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Advanced Quantum Technologies

Nonreciprocal Quantum Walks in Superconducting Quantum Circuits

Bo Li, Hao Wu, Wenhuan Wu, Yuan Zhou, Shuai Li, Yufeng Chen

Published Sep 1, 2026• 1 citation
Publisher

Abstract

ABSTRACT The phenomenon of quantum walks of quasiparticles under nonreciprocal coupling is not only pivotal for investigating a myriad of novel quantum phenomena in nonreciprocal many‐body quantum systems, but also holds promise for the construction of nonreciprocal quantum devices. To date, manipulating the dynamical behavior of quasiparticles in such systems remains a significant challenge. Leveraging the quantum interference effect between coherent and dissipative coupling channels, we propose an effective scheme for realizing nonreciprocal coupling between superconducting qubits, and based on this scheme, we present a one‐dimensional nonreciprocal quantum simulator. Through numerical simulations, we demonstrate that this quantum simulator is capable of precisely manipulating a variety of exotic quantum phenomena, including the non‐Hermitian skin effect, chiral currents, and chiral spin waves. Furthermore, we simulate the propagation dynamics of entangled states under nonreciprocal coupling; the results indicate that nonreciprocity significantly suppresses the propagation of entanglement. Moreover, we provide a phenomenological interpretation of these exotic physical phenomena emerging in the non‐Hermitian regime. The nonreciprocal coupling mechanism and the quantum simulator presented in this work provide a highly coherent, scalable, and controllable platform for the quantum simulation of nonreciprocal many‐body systems, paving the way for the design of nonreciprocal quantum devices.

