Showing 1–50 of 61 results for author: Murch, K

Mixed-State Symmetry-Protected Topology and Strong-to-Weak Spontaneous Symmetry Breaking in a Superconducting Qubit Array

Authors: Qian Cao, Zhen Bi, Kater W. Murch

Abstract: We experimentally investigate how symmetry-protected topological order in a one-dimensional cluster state is transformed by measurement and decoherence in a five-qubit superconducting array. We first characterize the state's nonlocal string order and show that controlled dephasing selectively suppresses one symmetry sector while leaving the other robust, consistent with average symmetry-protected… ▽ More We experimentally investigate how symmetry-protected topological order in a one-dimensional cluster state is transformed by measurement and decoherence in a five-qubit superconducting array. We first characterize the state's nonlocal string order and show that controlled dephasing selectively suppresses one symmetry sector while leaving the other robust, consistent with average symmetry-protected topological order. We then measure one sublattice in a tunable basis and show that the remaining qubits are driven between a long-range-entangled GHZ state and a paramagnetic state. When the measurement record is discarded, the conventional long-range correlator vanishes while a nonlinear fidelity correlator remains finite, providing a finite-size signature of strong-to-weak spontaneous symmetry breaking. These experiments demonstrate how conditioning, averaging, and decoherence reveal distinct manifestations of order encoded in the same underlying cluster state, and establish a superconducting-circuit setting for probing mixed-state symmetry and topology. △ Less

Submitted 25 August, 2026; originally announced August 2026.

Comments: 9 pages, 3 figures

Anomalous Microwave Response in YBCO Resonators beyond the Two-Level-System Model

Authors: Kaiwen Zheng, Nathan J. Johnson, Nathan T. Thobaben, Sidharth Duthaluru, Haochen Shen, Denae T. Cherry, David S. Wisbey, Kater W. Murch

Abstract: We report the microwave response of coplanar-waveguide (CPW) resonators fabricated from $\mathrm{YBa_2Cu_3O_{7-δ}}$ (YBCO) thin films over temperatures from approximately $70~\mathrm{mK}$ to $40~\mathrm{K}$. The resonators exhibit internal quality factors $Q_\mathrm{i}$ in the range of $4\times10^3$ to $10^4$ at 70 mK, which increase to a maximum of approximately $8\times10^3$ to $1.2\times10^4$ n… ▽ More We report the microwave response of coplanar-waveguide (CPW) resonators fabricated from $\mathrm{YBa_2Cu_3O_{7-δ}}$ (YBCO) thin films over temperatures from approximately $70~\mathrm{mK}$ to $40~\mathrm{K}$. The resonators exhibit internal quality factors $Q_\mathrm{i}$ in the range of $4\times10^3$ to $10^4$ at 70 mK, which increase to a maximum of approximately $8\times10^3$ to $1.2\times10^4$ near $6~\mathrm{K}$. At low temperatures, both $Q_\mathrm{i}$ and the fractional shift of the resonance frequency $Δf_\mathrm{r}/f_\mathrm{r}$ increases with temperature, qualitatively resembling behavior commonly associated with two-level-system (TLS) defects. However, neither response saturates on the temperature scale set by the resonator frequency, and the loss exhibits no observable microwave-power dependence. We show that low-temperature frequency upturn may be better described by an additional paramagnetic response associated with defect-induced local moments or Andreev bound states, while the low-temperature loss follows an approximately logarithmic temperature dependence whose microscopic origin remains unresolved. These measurements establish the millikelvin performance of patterned YBCO resonators and show that their low-temperature response cannot be understood within the conventional TLS framework alone. △ Less

Submitted 29 July, 2026; originally announced July 2026.

Comments: 8 pages, 5 figures

Real-time Monitoring of Neon Film Growth for Electron-on-Neon Qubits

Authors: Sidharth Duthaluru, Kaiwen Zheng, Erik A. Henriksen, Kater W. Murch

Abstract: Electron-on-neon (eNe) charge states coupled to superconducting circuits are a promising platform for quantum computing. Control over the formation of these charge states requires techniques to track and control the growth of solid Ne films on the circuit surface. We demonstrate a real-time Ne film-growth monitor using high-transition-temperature (high-$T_c$) YBCO microwave resonators. The high… ▽ More Electron-on-neon (eNe) charge states coupled to superconducting circuits are a promising platform for quantum computing. Control over the formation of these charge states requires techniques to track and control the growth of solid Ne films on the circuit surface. We demonstrate a real-time Ne film-growth monitor using high-transition-temperature (high-$T_c$) YBCO microwave resonators. The high $T_c$ enables tracking of the film thickness near Ne's triple temperature and below. Across more than 300 solidification experiments, we find that the final Ne thickness varies stochastically from a few nm to a few $μ$m for films solidified from the liquid phase. By increasing the driving power in the resonator, we consistently reduce the final thickness to below 100 nm. These results represent an important step toward controlled formation of Ne films for eNe qubits and highlight the broader utility of high-$T_c$ resonators for hybrid quantum systems. △ Less

Submitted 25 November, 2025; originally announced November 2025.

Comments: 7 pages, 5 figures

Journal ref: Phys. Rev. Applied 25, 044065 (2026)

Superconducting antiqubits achieve optimal phase estimation via unitary inversion

Authors: Xingrui Song, Surihan Sean Borjigin, Flavio Salvati, Yu-Xin Wang, Nicole Yunger Halpern, David R. M. Arvidsson-Shukur, Kater Murch

Abstract: A positron is equivalent to an electron traveling backward through time. Casting transmon superconducting qubits as akin to electrons, we simulate a positron with a transmon subject to particular resonant and off-resonant drives. We call positron-like transmons "antiqubits." An antiqubit's effective gyromagnetic ratio equals the negative of a qubit's. This fact enables us to time-invert a unitary… ▽ More A positron is equivalent to an electron traveling backward through time. Casting transmon superconducting qubits as akin to electrons, we simulate a positron with a transmon subject to particular resonant and off-resonant drives. We call positron-like transmons "antiqubits." An antiqubit's effective gyromagnetic ratio equals the negative of a qubit's. This fact enables us to time-invert a unitary implemented on a transmon by its environment. We apply this platform-specific unitary inversion, with qubit--antiqubit entanglement, to achieve a quantum advantage in phase estimation: consider measuring the strength of a field that points in an unknown direction. An entangled qubit--antiqubit sensor offers the greatest possible sensitivity (amount of Fisher information), per qubit, per application of the field. We prove this result theoretically and observe it experimentally. This work shows how antimatter, whether real or simulated, can enable platform-specific unitary inversion and benefit quantum information processing. △ Less

Submitted 4 June, 2025; originally announced June 2025.

Comments: 22 pages, 3 figures

Surface Morphology Assisted Trapping of Strongly Coupled Electron-on-Neon Charge States

Authors: Kaiwen Zheng, Xingrui Song, Kater W. Murch

Abstract: Single electrons confined to a free neon surface and manipulated through the circuit quantum electrodynamics (circuit QED) architecture is a promising novel quantum computing platform. Understanding the exact physical nature of the electron-on-neon (eNe) charge states is important for realizing this platform's potential for quantum technologies. We investigate how resonator trench depth and substr… ▽ More Single electrons confined to a free neon surface and manipulated through the circuit quantum electrodynamics (circuit QED) architecture is a promising novel quantum computing platform. Understanding the exact physical nature of the electron-on-neon (eNe) charge states is important for realizing this platform's potential for quantum technologies. We investigate how resonator trench depth and substrate surface properties influence the formation of eNe charge states and their coupling to microwave resonators. Through experimental observation supported by modeling, we find that shallow-depth etching of the resonator features maximizes coupling strength. By comparing the trapping statistics and surface morphology of devices with altered trench roughness, our work reveals the role of fabrication-induced surface features in the formation of strongly coupled eNe states. △ Less

Submitted 3 March, 2025; originally announced March 2025.

Comments: 7 pages, 6 figures

Sub-micron Circuit Fabrication on Diamond Anvils for Mesoscopic High-Pressure Experiments

Authors: Z. R. Rehfuss, K. Zheng, S. L. Gould, K. W. Murch, S. Ran

Abstract: We present a novel fabrication procedure to produce high-quality lift-off structures on diamond anvils extending from the culet down to the slanted facets. Feature sizes down to 500 nm are achieved through the use of a bi-layer resist stack and electron beam lithography. Device structures with strong adhesion to the diamond surface and high abrasion resistance are realized by optimizing the surfac… ▽ More We present a novel fabrication procedure to produce high-quality lift-off structures on diamond anvils extending from the culet down to the slanted facets. Feature sizes down to 500 nm are achieved through the use of a bi-layer resist stack and electron beam lithography. Device structures with strong adhesion to the diamond surface and high abrasion resistance are realized by optimizing the surface treatment. To benchmark our process, we fabricate a multi-lead tungsten circuit to measure changes of the superconducting transition temperature of zirconium across the structural phase transition at $\sim$30 GPa; showing a 4-fold jump of the critical temperature. Our process is easily reproducible in most traditional academic and industrial cleanroom facilities. This work paves the way for complex and high-precision fabrication and measurements inside diamond anvil cells and on other faceted crystalline samples. △ Less

Submitted 5 April, 2025; v1 submitted 6 January, 2025; originally announced January 2025.

