Showing 1–50 of 633 results for author: Nori, F

The Riemann Hypothesis Emerges in Dynamical Quantum Phase Transitions

Authors: ShiJie Wei, Yue Zhai, Quanfeng Lu, Wentao Yang, Pan Gao, Chao Wei, Junda Song, Franco Nori, Tao Xin, GuiLu Long

Abstract: The Riemann Hypothesis (RH), one of the most profound unsolved problems in mathematics, concerns the nontrivial zeros of the Riemann zeta function. Establishing connections between the RH and physical phenomena could offer new perspectives on its physical origin and verification. Here, we establish a direct correspondence between the nontrivial zeros of the zeta function and dynamical quantum phas… ▽ More The Riemann Hypothesis (RH), one of the most profound unsolved problems in mathematics, concerns the nontrivial zeros of the Riemann zeta function. Establishing connections between the RH and physical phenomena could offer new perspectives on its physical origin and verification. Here, we establish a direct correspondence between the nontrivial zeros of the zeta function and dynamical quantum phase transitions (DQPTs) in two realizable quantum systems, characterized by the averaged accumulated phase factor and the Loschmidt amplitude, respectively. This precise correspondence reveals that the RH can be viewed as the emergence of DQPTs at a specific temperature. We experimentally demonstrate this correspondence on a five-qubit spin-based system and further propose an universal quantum simulation framework for efficiently realizing both systems with polynomial resources, offering a quantum advantage for numerical verification of the RH. These findings uncover an intrinsic link between nonequilibrium critical dynamics and the RH, positioning quantum computing as a powerful platform for exploring one of mathematics' most enduring conjectures and beyond. △ Less

Submitted 14 November, 2025; originally announced November 2025.

Giant-Atom Quantum Batteries

Authors: Ke-Xiong Yan, Yang Liu, Yang Xiao, Jun-Hao Lin, Jie Song, Ye-Hong Chen, Franco Nori, Yan Xia

Abstract: Environmentally induced decoherence poses a fundamental challenge to quantum energy storage systems, causing irreversible energy dissipation and performance aging of quantum batteries (QBs). To address this issue, we propose a QB protocol utilizing the nonlocal coupling properties of giant atoms (GAs). In this architecture, both the QB and its charger are implemented as superconducting GAs with mu… ▽ More Environmentally induced decoherence poses a fundamental challenge to quantum energy storage systems, causing irreversible energy dissipation and performance aging of quantum batteries (QBs). To address this issue, we propose a QB protocol utilizing the nonlocal coupling properties of giant atoms (GAs). In this architecture, both the QB and its charger are implemented as superconducting GAs with multiple nonlocal coupling points to a shared microwave waveguide. By engineering these atoms in a braided configuration, where their coupling paths are spatially interleaved, we show the emergence of decoherence-immune interaction dynamics. This unique geometry enables destructive interference between decoherence channels while preserving coherent energy transfer between the charger and the QB, thereby effectively suppressing the aging effects induced by waveguide-mediated dissipation. The charging properties of separated and nested coupled configurations are investigated. The results show that these two configurations underperform the braided configuration. Additionally, we propose a long-range chiral charging scheme that facilitates unidirectional energy transfer between the charger and the battery, with the capability to reverse the flow direction by modulating the applied magnetic flux. Our result provides guidelines for implementing a decoherence-resistant charging protocol and remote chiral QBs in circuits with GAs engineering. △ Less

Submitted 26 October, 2025; originally announced October 2025.

Comments: 8 pages, 3 figures, Comments are welcome

Variational Quantum Algorithm for Unitary Dilation

Authors: S. X. Li, Keren Li, J. B. You, Y. -H. Chen, Clemens Gneiting, Franco Nori, X. Q. Shao

Abstract: We introduce a hybrid quantum-classical framework for efficiently implementing approximate unitary dilations of non-unitary operators with enhanced noise resilience. The method embeds a target non-unitary operator into a subblock of a unitary matrix generated by a parameterized quantum circuit with universal expressivity, while a classical optimizer adjusts circuit parameters under the global unit… ▽ More We introduce a hybrid quantum-classical framework for efficiently implementing approximate unitary dilations of non-unitary operators with enhanced noise resilience. The method embeds a target non-unitary operator into a subblock of a unitary matrix generated by a parameterized quantum circuit with universal expressivity, while a classical optimizer adjusts circuit parameters under the global unitary constraint. As a representative application, we consider the non-unitary propagator of a Lindbladian superoperator acting on the vectorized density matrix, which is relevant for simulating open quantum systems. We further validate the approach experimentally on superconducting devices in the Quafu quantum cloud computing cluster. Compared with standard dilation protocols, our method significantly reduces quantum resource requirements and improves robustness against device noise, achieving high-fidelity simulation. Its generality also enables compatibility with non-Markovian dynamics and Kraus-operator-based evolutions, providing a practical pathway for the noise-resilient simulation of non-unitary processes on near-term quantum hardware. △ Less

Submitted 21 October, 2025; originally announced October 2025.

Comments: 7 pages + 11 pages, 4 figures + 9 figures, comments are welcome

Resource efficient certification of system environment entanglement solely from reduced system dynamics

Authors: Jhen-Dong Lin, Pao-Wen Tu, Kuan-Yi Lee, Neill Lambert, Adam Miranowicz, Franco Nori, Yueh-Nan Chen

Abstract: Certifying nonclassical correlations typically requires access to all subsystems, presenting a major challenge in open quantum systems coupled to inaccessible environments. Recent works have shown that, in autonomous pure dephasing scenarios, quantum discord with the environment can be certified from system-only dynamics via the Hamiltonian ensemble formulation. However, this approach leaves open… ▽ More Certifying nonclassical correlations typically requires access to all subsystems, presenting a major challenge in open quantum systems coupled to inaccessible environments. Recent works have shown that, in autonomous pure dephasing scenarios, quantum discord with the environment can be certified from system-only dynamics via the Hamiltonian ensemble formulation. However, this approach leaves open whether stronger correlations, such as entanglement, can be certified. Moreover, its reliance on Fourier analysis requires full-time dynamics, which is experimentally resource-intensive and provides limited information about when such correlations are established during evolution. In this work, we present a method that enables the certification of system-environment quantum entanglement solely from the reduced dynamics of the system. The method is based on the theory of mixed-unitary channels and applies to general non-autonomous pure dephasing scenarios. Crucially, it relaxes the need for full-time dynamics, offering a resource-efficient approach that also reveals the precise timing of entanglement generation. We experimentally validate this method on a Quantinuum trapped-ion quantum processor with a controlled-dephasing model. Finally, we highlight its potential as a tool for certifying gravitationally induced entanglement. △ Less

Submitted 20 October, 2025; originally announced October 2025.

Comments: 12 pages, 4 figures

Disorder-Induced Strongly Correlated Photons in Waveguide QED

Authors: Guoqing Tian, Li-Li Zheng, Zhi-Ming Zhan, Franco Nori, Xin-You Lü

Abstract: Strongly correlated photons play a crucial role in modern quantum technologies. Here, we investigate the probability of generating strongly correlated photons in a chain of N qubits coupled to a one-dimensional (1D) waveguide. We found that disorder in the transition frequencies can induce photon antibunching, and especially nearly perfect photon blockade events in the transmission and reflection… ▽ More Strongly correlated photons play a crucial role in modern quantum technologies. Here, we investigate the probability of generating strongly correlated photons in a chain of N qubits coupled to a one-dimensional (1D) waveguide. We found that disorder in the transition frequencies can induce photon antibunching, and especially nearly perfect photon blockade events in the transmission and reflection outputs. As a comparison, in ordered chains, strongly correlated photons cannot be generated in the transmission output, and only weakly antibunched photons are found in the reflection output. The occurrence of nearly perfect photon blockade events stems from the disorder-induced near completely destructive interference of photon scattering paths. Our work highlights the impact of disorder on photon correlation generation and suggests that disorder can enhance the potential for achieving strongly correlated photon. △ Less

Submitted 13 October, 2025; originally announced October 2025.

Comments: 7 pages, 5 figures + Supplemental Material

Journal ref: Phys. Rev. Lett. 135, 153604 (2025)

Observation of Genuine Tripartite Non-Gaussian Entanglement from a Superconducting Three-Photon Spontaneous Parametric Down-Conversion Source

Authors: Benjamin Jarvis-Frain, Andy Schang, Fernando Quijandría, Ibrahim Nsanzineza, Dmytro Dubyna, C. W. Sandbo Chang, Franco Nori, C. M. Wilson

Abstract: The generation of entangled photons through Spontaneous Parametric Down-Conversion (SPDC) is a critical resource for many key experiments and technologies in the domain of quantum optics. Historically, SPDC was limited to the generation of photon pairs. However, the use of the strong nonlinearities in circuit quantum electrodynamics has recently enabled the observation of Three-Photon SPDC (3P-SPD… ▽ More The generation of entangled photons through Spontaneous Parametric Down-Conversion (SPDC) is a critical resource for many key experiments and technologies in the domain of quantum optics. Historically, SPDC was limited to the generation of photon pairs. However, the use of the strong nonlinearities in circuit quantum electrodynamics has recently enabled the observation of Three-Photon SPDC (3P-SPDC). Despite great interest in the entanglement structure of the resultant states, entanglement between photon triplets produced by a 3P-SPDC source has still has not been confirmed. Here, we report on the observation of genuine tripartite non-Gaussian entanglement in the steady-state output field of a 3P-SPDC source consisting of a superconducting parametric cavity coupled to a transmission line. We study this non-Gaussian tripartite entanglement using an entanglement witness built from three-mode correlation functions, and observe a maximum violation of the bound by 23 standard deviations of the statistical noise. Furthermore, we find strong agreement between the observed and the analytically predicted scaling of the entanglement witness. We then explore the impact of the temporal function used to define the photon mode on the observed value of the entanglement witness. △ Less

Submitted 17 November, 2025; v1 submitted 6 October, 2025; originally announced October 2025.

