency. More recently, quantum sensing has become a distinct and rapidly growing branch of research within the area of quantum science and technology. The field is expected to provide new opportunities, especially regarding high sensitivity and precision.… ▽ More In the rapidly evolving field of quantum computing, optimizing quantum circuits for specific tasks is crucial for enhancing performance and efficiency. More recently, quantum sensing has become a distinct and rapidly growing branch of research within the area of quantum science and technology. The field is expected to provide new opportunities, especially regarding high sensitivity and precision. Entanglement is one of the key factors in achieving high sensitivity and measurement precision [3]. This paper presents a novel approach utilizing quantum machine learning techniques to optimize entanglement distribution in quantum sensor circuits. By leveraging reinforcement learning within a quantum environment, we aim to optimize the entanglement layout to maximize Quantum Fisher Information (QFI) and entanglement entropy, which are key indicators of a quantum system's sensitivity and coherence, while minimizing circuit depth and gate counts. Our implementation, based on Qiskit, integrates noise models and error mitigation strategies to simulate realistic quantum environments. The results demonstrate significant improvements in circuit performance and sensitivity, highlighting the potential of machine learning in quantum circuit optimization by measuring high QFI and entropy in the range of 0.84-1.0 with depth and gate count reduction by 20-86%. △ Less

Submitted 28 August, 2025; originally announced August 2025.

Comments: 11 pages, 13 figures, 4 tables

MSC Class: 81P45; 68T07

HCQA: Hybrid Classical-Quantum Agent for Generating Optimal Quantum Sensor Circuits

Authors: Ahmad Alomari, Sathish A. P. Kumar

Abstract: This study proposes an HCQA for designing optimal Quantum Sensor Circuits (QSCs) to address complex quantum physics problems. The HCQA integrates computational intelligence techniques by leveraging a Deep Q-Network (DQN) for learning and policy optimization, enhanced by a quantum-based action selection mechanism based on the Q-values. A quantum circuit encodes the agent current state using Ry gate… ▽ More This study proposes an HCQA for designing optimal Quantum Sensor Circuits (QSCs) to address complex quantum physics problems. The HCQA integrates computational intelligence techniques by leveraging a Deep Q-Network (DQN) for learning and policy optimization, enhanced by a quantum-based action selection mechanism based on the Q-values. A quantum circuit encodes the agent current state using Ry gates, and then creates a superposition of possible actions. Measurement of the circuit results in probabilistic action outcomes, allowing the agent to generate optimal QSCs by selecting sequences of gates that maximize the Quantum Fisher Information (QFI) while minimizing the number of gates. This computational intelligence-driven HCQA enables the automated generation of entangled quantum states, specifically the squeezed states, with high QFI sensitivity for quantum state estimation and control. Evaluation of the HCQA on a QSC that consists of two qubits and a sequence of Rx, Ry, and S gates demonstrates its efficiency in generating optimal QSCs with a QFI of 1. This work highlights the synergy between AI-driven learning and quantum computation, illustrating how intelligent agents can autonomously discover optimal quantum circuit designs for enhanced sensing and estimation tasks. △ Less

Submitted 28 August, 2025; originally announced August 2025.

Comments: 9 pages, 9 figures

ACM Class: F.1.2; I.2.6; I.2.8

Above 99.9% Fidelity Single-Qubit Gates, Two-Qubit Gates, and Readout in a Single Superconducting Quantum Device

Authors: Fabian Marxer, Jakub Mrożek, Joona Andersson, Leonid Abdurakhimov, Janos Adam, Ville Bergholm, Rohit Beriwal, Chun Fai Chan, Saga Dahl, Soumya Ranjan Das, Frank Deppe, Olexiy Fedorets, Zheming Gao, Alejandro Gomez Frieiro, Daria Gusenkova, Andrew Guthrie, Tuukka Hiltunen, Hao Hsu, Eric Hyyppä, Joni Ikonen, Sinan Inel, Shan W. Jolin, Azad Karis, Seung-Goo Kim, William Kindel , et al. (42 additional authors not shown)