Comments: 6 pages, 3 figures

Experimental Realization of Discrete Time Quasi-Crystals

Authors: Guanghui He, Bingtian Ye, Ruotian Gong, Changyu Yao, Zhongyuan Liu, Kater W. Murch, Norman Y. Yao, Chong Zu

Abstract: Floquet (periodically driven) systems can give rise to unique non-equilibrium phases of matter without equilibrium analogs. The most prominent example is the realization of discrete time crystals. An intriguing question emerges: what other novel phases can manifest when the constraint of time periodicity is relaxed? In this study, we explore quantum systems subjected to a quasi-periodic drive. Lev… ▽ More Floquet (periodically driven) systems can give rise to unique non-equilibrium phases of matter without equilibrium analogs. The most prominent example is the realization of discrete time crystals. An intriguing question emerges: what other novel phases can manifest when the constraint of time periodicity is relaxed? In this study, we explore quantum systems subjected to a quasi-periodic drive. Leveraging a strongly interacting spin ensemble in diamond, we identify the emergence of long-lived discrete time quasi-crystals. Unlike conventional time crystals, time quasi-crystals exhibit robust sub-harmonic responses at multiple incommensurate frequencies. Furthermore, we show that the multi-frequency nature of the quasi-periodic drive allows for the formation of diverse patterns associated with different discrete time quasi-crystalline phases. Our findings demonstrate the existence of non-equilibrium phases in quasi-Floquet settings, significantly broadening the catalog of novel phenomena in driven many-body quantum systems. △ Less

Submitted 26 March, 2024; originally announced March 2024.

Comments: 7+5 pages, 4+5 figures

Journal ref: Physical Review X 15, 011055 (2025)

Agnostic Phase Estimation

Authors: Xingrui Song, Flavio Salvati, Chandrashekhar Gaikwad, Nicole Yunger Halpern, David R. M. Arvidsson-Shukur, Kater Murch

Abstract: The goal of quantum metrology is to improve measurements' sensitivities by harnessing quantum resources. Metrologists often aim to maximize the quantum Fisher information, which bounds the measurement setup's sensitivity. In studies of fundamental limits on metrology, a paradigmatic setup features a qubit (spin-half system) subject to an unknown rotation. One obtains the maximal quantum Fisher inf… ▽ More The goal of quantum metrology is to improve measurements' sensitivities by harnessing quantum resources. Metrologists often aim to maximize the quantum Fisher information, which bounds the measurement setup's sensitivity. In studies of fundamental limits on metrology, a paradigmatic setup features a qubit (spin-half system) subject to an unknown rotation. One obtains the maximal quantum Fisher information about the rotation if the spin begins in a state that maximizes the variance of the rotation-inducing operator. If the rotation axis is unknown, however, no optimal single-qubit sensor can be prepared. Inspired by simulations of closed timelike curves, we circumvent this limitation. We obtain the maximum quantum Fisher information about a rotation angle, regardless of the unknown rotation axis. To achieve this result, we initially entangle the probe qubit with an ancilla qubit. Then, we measure the pair in an entangled basis, obtaining more information about the rotation angle than any single-qubit sensor can achieve. We demonstrate this metrological advantage using a two-qubit superconducting quantum processor. Our measurement approach achieves a quantum advantage, outperforming every entanglement-free strategy. △ Less

Submitted 19 July, 2024; v1 submitted 29 February, 2024; originally announced March 2024.

Comments: 19 pages, 6 figures

Journal ref: Phys. Rev. Lett. 132, 260801 (2024)

Quantum energetics of a non-commuting measurement

Authors: Xiayu Linpeng, Nicolò Piccione, Maria Maffei, Léa Bresque, Samyak P. Prasad, Andrew N. Jordan, Alexia Auffèves, Kater W. Murch

Abstract: When a measurement observable does not commute with a quantum system's Hamiltonian, the energy of the measured system is typically not conserved during the measurement. Instead, energy can be transferred between the measured system and the meter. In this work, we experimentally investigate the energetics of non-commuting measurements in a circuit quantum electrodynamics system containing a transmo… ▽ More When a measurement observable does not commute with a quantum system's Hamiltonian, the energy of the measured system is typically not conserved during the measurement. Instead, energy can be transferred between the measured system and the meter. In this work, we experimentally investigate the energetics of non-commuting measurements in a circuit quantum electrodynamics system containing a transmon qubit embedded in a 3D microwave cavity. We show through spectral analysis of the cavity photons that a frequency shift is imparted on the probe, in balance with the associated energy changes of the qubit. Our experiment provides new insights into foundations of quantum measurement, as well as a better understanding of the key mechanisms at play in quantum energetics. △ Less

Submitted 27 November, 2023; v1 submitted 22 November, 2023; originally announced November 2023.

Comments: 9 pages, 4 figures

On-demand driven dissipation for cavity reset and cooling

Authors: Vivek Maurya, Haimeng Zhang, Daria Kowsari, Andre Kuo, Darian M. Hartsell, Clark Miyamoto, Jocelyn Liu, Sadman Shanto, Evangelos Vlachos, Azarin Zarassi, Kater W. Murch, Eli M. Levenson-Falk

Abstract: We present a superconducting circuit device that provides active, on-demand, tunable dissipation on a target mode of the electromagnetic field. Our device is based on a tunable "dissipator" that can be made lossy when tuned into resonance with a broadband filter mode. When driven parametrically, this dissipator induces loss on any mode coupled to it with energy detuning equal to the drive frequenc… ▽ More We present a superconducting circuit device that provides active, on-demand, tunable dissipation on a target mode of the electromagnetic field. Our device is based on a tunable "dissipator" that can be made lossy when tuned into resonance with a broadband filter mode. When driven parametrically, this dissipator induces loss on any mode coupled to it with energy detuning equal to the drive frequency. We demonstrate the use of this device to reset a superconducting qubit's readout cavity after a measurement, removing photons with a characteristic rate above $50\ μ\mathrm{s}^{-1}$. We also demonstrate that the dissipation can be driven constantly to simultaneously damp and cool the cavity, effectively eliminating thermal photon fluctuations as a relevant decoherence channel. Our results demonstrate the utility of our device as a modular tool for environmental engineering and entropy removal in circuit QED. △ Less

Submitted 27 March, 2024; v1 submitted 25 October, 2023; originally announced October 2023.

Comments: 12 pages, 6 figures

Chiral Bell-state transfer via dissipative Liouvillian dynamics

Authors: Shishir Khandelwal, Weijian Chen, Kater W. Murch, Géraldine Haack

Abstract: Chiral state transfer along closed loops in the vicinity of an exceptional point is one of the many counter-intuitive observations in non-Hermitian physics. The application of this property beyond proof-of-principle in quantum physics, is an open question. In this work, we demonstrate chiral state conversion between singlet and triplet Bell states through fully-quantum Liouvillian dynamics. Crucia… ▽ More Chiral state transfer along closed loops in the vicinity of an exceptional point is one of the many counter-intuitive observations in non-Hermitian physics. The application of this property beyond proof-of-principle in quantum physics, is an open question. In this work, we demonstrate chiral state conversion between singlet and triplet Bell states through fully-quantum Liouvillian dynamics. Crucially, we demonstrate that this property can be used for the chiral production of Bell states from separable states with a high fidelity and for a large range of parameters. Additionally, we show that the removal of quantum jumps from the dynamics through postselection can result in near-perfect Bell states from initially separable states. Our work presents the first application of chiral state transfer in quantum information processing and demonstrates a novel way to control entangled states by means of dissipation engineering. △ Less

Submitted 17 September, 2024; v1 submitted 17 October, 2023; originally announced October 2023.

Comments: 4+6 pages, 7 figures

Journal ref: Phys. Rev. Lett. 133, 070403 (2024)

Constraining work fluctuations of non-Hermitian dynamics across the exceptional point of a superconducting qubit

Authors: Serra Erdamar, Maryam Abbasi, Byung Ha, Weijian Chen, Jacob Muldoon, Yogesh Joglekar, Kater W. Murch

Abstract: Thermodynamics constrains changes to the energy of a system, both deliberate and random, via its first and second laws. When the system is not in equilibrium, fluctuation theorems such as the Jarzynski equality further restrict the distributions of deliberate work done. Such fluctuation theorems have been experimentally verified in small, non-equilibrium quantum systems undergoing unitary or decoh… ▽ More Thermodynamics constrains changes to the energy of a system, both deliberate and random, via its first and second laws. When the system is not in equilibrium, fluctuation theorems such as the Jarzynski equality further restrict the distributions of deliberate work done. Such fluctuation theorems have been experimentally verified in small, non-equilibrium quantum systems undergoing unitary or decohering dynamics. Yet, their validity in systems governed by a non-Hermitian Hamiltonian has long been contentious, due to the false premise of the Hamiltonian's dual and equivalent roles in dynamics and energetics. Here we show that work fluctuations in a non-Hermitian qubit obey the Jarzynski equality even if its Hamiltonian has complex or purely imaginary eigenvalues. With post-selection on a dissipative superconducting circuit undergoing a cyclic parameter sweep, we experimentally quantify the work distribution using projective energy measurements and show that the fate of the Jarzynski equality is determined by the parity-time symmetry of, and the energetics that result from, the corresponding non-Hermitian, Floquet Hamiltonian. By distinguishing the energetics from non-Hermitian dynamics, our results provide the recipe for investigating the non-equilibrium quantum thermodynamics of such open systems. △ Less

Submitted 21 September, 2023; originally announced September 2023.