Comments: Revised entanglement witness in eq. 3 & corresponding figs. (2,4). Results unchanged

Quantum Error Correction with Superpositions of Squeezed Fock States

Authors: Yexiong Zeng, Fernando Quijandría, Clemens Gneiting, Franco Nori

Abstract: Bosonic codes, leveraging infinite-dimensional Hilbert spaces for redundancy, offer great potential for encoding quantum information. However, the realization of a practical continuous-variable bosonic code that can simultaneously correct both single-photon loss and dephasing errors remains elusive, primarily due to the absence of exactly orthogonal codewords and the lack of an experiment-friendly… ▽ More Bosonic codes, leveraging infinite-dimensional Hilbert spaces for redundancy, offer great potential for encoding quantum information. However, the realization of a practical continuous-variable bosonic code that can simultaneously correct both single-photon loss and dephasing errors remains elusive, primarily due to the absence of exactly orthogonal codewords and the lack of an experiment-friendly state preparation scheme. Here, we propose a code based on the superposition of squeezed Fock states with an error-correcting capability that scales as $\propto\exp(-7r)$, where $r$ is the squeezing level. The codewords remain orthogonal at all squeezing levels. The Pauli-X operator acts as a rotation in phase space is an error-transparent gate, preventing correctable errors from propagating outside the code space during logical operations. In particular, this code achieves high-precision error correction for both single-photon loss and dephasing, even at moderate squeezing levels. Building on this code, we develop quantum error correction schemes that exceed the break-even threshold, supported by analytical derivations of all necessary quantum gates. Our code offers a competitive alternative to previous encodings for quantum computation using continuous bosonic qubits. △ Less

Submitted 5 October, 2025; originally announced October 2025.

Noise Protected Logical Qubit in a Open Chain of Superconducting Qubits with Ultrastrong Interactions

Authors: Roberto Stassi, Shilan Abo, Daniele Lamberto, Ye-Hong Chen, Adam Miranowicz, Salvatore Savasta, Franco Nori

Abstract: To achieve a fault-tolerant quantum computer, it is crucial to increase the coherence time of quantum bits. In this work, we theoretically investigate a system consisting of a series of superconducting qubits that alternate between XX and YY ultrastrong interactions. By considering the two-lowest energy eigenstates of this system as a {\it logical} qubit, we demonstrate that its coherence is signi… ▽ More To achieve a fault-tolerant quantum computer, it is crucial to increase the coherence time of quantum bits. In this work, we theoretically investigate a system consisting of a series of superconducting qubits that alternate between XX and YY ultrastrong interactions. By considering the two-lowest energy eigenstates of this system as a {\it logical} qubit, we demonstrate that its coherence is significantly enhanced: both its pure dephasing and relaxation times are extended beyond those of individual {\it physical} qubits. Specifically, we show that by increasing either the interaction strength or the number of physical qubits in the chain, the logical qubit's pure dephasing rate is suppressed to zero, and its relaxation rate is reduced to half the relaxation rate of a single physical qubit. Single qubit and two-qubit gates can be performed with a high fidelity. △ Less

Submitted 22 September, 2025; originally announced September 2025.

Exotic quantum light-matter interactions in bilayer square lattices

Authors: Xing-Liang Dong, Peng-Bo Li, Jia-Qiang Chen, Fu-Li Li, Franco Nori

Abstract: We investigate quantum emitters (QEs) interacting with a photonic structured bath made of bilayer square lattices, where the resonance anti-crossing between the energy bands opens a symmetric middle energy gap. Due to the intrinsic chiral symmetry of the bath and interactions with the square-like band-edges, the QE-photon dressed states generated in this inner bandgap are odd-neighbor, robust, and… ▽ More We investigate quantum emitters (QEs) interacting with a photonic structured bath made of bilayer square lattices, where the resonance anti-crossing between the energy bands opens a symmetric middle energy gap. Due to the intrinsic chiral symmetry of the bath and interactions with the square-like band-edges, the QE-photon dressed states generated in this inner bandgap are odd-neighbor, robust, and anisotropic, when the emitters' transition frequencies lie in the middle of the bandgap. We also use giant artificial atoms to engineer and modify the dressed states' patterns. Exotic bound states can lead to spin models with symmetry protection, resulting in fascinating many-body phases. As an example, we show that this proposal can be used to generate both edge states and corner states in the generalized 2D Su-Schrieffer-Heeger (SSH) model. This work opens up new avenues for research into innovative quantum many-body physics and quantum simulations with photonic or phononic multilayer structures. △ Less

Submitted 14 September, 2025; v1 submitted 11 September, 2025; originally announced September 2025.

Comments: 12 pages, 7 figures

Journal ref: Phys. Rev. B 108, 045407 (2023)

Unified formalism and adaptive algorithms for optimal quantum state, detector and process tomography

Authors: Shuixin Xiao, Xiangyu Wang, Yuanlong Wang, Zhibo Hou, Jun Zhang, Ian R. Petersen, Wen-Zhe Yan, Hidehiro Yonezawa, Franco Nori, Guo-Yong Xiang, Daoyi Dong

Abstract: Quantum tomography is a standard technique for characterizing, benchmarking and verifying quantum systems/devices and plays a vital role in advancing quantum technology and understanding the foundations of quantum mechanics. Achieving the highest possible tomography accuracy remains a central challenge. Here we unify the infidelity metrics for quantum state, detector and process tomography in a si… ▽ More Quantum tomography is a standard technique for characterizing, benchmarking and verifying quantum systems/devices and plays a vital role in advancing quantum technology and understanding the foundations of quantum mechanics. Achieving the highest possible tomography accuracy remains a central challenge. Here we unify the infidelity metrics for quantum state, detector and process tomography in a single index $1-F(\hat S,S)$, where $S$ represents the true density matrix, POVM element, or process matrix, and $\hat S$ is its estimator. We establish a sufficient and necessary condition for any tomography protocol to attain the optimal scaling $1-F= O(1/N) $ where $N$ is the number of state copies consumed, in contrast to the $O(1/\sqrt{N})$ worst-case scaling of static methods. Guided by this result, we propose adaptive algorithms with provably optimal infidelity scalings for state, detector, and process tomography. Numerical simulations and quantum optical experiments validate the proposed methods, with our experiments reaching, for the first time, the optimal infidelity scaling in ancilla-assisted process tomography. △ Less

Submitted 7 September, 2025; originally announced September 2025.

Comments: 7+27 pages

Dynamically encircling an exceptional point through phase-tracked closed-loop control

Authors: Sen Zhang, Yangyu Huang, Lei Yu, Kaixuan He, Ning Zhou, Dingbang Xiao, Xuezhong Wu, Franco Nori, Hui Jing, Xin Zhou

Abstract: The intricate complex eigenvalues of non-Hermitian Hamiltonians manifest as Riemann surfaces in control parameter spaces. At the exceptional points (EPs), the degeneracy of both eigenvalues and eigenvectors introduces noteworthy topological features, particularly during the encirclement of the EPs. Traditional methods for probing the state information on the Riemann surfaces involve static measure… ▽ More The intricate complex eigenvalues of non-Hermitian Hamiltonians manifest as Riemann surfaces in control parameter spaces. At the exceptional points (EPs), the degeneracy of both eigenvalues and eigenvectors introduces noteworthy topological features, particularly during the encirclement of the EPs. Traditional methods for probing the state information on the Riemann surfaces involve static measurements; however, realizing continuous encircling remains a formidable challenge due to non-adiabatic transitions that disrupt the transport paths. Here we propose an approach leveraging the phase-locked loop (PLL) technique to facilitate smooth, dynamic encircling of EPs while maintaining resonance. Our methodology strategically ties the excitation frequencies of steady states to their response phases, enabling controlled traversal along the Riemann surfaces of real eigenvalues. This study advances the concept of phase-tracked dynamical encircling and explores its practical implementation within a fully electrically controlled non-Hermitian microelectromechanical system, highlighting robust in-situ tunability and providing methods for exploring non-Hermitian topologies. △ Less

Submitted 5 September, 2025; originally announced September 2025.

Comments: 43 pages, 12 figures

Journal ref: Commun Phys 8, 344 (2025)

Cavity QED based on strongly localized modes: exponentially enhancing single-atom cooperativity

Authors: Qian Bin, Ying Wu, Jin-Hua Gao, Aixi Chen, Franco Nori, Xin-You Lü

Abstract: Large single-atom cooperativity in quantum systems is important for quantum information processing. Here, we propose to exponentially enhance the single-atom cooperativity parameter by exploiting the strongly localized effect of modes in cavity quantum electrodynamics (QED) systems. By increasing the wing width of a cavity with special geometry symmetry, the interference property allows us to expo… ▽ More Large single-atom cooperativity in quantum systems is important for quantum information processing. Here, we propose to exponentially enhance the single-atom cooperativity parameter by exploiting the strongly localized effect of modes in cavity quantum electrodynamics (QED) systems. By increasing the wing width of a cavity with special geometry symmetry, the interference property allows us to exponentially improves the quality factor Q without altering the mode volume V for cavities supporting subwavelength light modes. This effectively overcomes the trade-off between Q and V in conventional subwavelength Fabry-Perot cavities. Consequently, we demonstrate the occurrence of ultra-long vacuum Rabi oscillations and the generation of strong photon blockade by enhancing the single-atom cooperativity parameter. This work offers a promising approach for advancing coherent manipulation and holds significant potential for applications in establishing longer-distance quantum communication networks, enhancing the precision and stability of quantum sensors, and improving the efficiency of quantum algorithms. △ Less

Submitted 4 September, 2025; originally announced September 2025.