Abstract: Achieving high-fidelity single-qubit gates, two-qubit gates, and qubit readout is critical for building scalable, error-corrected quantum computers. However, device parameters that enhance one operation often degrade the others, making simultaneous optimization challenging. Here, we demonstrate that careful tuning of qubit-coupler coupling strengths in a superconducting circuit with two transmon q… ▽ More Achieving high-fidelity single-qubit gates, two-qubit gates, and qubit readout is critical for building scalable, error-corrected quantum computers. However, device parameters that enhance one operation often degrade the others, making simultaneous optimization challenging. Here, we demonstrate that careful tuning of qubit-coupler coupling strengths in a superconducting circuit with two transmon qubits coupled via a tunable coupler enables high-fidelity single- and two-qubit gates, without compromising readout performance. As a result, we achieve a 40h-averaged CZ gate fidelity of 99.93%, simultaneous single-qubit gate fidelities of 99.98%, and readout fidelities over 99.94% in a single device. These results are enabled by optimized coupling parameters, an efficient CZ gate calibration experiment based on our new Phased-Averaged Leakage Error Amplification (PALEA) protocol, and a readout configuration compatible with high coherence qubits. Our results demonstrate a viable path toward scaling up superconducting quantum processors while maintaining consistently high fidelities across all core operations. △ Less

Submitted 22 August, 2025; originally announced August 2025.

Journal ref: PRX Quantum 7, 020333 (2026)

Portfolio construction using a sampling-based variational quantum scheme

Authors: Gabriele Agliardi, Dimitris Alevras, Vaibhaw Kumar, Roberto Lo Nardo, Gabriele Compostella, Sumit Kumar, Manuel Proissl, Bimal Mehta

Abstract: The efficient and effective construction of portfolios that adhere to real-world constraints is a challenging optimization task in finance. We investigate a concrete representation of the problem with a focus on design proposals of an Exchange Traded Fund. We evaluate the sampling-based CVaR Variational Quantum Algorithm (VQA), combined with a local-search post-processing, for solving problem inst… ▽ More The efficient and effective construction of portfolios that adhere to real-world constraints is a challenging optimization task in finance. We investigate a concrete representation of the problem with a focus on design proposals of an Exchange Traded Fund. We evaluate the sampling-based CVaR Variational Quantum Algorithm (VQA), combined with a local-search post-processing, for solving problem instances that beyond a certain size become classically hard. We also propose a problem formulation that is suited for sampling-based VQA. Our utility-scale experiments on IBM Heron processors involve 109 qubits and up to 4200 gates, achieving a relative solution error of 0.49%. Results indicate that a combined quantum-classical workflow achieves better accuracy compared to purely classical local search, and that hard-to-simulate quantum circuits may lead to better convergence than simpler circuits. Our work paves the path to further explore portfolio construction with quantum computers. △ Less

Submitted 7 November, 2025; v1 submitted 19 August, 2025; originally announced August 2025.

A Symmetry-Based Taxonomy of Quantum Algorithms

Authors: Sakshi Kumar, Sumit Chilkoti, Mrittunjoy Guha Majumdar

Abstract: We propose a taxonomy for quantum algorithms grounded in the fundamental symmetries, both continuous and discrete, underlying quantum state spaces, oracles, and circuit dynamics. By organizing algorithms according to their symmetry groups and invariants, we define distinct algorithm classes whose behavior, verification, and complexity can be characterized by the symmetries they preserve or exploit… ▽ More We propose a taxonomy for quantum algorithms grounded in the fundamental symmetries, both continuous and discrete, underlying quantum state spaces, oracles, and circuit dynamics. By organizing algorithms according to their symmetry groups and invariants, we define distinct algorithm classes whose behavior, verification, and complexity can be characterized by the symmetries they preserve or exploit. This symmetry-centric classification not only reflects the deep connection between symmetries and conservation laws in physics, but also yields practical benefits for scalable and reliable quantum computation. △ Less

Submitted 12 August, 2025; originally announced August 2025.