Comments: 7 pages, 5 figures

Superconducting Non-Reciprocity Based on Time-Modulated Coupled-Resonator Systems

Authors: Yi Zhuang, Chandrashekhar Gaikwad, Daria Kowsari, Kater Murch, Aravind Nagulu

Abstract: We present a unified approach for designing a diverse range of superconducting non-reciprocal components, including circulators, isolators, and uni-directional amplifiers, based on temporally-modulated coupled resonator networks. Our method leverages standard SQUID-based resonators as building blocks, arranged in various configurations such as series-coupled, wye-connected, and lattice-coupled res… ▽ More We present a unified approach for designing a diverse range of superconducting non-reciprocal components, including circulators, isolators, and uni-directional amplifiers, based on temporally-modulated coupled resonator networks. Our method leverages standard SQUID-based resonators as building blocks, arranged in various configurations such as series-coupled, wye-connected, and lattice-coupled resonators, to realize a wide range of on-chip non-reciprocal devices. Our theoretical studies demonstrated the effectiveness of the proposed approach, achieving circulators and isolators with near-zero insertion losses and isolation greater than 20 dB, and directional amplifiers with forward gain exceeding 10 dB and reverse isolation greater than 20 dB. To validate our findings, we implemented and measured a series-coupled three-resonator superconducting isolator using a single-layer superconducting process. At a base temperature of 20 mK, our device exhibited insertion loss of 1.3 dB in the forward direction, and isolation of up to 25 dB at the center frequency and greater than 15 dB across a bandwidth of 250 MHz in the reverse direction. Our approach promises to enable the design of a broad range of high-performance non-reciprocal devices for superconducting circuits. △ Less

Submitted 4 July, 2023; originally announced July 2023.

Comments: 14 pages, 10 figures

Observing Parity Time Symmetry Breaking in a Josephson Parametric Amplifier

Authors: Chandrashekhar Gaikwad, Daria Kowsari, Weijian Chen, Kater W. Murch

Abstract: A coupled two-mode system with balanced gain and loss is a paradigmatic example of an open quantum system that can exhibit real spectra despite being described by a non-Hermitian Hamiltonian. We utilize a degenerate parametric amplifier operating in three-wave mixing mode to realize such a system of balanced gain and loss between the two quadrature modes of the amplifier. By examining the time-dom… ▽ More A coupled two-mode system with balanced gain and loss is a paradigmatic example of an open quantum system that can exhibit real spectra despite being described by a non-Hermitian Hamiltonian. We utilize a degenerate parametric amplifier operating in three-wave mixing mode to realize such a system of balanced gain and loss between the two quadrature modes of the amplifier. By examining the time-domain response of the amplifier, we observe a characteristic transition from real-to-imaginary energy eigenvalues associated with the Parity-Time-symmetry-breaking transition. △ Less

Submitted 26 June, 2023; originally announced June 2023.

Comments: 6 pages, 4 figures

Dissipative preparation and stabilization of many-body quantum states in a superconducting qutrit array

Authors: Yunzhao Wang, Kyrylo Snizhko, Alessandro Romito, Yuval Gefen, Kater Murch

Abstract: We present and analyze a protocol for driven-dissipatively preparing and stabilizing a manifold of quantum manybody entangled states with symmetry-protected topological order. Specifically, we consider the experimental platform consisting of superconducting transmon circuits and linear microwave resonators. We perform theoretical modeling of this platform via pulse-level simulations based on phy… ▽ More We present and analyze a protocol for driven-dissipatively preparing and stabilizing a manifold of quantum manybody entangled states with symmetry-protected topological order. Specifically, we consider the experimental platform consisting of superconducting transmon circuits and linear microwave resonators. We perform theoretical modeling of this platform via pulse-level simulations based on physical features of real devices. In our protocol, transmon qutrits are mapped onto spin-1 systems. The qutrits' sharing of nearest-neighbor dispersive coupling to a dissipative microwave resonator enables elimination of state population in the $S^\mathrm{total}=2$ subspace for each adjacent pair, and thus, the stabilization of the manybody system into the Affleck, Kennedy, Lieb, and Tasaki (AKLT) state up to the edge mode configuration. We also analyze the performance of our protocol as the system size scales up to four qutrits, in terms of its fidelity as well as the stabilization time. Our work shows the capacity of driven-dissipative superconducting cQED systems to host robust and self-corrected quantum manybody states that are topologically non-trivial. △ Less

Submitted 22 June, 2023; v1 submitted 21 March, 2023; originally announced March 2023.

Comments: 19 pages, 12 figures

Journal ref: Phys. Rev. A 108, 013712 (2023)

Quantum acoustic Fano interference of surface phonons

Authors: J. M. Kitzman, J. R. Lane, C. Undershute, N. R. Beysengulov, C. A. Mikolas, K. W. Murch, J. Pollanen

Abstract: Quantum acoustic systems, which integrate surface or bulk phonons with superconducting qubits, offer a unique opportunity to investigate phononic $interference$ and $scattering$ processes in the quantum regime. In particular the interaction between a superconducting qubit and a phononic oscillator allows the qubit to sense the oscillator's excitation spectrum and underlying interference effects. H… ▽ More Quantum acoustic systems, which integrate surface or bulk phonons with superconducting qubits, offer a unique opportunity to investigate phononic $interference$ and $scattering$ processes in the quantum regime. In particular the interaction between a superconducting qubit and a phononic oscillator allows the qubit to sense the oscillator's excitation spectrum and underlying interference effects. Here we present measurements revealing Fano interference of a resonantly trapped piezoelectric surface acoustic wave (SAW) mode with a broad continuum of surface phonons in a system consisting of a SAW resonator coupled to a superconducting qubit. The experiments highlight the existence of additional weakly coupled mechanical modes and their influence on the qubit-phonon interaction and underscore the importance of phononic interference in quantum acoustic architectures that have been proposed for quantum information processing applications. △ Less

Submitted 24 July, 2023; v1 submitted 2 February, 2023; originally announced February 2023.

Comments: 7 pages, 5 figures. Updated title and abstract

Journal ref: Physical Review A 108, L010601 (2023)

Quasi-Floquet prethermalization in a disordered dipolar spin ensemble in diamond

Authors: Guanghui He, Bingtian Ye, Ruotian Gong, Zhongyuan Liu, Kater W. Murch, Norman Y. Yao, Chong Zu

Abstract: Floquet (periodic) driving has recently emerged as a powerful technique for engineering quantum systems and realizing non-equilibrium phases of matter. A central challenge to stabilizing quantum phenomena in such systems is the need to prevent energy absorption from the driving field. Fortunately, when the frequency of the drive is significantly larger than the local energy scales of the many-body… ▽ More Floquet (periodic) driving has recently emerged as a powerful technique for engineering quantum systems and realizing non-equilibrium phases of matter. A central challenge to stabilizing quantum phenomena in such systems is the need to prevent energy absorption from the driving field. Fortunately, when the frequency of the drive is significantly larger than the local energy scales of the many-body system, energy absorption is suppressed. The existence of this so-called prethermal regime depends sensitively on the range of interactions and the presence of multiple driving frequencies. Here, we report the observation of Floquet prethermalization in a strongly interacting dipolar spin ensemble in diamond, where the angular dependence of the dipolar coupling helps to mitigate the long-ranged nature of the interaction. Moreover, we extend our experimental observation to quasi-Floquet drives with multiple incommensurate frequencies. In contrast to a single-frequency drive, we find that the existence of prethermalization is extremely sensitive to the smoothness of the applied field. Our results open the door to stabilizing and characterizing non-equilibrium phenomena in quasi-periodically driven systems. △ Less

Submitted 28 September, 2023; v1 submitted 21 December, 2022; originally announced December 2022.

Comments: 7+13 pages, 3+8 figures

Journal ref: Physical Review Letters 131, 130401 (2023)

Speeding up entanglement generation by proximity to higher-order exceptional points

Authors: Zeng-Zhao Li, Weijian Chen, Maryam Abbasi, Kater W. Murch, K. Birgitta Whaley

Abstract: Entanglement is a key resource for quantum information technologies ranging from quantum sensing to quantum computing. Conventionally, the entanglement between two coupled qubits is established at the time scale of the inverse of the coupling strength. In this work, we study two weakly coupled non-Hermitian qubits and observe entanglement generation at a significantly shorter time scale by proximi… ▽ More Entanglement is a key resource for quantum information technologies ranging from quantum sensing to quantum computing. Conventionally, the entanglement between two coupled qubits is established at the time scale of the inverse of the coupling strength. In this work, we study two weakly coupled non-Hermitian qubits and observe entanglement generation at a significantly shorter time scale by proximity to a higher-order exceptional point. We establish a non-Hermitian perturbation theory based on constructing a biorthogonal complete basis and further identify the optimal condition to obtain the maximally entangled state. Our study of speeding up entanglement generation in non-Hermitian quantum systems opens new avenues for harnessing coherent nonunitary dissipation for quantum technologies. △ Less

Submitted 8 September, 2023; v1 submitted 10 October, 2022; originally announced October 2022.