Comments: 7 pages, 4 figures

Journal ref: Phys. Rev. Lett. 135, 103602 (2025)

Observation of Inelastic Meson Scattering in a Floquet System using a Digital Quantum Simulator

Authors: Ziting Wang, Zi-Yong Ge, Yun-Hao Shi, Zheng-An Wang, Si-Yun Zhou, Hao Li, Kui Zhao, Yue-Shan Xu, Wei-Guo Ma, Hao-Tian Liu, Cai-Ping Fang, Jia-Cheng Song, Tian-Ming Li, Jia-Chi Zhang, Yu Liu, Cheng-Lin Deng, Guangming Xue, Haifeng Yu, Kai Xu, Kaixuan Huang, Franco Nori, Heng Fan

Abstract: Lattice gauge theories provide a non-perturbative framework for understanding confinement and hadronic physics, but their real-time dynamics remain challenging for classical computations. However, quantum simulators offer a promising alternative for exploring such dynamics beyond classical capabilities. Here, we experimentally investigate meson scattering using a superconducting quantum processor.… ▽ More Lattice gauge theories provide a non-perturbative framework for understanding confinement and hadronic physics, but their real-time dynamics remain challenging for classical computations. However, quantum simulators offer a promising alternative for exploring such dynamics beyond classical capabilities. Here, we experimentally investigate meson scattering using a superconducting quantum processor. Employing a digital protocol, we realize a Floquet spin chain equivalent to a one-dimensional Floquet $\mathbb{Z}_2$ lattice gauge theory. We observe Bloch oscillations of single kinks and strong binding between adjacent kinks, signaling confinement and the formation of stable mesons in this Floquet system. Using full-system joint readout, we resolve meson populations by string length, enabling identification of meson scattering channels. Our results reveal the fragmentation of a long-string meson into multiple short-string mesons, which is also an experimental signature of string breaking. Moreover, we directly observe inelastic meson scattering, where two short-string mesons can merge into a longer one. Our results pave the way for studying interacting gauge particles and composite excitations on digital quantum simulators. △ Less

Submitted 28 August, 2025; originally announced August 2025.

Comments: 7+3 pages, 4+2 figures

Quantum circuit complexity and unsupervised machine learning of topological order

Authors: Yanming Che, Clemens Gneiting, Xiaoguang Wang, Franco Nori

Abstract: Inspired by the close relationship between Kolmogorov complexity and unsupervised machine learning, we explore quantum circuit complexity, an important concept in quantum computation and quantum information science, as a pivot to understand and to build interpretable and efficient unsupervised machine learning for topological order in quantum many-body systems. To span a bridge from conceptual pow… ▽ More Inspired by the close relationship between Kolmogorov complexity and unsupervised machine learning, we explore quantum circuit complexity, an important concept in quantum computation and quantum information science, as a pivot to understand and to build interpretable and efficient unsupervised machine learning for topological order in quantum many-body systems. To span a bridge from conceptual power to practical applicability, we present two theorems that connect Nielsen's quantum circuit complexity for the quantum path planning between two arbitrary quantum many-body states with fidelity change and entanglement generation, respectively. Leveraging these connections, fidelity-based and entanglement-based similarity measures or kernels, which are more practical for implementation, are formulated. Using the two proposed kernels, numerical experiments targeting the unsupervised clustering of quantum phases of the bond-alternating XXZ spin chain, the ground state of Kitaev's toric code and random product states, are conducted, demonstrating their superior performance. Relations with classical shadow tomography and shadow kernel learning are also discussed, where the latter can be naturally derived and understood from our approach. Our results establish connections between key concepts and tools of quantum circuit computation, quantum complexity, and machine learning of topological quantum order. △ Less

Submitted 6 August, 2025; originally announced August 2025.

Comments: 17 pages, with appendix; 4 figures. Code is available upon reasonable request, and will be open-sourced along with the publication. Comments are welcome

Probing strongly driven and strongly coupled superconducting qubit-resonator system

Authors: Oleh V. Ivakhnenko, Christoforus Dimas Satrya, Yu-Cheng Chang, Rishabh Upadhyay, Joonas T. Peltonen, Sergey N. Shevchenko, Franco Nori, Jukka P. Pekola

Abstract: We investigated a strongly driven qubit strongly connected to a quantum resonator. The measured system was a superconducting flux qubit coupled to a coplanar-waveguide resonator which is weakly coupled to a probing feedline. This hybrid qubit-resonator system was driven by a magnetic flux and probed with a weak probe signal through the feedline. We observed and theoretically described the quantum… ▽ More We investigated a strongly driven qubit strongly connected to a quantum resonator. The measured system was a superconducting flux qubit coupled to a coplanar-waveguide resonator which is weakly coupled to a probing feedline. This hybrid qubit-resonator system was driven by a magnetic flux and probed with a weak probe signal through the feedline. We observed and theoretically described the quantum interference effects, deviating from the usual single-qubit Landau-Zener-Stückelberg-Majorana interferometry, because the strong coupling distorts the qubit energy levels. △ Less

Submitted 5 August, 2025; originally announced August 2025.

Comments: 12 pages, 9 figures

Efficiently Generation of Cluster States via Time-Delayed Feedback in Matrix Representation

Authors: Jia-Jin Zou, Jian-Wei Qin, Franco Nori, Ze-Liang Xiang

Abstract: Cluster states, as highly entangled multi-qubit states, are widely used as essential resources for quantum communication and quantum computing. However, due to the diverse requirements of applications for cluster states with specific entanglement structures, a universal generation protocol is still lacking. Here we develop a matrix representation according to the characteristics of time-delayed fe… ▽ More Cluster states, as highly entangled multi-qubit states, are widely used as essential resources for quantum communication and quantum computing. However, due to the diverse requirements of applications for cluster states with specific entanglement structures, a universal generation protocol is still lacking. Here we develop a matrix representation according to the characteristics of time-delayed feedback (TDF) and propose a protocol for generating arbitrary cluster states with multiple TDFs. The matrix representation also allows us to optimize the generation process to reduce TDF usage, thus improving efficiency. In particular, we demonstrate a tree-cluster-state generation process that requires only one TDF. Moreover, accounting for the critical loss mechanisms and imperfections in our protocol, we discuss the additional losses caused by multiple TDFs and evaluate the fidelity of the resulting cluster states. △ Less

Submitted 21 July, 2025; originally announced July 2025.

Comments: 11 pages, 5 figures (1 figure in Appendix). Comments welcome

Scattering States in One-Dimensional Non-Hermitian Baths

Authors: Jimin Li, Yuwen E. Zhang, Franco Nori, Zongping Gong

Abstract: A single quantum emitter coupled to a structured non-Hermitian environment shows anomalous bound states and real-time dynamics without Hermitian counterparts, as shown in [Gong et al., Phys. Rev. Lett. 129, 223601 (2022)]. In this work, we establish a general approach for studying the scattering states of a single quantum emitter coupled to one-dimensional non-Hermitian single-band baths. We forma… ▽ More A single quantum emitter coupled to a structured non-Hermitian environment shows anomalous bound states and real-time dynamics without Hermitian counterparts, as shown in [Gong et al., Phys. Rev. Lett. 129, 223601 (2022)]. In this work, we establish a general approach for studying the scattering states of a single quantum emitter coupled to one-dimensional non-Hermitian single-band baths. We formally solve the exact eigenvalue equation for all the scattering states defined on finite periodic lattices. In the thermodynamic limit, the formal solution reduces to the celebrated Lippmann-Schwinger equation for generic baths. In this case, we find that the scattering states are no longer linear superpositions of plane waves in general, unlike those in Hermitian systems; Instead, the wave functions exhibit a large, yet finite localization length proportional to the lattice size. Furthermore, we show and discuss the cases where the Lippmann-Schwinger equation breaks down. We find the analytical solutions for the Hatano-Nelson and unidirectional next-to-nearest-neighbor baths in the thermodynamic limit. △ Less

Submitted 11 November, 2025; v1 submitted 2 July, 2025; originally announced July 2025.

Journal ref: Phys. Rev. A 112, 053707 (2025)

Efficient implementation of quantum signal processing via the adiabatic-impulse model

Authors: D. O. Shendryk, O. V. Ivakhnenko, S. N. Shevchenko, Franco Nori

Abstract: Here we investigate analogy between quantum signal processing (QSP) and the adiabatic-impulse model (AIM) in order to implement the QSP algorithm with fast quantum logic gates. QSP is an algorithm that uses single-qubit dynamics to perform a polynomial function transformation. AIM effectively describes the evolution of a two-level quantum system under strong external driving field. We can map para… ▽ More Here we investigate analogy between quantum signal processing (QSP) and the adiabatic-impulse model (AIM) in order to implement the QSP algorithm with fast quantum logic gates. QSP is an algorithm that uses single-qubit dynamics to perform a polynomial function transformation. AIM effectively describes the evolution of a two-level quantum system under strong external driving field. We can map parameters from QSP to AIM to implement QSP-like evolution with nonadiabatic, high-amplitude external drives. By choosing AIM parameters that control non-adiabatic transition parameters (such as driving amplitude $A$, frequency $ω$, and signal timing), one can achieve polynomial approximations and increase robustness in quantum circuits. The analogy presented here between QSP and AIM can be useful as a way to directly implement the QSP algorithm on quantum systems and obtain all the benefits from the fast Landau-Zener-Stuckelberg-Majotana (LZSM) quantum logic gates. △ Less

Submitted 1 December, 2025; v1 submitted 26 June, 2025; originally announced June 2025.

Comments: 12 pages, 5 figures

Journal ref: Phys. Rev. A 112, 042437 (2025)

Characterizing Many-body Dynamics with Projected Ensembles on a Superconducting Quantum Processor

Authors: Zhiguang Yan, Zi-Yong Ge, Rui Li, Yu-Ran Zhang, Franco Nori, Yasunobu Nakamura

Abstract: Quantum simulators offer a new opportunity for the experimental exploration of non-equilibrium quantum many-body dynamics, which have traditionally been characterized through expectation values or entanglement measures, based on density matrices of the system. Recently, a more general framework for studying quantum many-body systems based on projected ensembles has been introduced, revealing novel… ▽ More Quantum simulators offer a new opportunity for the experimental exploration of non-equilibrium quantum many-body dynamics, which have traditionally been characterized through expectation values or entanglement measures, based on density matrices of the system. Recently, a more general framework for studying quantum many-body systems based on projected ensembles has been introduced, revealing novel quantum phenomena, such as deep thermalization in chaotic systems. Here, we experimentally investigate a chaotic quantum many-body system using projected ensembles on a superconducting processor with 16 qubits on a square lattice. Our results provide direct evidence of deep thermalization by observing a Haar-distributed projected ensemble for the steady states within a charge-conserved sector. Moreover, by introducing an ensemble-averaged entropy as a metric, we establish a benchmark for many-body information leakage from the system to its environment. Our work paves the way for studying quantum many-body dynamics using projected ensembles and shows a potential implication for advancing quantum simulation techniques. △ Less

Submitted 26 June, 2025; originally announced June 2025.