DOI
10.1002/qute.70449

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ABSTRACT The phenomenon of quantum walks of quasiparticles under nonreciprocal coupling is not only pivotal for investigating a myriad of novel quantum phenomena in nonreciprocal many‐body quantum systems, but also holds promise for the construction of nonreciprocal quantum devices. To date, manipulating the dynamical behavior of quasiparticles in such systems remains a significant challenge. Leveraging the quantum interference effect between coherent and dissipative coupling channels, we propose an effective scheme for realizing nonreciprocal coupling between superconducting qubits, and based on this scheme, we present a one‐dimensional nonreciprocal quantum simulator. Through numerical simulations, we demonstrate that this quantum simulator is capable of precisely manipulating a variety of exotic quantum phenomena, including the non‐Hermitian skin effect, chiral currents, and chiral spin waves. Furthermore, we simulate the propagation dynamics of entangled states under nonreciprocal coupling; the results indicate that nonreciprocity significantly suppresses the propagation of entanglement. Moreover, we provide a phenomenological interpretation of these exotic physical phenomena emerging in the non‐Hermitian regime. The nonreciprocal coupling mechanism and the quantum simulator presented in this work provide a highly coherent, scalable, and controllable platform for the quantum simulation of nonreciprocal many‐body systems, paving the way for the design of nonreciprocal quantum devices.
The non-Hermitian skin effect (NHSE), referring to the anomalous localization of eigenstates, has been widely discussed in single-particle systems. However, the interplay between the interacting NHSE and the quantum optical phenomena remains elusive. Here, We present the non-Hermitian quantum walks of strongly correlated photon pairs. Through studying the dynamic behavior of the photon pairs in the nonreciprocal Bose-Hubbard chain, we theoretically demonstrate the interplay between interactions and the NHSE. When launching two non-entangled photons, the photons exhibit the nonreciprocal bunching effect, which can be suppressed by the interactions. When launching two path-entangled photons, the interactions can not only suppress but reverse the nonreciprocal bunching. In stark contrast to the common wisdom that the NHSE suppresses the entanglement of the quantum many-body system, the entanglement of the photons here can be significantly enhanced. Our results reveal that many-body interactions and entanglement significantly restrain the non-Hermitian nonreciprocal quantum dynamics, offering insights into the interplay between many-body quantum entanglement and non-Hermitian physics. The non-Hermitian skin effect, which nonreciprocally accumulates particles at the boundary, generally suppresses entanglement by constraining global correlations. Here, the authors demonstrate that interactions can suppress nonreciprocal bunching, while entangled photons can reverse bunching direction, leading to enhanced entanglement.
We present a scheme to efficiently simulate nonreciprocal many-body spin models using a non-Markovian open system. Our scheme utilizes a single quantum emitter coupled to a waveguide mode that is fed back to the same emitter after a time delay. With this coherent delayed feedback, an effective nonreciprocal interaction can be engineered, wherein the emitter at earlier times affects itself at later times, creating a scalable spin chain. We show that all the steady-state quantities of such spin models can be accessed through measurement of the emitter and the output field. Furthermore, using classical feedback that resets the emitter, quench dynamics from arbitrary product states can be simulated. We demonstrate striking features of nonreciprocal models, such as the Liouvillian skin effect, anomalous relaxation dynamics, and quasi-long-range order of output photons. Finally, we show that the proposal is amenable to experimental realization and robust to imperfections. Our work establishes a feasible way to scale up a nonreciprocal many-body system, and provides a new route towards generation of exotic states of light.
This article delves into the intricacies of nonreciprocity in a system composed of two superconducting qubits operating in a transmon regime and coupled to a 1-D waveguide. Nonreciprocity, characterized by asymmetric signal transmission, is a phenomenon of increasing interest in quantum technologies, as it enables one-way signal propagation and isolation. Our investigation derives the conditions to obtain nonreciprocity in a qubit-loaded waveguide and discusses how common approximations may impact the observed nonreciprocal effects, highlighting crucial factors that must be considered in designing such systems. By employing realistic system parameters, we unveil significant effects on the system dynamics when higher energy levels are included in the transmon regime. This leads to a reduction of nonreciprocity compared to the ideal scenario, with the transmission ratio falling below the two-level approximation limit of2/32/32/3. Additionally, the statistical properties of the system undergo significant changes, particularly in the coherent functions for transmitted and reflected signals. The emergence of additional population transfer channels and the critical role of quantum correlations between qubits underscore the necessity of careful modeling. Our findings have implications for quantum device engineering, offering insights that can enhance the design of more efficient quantum technology and advanced quantum communication systems.
The anomalous non-Hermitian dynamical phenomenon with the non-Hermitian skin effect (NHSE) attracts wide attention due to its novel physics and promising applications. Here, we propose a new type of non-unitary discrete-time quantum walk system demonstrating the NHSE and anomalous non-Hermitian dynamical phenomena, including the dynamical chiral phenomenon, the funneling phenomenon on the domain wall, and the anomalous reflection on the phase impurity. Furthermore, we design the quantum circuit experiments of these quantum walk systems and numerically simulate them with quantum noises to verify the robustness of the non-Hermitian dynamical phenomenon on the noisy intermediate-scale quantum (NISQ) devices. Our work paves the way for implementing the non-Hermitian dynamical phenomenon on the quantum circuit.
Nonreciprocal microwave routing plays a crucial role in measuring quantum circuits, and allows for realizing cascaded quantum systems for generating and stabilizing entanglement between noninteracting qubits. The most commonly used tools for implementing directionality are ferrite-based circulators. These devices are versatile, but suffer from excess loss, a large footprint, and fixed directionality. For utilizing nonreciprocity in scalable quantum circuits it is desirable to develop efficient integration of low-loss and controllable directional elements. Here, we report the design and experimental realization of a minimal controllable directional interface that can be directly coupled to superconducting qubits. In the device presented, nonreciprocity is realized through a combination of interference and phase-controlled parametric pumping. We have achieved a maximum directionality of around 30 dB, and the performance of the device is predicted quantitatively from independent calibration measurements. Using the excellent agreement of model and experiment, we predict that the circuit will be useable as a chiral qubit interface with inefficiencies at the 1 % level or below. Our work offers a promising route for realizing high-fidelity signal routing and entanglement generation in all-to-all connected microwave quantum networks, and provides a path for isolator-free qubit readout schemes. Published by the American Physical Society 2024
Quantum many-body systems in which time-reversal symmetry is broken give rise to a wealth of exotic phases, and thus constitute one of the frontiers of modern condensed matter physics. Quantum simulation allows us to better understand many-body systems with huge Hilbert space, where classical simulation is usually inefficient. With superconducting quantum circuit as a platform for quantum simulation, we realize synthetic Abelian gauge fields by using microwave drive and tunable coupling in loop configurations to break the time-reversal symmetry of the system. Based on high-precision manipulation and readout of circuit-QED architecture, we demonstrate the chiral ground spin current of a time-reversal symmetry broken system with nontrivial interactions. Our work is a significant attempt to simulate quantum many-body systems with time-reversal symmetry breaking in multi-qubit superconducting processors.