Comments: 6+18 pages, 4+12 figures. Zeng-Zhao Li and Weijian Chen contributed equally to this work

Journal ref: Phys. Rev. Lett. 131, 100202 (2023)

Phononic bath engineering of a superconducting qubit

Authors: J. M. Kitzman, J. R. Lane, C. Undershute, P. M. Harrington, N. R. Beysengulov, C. A. Mikolas, K. W. Murch, J. Pollanen

Abstract: Phonons, the ubiquitous quanta of vibrational energy, play a vital role in the performance of quantum technologies. Conversely, unintended coupling to phonons degrades qubit performance and can lead to correlated errors in superconducting qubit systems. Regardless of whether phonons play an enabling or deleterious role, they do not typically admit control over their spectral properties, nor the po… ▽ More Phonons, the ubiquitous quanta of vibrational energy, play a vital role in the performance of quantum technologies. Conversely, unintended coupling to phonons degrades qubit performance and can lead to correlated errors in superconducting qubit systems. Regardless of whether phonons play an enabling or deleterious role, they do not typically admit control over their spectral properties, nor the possibility of engineering their dissipation to be used as a resource. Here we show that coupling a superconducting qubit to a bath of piezoelectric surface acoustic wave phonons enables a novel platform for investigating open quantum systems. By shaping the loss spectrum of the qubit via the bath of lossy surface phonons, we demonstrate preparation and dynamical stabilization of superposition states through the combined effects of drive and dissipation. These experiments highlight the versatility of engineered phononic dissipation and advance the understanding of mechanical losses in superconducting qubit systems. △ Less

Submitted 24 July, 2023; v1 submitted 15 August, 2022; originally announced August 2022.

Comments: Main text: 17 pages, 5 figures. Supplementary Information: 7 pages, 5 figures

Journal ref: Nature Communications 14, 3910 (2023)

Evidence of dual Shapiro steps in a Josephson junctions array

Authors: Nicolò Crescini, Samuel Cailleaux, Wiebke Guichard, Cécile Naud, Olivier Buisson, Kater Murch, Nicolas Roch

Abstract: The modern primary voltage standard is based on the AC Josephson effect and the ensuing Shapiro steps, where a microwave tone applied to a Josephson junction yields a constant voltage $hf/2e$ ($h$ is Planck's constant and $e$ the electron charge) determined by only the microwave frequency $f$ and fundamental constants. Duality arguments for current and voltage have long suggested the possibility o… ▽ More The modern primary voltage standard is based on the AC Josephson effect and the ensuing Shapiro steps, where a microwave tone applied to a Josephson junction yields a constant voltage $hf/2e$ ($h$ is Planck's constant and $e$ the electron charge) determined by only the microwave frequency $f$ and fundamental constants. Duality arguments for current and voltage have long suggested the possibility of dual Shapiro steps -- that a Josephson junction device could produce current steps with heights determined only on the applied frequency. In this report, we embed an ultrasmall Josephson junction in a high impedance array of larger junctions to reveal dual Shapiro steps. For multiple frequencies, we detect that the AC response of the circuit is synchronised with the microwave tone at frequency $f$, and the corresponding emergence of flat steps in the DC response with current $2ef$, equal to the tunnelling of a Cooper pair per tone period. This work sheds new light on phase-charge duality, omnipresent in condensed matter physics, and extends it to Josephson circuits. Looking forward, it opens a broad range of possibilities for new experiments in the field of circuit quantum electrodynamics and is an important step towards the long-sought closure of the quantum metrology electrical triangle. △ Less

Submitted 19 July, 2022; originally announced July 2022.

Comments: 14 pages, 11 figures

Energetic cost of measurements using quantum, coherent, and thermal light

Authors: Xiayu Linpeng, Léa Bresque, Maria Maffei, Andrew N. Jordan, Alexia Auffèves, Kater W. Murch

Abstract: Quantum measurements are basic operations that play a critical role in the study and application of quantum information. We study how the use of quantum, coherent, and classical thermal states of light in a circuit quantum electrodynamics setup impacts the performance of quantum measurements, by comparing their respective measurement backaction and measurement signal to noise ratio per photon. In… ▽ More Quantum measurements are basic operations that play a critical role in the study and application of quantum information. We study how the use of quantum, coherent, and classical thermal states of light in a circuit quantum electrodynamics setup impacts the performance of quantum measurements, by comparing their respective measurement backaction and measurement signal to noise ratio per photon. In the strong dispersive limit, we find that thermal light is capable of performing quantum measurements with comparable efficiency to coherent light, both being outperformed by single-photon light. We then analyze the thermodynamic cost of each measurement scheme. We show that single-photon light shows an advantage in terms of energy cost per information gain, reaching the fundamental thermodynamic cost. △ Less

Submitted 6 June, 2022; v1 submitted 2 March, 2022; originally announced March 2022.

Comments: 15 pages, 7 figures

Journal ref: Phys. Rev. Lett. 128, 220506 (2022)

Engineered Dissipation for Quantum Information Science

Authors: Patrick M. Harrington, Erich Mueller, Kater Murch

Abstract: Quantum information processing relies on precise control of non-classical states in the presence of many uncontrolled environmental degrees of freedom -- requiring careful orchestration of how the relevant degrees of freedom interact with that environment. These interactions are often viewed as detrimental, as they dissipate energy and decohere quantum states. Nonetheless, when controlled, dissipa… ▽ More Quantum information processing relies on precise control of non-classical states in the presence of many uncontrolled environmental degrees of freedom -- requiring careful orchestration of how the relevant degrees of freedom interact with that environment. These interactions are often viewed as detrimental, as they dissipate energy and decohere quantum states. Nonetheless, when controlled, dissipation is an essential tool for manipulating quantum information: Dissipation engineering enables quantum measurement, quantum state preparation, and quantum state stabilization. The progress of quantum device technology, marked by improvements of characteristic coherence times and extensible architectures for quantum control, has coincided with the development of such dissipation engineering tools which interface quantum and classical degrees of freedom. This Review presents dissipation as a fundamental aspect of the measurement and control of quantum devices and highlights the role of dissipation engineering for quantum error correction and quantum simulation that enables quantum information processing on a practical scale. △ Less

Submitted 30 May, 2022; v1 submitted 10 February, 2022; originally announced February 2022.

Comments: Review article: 32 pages, 4 figures, 2 boxes

Journal ref: Nat Rev Phys 4, 660-671 (2022)

Nitrogen Plasma Passivated Niobium Resonators for Superconducting Quantum Circuits

Authors: K. Zheng, D. Kowsari, N. J. Thobaben, X. Du, X. Song, S. Ran, E. A. Henriksen, D. S. Wisbey, K. W. Murch

Abstract: Microwave loss in niobium metallic structures used for superconducting quantum circuits is limited by a native surface oxide layer formed over a timescale of minutes when exposed to an ambient environment. In this work, we show that nitrogen plasma treatment forms a niobium nitride layer at the metal-air interface which prevents such oxidation. X-ray photoelectron spectroscopy confirms the doping… ▽ More Microwave loss in niobium metallic structures used for superconducting quantum circuits is limited by a native surface oxide layer formed over a timescale of minutes when exposed to an ambient environment. In this work, we show that nitrogen plasma treatment forms a niobium nitride layer at the metal-air interface which prevents such oxidation. X-ray photoelectron spectroscopy confirms the doping of nitrogen more than 5 nm into the surface and a suppressed oxygen presence. This passivation remains stable after aging for 15 days in an ambient environment. Cryogenic microwave characterization shows an average filling factor adjusted two-level-system loss tangent $\rm{Fδ_{TLS}}$ of $(2.9\pm0.5)\cdot10^{-7}$ for resonators with 3 $\rmμ$m center strip and $(1.0\pm0.3)\cdot10^{-7}$ for 20 $\rmμ$m center strip, exceeding the performance of unpassivated samples by a factor of four. △ Less

Submitted 18 February, 2022; v1 submitted 5 January, 2022; originally announced January 2022.

Comments: 7 pages, 5 figures

Journal ref: Appl. Phys. Lett. 120, 102601 (2022)

Decoherence Induced Exceptional Points in a Dissipative Superconducting Qubit

Authors: Weijian Chen, Maryam Abbasi, Byung Ha, Serra Erdamar, Yogesh N. Joglekar, Kater W. Murch

Abstract: Open quantum systems interacting with an environment exhibit dynamics described by the combination of dissipation and coherent Hamiltonian evolution. Taken together, these effects are captured by a Liouvillian superoperator. The degeneracies of the (generically non-Hermitian) Liouvillian are exceptional points, which are associated with critical dynamics as the system approaches steady state. We u… ▽ More Open quantum systems interacting with an environment exhibit dynamics described by the combination of dissipation and coherent Hamiltonian evolution. Taken together, these effects are captured by a Liouvillian superoperator. The degeneracies of the (generically non-Hermitian) Liouvillian are exceptional points, which are associated with critical dynamics as the system approaches steady state. We use a superconducting transmon circuit coupled to an engineered environment to observe two different types of Liouvillian exceptional points that arise either from the interplay of energy loss and decoherence or purely due to decoherence. By dynamically tuning the Liouvillian superoperators in real time we observe a non-Hermiticity-induced chiral state transfer. Our study motivates a new look at open quantum system dynamics from the vantage of Liouvillian exceptional points, enabling applications of non-Hermitian dynamics in the understanding and control of open quantum systems. △ Less

Submitted 8 November, 2021; originally announced November 2021.