Comments: 24+31 pages, 4 figures

Theory of multi-qubit superradiance in a waveguide in the presence of finite delay times

Authors: Sofia Arranz Regidor, Franco Nori, Stephen Hughes

Abstract: We study the quantum dynamics of multiple two-level atoms (qubits) in a waveguide quantum electrodynamics system, with a focus on modified superradiance effects between two or four atoms with finite delay times. Using a numerically exact matrix product approach, we explore both Markovian and non-Markovian regimes, and highlight the significant influence of time-delayed feedback effects and the cle… ▽ More We study the quantum dynamics of multiple two-level atoms (qubits) in a waveguide quantum electrodynamics system, with a focus on modified superradiance effects between two or four atoms with finite delay times. Using a numerically exact matrix product approach, we explore both Markovian and non-Markovian regimes, and highlight the significant influence of time-delayed feedback effects and the clear breakdown of assuming instantaneous coupling dynamics. We first show a system composed of two spatially separated qubits, prepared in a doubly excited state (both fully excited), and provide a comprehensive study of how delayed feedback influences the collective system decay rates, as well as the quantum correlations between waveguide photons, atoms, and between atom and photons. The system is then extended to include two additional qubits located next to the initial ones (four qubits in total), and we demonstrate, by manipulating the initial excitations and the time-delay effects, how long-term quantum correlations and light-matter entangled states can be established. △ Less

Submitted 19 June, 2025; originally announced June 2025.

Comments: 27 pages, 7 figures

A Time-Symmetric Quantum Algorithm for Direct Eigenstate Determination

Authors: Shijie Wei, Jingwei Wen, Xiaogang Li, Peijie Chang, Bozhi Wang, Franco Nori, Guilu Long

Abstract: Time symmetry in quantum mechanics, where the current quantum state is determined jointly by both the past and the future, offers a more comprehensive description of physical phenomena. This symmetry facilitates both forward and backward time evolution, providing a computational advantage over methods that rely on a fixed time direction. In this work, we present a nonvariational and \textit{time-s… ▽ More Time symmetry in quantum mechanics, where the current quantum state is determined jointly by both the past and the future, offers a more comprehensive description of physical phenomena. This symmetry facilitates both forward and backward time evolution, providing a computational advantage over methods that rely on a fixed time direction. In this work, we present a nonvariational and \textit{time-symmetric quantum algorithm} for addressing the eigenvalue problem of the Hamiltonian, leveraging the coherence between forward and backward time evolution. Our approach enables the simultaneous determination of both the ground state and the highest excited state, as well as the direct identification of arbitrary eigenstates of the Hamiltonian. Unlike existing methods, our algorithm eliminates the need for prior computation of lower eigenstates, allowing for the direct extraction of any eigenstate and energy bandwidth while avoiding error accumulation. Its non-variational nature ensures convergence to target states without encountering the barren plateau problem. We demonstrate the feasibility of implementing the non-unitary evolution using both the linear combination of unitaries and quantum Monte Carlo methods. Our algorithm is applied to compute the energy bandwidth and spectrum of various molecular systems, as well as to identify topological states in condensed matter systems, including the Kane-Mele model and the Su-Schrieffer-Heeger model. We anticipate that this algorithm will provide an efficient solution for eigenvalue problems, particularly in distinguishing quantum phases and calculating energy bands. △ Less

Submitted 11 June, 2025; originally announced June 2025.

Upper Bound on Quantum Fisher Information in Pseudo-Hermitian Systems

Authors: Ievgen I. Arkhipov, Franco Nori, Şahin K. Özdemir

Abstract: Non-Hermitian systems have attracted considerable interest over the last few decades due to their unique spectral and dynamical properties not encountered in Hermitian counterparts. An intensely debated question is whether non-Hermitian systems, described by pseudo-Hermitian Hamiltonians with real spectra, can offer enhanced sensitivity for parameter estimation when they are operated at or close t… ▽ More Non-Hermitian systems have attracted considerable interest over the last few decades due to their unique spectral and dynamical properties not encountered in Hermitian counterparts. An intensely debated question is whether non-Hermitian systems, described by pseudo-Hermitian Hamiltonians with real spectra, can offer enhanced sensitivity for parameter estimation when they are operated at or close to exceptional points. However, much of the current analysis and conclusions are based on mathematical formalism developed for Hermitian quantum systems, which is questionable when applied to pseudo-Hermitian Hamiltonians, whose Hilbert space is intrinsically curved. Here, we develop a covariant formulation of quantum Fisher information (QFI) defined on the curved Hilbert space of pseudo-Hermitian Hamiltonians. This covariant framework ensures the preservation of the state norm and enables a consistent treatment of parameter sensitivity. We further show that the covariant QFI of pseudo-Hermitian systems can be mapped to the ordinary QFI of corresponding Hermitian systems, and establish conditions when they become dual to each other, thus revealing a deeper geometric connection between the two. Importantly, this correspondence naturally imposes an upper bound on the covariant QFI and identifies the criteria under which quantum sensing in pseudo-Hermitian systems can exhibit supremacy over Hermitian ones. △ Less

Submitted 12 June, 2025; v1 submitted 11 June, 2025; originally announced June 2025.

Comments: 9 pages, 1 figure

Non-Hermitian sensing from the perspective of post-selected measurements

Authors: Neng Zeng, Tao Liu, Keyu Xia, Yu-Ran Zhang, Franco Nori

Abstract: By employing the Naimark dilation, we establish a fundamental connection between non-Hermitian quantum sensing and post-selected measurements. The sensitivity of non-Hermitian quantum sensors is determined by the effective quantum Fisher information (QFI), which incorporates the success probability of post-selection. We demonstrate that non-Hermitian sensors cannot outperform their Hermitian count… ▽ More By employing the Naimark dilation, we establish a fundamental connection between non-Hermitian quantum sensing and post-selected measurements. The sensitivity of non-Hermitian quantum sensors is determined by the effective quantum Fisher information (QFI), which incorporates the success probability of post-selection. We demonstrate that non-Hermitian sensors cannot outperform their Hermitian counterpart when all information is harnessed, since the total QFI for the extended system constrains the effective QFI of the non-Hermitian subsystem. Moreover, we quantify the efficiency of non-Hermitian sensors with the ratio of the effective QFI to the total QFI, which can be optimized within the framework of post-selected measurements with minimal experimental trials. Our work provides a distinctive theoretical framework for investigating non-Hermitian quantum sensing and designing noise-resilient quantum metrological protocols. △ Less

Submitted 12 May, 2025; v1 submitted 8 May, 2025; originally announced May 2025.

Comments: 7+6 pages, 3+4 figures

QuantumToolbox.jl: An efficient Julia framework for simulating open quantum systems

Authors: Alberto Mercurio, Yi-Te Huang, Li-Xun Cai, Yueh-Nan Chen, Vincenzo Savona, Franco Nori

Abstract: We present QuantumToolbox$.$jl, an open-source Julia package for simulating open quantum systems. Designed with a syntax familiar to users of QuTiP (Quantum Toolbox in Python), it harnesses Julia's high-performance ecosystem to deliver fast and scalable simulations. The package includes a suite of time-evolution solvers supporting distributed computing and GPU acceleration, enabling efficient simu… ▽ More We present QuantumToolbox$.$jl, an open-source Julia package for simulating open quantum systems. Designed with a syntax familiar to users of QuTiP (Quantum Toolbox in Python), it harnesses Julia's high-performance ecosystem to deliver fast and scalable simulations. The package includes a suite of time-evolution solvers supporting distributed computing and GPU acceleration, enabling efficient simulation of large-scale quantum systems. We also show how QuantumToolbox$.$jl can integrate with automatic differentiation tools, making it well-suited for gradient-based optimization tasks such as quantum optimal control. Benchmark comparisons demonstrate substantial performance gains over existing frameworks. With its flexible design and computational efficiency, QuantumToolbox$.$jl serves as a powerful tool for both theoretical studies and practical applications in quantum science. △ Less

Submitted 20 September, 2025; v1 submitted 30 April, 2025; originally announced April 2025.

Comments: 26 pages, 8 figures

Journal ref: Quantum 9, 1866 (2025)

Quantum excitation transfer in bosonic networks: a bipartite-graph framework

Authors: Cheng Liu, Yu-Hong Liu, Le-Man Kuang, Franco Nori, Jie-Qiao Liao

Abstract: Highly efficient transfer of quantum resources including quantum excitations, states, and information on quantum networks is an important task in quantum information science. Here, we propose a bipartite-graph framework for studying quantum excitation transfer in bosonic networks by diagonalizing the intermediate sub-network between the sender and the receiver to construct a bipartite-graph config… ▽ More Highly efficient transfer of quantum resources including quantum excitations, states, and information on quantum networks is an important task in quantum information science. Here, we propose a bipartite-graph framework for studying quantum excitation transfer in bosonic networks by diagonalizing the intermediate sub-network between the sender and the receiver to construct a bipartite-graph configuration. We examine the statistical properties of the bosonic networks in both the original and bipartite-graph representations. In particular, we investigate quantum excitation transfer in both the finite and infinite intermediate-normal-mode cases, and show the dependence of the transfer efficiency on the network configurations and system parameters. We find the bound of maximally transferred excitations for various network configurations and reveal the underlying physical mechanisms. We also find that the dark-mode effect will degrade the excitation transfer efficiency. Our findings provide a new insight for the design and optimization of quantum networks. △ Less

Submitted 22 April, 2025; originally announced April 2025.

