Abstract

Nonreciprocal microwave devices, such as circulators and isolators, are needed in high-fidelity qubit readout schemes to unidirectionally route the readout signals and protect the qubits against noise coming from the output chain. However, cryogenic circulators and isolators are prohibitive in scalable superconducting architectures because they rely on magnetic materials. Here, we perform a fast (750 ns) high-fidelity (95%) quantum nondemolition readout of a coherent superconducting qubit ($T_{1}=52$ $\mu s$, $T_{\rm{2E}}=35$ $\mu s$) without any nonreciprocal magnetic devices. We employ in our readout chain a microwave-controlled qubit-Readout Multi-Chip Module (qRMCM) that integrates interferometric directional Josephson devices consisting of an isolator and a reconfigurable isolator/amplifier device and an off-chip low-pass filter. Using the qRMCM, we demonstrate isolation up to 45 dB within 13 MHz, when both directional devices are operated as isolators, and low-noise amplification in excess of 10 dB within a dynamical bandwidth of $10$ MHz, when the reconfigurable device is operated as an amplifier. We also demonstrate using the variable isolation of the qRMCM an in-situ enhancement of the qubit coherence times $T_{\rm{\varphi}}$ and $T_{\rm{2E}}$ by two orders of magnitude (i.e., from $T_{\rm{\varphi}}=T_{\rm{2E}}=0.5$ $\mu s$ to $T_{\rm{\varphi}}=90$ $\mu s$ and $T_{\rm{2E}}=50$ $\mu s$). Furthermore, by directly comparing the qRMCM performance to a state-of-art configuration (with $T_{\rm{2E}}\approx 2T_{1}$) that employs a pair of wideband magnetic isolators, we find that the excess pure dephasing measured with the qRMCM (for which $T_{\rm{2E}}\approx T_{1}$) is likely limited by residual thermal photon population in the readout resonator. Improved versions of the qRMCM could replace magnetic circulators and isolators in large superconducting quantum processors.

Highlights

  • Nonreciprocity breaks the transmission-coefficient symmetry for light upon exchange of sources and detectors

  • The qubit-qubit readout multichip module (QRMCM) experiment differs from the qubit-multipath interoferometric Josephson isolator (MPIJIS) experiment reported in Ref. [35] in several important aspects: it (1) realizes an on-chip MPIJIS device operated with a single pump; it (2) introduces a working QRMCM operated in continuous mode, which integrates a Purcell filter, a superconducting directional coupler, MPIJIS devices, and a reconfigurable multipath interoferometric Josephson directional amplifier (MPIJDA) or MPIJIS device; (3) it demonstrates a high-fidelity, high-coherence, dispersive qubit measurement without any magnetic isolators and circulators in the output chain; (4) it achieves an isolation of more than 40 dB at the readout frequency using two MPIJIS devices in series; and (5) it can be used to investigate the dependence of the qubit coherence on the MPIJIS response, varied in situ with use of the pump tone

  • The QRMCM includes a Purcell filter, a superconducting directional coupler, two MPIJIS devices, and one reconfigurable directional Josephson device integrated into one printed circuit board (PCB)

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Summary

INTRODUCTION

Nonreciprocity breaks the transmission-coefficient symmetry for light upon exchange of sources and detectors. In an attempt to solve this scalability challenge, a variety of viable alternative circulator and isolator schemes have been proposed and realized, which use photonic transitions between coupled resonance modes [21,22], the Hall effect [6], frequency conversion in nonlinear transmission lines [23], frequency conversion combined with delay lines [24,25], dynamical modulation of transfer switches incorporated with delay lines [26], and reservoir engineered optomechanical interactions [27,28,29] Despite this great progress in the development of alternative directional devices and readout schemes, demonstrating high-fidelity dispersive readout of a superconducting qubit with relatively high coherence. VI, we conclude by providing a summary of the results and a brief outlook

THE MPIJIS DEVICE
Pump 1
Scattering parameters
Pump power dependence
Flux parity dependence
QUBIT READOUT MULTICHIP MODULE
MPIJIS-MPIJDA experiment
Readout
MPIJIS-MPIJIS experiment
Variable isolation experiment
QRMCM versus two magnetic isolators
DISCUSSION
CONCLUSION
OUT NA
Findings
QRMCM versus magnetic isolator setup
Qubit measurement parameters
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