Comments: 11 pages, 10 figures

Journal ref: Phys. Rev. Lett. 128, 110402 (2022)

Topological quantum state control through exceptional-point proximity

Authors: Maryam Abbasi, Weijian Chen, Mahdi Naghiloo, Yogesh N. Joglekar, Kater W. Murch

Abstract: We study the quantum evolution of a non-Hermitian qubit realized as a submanifold of a dissipative superconducting transmon circuit. Real-time tuning of the system parameters to encircle an exceptional point results in non-reciprocal quantum state transfer. We further observe chiral geometric phases accumulated under state transport, verifying the quantum coherent nature of the evolution in the co… ▽ More We study the quantum evolution of a non-Hermitian qubit realized as a submanifold of a dissipative superconducting transmon circuit. Real-time tuning of the system parameters to encircle an exceptional point results in non-reciprocal quantum state transfer. We further observe chiral geometric phases accumulated under state transport, verifying the quantum coherent nature of the evolution in the complex energy landscape and distinguishing between coherent and incoherent effects associated with exceptional point encircling. Our work demonstrates an entirely new method for control over quantum state vectors, highlighting new facets of quantum bath engineering enabled through dynamical non-Hermitian control. △ Less

Submitted 14 January, 2022; v1 submitted 11 August, 2021; originally announced August 2021.

Comments: 9 pages, 8 figures

Fabrication and surface treatment of electron-beam evaporated niobium for low-loss coplanar waveguide resonators

Authors: D. Kowsari, K. Zheng, J. T. Monroe, N. J. Thobaben, X. Du, P. M. Harrington, E. A. Henriksen, D. S. Wisbey, K. W. Murch

Abstract: We characterize low-loss electron-beam evaporated niobium thin films deposited under ultra-high vacuum conditions. Slow deposition yields films with a high superconducting transition temperature ($9.20 \pm 0.06 \rm ~K$) as well as a residual resistivity ratio of $4.8$. We fabricate the films into coplanar waveguide resonators to extract the intrinsic loss due to the presence of two-level-system fl… ▽ More We characterize low-loss electron-beam evaporated niobium thin films deposited under ultra-high vacuum conditions. Slow deposition yields films with a high superconducting transition temperature ($9.20 \pm 0.06 \rm ~K$) as well as a residual resistivity ratio of $4.8$. We fabricate the films into coplanar waveguide resonators to extract the intrinsic loss due to the presence of two-level-system fluctuators using microwave measurements. For a coplanar waveguide resonator gap of $2~μ\rm m$, the films exhibit filling-factor-adjusted two-level-system loss tangents as low as $1.5 \times 10^{-7}$ with single-photon regime internal quality factors in excess of one million after removing native surface oxides of the niobium. △ Less

Submitted 4 October, 2021; v1 submitted 11 August, 2021; originally announced August 2021.

Comments: 5 pages, 3 figures

Journal ref: Appl. Phys. Lett. 120, 102601 (2022)

Efficiently Fuelling a Quantum Engine with Incompatible Measurements

Authors: Sreenath K. Manikandan, Cyril Elouard, Kater W. Murch, Alexia Auffèves, Andrew N. Jordan

Abstract: We propose a quantum harmonic oscillator measurement engine fueled by simultaneous quantum measurements of the non-commuting position and momentum quadratures of the quantum oscillator. The engine extracts work by moving the harmonic trap suddenly, conditioned on the measurement outcomes. We present two protocols for work extraction, respectively based on single-shot and time-continuous quantum me… ▽ More We propose a quantum harmonic oscillator measurement engine fueled by simultaneous quantum measurements of the non-commuting position and momentum quadratures of the quantum oscillator. The engine extracts work by moving the harmonic trap suddenly, conditioned on the measurement outcomes. We present two protocols for work extraction, respectively based on single-shot and time-continuous quantum measurements. In the single-shot limit, the oscillator is measured in a coherent state basis; the measurement adds an average of one quantum of energy to the oscillator, which is then extracted in the feedback step. In the time-continuous limit, continuous weak quantum measurements of both position and momentum of the quantum oscillator result in a coherent state, whose coordinates diffuse in time. We relate the extractable work to the noise added by quadrature measurements, and present exact results for the work distribution at arbitrary finite time. Both protocols can achieve unit work conversion efficiency in principle. △ Less

Submitted 28 April, 2022; v1 submitted 28 July, 2021; originally announced July 2021.

Comments: 13 pages, 5 figures

Single electrons on solid neon as a solid-state qubit platform

Authors: Xianjing Zhou, Gerwin Koolstra, Xufeng Zhang, Ge Yang, Xu Han, Brennan Dizdar, Xinhao Li, Divan Ralu, Wei Guo, Kater W. Murch, David I. Schuster, Dafei Jin

Abstract: Progress toward the realization of quantum computers requires persistent advances in their constituent building blocks - qubits. Novel qubit platforms that simultaneously embody long coherence, fast operation, and large scalability offer compelling advantages in the construction of quantum computers and many other quantum information systems. Electrons, ubiquitous elementary particles of nonzero c… ▽ More Progress toward the realization of quantum computers requires persistent advances in their constituent building blocks - qubits. Novel qubit platforms that simultaneously embody long coherence, fast operation, and large scalability offer compelling advantages in the construction of quantum computers and many other quantum information systems. Electrons, ubiquitous elementary particles of nonzero charge, spin, and mass, have commonly been perceived as paradigmatic local quantum information carriers. Despite superior controllability and configurability, their practical performance as qubits via either motional or spin states depends critically on their material environment. Here we report our experimental realization of a new qubit platform based upon isolated single electrons trapped on an ultraclean solid neon surface in vacuum. By integrating an electron trap in a circuit quantum electrodynamics architecture, we achieve strong coupling between the motional states of a single electron and a single microwave photon in an on-chip superconducting resonator. Qubit gate operations and dispersive readout are implemented to measure the energy relaxation time $T_1$ of $15~μ$s and phase coherence time $T_2$ over $200~$ns. These results indicate that the electron-on-solid-neon qubit already performs near the state of the art as a charge qubit. △ Less

Submitted 27 February, 2022; v1 submitted 18 June, 2021; originally announced June 2021.

Comments: 16 pages, 11 figures

Journal ref: Nature 605, 46-50 (2022)

Optical Direct Write of Dolan--Niemeyer-Bridge Junctions for Transmon Qubits

Authors: J. T. Monroe, D. Kowsari, K. Zheng, C. Gaikwad, J. Brewster, D. S. Wisbey, K. W. Murch

Abstract: We characterize highly coherent transmon qubits fabricated with a direct-write photolithography system. Multi-layer evaporation and oxidation allows us to change the critical current density by reducing the effective tunneling area and increasing the barrier thickness. Surface treatments before resist application and again before evaporation result in high coherence devices. With optimized surface… ▽ More We characterize highly coherent transmon qubits fabricated with a direct-write photolithography system. Multi-layer evaporation and oxidation allows us to change the critical current density by reducing the effective tunneling area and increasing the barrier thickness. Surface treatments before resist application and again before evaporation result in high coherence devices. With optimized surface treatments we achieve energy relaxation $T_1$ times in excess of $80\ μ$s for three dimensional transmon qubits with Josephson junction lithographic areas of 2 $μ\mathrm{m}^2$. △ Less

Submitted 9 August, 2021; v1 submitted 16 June, 2021; originally announced June 2021.

Comments: 7 pages, 3 figures

Journal ref: Appl. Phys. Lett. 119, 062601 (2021)

Quantum jumps in the non-Hermitian dynamics of a superconducting qubit

Authors: Weijian Chen, Maryam Abbasi, Yogesh N. Joglekar, Kater W. Murch

Abstract: We study the dynamics of a driven non-Hermitian superconducting qubit which is perturbed by quantum jumps between energy levels, a purely quantum effect with no classical correspondence. The quantum jumps mix the qubit states leading to decoherence. We observe that this decoherence rate is enhanced near the exceptional point, owing to the cube-root topology of the non-Hermitian eigenenergies. Toge… ▽ More We study the dynamics of a driven non-Hermitian superconducting qubit which is perturbed by quantum jumps between energy levels, a purely quantum effect with no classical correspondence. The quantum jumps mix the qubit states leading to decoherence. We observe that this decoherence rate is enhanced near the exceptional point, owing to the cube-root topology of the non-Hermitian eigenenergies. Together with the effect of non-Hermitian gain/loss, quantum jumps can also lead to a breakdown of adiabatic evolution under the slow-driving limit. Our study shows the critical role of quantum jumps in generalizing the applications of classical non-Hermitian systems to open quantum systems for sensing and control. △ Less

Submitted 16 September, 2021; v1 submitted 10 March, 2021; originally announced March 2021.

Comments: 12 pages, 10 figures

Journal ref: Phys. Rev. Lett. 127, 140504 (2021)

Observing a Topological Transition in Weak-Measurement-Induced Geometric Phases

Authors: Yunzhao Wang, Kyrylo Snizhko, Alessandro Romito, Yuval Gefen, Kater Murch

Abstract: Measurement plays a quintessential role in the control of quantum systems. Beyond initialization and readout which pertain to projective measurements, weak measurements in particular, through their back-action on the system, may enable various levels of coherent control. The latter ranges from observing quantum trajectories to state dragging and steering. Furthermore, just like the adiabatic evolu… ▽ More Measurement plays a quintessential role in the control of quantum systems. Beyond initialization and readout which pertain to projective measurements, weak measurements in particular, through their back-action on the system, may enable various levels of coherent control. The latter ranges from observing quantum trajectories to state dragging and steering. Furthermore, just like the adiabatic evolution of quantum states that is known to induce the Berry phase, sequential weak measurements may lead to path-dependent geometric phases. Here we measure the geometric phases induced by sequences of weak measurements and demonstrate a topological transition in the geometric phase controlled by measurement strength. This connection between weak measurement induced quantum dynamics and topological transitions reveals subtle topological features in measurement-based manipulation of quantum systems. Our protocol could be implemented for classes of operations (e.g. braiding) which are topological in nature. Furthermore, our results open new horizons for measurement-enabled quantum control of many-body topological states. △ Less

Submitted 6 June, 2022; v1 submitted 10 February, 2021; originally announced February 2021.