Comments: 12 pages, 5 figures

Chirality-induced quantum nonreciprocity

Authors: Zimo Zhang, Zhongxiao Xu, Ran Huang, Xingda Lu, Fengbo Zhang, Donghao Li, Şahin K. Özdemir, Franco Nori, Han Bao, Yanhong Xiao, Bing Chen, Hui Jing, Heng Shen

Abstract: Chirality, nonreciprocity, and quantum correlations are at the center of a wide range of intriguing effects and applications across natural sciences and emerging quantum technologies. However, the direct link combining these three essential concepts has remained unknown till now. Here, we establish a chiral non-Hermitian platform with flying atoms and demonstrate chirality-induced nonreciprocal bi… ▽ More Chirality, nonreciprocity, and quantum correlations are at the center of a wide range of intriguing effects and applications across natural sciences and emerging quantum technologies. However, the direct link combining these three essential concepts has remained unknown till now. Here, we establish a chiral non-Hermitian platform with flying atoms and demonstrate chirality-induced nonreciprocal bipartite quantum correlations between two channels: Quantum correlation emerges when two spatially separated light beams of the same polarization propagate in opposite directions in the atomic cloud, and it becomes zero when they travel in the same direction. Thus, just by flipping the propagation direction of one of the beams while keeping its polarization the same as the other beam, we can create or annihilate quantum correlations between two channels. We also show that this nonreciprocal quantum correlation can be extended to multi-color sidebands with Floquet engineering. Our findings may pave the road for realizing one-way quantum effects, such as nonreciprocal squeezing or entanglement, with a variety of chiral devices, for the emerging applications of e.g., directional quantum network or nonreciprocal quantum metrology. △ Less

Submitted 21 April, 2025; v1 submitted 17 April, 2025; originally announced April 2025.

Comments: To be published

Waveguide QED with dissipative light-matter couplings

Authors: Xing-Liang Dong, Peng-Bo Li, Zongping Gong, Franco Nori

Abstract: Dissipative light-matter coupling plays a vital role in non-Hermitian physics, but it remains largely unexplored in waveguide QED systems. In this work, we find that by employing pseudo-Hermitian symmetry rather than anti-PT symmetry, the concept of dissipative coupling could be generalized and applied to the field of waveguide QED. This leads to a series of intriguing results, such as spontaneous… ▽ More Dissipative light-matter coupling plays a vital role in non-Hermitian physics, but it remains largely unexplored in waveguide QED systems. In this work, we find that by employing pseudo-Hermitian symmetry rather than anti-PT symmetry, the concept of dissipative coupling could be generalized and applied to the field of waveguide QED. This leads to a series of intriguing results, such as spontaneous breaking of pseudo-Hermitian symmetry across the exceptional points (EPs), level attraction between the bound states, and critical transition across the EPs for the population of quantum emitters in the bound state. Thanks to the tunability of photonic bands in crystal waveguides, we also demonstrate that dissipative light-matter coupling leads to the emergence of nonstandard third-order exceptional points with chiral spatial profiles in a topological waveguide QED system. This work provides a promising paradigm for studying non-Hermitian quantum phenomena in waveguide QED systems. △ Less

Submitted 19 March, 2025; originally announced March 2025.

Comments: 7 pages, 4 figures

Journal ref: Phys. Rev. Research 7, L012036 (2025)

Exploring Dynamics of Open Quantum Systems in Naturally Inaccessible Regimes

Authors: Xu-Ke Gu, Li-Zhou Tan, Franco Nori, J. Q. You

Abstract: Markovian open quantum systems are governed by the Lindblad master equation where the dissipation contains two parts, i.e., the anti-Hermitian operator and the quantum jumps, which share a common dissipation rate. We generalize the Lindblad master equation via postselection to a generalized Liouvillian formalism in which the effective damping rate of the anti-Hermitian operator can be different fr… ▽ More Markovian open quantum systems are governed by the Lindblad master equation where the dissipation contains two parts, i.e., the anti-Hermitian operator and the quantum jumps, which share a common dissipation rate. We generalize the Lindblad master equation via postselection to a generalized Liouvillian formalism in which the effective damping rate of the anti-Hermitian operator can be different from the quantum jump rate. Our formalism provides a parameter space with regimes inaccessible in naturally-occurring systems. We explore these new regimes and find several interesting results including negative damping rates and generalized Liouvillian exceptional points. In a previously unexplored zero-damping Liouvillian regime where the damping rate is negligible, we investigate the effect only due to the quantum jumps and show an unusual polynomial decay of the excited state. This generalized Liouvillian formalism offers opportunities to explore novel phenomena and quantum technologies associated with the peculiar behavior of quantum jumps. △ Less

Submitted 10 March, 2025; originally announced March 2025.

Comments: 6 pages, 3 figures

Heisenberg and Heisenberg-Like Representations via Hilbert Space Bundle Geometry in the Non-Hermitian Regime

Authors: Chia-Yi Ju, Adam Miranowicz, Jacob Barnett, Guang-Yin Chen, Franco Nori

Abstract: The equivalence between the Schrödinger and Heisenberg representations is a cornerstone of quantum mechanics. However, this relationship remains unclear in the non-Hermitian regime, particularly when the Hamiltonian is time-dependent. In this study, we address this gap by establishing the connection between the two representations, incorporating the metric of the Hilbert space bundle. We not only… ▽ More The equivalence between the Schrödinger and Heisenberg representations is a cornerstone of quantum mechanics. However, this relationship remains unclear in the non-Hermitian regime, particularly when the Hamiltonian is time-dependent. In this study, we address this gap by establishing the connection between the two representations, incorporating the metric of the Hilbert space bundle. We not only demonstrate the consistency between the Schrödinger and Heisenberg representations but also present a Heisenberg-like representation grounded in the generalized vielbein formalism, which provides a clear and intuitive geometric interpretation. Unlike the standard Heisenberg representation, where the metric of the Hilbert space is encoded solely in the dual states, the Heisenberg-like representation distributes the metric information between both the states and the dual states. Despite this distinction, it retains the same Heisenberg equation of motion for operators. Within this formalism, the Hamiltonian is replaced by a Hermitian counterpart, while the "non-Hermiticity" is transferred to the operators. Moreover, this approach extends to regimes with a dynamical metric (beyond the pseudo-Hermitian framework) and to systems governed by time-dependent Hamiltonians. △ Less

Submitted 13 May, 2025; v1 submitted 5 March, 2025; originally announced March 2025.

Comments: 8 pages, 3 tables

Precise Quantum Control of Molecular Rotation Toward a Desired Orientation

Authors: Qian-Qian Hong, Daoyi Dong, Niels E. Henriksen, Franco Nori, Jun He, Chuan-Cun Shu

Abstract: The lack of a direct map between control fields and desired control objectives poses a significant challenge in applying quantum control theory to quantum technologies. Here, we propose an analytical framework to precisely control a limited set of quantum states and construct desired coherent superpositions using a well-designed laser pulse sequence with optimal amplitudes, phases, and delays. Thi… ▽ More The lack of a direct map between control fields and desired control objectives poses a significant challenge in applying quantum control theory to quantum technologies. Here, we propose an analytical framework to precisely control a limited set of quantum states and construct desired coherent superpositions using a well-designed laser pulse sequence with optimal amplitudes, phases, and delays. This theoretical framework that corresponds to a multi-level pulse-area theorem establishes a straightforward mapping between the control parameters of the pulse sequence and the amplitudes and phases of rotational states within a specific subspace. As an example, we utilize this approach to generate 15 distinct and desired rotational superpositions of ultracold polar molecules, leading to 15 desired field-free molecular orientations. By optimizing the superposition of the lowest 16 rotational states, we demonstrate that this approach can achieve a maximum orientation value of $|\langle\cosθ\rangle|_{\rm{max}}$ above 0.99, which is very close to the global optimal value of 1 that could be achieved in an infinite-dimensional state space. This work marks a significant advancement in achieving precise control over multi-level subsystems within molecules. It holds potential applications in molecular alignment and orientation, as well as in various interdisciplinary fields related to the precise quantum control of ultracold polar molecules, opening up considerable opportunities in molecular-based quantum techniques. △ Less

Submitted 14 February, 2025; originally announced February 2025.

Comments: The manuscript has been accepted for publication in Phys. Rev. Research as a Letter

Decoding scrambled quantum information that was never encoded: An experimental demonstration

Authors: Yi-Te Huang, Siang-Wei Huang, Jhen-Dong Lin, Adam Miranowicz, Neill Lambert, Guang-Yin Chen, Franco Nori, Yueh-Nan Chen

Abstract: Quantum information scrambling (QIS) describes the rapid spread of initially localized information across an entire quantum many-body system through entanglement generation. Once scrambled, the original local information becomes encoded globally, inaccessible from any single subsystem. In this work, we introduce a protocol that enables information scrambling into the past, allowing decoding even b… ▽ More Quantum information scrambling (QIS) describes the rapid spread of initially localized information across an entire quantum many-body system through entanglement generation. Once scrambled, the original local information becomes encoded globally, inaccessible from any single subsystem. In this work, we introduce a protocol that enables information scrambling into the past, allowing decoding even before the original information is generated. This protocol is achieved by simulating a closed timelike curve (a theoretical construct in which particles can traverse backward in time) using postselection methods. Remarkably, the postselected outcome corresponds to a paradox-free trajectory that enables consistent time travel and reliable information recovery. Furthermore, the success probability is governed by out-of-time-ordered correlations, which is a standard measure of QIS. We present a quantum circuit design and experimentally implement our protocol on cloud-based Quantinuum and IBM quantum processors. Our approach illuminates a unique quantum task: retrieving information encoded in the future without altering the past. △ Less

Submitted 28 July, 2025; v1 submitted 4 January, 2025; originally announced January 2025.