Design and Experimental Realization of Various Protocols for Secure Quantum Computation and Communication

Authors: Satish Kumar

Abstract: A set of new schemes for quantum computation and communication have been either designed or experimentally realized using optimal quantum resources. A multi-output quantum teleportation scheme, where a sender (Alice) teleports an m and m+1-qubit GHZ-like unknown state to a receiver (Bob), has been demonstrated using two copies of the Bell state instead of a five-qubit cluster state and implemented… ▽ More A set of new schemes for quantum computation and communication have been either designed or experimentally realized using optimal quantum resources. A multi-output quantum teleportation scheme, where a sender (Alice) teleports an m and m+1-qubit GHZ-like unknown state to a receiver (Bob), has been demonstrated using two copies of the Bell state instead of a five-qubit cluster state and implemented on IBM's quantum computer for the m=1 case. Another scheme, known as quantum broadcasting where a known state is sent to two spatially separated parties (Bob and Charlie) has also been realized using two Bell states. It is shown that existing quantum broadcasting schemes can be reduced to multiparty remote state preparation. After achieving teleportation of unknown and known states, sending a quantum operator becomes the next step. A scheme for remote implementation of operators (RIO), specifically a controlled joint-RIO (CJRIO), has been proposed using a four-qubit hyper-entangled state involving spatial and polarization degrees of freedom. In this direction, two more variants, remote implementation of hidden and partially unknown operators (RIHO and RIPUO) have also been proposed. Their success probabilities are analyzed considering dissipation of an auxiliary coherent state interacting with the environment. For secure multiparty tasks like quantum voting or auction, secure multiparty quantum computation (SMQC) becomes essential. A quantum anonymous voting (QAV) scheme has been experimentally implemented on IBM's quantum computer. Finally, two quantum key distribution (QKD) protocols, coherent one-way (COW) and differential phase shift (DPS), are experimentally demonstrated and the key rates are analyzed as functions of post-processing parameters and detector dead times across various distances. △ Less

Submitted 13 July, 2025; originally announced July 2025.

High-power readout of a transmon qubit using a nonlinear coupling

Authors: Cyril Mori, Vladimir Milchakov, Francesca D'Esposito, Lucas Ruela, Shelender Kumar, Vishnu Narayanan Suresh, Waël Ardati, Dorian Nicolas, Giulio Cappelli, Arpit Ranadive, Gwenael Le Gal, Martina Esposito, Quentin Ficheux, Nicolas Roch, Tomás Ramos, Olivier Buisson

Abstract: The field of superconducting qubits is constantly evolving with new circuit designs. However, when it comes to qubit readout, the use of simple transverse linear coupling remains overwhelmingly prevalent. This standard readout scheme has significant drawbacks: in addition to the Purcell effect, it suffers from a limitation on the maximal number of photons in the readout mode, which restricts the s… ▽ More The field of superconducting qubits is constantly evolving with new circuit designs. However, when it comes to qubit readout, the use of simple transverse linear coupling remains overwhelmingly prevalent. This standard readout scheme has significant drawbacks: in addition to the Purcell effect, it suffers from a limitation on the maximal number of photons in the readout mode, which restricts the signal-to-noise ratio (SNR) and the Quantum Non-Demolition (QND) nature of the readout. Here, we explore the high-power regime by engineering a nonlinear coupling between a transmon qubit and its readout mode. Our approach builds upon previous work by Dassonneville et al. [Physical Review X 10, 011045 (2020)], on qubit readout with a non-perturbative cross-Kerr coupling in a transmon molecule. We demonstrate a readout fidelity of 99.21% with 89 photons utilizing a parametric amplifier. At this elevated photon number, the QND nature remains high at 96.7%. Even with up to 300 photons, the QNDness is only reduced by a few percent. This is qualitatively explained by deriving a critical number of photons associated with the nonlinear coupling, yielding a theoretical value of $\bar{n}_r^\text{crit} = 377$ photons for our sample's parameters. These results highlight the promising performance of the transmon molecule in the high-power regime, establishing it as a compelling platform for high-fidelity qubit readout. △ Less

Submitted 5 August, 2025; v1 submitted 4 July, 2025; originally announced July 2025.