Comments: 8 pages, 5 figures

Journal ref: Phys. Rev. Research 4, 023179 (2022)

Quantum process inference for a single qubit Maxwell's demon

Authors: Xingrui Song, Mahdi Naghiloo, Kater Murch

Abstract: While quantum measurement theories are built around density matrices and observables, the laws of thermodynamics are based on processes such as are used in heat engines and refrigerators. The study of quantum thermodynamics fuses these two distinct paradigms. In this article, we highlight the usage of quantum process matrices as a unified language for describing thermodynamic processes in the quan… ▽ More While quantum measurement theories are built around density matrices and observables, the laws of thermodynamics are based on processes such as are used in heat engines and refrigerators. The study of quantum thermodynamics fuses these two distinct paradigms. In this article, we highlight the usage of quantum process matrices as a unified language for describing thermodynamic processes in the quantum regime. We experimentally demonstrate this in the context of a quantum Maxwell's demon, where two major quantities are commonly investigated; the average work extraction $\langle W \rangle$ and the efficacy $γ$ which measures how efficiently the feedback operation uses the obtained information. Using the tool of quantum process matrices, we develop the optimal feedback protocols for these two quantities and experimentally investigate them in a superconducting circuit QED setup. △ Less

Submitted 17 June, 2021; v1 submitted 1 February, 2021; originally announced February 2021.

Comments: 11 pages, 5 figures

Journal ref: Phys. Rev. A 104, 022211 (2021)

Quantum Simulators: Architectures and Opportunities

Authors: Ehud Altman, Kenneth R. Brown, Giuseppe Carleo, Lincoln D. Carr, Eugene Demler, Cheng Chin, Brian DeMarco, Sophia E. Economou, Mark A. Eriksson, Kai-Mei C. Fu, Markus Greiner, Kaden R. A. Hazzard, Randall G. Hulet, Alicia J. Kollar, Benjamin L. Lev, Mikhail D. Lukin, Ruichao Ma, Xiao Mi, Shashank Misra, Christopher Monroe, Kater Murch, Zaira Nazario, Kang-Kuen Ni, Andrew C. Potter, Pedram Roushan , et al. (12 additional authors not shown)

Abstract: Quantum simulators are a promising technology on the spectrum of quantum devices from specialized quantum experiments to universal quantum computers. These quantum devices utilize entanglement and many-particle behaviors to explore and solve hard scientific, engineering, and computational problems. Rapid development over the last two decades has produced more than 300 quantum simulators in operati… ▽ More Quantum simulators are a promising technology on the spectrum of quantum devices from specialized quantum experiments to universal quantum computers. These quantum devices utilize entanglement and many-particle behaviors to explore and solve hard scientific, engineering, and computational problems. Rapid development over the last two decades has produced more than 300 quantum simulators in operation worldwide using a wide variety of experimental platforms. Recent advances in several physical architectures promise a golden age of quantum simulators ranging from highly optimized special purpose simulators to flexible programmable devices. These developments have enabled a convergence of ideas drawn from fundamental physics, computer science, and device engineering. They have strong potential to address problems of societal importance, ranging from understanding vital chemical processes, to enabling the design of new materials with enhanced performance, to solving complex computational problems. It is the position of the community, as represented by participants of the NSF workshop on "Programmable Quantum Simulators," that investment in a national quantum simulator program is a high priority in order to accelerate the progress in this field and to result in the first practical applications of quantum machines. Such a program should address two areas of emphasis: (1) support for creating quantum simulator prototypes usable by the broader scientific community, complementary to the present universal quantum computer effort in industry; and (2) support for fundamental research carried out by a blend of multi-investigator, multi-disciplinary collaborations with resources for quantum simulator software, hardware, and education. △ Less

Submitted 20 December, 2019; v1 submitted 14 December, 2019; originally announced December 2019.

Comments: 41 pages -- references and acknowledgments added in v2

Journal ref: PRX Quantum 2, 017003 (2021)

Integrating superfluids with superconducting qubit systems

Authors: J. R. Lane, D. Tan, N. R. Beysengulov, K. Nasyedkin, E. Brook, L. Zhang, T. Stefanski, H. Byeon, K. W. Murch, J. Pollanen

Abstract: Superfluid helium's low-loss dielectric properties, excellent thermal conductivity, and unique collective excitations make it an attractive candidate to incorporate into superconducting qubit systems. We controllably immerse a three-dimensional superconducting transmon qubit in superfluid helium-4 and measure the spectroscopic and coherence properties of the system. We find that the cavity, the qu… ▽ More Superfluid helium's low-loss dielectric properties, excellent thermal conductivity, and unique collective excitations make it an attractive candidate to incorporate into superconducting qubit systems. We controllably immerse a three-dimensional superconducting transmon qubit in superfluid helium-4 and measure the spectroscopic and coherence properties of the system. We find that the cavity, the qubit, and their coupling are all modified by the superfluid, which we analyze within the framework of circuit quantum electrodynamics (cQED). At at temperatures relevant to quantum computing experiments, the energy relaxation time of the qubit is not significantly changed by the presence of the superfluid, while the pure dephasing time modestly increases, which we attribute to improved thermalization of the microwave environment via the superfluid. △ Less

Submitted 18 November, 2019; v1 submitted 17 July, 2019; originally announced July 2019.

Comments: 11 pages, 8 figures

Journal ref: Phys. Rev. A 101, 012336 (2020)

Quantum state tomography across the exceptional point in a single dissipative qubit

Authors: M. Naghiloo, M. Abbasi, Yogesh N. Joglekar, K. W. Murch

Abstract: Open systems with gain and loss, described by non-trace-preserving, non-Hermitian Hamiltonians, have been a subject of intense research recently. The effect of exceptional-point degeneracies on the dynamics of classical systems has been observed through remarkable phenomena such as the parity-time symmetry breaking transition, asymmetric mode switching, and optimal energy transfer. On the other ha… ▽ More Open systems with gain and loss, described by non-trace-preserving, non-Hermitian Hamiltonians, have been a subject of intense research recently. The effect of exceptional-point degeneracies on the dynamics of classical systems has been observed through remarkable phenomena such as the parity-time symmetry breaking transition, asymmetric mode switching, and optimal energy transfer. On the other hand, consequences of an exceptional point for quantum evolution and decoherence are hitherto unexplored. Here, we use post-selection on a three-level superconducting transmon circuit with tunable Rabi drive, dissipation, and detuning to carry out quantum state tomography of a single dissipative qubit in the vicinity of its exceptional point. Quantum state tomography reveals the PT symmetry breaking transition at zero detuning, decoherence enhancement at finite detuning, and a quantum signature of the exceptional point in the qubit relaxation state. Our observations demonstrate rich phenomena associated with non-Hermitian physics such as non-orthogonality of eigenstates in a fully quantum regime and open routes to explore and harness exceptional point degeneracies for enhanced sensing and quantum information processing. △ Less

Submitted 29 April, 2019; v1 submitted 23 January, 2019; originally announced January 2019.

Comments: 10 pages, 8 figures

Bath engineering of a fluorescing artificial atom with a photonic crystal

Authors: P. M. Harrington, M. Naghiloo, D. Tan, K. W. Murch

Abstract: We demonstrate how the dissipative interaction between a superconducting qubit and a microwave photonic crystal can be used for quantum bath engineering. The photonic crystal is created with a step-impedance transmission line which suppresses and enhances the quantum spectral density of states, influencing decay transitions of a transmon circuit. The qubit interacts with the transmission line indi… ▽ More We demonstrate how the dissipative interaction between a superconducting qubit and a microwave photonic crystal can be used for quantum bath engineering. The photonic crystal is created with a step-impedance transmission line which suppresses and enhances the quantum spectral density of states, influencing decay transitions of a transmon circuit. The qubit interacts with the transmission line indirectly via dispersive coupling to a cavity. We characterize the photonic crystal density of states from both the unitary and dissipative dynamics of the qubit. When the qubit is driven, it dissipates into the frequency dependent density of states of the photonic crystal. Our result is the deterministic preparation of qubit superposition states as the steady-state of coherent driving and dissipation near by the photonic crystal band edge, which we characterize with quantum state tomography. Our results highlight how the multimode environment from the photonic crystal forms a resource for quantum control. △ Less

Submitted 29 April, 2019; v1 submitted 10 December, 2018; originally announced December 2018.

Comments: 9 pages 7 figures

Journal ref: Phys. Rev. A 99, 052126 (2019)

Characterizing a statistical arrow of time in quantum measurement dynamics

Authors: P. M. Harrington, D. Tan, M. Naghiloo, K. W. Murch

Abstract: In both thermodynamics and quantum mechanics the arrow of time is characterized by the statistical likelihood of physical processes. We characterize this arrow of time for the continuous quantum measurement dynamics of a superconducting qubit. By experimentally tracking individual weak measurement trajectories, we compare the path probabilities of forward and backward-in-time evolution to develop… ▽ More In both thermodynamics and quantum mechanics the arrow of time is characterized by the statistical likelihood of physical processes. We characterize this arrow of time for the continuous quantum measurement dynamics of a superconducting qubit. By experimentally tracking individual weak measurement trajectories, we compare the path probabilities of forward and backward-in-time evolution to develop an arrow of time statistic associated with measurement dynamics. We compare the statistics of individual trajectories to ensemble properties showing that the measurement dynamics obeys both detailed and integral fluctuation theorems thus establishing the consistency between microscopic and macroscopic measurement dynamics. △ Less

Submitted 10 April, 2019; v1 submitted 19 November, 2018; originally announced November 2018.