Comments: 14 pages, 6 figures, 4 tables

Topological state permutations in time-modulated non-Hermitian multiqubit systems with suppressed non-adiabatic transitions

Authors: Ievgen I. Arkhipov, Philippe Lewalle, Franco Nori, Şahin K. Özdemir, K. Birgitta Whaley

Abstract: Non-Hermitian systems have been at the center of intense research for over a decade, partly due to their nontrivial energy topology formed by intersecting Riemann manifolds with branch points known as exceptional points (EPs). This spectral property can be exploited, e.g., to achieve topologically controlled state permutations that are necessary for implementing a wide class of classical and quant… ▽ More Non-Hermitian systems have been at the center of intense research for over a decade, partly due to their nontrivial energy topology formed by intersecting Riemann manifolds with branch points known as exceptional points (EPs). This spectral property can be exploited, e.g., to achieve topologically controlled state permutations that are necessary for implementing a wide class of classical and quantum information protocols. However, the complex-valued spectra of typical non-Hermitian systems lead to instabilities, losses, and breakdown of adiabaticity, which impedes the practical use of EP-induced energy topologies in quantum information protocols based on state permutation symmetries. Indeed, in a given non-Hermitian multiqubit system, the dynamical winding around EPs always results in a predetermined set of attenuated final eigenstates, due to the interplay of decoherence and non-adiabatic transitions, irrespective of the initial conditions. In this work, we address this long-standing problem by introducing a model of interacting qubits governed by an effective non-Hermitian Hamiltonian that hosts novel types of EPs while maintaining a completely real energy spectrum, ensuring the absence of losses in the system's dynamics. We demonstrate that such non-Hermitian Hamiltonians enable the realization of genuine, in general, non-Abelian permutation groups in the multiqubit system's eigenspace while dynamically encircling these EPs. Our findings indicate that, contrary to previous beliefs, non-Hermiticity can be utilized to achieve controlled topological state permutations in time-modulated multiqubit systems, thus paving the way for the advancement and development of novel quantum information protocols in real-world non-Hermitian quantum systems. △ Less

Submitted 11 September, 2025; v1 submitted 27 January, 2025; originally announced January 2025.

Comments: 26 pages, 15 figures

Journal ref: Phys. Rev. Research 7, 033242 (2025)

Achieving Robust Single-Photon Blockade with a Single Nanotip

Authors: Jian Tang, Yun-Lan Zuo, Xun-Wei Xu, Ran Huang, Adam Miranowicz, Franco Nori, Hui Jing

Abstract: Backscattering losses, due to intrinsic imperfections or external perturbations that are unavoidable in optical resonators, can severely affect the performance of practical photonic devices. In particular, for quantum single-photon devices, robust quantum correlations against backscattering losses, which are highly desirable for diverse applications, have remained largely unexplored. Here, we show… ▽ More Backscattering losses, due to intrinsic imperfections or external perturbations that are unavoidable in optical resonators, can severely affect the performance of practical photonic devices. In particular, for quantum single-photon devices, robust quantum correlations against backscattering losses, which are highly desirable for diverse applications, have remained largely unexplored. Here, we show that single-photon blockade against backscattering loss, an important purely quantum effect, can be achieved by introducing a nanotip near a Kerr nonlinear resonator with intrinsic defects. We find that the quantum correlation of single photons can approach that of a lossless cavity even in the presence of strong backscattering losses. Moreover, the behavior of such quantum correlation is distinct from that of the classical mean-photon number with different strengths of the nonlinearity, due to the interplay of the resonator nonlinearity and the tip-induced optical coupling. Our work sheds new light on protecting and engineering fragile quantum devices against imperfections, for applications in robust single-photon sources and backscattering-immune quantum devices. △ Less

Submitted 27 December, 2024; originally announced December 2024.

Comments: 4 figures

QuTiP 5: The Quantum Toolbox in Python

Authors: Neill Lambert, Eric Giguère, Paul Menczel, Boxi Li, Patrick Hopf, Gerardo Suárez, Marc Gali, Jake Lishman, Rushiraj Gadhvi, Rochisha Agarwal, Asier Galicia, Nathan Shammah, Paul Nation, J. R. Johansson, Shahnawaz Ahmed, Simon Cross, Alexander Pitchford, Franco Nori

Abstract: QuTiP, the Quantum Toolbox in Python, has been at the forefront of open-source quantum software for the past 13 years. It is used as a research, teaching, and industrial tool, and has been downloaded millions of times by users around the world. Here we introduce the latest developments in QuTiP v5, which are set to have a large impact on the future of QuTiP and enable it to be a modern, continuous… ▽ More QuTiP, the Quantum Toolbox in Python, has been at the forefront of open-source quantum software for the past 13 years. It is used as a research, teaching, and industrial tool, and has been downloaded millions of times by users around the world. Here we introduce the latest developments in QuTiP v5, which are set to have a large impact on the future of QuTiP and enable it to be a modern, continuously developed and popular tool for another decade and more. We summarize the code design and fundamental data layer changes as well as efficiency improvements, new solvers, applications to quantum circuits with QuTiP-QIP, and new quantum control tools with QuTiP-QOC. Additional flexibility in the data layer underlying all ``quantum objects'' in QuTiP allows us to harness the power of state-of-the-art data formats and packages like JAX, CuPy, and more. We explain these new features with a series of both well-known and new examples. The code for these examples is available in a static form on GitHub and as continuously updated and documented notebooks in the qutip-tutorials package. △ Less

Submitted 1 October, 2025; v1 submitted 5 December, 2024; originally announced December 2024.

Comments: 82 pages, 25 figures. Updated with additional tables, information on environment class, and formatting fixes

Tunable quantum router with giant atoms, implementing quantum gates, teleportation, non-reciprocity, and circulators

Authors: Rui-Yang Gong, Zi-Yu He, Cheng-He Yu, Ge-Fei Zhang, Franco Nori, Ze-Liang Xiang

Abstract: The unique photon-scattering phenomena of giant-atom systems offer a novel paradigm for exploring innovative quantum optics phenomena and applications. Here, we investigate a giant-atom configuration embedded in a dual-rail waveguide, whose scattering behavior is analytically derived based on a four-port model and affected by both waveguide-induced and interatomic interaction phases. One can modul… ▽ More The unique photon-scattering phenomena of giant-atom systems offer a novel paradigm for exploring innovative quantum optics phenomena and applications. Here, we investigate a giant-atom configuration embedded in a dual-rail waveguide, whose scattering behavior is analytically derived based on a four-port model and affected by both waveguide-induced and interatomic interaction phases. One can modulate these phases to achieve targeted routing and non-reciprocal scattering of photons. Furthermore, using such a configuration, we propose quantum applications such as quantum storage, path-encoded quantum gates (e.g., CNOT gate), quantum teleportation, and quantum circulators. This configuration can be implemented with state-of-the-art solid-state quantum systems, enabling a wide range of quantum applications and facilitating the development of quantum networks. △ Less

Submitted 28 November, 2024; originally announced November 2024.

Comments: 18 pages,11 figures

A universal framework for the quantum simulation of Yang-Mills theory

Authors: Jad C. Halimeh, Masanori Hanada, Shunji Matsuura, Franco Nori, Enrico Rinaldi, Andreas Schäfer

Abstract: We provide a universal framework for the quantum simulation of SU(N) Yang--Mills theories on fault-tolerant digital quantum computers adopting the orbifold lattice formulation. As warm-up examples, we also consider simple models, including scalar field theory and the Yang--Mills matrix model, to illustrate the universality of our formulation, which shows up in the fact that the truncated Hamiltoni… ▽ More We provide a universal framework for the quantum simulation of SU(N) Yang--Mills theories on fault-tolerant digital quantum computers adopting the orbifold lattice formulation. As warm-up examples, we also consider simple models, including scalar field theory and the Yang--Mills matrix model, to illustrate the universality of our formulation, which shows up in the fact that the truncated Hamiltonian can be expressed in the same simple form for any N, any dimension, and any lattice size, in stark contrast to the popular approach based on the Kogut--Susskind formulation. In all these cases, the truncated Hamiltonian can be programmed on a quantum computer using only standard tools well-established in the field of quantum computation. As a concrete application of this universal framework, we consider Hamiltonian time evolution by Suzuki--Trotter decomposition. This turns out to be a straightforward task due to the simplicity of the truncated Hamiltonian. We also provide a simple circuit structure that contains only CNOT and one-qubit gates, independent of the details of the theory investigated. △ Less

Submitted 15 November, 2025; v1 submitted 20 November, 2024; originally announced November 2024.

Comments: v2: to appear in Communications Physics

Report number: RIKEN-iTHEMS-Report-24

Versatile Control of Nonlinear Topological States in Non-Hermitian Systems

Authors: Zhao-Fan Cai, Yu-Chun Wang, Yu-Ran Zhang, Tao Liu, Franco Nori

Abstract: The non-Hermitian skin effect (NHSE) and nonlinearity can both delocalize topological modes (TMs) from the interface. However, the NHSE requires precise parameter tuning, while nonlinearity in Hermitian systems results in partial delocalization with limited mode capacity. To overcome these limitations, we propose a non-Hermitian nonlinear topological interface model that integrates Hermitian and n… ▽ More The non-Hermitian skin effect (NHSE) and nonlinearity can both delocalize topological modes (TMs) from the interface. However, the NHSE requires precise parameter tuning, while nonlinearity in Hermitian systems results in partial delocalization with limited mode capacity. To overcome these limitations, we propose a non-Hermitian nonlinear topological interface model that integrates Hermitian and non-Hermitian lattices with nonreciprocal hopping and nonlinearity. This system enables the complete delocalization of TMs across the entire lattice without fine-tuning, while allowing precise control over the wavefunction profile and spatial distribution through the intrinsic configuration and intensity of the nonlinearity. Using the spectral localizer, we demonstrate the topological protection and robustness of these extended non-Hermitian TMs against disorder. Furthermore, we show that under external pumping, localized excitations evolve into predefined profiles and generate long-range patterns, an effect unattainable in Hermitian systems. These findings reveal how the interplay of nonlinearity and NHSE shapes topological states, paving the way for compact topological devices. △ Less

Submitted 30 August, 2025; v1 submitted 15 November, 2024; originally announced November 2024.