Comments: 17 pages, 8 figures

Multimode and Random-Access Optical Quantum Memory via Adiabatic Phase Imprinting

Authors: Nasser Gohari Kamel, Sourabh Kumar, Ujjwal Gautam, Erhan Saglamyurek, Vahid Salari, Daniel Oblak

Abstract: A photonic quantum memory capable of simultaneously storing multiple qubits and subsequently recalling any randomly selected subset of the qubits, is essential for large-scale quantum networking and computing. Such functionality, akin to classical Random-Access Memory (RAM), has proven difficult to implement due to the absence of a versatile random-access mechanism and limited multimode capacity i… ▽ More A photonic quantum memory capable of simultaneously storing multiple qubits and subsequently recalling any randomly selected subset of the qubits, is essential for large-scale quantum networking and computing. Such functionality, akin to classical Random-Access Memory (RAM), has proven difficult to implement due to the absence of a versatile random-access mechanism and limited multimode capacity in existing quantum memory protocols. A potential path to developing the quantum analog to RAM is offered by photon-echo protocols in rare-earth ion-doped materials, such as Revival Of Silenced Echo. These can utilize optical rephasing pulses to selectively read-out frequency multiplexed photonic qubits within an inhomogeneously broadened optical transition. However, the conventional non-adiabatic nature of the rephasing pulses requires intense, short-duration pulses, impeding their fidelity and multimode capacity. To address these critical limitations, we introduce an alternate protocol that employs Rapid Adiabatic Passage (RAP) rephasing pulses, to realize quantum memory, which invokes phase-imprints to suppress undesirable echoes. Using the optical transitions of a $^{171}{\rm Yb}^{3+}$:${\rm Y}_2{\rm SiO}_5$ crystal, we demonstrate the storage and retrieval of multiple intricate spectro-temporally photonic modes and achieve optical random access memory across eight distinct spectral modes. This protocol yields greatly enhanced mode-mapping versatility while substantially lowering the required rephasing pulse intensity, providing a more efficient and reliable approach for high-fidelity qubit storage and retrieval. △ Less

Submitted 13 June, 2025; originally announced June 2025.

Comments: 12 pages, 5 figures

RhoDARTS: Differentiable Quantum Architecture Search with Density Matrix Simulations

Authors: Swagat Kumar, Jan-Nico Zaech, Colin Michael Wilmott, Luc Van Gool

Abstract: Variational Quantum Algorithms (VQAs) are a promising approach to leverage Noisy Intermediate-Scale Quantum (NISQ) computers. However, choosing optimal quantum circuits that efficiently solve a given VQA problem is a non-trivial task. Quantum Architecture Search (QAS) algorithms enable automatic generation of quantum circuits tailored to the provided problem. Existing QAS approaches typically adap… ▽ More Variational Quantum Algorithms (VQAs) are a promising approach to leverage Noisy Intermediate-Scale Quantum (NISQ) computers. However, choosing optimal quantum circuits that efficiently solve a given VQA problem is a non-trivial task. Quantum Architecture Search (QAS) algorithms enable automatic generation of quantum circuits tailored to the provided problem. Existing QAS approaches typically adapt classical neural architecture search techniques, training machine learning models to sample relevant circuits, but often overlook the inherent quantum nature of the circuits they produce. By reformulating QAS from a quantum perspective, we propose a sampling-free differentiable QAS algorithm that models the search process as the evolution of a quantum mixed state, which emerges from the search space of quantum circuits. The mixed state formulation also enables our method to incorporate generic noise models, for example the depolarizing channel, which cannot be modeled by state vector simulation. We validate our method by finding circuits for state initialization and Hamiltonian optimization tasks, namely the variational quantum eigensolver and the unweighted max-cut problems. We show our approach to be comparable to, if not outperform, existing QAS techniques while requiring significantly fewer quantum simulations during training, and also show improved robustness levels to noise. △ Less

Submitted 6 October, 2025; v1 submitted 4 June, 2025; originally announced June 2025.