Comments: 10 pages 5 figures

Journal ref: Phys. Rev. Lett. 123, 020502 (2019)

Understanding the saturation power of Josephson Parametric Amplifiers made from SQUIDs arrays

Authors: Luca Planat, Remy Dassonneville, Javier Puertas Martinez, Farshad Foroughi, Olivier Buisson, Wiebke Hasch-Guichard, Cecile Naud, R. Vijay, Kater Murch, Nicolas Roch

Abstract: We report on the implementation and detailed modelling of a Josephson Parametric Amplifier (JPA) made from an array of eighty Superconducting QUantum Interference Devices (SQUIDs), forming a non-linear quarter-wave resonator. This device was fabricated using a very simple single step fabrication process. It shows a large bandwidth (45 MHz), an operating frequency tunable between 5.9 GHz and 6.8 GH… ▽ More We report on the implementation and detailed modelling of a Josephson Parametric Amplifier (JPA) made from an array of eighty Superconducting QUantum Interference Devices (SQUIDs), forming a non-linear quarter-wave resonator. This device was fabricated using a very simple single step fabrication process. It shows a large bandwidth (45 MHz), an operating frequency tunable between 5.9 GHz and 6.8 GHz and a large input saturation power (-117 dBm) when biased to obtain 20 dB of gain. Despite the length of the SQUID array being comparable to the wavelength, we present a model based on an effective non-linear LC series resonator that quantitatively describes these figures of merit without fitting parameters. Our work illustrates the advantage of using array-based JPA since a single-SQUID device showing the same bandwidth and resonant frequency would display a saturation power 15 dB lower. △ Less

Submitted 3 April, 2019; v1 submitted 22 September, 2018; originally announced September 2018.

Comments: 12 pages, 9 figures, Appendices included

Journal ref: Phys. Rev. Applied 11, 034014 (2019)

Information gain and loss for a quantum Maxwell's demon

Authors: M. Naghiloo, J. J. Alonso, A. Romito, E. Lutz, K. W. Murch

Abstract: We use continuous weak measurements of a driven superconducting qubit to experimentally study the information dynamics of a quantum Maxwell's demon. We show how information gained by a demon who can track single quantum trajectories of the qubit can be converted into work using quantum coherent feedback. We verify the validity of a quantum fluctuation theorem with feedback by utilizing information… ▽ More We use continuous weak measurements of a driven superconducting qubit to experimentally study the information dynamics of a quantum Maxwell's demon. We show how information gained by a demon who can track single quantum trajectories of the qubit can be converted into work using quantum coherent feedback. We verify the validity of a quantum fluctuation theorem with feedback by utilizing information obtained along single trajectories. We demonstrate, in particular, that quantum backaction can lead to a loss of information in imperfect measurements. We furthermore probe the transition between information gain and loss by varying the initial purity of the qubit. △ Less

Submitted 20 February, 2018; originally announced February 2018.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. Lett. 121, 030604 (2018)

Homodyne monitoring of post-selected decay

Authors: D. Tan, N. Foroozani, M. Naghiloo, A. H. Kiilerich, K. Mølmer, K. W. Murch

Abstract: We use homodyne detection to monitor the radiative decay of a superconducting qubit. According to the classical theory of conditional probabilities, the excited state population differs from an exponential decay law if it is conditioned upon a later projective qubit measurement. Quantum trajectory theory accounts for the expectation values of general observables, and we use experimental data to sh… ▽ More We use homodyne detection to monitor the radiative decay of a superconducting qubit. According to the classical theory of conditional probabilities, the excited state population differs from an exponential decay law if it is conditioned upon a later projective qubit measurement. Quantum trajectory theory accounts for the expectation values of general observables, and we use experimental data to show how a homodyne detection signal is conditioned upon both the initial state and the finally projected state of a decaying qubit. We observe, in particular, how anomalous weak values occur in continuous weak measurement for certain pre- and post-selected states. Subject to homodyne detection, the density matrix evolves in a stochastic manner, but it is restricted to a specific surface in the Bloch sphere. We show that a similar restriction applies to the information associated with the post-selection, and thus bounds the predictions of the theory. △ Less

Submitted 11 May, 2017; originally announced May 2017.

Comments: 11 pages, 8 figures

Journal ref: Phys. Rev. A 96, 022104 (2017)

Quantum Zeno effects from measurement controlled qubit-bath interactions

Authors: P. M. Harrington, J. T. Monroe, K. W. Murch

Abstract: The Zeno and anti-Zeno effects are features of measurement-driven quantum evolution where frequent measurement inhibits or accelerates the decay of a quantum state. Either type of evolution can emerge depending on the system-environment interaction and measurement method. In this experiment, we use a superconducting qubit to map out both types of Zeno effect in the presence of structured noise bat… ▽ More The Zeno and anti-Zeno effects are features of measurement-driven quantum evolution where frequent measurement inhibits or accelerates the decay of a quantum state. Either type of evolution can emerge depending on the system-environment interaction and measurement method. In this experiment, we use a superconducting qubit to map out both types of Zeno effect in the presence of structured noise baths and variable measurement rates. We observe both the suppression and acceleration of qubit decay as repeated measurements are used to modulate the qubit spectrum causing the qubit to sample different portions of the bath. We compare the Zeno effects arising from dispersive energy measurements and purely-dephasing `quasi'-measurements, showing energy measurements are not necessary to accelerate or suppress the decay process. △ Less

Submitted 25 April, 2017; v1 submitted 24 March, 2017; originally announced March 2017.

Comments: 7 pages, 8 figures

Journal ref: Phys. Rev. Lett. 118, 240401 (2017)

Heat and work along individual trajectories of a quantum bit

Authors: M. Naghiloo, D. Tan, P. M. Harrington, J. J. Alonso, E. Lutz, A. Romito, K. W. Murch

Abstract: We use a near quantum limited detector to experimentally track individual quantum state trajectories of a driven qubit formed by the hybridization of a waveguide cavity and a transmon circuit. For each measured quantum coherent trajectory, we separately identify energy changes of the qubit as heat and work, and verify the first law of thermodynamics for an open quantum system. We further establish… ▽ More We use a near quantum limited detector to experimentally track individual quantum state trajectories of a driven qubit formed by the hybridization of a waveguide cavity and a transmon circuit. For each measured quantum coherent trajectory, we separately identify energy changes of the qubit as heat and work, and verify the first law of thermodynamics for an open quantum system. We further establish the consistency of these results by comparison with the master equation approach and the two-projective-measurement scheme, both for open and closed dynamics, with the help of a quantum feedback loop that compensates for the exchanged heat and effectively isolates the qubit. △ Less

Submitted 29 April, 2019; v1 submitted 17 March, 2017; originally announced March 2017.

Comments: 11 pages, 7 figures

Journal ref: Phys. Rev. Lett. 124, 110604 (2020)

Quantum caustics in resonance fluorescence trajectories

Authors: M. Naghiloo, D. Tan, P. M. Harrington, P. Lewalle, A. N. Jordan, K. W. Murch

Abstract: We employ phase-sensitive amplification to perform homodyne detection of the resonance fluorescence from a driven superconducting artificial atom. Entanglement between the emitter and its fluorescence allows us to track the individual quantum state trajectories of the emitter conditioned on the outcomes of the field measurements. We analyze the ensemble properties of these trajectories by consider… ▽ More We employ phase-sensitive amplification to perform homodyne detection of the resonance fluorescence from a driven superconducting artificial atom. Entanglement between the emitter and its fluorescence allows us to track the individual quantum state trajectories of the emitter conditioned on the outcomes of the field measurements. We analyze the ensemble properties of these trajectories by considering trajectories that connect specific initial and final states. By applying the stochastic path integral formalism, we calculate equations-of-motion for the most likely path between two quantum states and compare these predicted paths to experimental data. Drawing on the mathematical similarity between the action formalism of the most likely quantum paths and ray optics we study the emergence of caustics in quantum trajectories---places where multiple extrema in the stochastic action occur. We observe such multiple most likely paths in experimental data and find these paths to be in reasonable quantitative agreement with theoretical calculations. △ Less

Submitted 8 September, 2017; v1 submitted 9 December, 2016; originally announced December 2016.