Comments: 21 pages, 16 figures

Journal ref: Commun Phys 8, 360 (2025)

Resource-efficient quantum correlation measurements via multicopy neural network methods

Authors: Patrycja Tulewicz, Karol Bartkiewicz, Adam Miranowicz, Franco Nori

Abstract: Measuring complex properties in quantum systems, such as measures of quantum entanglement and Bell nonlocality, is inherently challenging. Traditional methods, like quantum state tomography (QST), necessitate a full reconstruction of the density matrix for a given system and demand resources that scale exponentially with system size. We propose an alternative strategy that reduces the required inf… ▽ More Measuring complex properties in quantum systems, such as measures of quantum entanglement and Bell nonlocality, is inherently challenging. Traditional methods, like quantum state tomography (QST), necessitate a full reconstruction of the density matrix for a given system and demand resources that scale exponentially with system size. We propose an alternative strategy that reduces the required information by combining multicopy measurements with artificial neural networks (ANNs), resulting in a 67\% reduction in measurement requirements compared to QST. We have successfully measured two-qubit quantum correlations of Bell states subjected to a depolarizing channel (resulting in Werner states) and an amplitude damping channel (leading to Horodecki states) using the multicopy approach on real quantum hardware. Our experiments, conducted with transmon qubits on IBMQ processors, quantified the violation of Bell's inequality and the negativity of two-qubit entangled states. We compared these results with those obtained from the standard QST approach and applied a maximum likelihood method to mitigate noise. We trained ANNs to estimate entanglement and nonlocality measures using optimized sets of projections identified through Shapley's (SHAP) analysis for the Werner and Horodecki states. The ANN output, based on this reduced set of projections, aligns well with expected values and enhances noise robustness. This approach simplifies and improves the error-robustness of multicopy measurements, eliminating the need for complex nonlinear equation analysis. It represents a significant advancement in AI-assisted quantum measurements, making practical implementation on current quantum hardware more feasible. △ Less

Submitted 24 November, 2025; v1 submitted 8 November, 2024; originally announced November 2024.

MSC Class: 81P42; 81P15; 81P68

Journal ref: Scientific Reports 15, 40868 (2025)

Suppressed Energy Relaxation in the Quantum Rabi Model at the Critical Point

Authors: Ye-Hong Chen, Zhi-Cheng Shi, Yu-Ran Zhang, Franco Nori, Yan Xia

Abstract: We derive a modified master equation for the quantum Rabi model in the parameter regime where quantum criticality can occur. The modified master equation can avoid some unphysical predictions, such as excitations in the system at zero temperature and emission of ground-state photons. Due to spectrum collapse, we find that there is mostly no energy relaxation in the system at the critical point. Fo… ▽ More We derive a modified master equation for the quantum Rabi model in the parameter regime where quantum criticality can occur. The modified master equation can avoid some unphysical predictions, such as excitations in the system at zero temperature and emission of ground-state photons. Due to spectrum collapse, we find that there is mostly no energy relaxation in the system at the critical point. For the same reason, phase coherence rapidly reduces and vanishes at the critical point. We analyze the quantum metrological limits of the system in the presence of dephasing. These results show a strong limitation on the precision of phase-shift estimation. △ Less

Submitted 6 November, 2024; originally announced November 2024.

Comments: 7 pages, two figures, comments are welcome

Neural Network-Based Design of Approximate Gottesman-Kitaev-Preskill Code

Authors: Yexiong Zeng, Wei Qin, Ye-Hong Chen, Clemens Gneiting, Franco Nori

Abstract: Gottesman-Kitaev-Preskill (GKP) encoding holds promise for continuous-variable fault-tolerant quantum computing. While an ideal GKP encoding is abstract and impractical due to its nonphysical nature, approximate versions provide viable alternatives. Conventional approximate GKP codewords are superpositions of multiple {large-amplitude} squeezed coherent states. This feature ensures correctability… ▽ More Gottesman-Kitaev-Preskill (GKP) encoding holds promise for continuous-variable fault-tolerant quantum computing. While an ideal GKP encoding is abstract and impractical due to its nonphysical nature, approximate versions provide viable alternatives. Conventional approximate GKP codewords are superpositions of multiple {large-amplitude} squeezed coherent states. This feature ensures correctability against single-photon loss and dephasing {at short times}, but also increases the difficulty of preparing the codewords. To minimize this trade-off, we utilize a neural network to generate optimal approximate GKP states, allowing effective error correction with just a few squeezed coherent states. We find that such optimized GKP codes outperform the best conventional ones, requiring fewer squeezed coherent states, while maintaining simple and generalized stabilizer operators. Specifically, the former outperform the latter with just \textit{one third} of the number of squeezed coherent states at a squeezing level of 9.55 dB. This optimization drastically decreases the complexity of codewords while improving error correctability. △ Less

Submitted 2 November, 2024; originally announced November 2024.

Report number: Yexiong Zeng, Wei Qin, Ye-Hong Chen, Clemens Gneiting, Franco Nori, Phys. Rev. Lett. 134, 060601 (2025)

Journal ref: Phys. Rev. Lett. 134, 060601 (2025)

Efficient charging of multiple open quantum batteries through dissipation and pumping

Authors: Josephine Dias, Hui Wang, Kae Nemoto, Franco Nori, William J. Munro

Abstract: We explore a protocol that efficiently charges multiple open quantum batteries in parallel using a single charger. This protocol shows super-extensive charging through collective coupling of the charger and the battery to the same thermal reservoir. When applied to multiple quantum batteries, each coupled to different thermal reservoirs, the energy cannot be efficiently transferred from the charge… ▽ More We explore a protocol that efficiently charges multiple open quantum batteries in parallel using a single charger. This protocol shows super-extensive charging through collective coupling of the charger and the battery to the same thermal reservoir. When applied to multiple quantum batteries, each coupled to different thermal reservoirs, the energy cannot be efficiently transferred from the charger to the battery via collective dissipation alone. We show that the counter-intuitive act of incorporating both dissipation and incoherent collective pumping on the charger enables efficient parallel charging of many quantum batteries. △ Less

Submitted 25 October, 2024; originally announced October 2024.

Comments: 5 pages, 2 figures

Engineering the Environment of a Superconducting Qubit with an Artificial Giant Atom

Authors: Jingjing Hu, Dengfeng Li, Yufan Qie, Zelong Yin, Anton Frisk Kockum, Franco Nori, Shuoming An

Abstract: In quantum computing, precise control of system-environment coupling is essential for high-fidelity gates, measurements, and networking. We present an architecture that employs an artificial giant atom from waveguide quantum electrodynamics to tailor the interaction between a superconducting qubit and its environment. This frequency-tunable giant atom exhibits both frequency and power selectivity… ▽ More In quantum computing, precise control of system-environment coupling is essential for high-fidelity gates, measurements, and networking. We present an architecture that employs an artificial giant atom from waveguide quantum electrodynamics to tailor the interaction between a superconducting qubit and its environment. This frequency-tunable giant atom exhibits both frequency and power selectivity for photons: when resonant with the qubit, it reflects single photons emitted from the qubit while remaining transparent to strong microwave signals for readout and control. This approach surpasses the Purcell limit and significantly extends the qubit's lifetime by ten times while maintaining the readout speed, thereby improving both gate operations and readout. Our architecture holds promise for bridging circuit and waveguide quantum electrodynamics systems in quantum technology applications. △ Less

Submitted 20 October, 2024; originally announced October 2024.

How to realize compact and non-compact localized states in disorder-free hypercube networks

Authors: Ievgen I. Arkhipov, Fabrizio Minganti, Franco Nori

Abstract: We present a method for realizing various zero-energy localized states on disorder-free hypercube graphs. Previous works have already indicated that disorder is not essential for observing localization phenomena in noninteracting systems, with some prominent examples including the 1D Aubry-André model, characterized solely by incommensurate potentials, or 2D incommensurate Moiré lattices, which ex… ▽ More We present a method for realizing various zero-energy localized states on disorder-free hypercube graphs. Previous works have already indicated that disorder is not essential for observing localization phenomena in noninteracting systems, with some prominent examples including the 1D Aubry-André model, characterized solely by incommensurate potentials, or 2D incommensurate Moiré lattices, which exhibit localization due to the flat band spectrum. Moreover, flat band systems with translational invariance can also possess so-called compact localized states, characterized by exactly zero amplitude outside a finite region of the lattice. Here, we demonstrate that both compact and non-compact (i.e., Anderson-like) localized states naturally emerge in disorder-free hypercubes, which can be systematically constructed using Cartan products. This construction ensures the robustness of these localized states against perturbations. Furthermore, we show that the hypercubes can be associated with the Fock space of interacting spin systems exhibiting localization. Viewing localization from the hypercube perspective, with its inherently simple eigenspace structure, offers a clearer and more intuitive understanding of the underlying Fock-space many-body localization phenomena. Our findings can be readily tested on existing experimental platforms, where hypercube graphs can be emulated, e.g., by photonic networks of coupled optical cavities or waveguides. The results can pave the way for the development of novel quantum information protocols and enable effective simulation of quantum many-body localization phenomena. △ Less

Submitted 1 July, 2025; v1 submitted 14 October, 2024; originally announced October 2024.

Comments: 14 pages

Journal ref: Phys. Rev. Research 7, 033002 (2025)

Quantum heat engine based on quantum interferometry: the SU(1,1) Otto cycle

Authors: Alessandro Ferreri, Hui Wang, Franco Nori, Frank K. Wilhelm, David Edward Bruschi

Abstract: We present a quantum heat engine based on a quantum Otto cycle, whose working substance reproduces the same outcomes of a SU(1,1) interference process at the end of each adiabatic transformation. This device takes advantage of the extraordinary quantum metrological features of the SU(1,1) interferometer to better discriminate the sources of uncertainty of relevant observables during each adiabatic… ▽ More We present a quantum heat engine based on a quantum Otto cycle, whose working substance reproduces the same outcomes of a SU(1,1) interference process at the end of each adiabatic transformation. This device takes advantage of the extraordinary quantum metrological features of the SU(1,1) interferometer to better discriminate the sources of uncertainty of relevant observables during each adiabatic stroke of the cycle. Applications to circuit QED platforms are also discussed. △ Less

Submitted 24 September, 2024; v1 submitted 20 September, 2024; originally announced September 2024.