Comments: 27 pages, 19 figures

Multi Moire Networks in Engineered Lateral Hetero-Bilayers: Programmable Phononic Reconfiguration and Second Harmonic Generation

Authors: Suman Kumar Chakraborty, Frederico B. Sousa, Chakradhar Sahoo, Indrajeet Dhananjay Prasad, Shneha Biswas, Purbasha Ray, Biswajeet Nayak, Rafael Rojas, Baisali Kundu, Alfred J. H. Jones, Jill A. Miwa, Søren Ulstrup, Sudipta Dutta, Santosh Kumar, Leandro M. Malard, Gopal K. Pradhan, Prasana Kumar Sahoo

Abstract: Moire engineering in two-dimensional transition metal dichalcogenides enables access to correlated quantum phenomena. Realizing such effects demands simultaneous control over twist angle and material composition to modulate phonons, excitons, and their interactions. However, most studies rely on exfoliated flakes, limiting scalability and systematic exploration. Here, we demonstrate a scalable mul… ▽ More Moire engineering in two-dimensional transition metal dichalcogenides enables access to correlated quantum phenomena. Realizing such effects demands simultaneous control over twist angle and material composition to modulate phonons, excitons, and their interactions. However, most studies rely on exfoliated flakes, limiting scalability and systematic exploration. Here, we demonstrate a scalable multi-moire network by vertically stacking CVD-grown monolayer lateral heterostructures. Signatures of moire non-rigidity, including phonon frequency softening, linewidth broadening, and strain localization, are attributed to two lattice relaxation modes; rotational reconstruction and volumetric dilation. Micro-angle-resolved photoemission spectroscopy reveals that interfacial orbital interactions modulate interlayer coupling. At aligned angles, molybdenum diselenides exhibit reduced valley polarization and Davydov splitting, indicating strain-induced symmetry breaking and chiral phonon effects. Notably, SHG modulation was obderved with variation in twist angle due to lower coherence and band-offset-driven phase delay. First-principles calculations support these findings. This work provides a route to programmable, scalable multi-moire platforms for opto-straintronics, quantum sensing, and on-chip photonics. △ Less

Submitted 2 May, 2025; originally announced May 2025.

Magnetic field orientation dependence of continuous-wave optically detected magnetic resonance with nitrogen-vacancy ensembles

Authors: Pralekh Dubey, Shashank Kumar, Chinmaya Singh, Jemish Naliyapara, Monish A Poojar, Harikrishnan K B, Anshul Poonia, Phani Peddibhotla

Abstract: Continuous-wave optically detected magnetic resonance (CW-ODMR) measurements with nitrogen-vacancy (NV) spins in diamond are used for sensing DC magnetic fields from nearby magnetic targets. However, this technique suffers from ambiguities in the extraction of the magnetic field components when resonances due to different NV orientation classes overlap with each other. Here, we perform detailed ex… ▽ More Continuous-wave optically detected magnetic resonance (CW-ODMR) measurements with nitrogen-vacancy (NV) spins in diamond are used for sensing DC magnetic fields from nearby magnetic targets. However, this technique suffers from ambiguities in the extraction of the magnetic field components when resonances due to different NV orientation classes overlap with each other. Here, we perform detailed experimental and theoretical studies of such effects on NV ensembles experiencing low bias magnetic fields. In particular, through symmetry considerations, we systematically examine the ODMR response of different NV orientation classes as a function of the orientation of the magnetic field vector. Our studies are of importance for performing a careful and detailed analysis of the ODMR spectra in order to infer the vector magnetic field information. Our results find application in the studies of magnetic samples that require a low applied bias field and also can be potentially adapted to defect spins in other solid-state systems. △ Less

Submitted 19 July, 2025; v1 submitted 25 April, 2025; originally announced April 2025.