Comments: 12 pages, 10 figures

Journal ref: Phys. Rev. A 96, 053807 (2017)

Quantum smoothing for classical mixtures

Authors: D. Tan, M. Naghiloo, K. Mølmer, K. W. Murch

Abstract: In quantum mechanics, wave functions and density matrices represent our knowledge about a quantum system and give probabilities for the outcomes of measurements. If the combined dynamics and measurements on a system lead to a density matrix $ρ(t)$ with only diagonal elements in a given basis $\{|n\rangle\}$, it may be treated as a classical mixture, i.e., a system which randomly occupies the basis… ▽ More In quantum mechanics, wave functions and density matrices represent our knowledge about a quantum system and give probabilities for the outcomes of measurements. If the combined dynamics and measurements on a system lead to a density matrix $ρ(t)$ with only diagonal elements in a given basis $\{|n\rangle\}$, it may be treated as a classical mixture, i.e., a system which randomly occupies the basis states $|n\rangle$ with probabilities $ρ_{nn}(t)$. Fully equivalent to so-called smoothing in classical probability theory, subsequent probing of the occupation of the states $|n\rangle$ improves our ability to retrodict what was the outcome of a projective state measurement at time $t$. Here, we show with experiments on a superconducting qubit that the smoothed probabilities do not, in the same way as the diagonal elements of $ρ$, permit a classical mixture interpretation of the state of the system at the past time $t$. △ Less

Submitted 30 August, 2016; v1 submitted 1 July, 2016; originally announced July 2016.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. A 94, 050102 (2016)

Mapping quantum state dynamics in spontaneous emission

Authors: M. Naghiloo, N. Foroozani, D. Tan, A. Jadbabaie, K. W. Murch

Abstract: The evolution of a quantum state undergoing radiative decay depends on how the emission is detected. We employ phase-sensitive amplification to perform homodyne detection of the spontaneous emission from a superconducting artificial atom. Using quantum state tomography, we characterize the correlation between the detected homodyne signal and the emitter's state, and map out the conditional back-ac… ▽ More The evolution of a quantum state undergoing radiative decay depends on how the emission is detected. We employ phase-sensitive amplification to perform homodyne detection of the spontaneous emission from a superconducting artificial atom. Using quantum state tomography, we characterize the correlation between the detected homodyne signal and the emitter's state, and map out the conditional back-action of homodyne measurement. By tracking the diffusive quantum trajectories of the state as it decays, we characterize selective stochastic excitation induced by the choice of measurement basis. Our results demonstrate dramatic differences from the quantum jump evolution that is associated with photodetection and highlight how continuous field detection can be harnessed to control quantum evolution. △ Less

Submitted 24 March, 2016; v1 submitted 7 December, 2015; originally announced December 2015.

Comments: 8 pages, 8 figures

Journal ref: Nature Communications 7, Article number: 11527 (2016)

Correlations of the time dependent signal and the state of a continuously monitored quantum system

Authors: N. Foroozani, M. Naghiloo, D. Tan, K. Mølmer, K. W. Murch

Abstract: In quantum physics, measurements give random results and yield a corresponding random back action on the state of the system subject to measurement. If a quantum system is probed continuously over time, its state evolves along a stochastic quantum trajectory. To investigate the characteristic properties of such dynamics, we perform weak continuous measurements on a superconducting qubit that is dr… ▽ More In quantum physics, measurements give random results and yield a corresponding random back action on the state of the system subject to measurement. If a quantum system is probed continuously over time, its state evolves along a stochastic quantum trajectory. To investigate the characteristic properties of such dynamics, we perform weak continuous measurements on a superconducting qubit that is driven to undergo Rabi oscillations. From the data we observe a number of striking temporal correlations within the time dependent signals and the quantum trajectories of the qubit, and we discuss their explanation in terms of quantum measurement and photodetection theory. △ Less

Submitted 12 October, 2015; v1 submitted 5 August, 2015; originally announced August 2015.

Comments: 8 pages 5 figures

Journal ref: Phys. Rev. Lett. 116, 110401 (2016)

Prediction and retrodiction for a continuously monitored superconducting qubit

Authors: D. Tan, S. Weber, I. Siddiqi, K. Mølmer, K. W. Murch

Abstract: The quantum state of a superconducting transmon qubit inside a three-dimensional cavity is monitored by reflection of a microwave field on the cavity. The information inferred from the measurement record is incorporated in a density matrix $ρ_t$, which is conditioned on probe results until $t$, and in an auxiliary matrix $E_t$, which is conditioned on probe results obtained after $t$. Here, we obt… ▽ More The quantum state of a superconducting transmon qubit inside a three-dimensional cavity is monitored by reflection of a microwave field on the cavity. The information inferred from the measurement record is incorporated in a density matrix $ρ_t$, which is conditioned on probe results until $t$, and in an auxiliary matrix $E_t$, which is conditioned on probe results obtained after $t$. Here, we obtain these matrices from experimental data and we illustrate their application to predict and retrodict the outcome of weak and strong qubit measurements. △ Less

Submitted 10 November, 2014; v1 submitted 1 September, 2014; originally announced September 2014.

Comments: 9 pages, 8 figures

Journal ref: Phys. Rev. Lett. 114, 090403 (2015)

Mapping the optimal route between two quantum states

Authors: S. J. Weber, A. Chantasri, J. Dressel, A. N. Jordan, K. W. Murch, I. Siddiqi

Abstract: A central feature of quantum mechanics is that a measurement is intrinsically probabilistic. As a result, continuously monitoring a quantum system will randomly perturb its natural unitary evolution. The ability to control a quantum system in the presence of these fluctuations is of increasing importance in quantum information processing and finds application in fields ranging from nuclear magneti… ▽ More A central feature of quantum mechanics is that a measurement is intrinsically probabilistic. As a result, continuously monitoring a quantum system will randomly perturb its natural unitary evolution. The ability to control a quantum system in the presence of these fluctuations is of increasing importance in quantum information processing and finds application in fields ranging from nuclear magnetic resonance to chemical synthesis. A detailed understanding of this stochastic evolution is essential for the development of optimized control methods. Here we reconstruct the individual quantum trajectories of a superconducting circuit that evolves in competition between continuous weak measurement and driven unitary evolution. By tracking individual trajectories that evolve between an arbitrary choice of initial and final states we can deduce the most probable path through quantum state space. These pre- and post-selected quantum trajectories also reveal the optimal detector signal in the form of a smooth time-continuous function that connects the desired boundary conditions. Our investigation reveals the rich interplay between measurement dynamics, typically associated with wave function collapse, and unitary evolution of the quantum state as described by the Schrodinger equation. These results and the underlying theory, based on a principle of least action, reveal the optimal route from initial to final states, and may enable new quantum control methods for state steering and information processing. △ Less

Submitted 19 March, 2014; originally announced March 2014.

Comments: 12 pages, 9 figures

Journal ref: Nature 511, 570-573 (31 July 2014)

Observing single quantum trajectories of a superconducting qubit

Authors: K. W. Murch, S. J. Weber, C. Macklin, I. Siddiqi

Abstract: The length of time that a quantum system can exist in a superposition state is determined by how strongly it interacts with its environment. This interaction entangles the quantum state with the inherent fluctuations of the environment. If these fluctuations are not measured, the environment can be viewed as a source of noise, causing random evolution of the quantum system from an initially pure s… ▽ More The length of time that a quantum system can exist in a superposition state is determined by how strongly it interacts with its environment. This interaction entangles the quantum state with the inherent fluctuations of the environment. If these fluctuations are not measured, the environment can be viewed as a source of noise, causing random evolution of the quantum system from an initially pure state into a statistical mixture-a process known as decoherence. However, by accurately measuring the environment in real time, the quantum system can be maintained in a pure state and its time evolution described by a quantum trajectory conditioned on the measurement outcome. We employ weak measurements to monitor a microwave cavity embedding a superconducting qubit and track the individual quantum trajectories of the system. In this architecture, the environment is dominated by the fluctuations of a single electromagnetic mode of the cavity. Using a near-quantum-limited parametric amplifier, we selectively measure either the phase or amplitude of the cavity field, and thereby confine trajectories to either the equator or a meridian of the Bloch sphere. We perform quantum state tomography at discrete times along the trajectory to verify that we have faithfully tracked the state of the quantum system as it diffuses on the surface of the Bloch sphere. Our results demonstrate that decoherence can be mitigated by environmental monitoring and validate the foundations of quantum feedback approaches based on Bayesian statistics. Moreover, our experiments suggest a new route for implementing what Schrodinger termed "quantum steering"-harnessing action at a distance to manipulate quantum states via measurement. △ Less

Submitted 14 October, 2013; v1 submitted 30 May, 2013; originally announced May 2013.

Comments: 7 pages, 6 figures

Journal ref: Nature 502, 211 (10 October 2013)

Suppression of the radiative decay of atomic coherence in squeezed vacuum

Authors: K. W. Murch, S. J. Weber, K. M. Beck, Eran Ginossar, I. Siddiqi

Abstract: Quantum fluctuations of the electromagnetic vacuum are responsible for physical effects such as the Casimir force and the radiative decay of atoms, and set fundamental limits on the sensitivity of measurements. Entanglement between photons can produce correlations that result in a reduction of these fluctuations below the vacuum level allowing measurements that surpass the standard quantum limit i… ▽ More Quantum fluctuations of the electromagnetic vacuum are responsible for physical effects such as the Casimir force and the radiative decay of atoms, and set fundamental limits on the sensitivity of measurements. Entanglement between photons can produce correlations that result in a reduction of these fluctuations below the vacuum level allowing measurements that surpass the standard quantum limit in sensitivity. Here we demonstrate that the radiative decay rate of an atom that is coupled to quadrature squeezed electromagnetic vacuum can be reduced below its natural linewidth. We observe a two-fold reduction of the transverse radiative decay rate of a superconducting artificial atom coupled to continuum squeezed vacuum generated by a Josephson parametric amplifier, allowing the transverse coherence time T_2 to exceed the vacuum decay limit of 2T_1. We demonstrate that the measured radiative decay dynamics can be used to tomographically reconstruct the Wigner distribution of the the itinerant squeezed state. Our results are the first confirmation of a canonical prediction of quantum optics and open the door to new studies of the quantum light-matter interaction. △ Less

Submitted 22 April, 2013; v1 submitted 26 January, 2013; originally announced January 2013.

Comments: 9 pages, 5 figures

Journal ref: Nature 499, 62-65 (04 July 2013)