Journal ref: Phys. Rev. Research 7, 013284 (2025)

Dissipation and Interaction-Controlled Non-Hermitian Skin Effects

Authors: Yang Li, Zhao-Fan Cai, Tao Liu, Franco Nori

Abstract: Non-Hermitian skin effects (NHSEs) have recently been extensively studied at the single-particle level. When many-body interactions become dominant, novel non-Hermitian phenomena can emerge. In this work, we propose an experimentally accessible mechanism to induce and control NHSEs in interacting and reciprocal dissipative systems. We consider both 1D and 2D Bose-Hubbard lattices subject to stagge… ▽ More Non-Hermitian skin effects (NHSEs) have recently been extensively studied at the single-particle level. When many-body interactions become dominant, novel non-Hermitian phenomena can emerge. In this work, we propose an experimentally accessible mechanism to induce and control NHSEs in interacting and reciprocal dissipative systems. We consider both 1D and 2D Bose-Hubbard lattices subject to staggered two-particle loss combined with synthetic magnetic flux and long-range hopping. When the two-particle loss is small, the bound eigenstates (e.g., doublons and triplons) are all localized at the same boundary due to the interplay between the magnetic flux and staggered two-particle loss. In contrast, for strong two-particle loss, the skin-mode localization direction of the bound particles is unexpectedly reversed. This reversal stems from the combined effect of the staggered two-particle loss, synthetic magnetic flux, and long-range hopping, through which virtual second-order and third-order hopping processes induce effectively strong nonreciprocal hopping of doublons. Our results open up a new avenue for exploring novel non-Hermitian phenomena in many-body systems. △ Less

Submitted 10 July, 2025; v1 submitted 22 August, 2024; originally announced August 2024.

Comments: 16 pages, 12 figures; New model

Identifying Entanglement Phases with Bipartite Projected Ensembles

Authors: Zi-Yong Ge, Franco Nori

Abstract: We introduce bipartite projected ensembles (BPEs) for quantum many-body wave functions, which consist of pure states supported on two local subsystems, with each state associated with the outcome of a projective measurement of the complementary subsystem in a fixed local basis. We demonstrate that the corresponding ensemble-averaged entanglements (EAEs) between two subsystems can effectively ident… ▽ More We introduce bipartite projected ensembles (BPEs) for quantum many-body wave functions, which consist of pure states supported on two local subsystems, with each state associated with the outcome of a projective measurement of the complementary subsystem in a fixed local basis. We demonstrate that the corresponding ensemble-averaged entanglements (EAEs) between two subsystems can effectively identify entanglement phases. In volume-law entangled states, EAE converges to a nonzero value with increasing distance between subsystems. For critical systems, EAE exhibits power-law decay, and it decays exponentially for area-law systems. Thus, entanglement phase transitions can be viewed as a disordered-ordered phase transition. We also apply BPE and EAE to measured random Clifford circuits to probe measurement-induced phase transitions. We show that EAE serves not only as a witness to phase transitions, but also unveils additional critical phenomena properties, including dynamical scaling and surface critical exponents. Our findings provide an alternative approach to diagnosing entanglement laws, thus enhancing the understanding of entanglement phase transitions. Moreover, given the accessibility of measuring EAE in quantum simulators, our results hold promise for impacting quantum simulations. △ Less

Submitted 1 November, 2025; v1 submitted 15 August, 2024; originally announced August 2024.

Comments: 14 pages, 5+2 figures

Journal ref: Phys. Rev. Research 7, 043022(2025)

Quantum Cross Nonlinearity for Photon-Number-Resolving Nondestructive Detection

Authors: Jiang-Shan Tang, Mingyuan Chen, Miao Cai, Lei Tang, Yanqing Lu, Keyu Xia, Franco Nori

Abstract: We present an unconventional mechanism for quantum nonlinearity in a system comprising of a V-type quantum emitter (QE) and two Fabry-Perot cavities. The two transitions of the V-type QE are effectively coupled with two independent cavity modes. The system exhibits a strong quantum nonlinear control in the transmission even at the single-photon level, which we refer to as quantum cross nonlinearit… ▽ More We present an unconventional mechanism for quantum nonlinearity in a system comprising of a V-type quantum emitter (QE) and two Fabry-Perot cavities. The two transitions of the V-type QE are effectively coupled with two independent cavity modes. The system exhibits a strong quantum nonlinear control in the transmission even at the single-photon level, which we refer to as quantum cross nonlinearity. The underlying physics can be understood as quantum competition between the two transitions of the QE sharing a common ground state. By leveraging this quantum cross nonlinearity, we further show photon-number-resolving quantum nondestructive detection. Owing to the widespread nature of this V-type configuration, our approach can be readily extended to diverse cavity quantum electrodynamic systems beyond the realm of optics, encompassing, e.g., microwave photons and acoustic wave phonons. This versatility may facilitate numerous unique applications for quantum information processing. △ Less

Submitted 14 August, 2024; originally announced August 2024.

Chiral-Extended Photon-Emitter Dressed States in Non-Hermitian Topological Baths

Authors: Zhao-Fan Cai, Xin Wang, Zi-Xuan Liang, Tao Liu, Franco Nori

Abstract: The interplay of quantum emitters and non-Hermitian structured baths has received increasing attention in recent years. Here, we predict unconventional quantum optical behaviors of quantum emitters coupled to a non-Hermitian topological bath, which is realized in a 1D Su-Schrieffer-Heeger photonic chain subjected to nonlocal dissipation. In addition to the Hermitian-like chiral bound states in the… ▽ More The interplay of quantum emitters and non-Hermitian structured baths has received increasing attention in recent years. Here, we predict unconventional quantum optical behaviors of quantum emitters coupled to a non-Hermitian topological bath, which is realized in a 1D Su-Schrieffer-Heeger photonic chain subjected to nonlocal dissipation. In addition to the Hermitian-like chiral bound states in the middle line gap and skin-mode-like hidden bound states inside the point gap, we identify peculiar in-gap chiral and extended photon-emitter dressed states. This is due to the competition of topological-edge localization and non-Hermitian skin-mode localization in combination with the non-Bloch bulk-boundary correspondence. Strikingly, dissipation can shape the wavefunction profile of the dressed state. Furthermore, when two emitters are coupled to the same bath, such in-gap dressed states can mediate the nonreciprocal long-range emitter-emitter interactions, with the interaction range limited only by the dissipation of the bath. Our work opens the door to further study rich quantum optical phenomena and exotic many-body physics utilizing quantum emitters coupled to non-Hermitian baths. △ Less

Submitted 10 July, 2025; v1 submitted 14 August, 2024; originally announced August 2024.

Comments: 23 pages, 10 figures; Published Version in PRA Letters

Journal ref: Phys. Rev. A 111, L061701 (2025)

Floquet engineering the quantum Rabi model in the ultrastrong coupling regime

Authors: Kamran Akbari, Franco Nori, Stephen Hughes

Abstract: We study the quantum Rabi model for a two-level system coupled to a quantized cavity mode under periodic modulation of the cavity-dipole coupling in the ultrastrong coupling regime, leading to rich Floquet states. As an application of the theory, we show how purely mechanical driving can produce real photons, depending on the strength and frequency of the periodic coupling rate. We study the quantum Rabi model for a two-level system coupled to a quantized cavity mode under periodic modulation of the cavity-dipole coupling in the ultrastrong coupling regime, leading to rich Floquet states. As an application of the theory, we show how purely mechanical driving can produce real photons, depending on the strength and frequency of the periodic coupling rate. △ Less

Submitted 23 July, 2024; originally announced July 2024.

Comments: 8 pages, 3 figures

Journal ref: Phys. Rev. Lett. 134, 063602 (2025)

Using PT-symmetric Qubits to Break the Tradeoff Between Fidelity and the Degree of Quantum Entanglement

Authors: B. -B. Liu, Shi-Lei Su, Y. -L. Zuo, Qiongyi He, Gang Chen, F. Nori, H. Jing

Abstract: A noteworthy discovery is that the minimal evolution time is smaller for parity-time ($\mathcal{PT}$) symmetric systems compared to Hermitian setups. Moreover, there is a significant acceleration of two-qubit quantum entanglement preparation near the exceptional point (EP), or spectral coalescence, within such system. Nevertheless, an important problem often overlooked for quantum EP-based devices… ▽ More A noteworthy discovery is that the minimal evolution time is smaller for parity-time ($\mathcal{PT}$) symmetric systems compared to Hermitian setups. Moreover, there is a significant acceleration of two-qubit quantum entanglement preparation near the exceptional point (EP), or spectral coalescence, within such system. Nevertheless, an important problem often overlooked for quantum EP-based devices is their fidelity, greatly affected by the process of dissipation or post-selection, creating an inherent trade-off relation between the degree of entanglement and fidelity. Our study demonstrates that this limitation can be effectively overcome by harnessing an active $\mathcal{PT}$-symmetric system, which possesses balanced gain and loss, enabling maximal entanglement with rapid speed, high fidelity, and greater resilience to non-resonant errors. This new approach can efficiently prepare multi-qubit entanglement and use not only bipartite but also tripartite entanglement, as illustrative examples, even when the precise gain-loss balance is not strictly maintained. Our analytical findings are in excellent agreement with numerical simulations, confirming the potential of truly $\mathcal{PT}$-devices as a powerful tool for creating and engineering diverse quantum resources for applications in quantum information technology △ Less

Submitted 30 September, 2024; v1 submitted 11 July, 2024; originally announced July 2024.

Comments: 4 figures, 7 pages for main text. Totally 13 figures and 19 pages including appendix