Abstract

We report on the design and performance of an on-chip microwave circulator with a widely (GHz) tunable operation frequency. Non-reciprocity is created with a combination of frequency conversion and delay, and requires neither permanent magnets nor microwave bias tones, allowing on-chip integration with other superconducting circuits without the need for high-bandwidth control lines. Isolation in the device exceeds 20 dB over a bandwidth of tens of MHz, and its insertion loss is small, reaching as low as 0.9 dB at select operation frequencies. Furthermore, the device is linear with respect to input power for signal powers up to hundreds of fW ($\approx 10^3$ circulating photons), and the direction of circulation can be dynamically reconfigured. We demonstrate its operation at a selection of frequencies between 4 and 6 GHz.

Highlights

  • Experiments on one or several superconducting qubits have shown that the circuit quantum electrodynamics architecture [1] is a viable platform for the realization of a quantum information processor [2,3]

  • As superconducting qubit experiments scale in complexity, further signal processing innovations are needed to preserve the high level of control demonstrated in few-qubit experiments

  • Lorentz reciprocity is broken in the circuit with sequential translations in frequency and time, which we show with a simple model system composed of just two components: multiplying elements and delays

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Summary

INTRODUCTION

Experiments on one or several superconducting qubits have shown that the circuit quantum electrodynamics architecture [1] is a viable platform for the realization of a quantum information processor [2,3]. Enforcing one-way signal propagation is critical, for example, in the separation of incoming and outgoing fields for quantum-limited reflection amplifiers, or the isolation of sensitive quantum devices from the backaction of the microwave receiver (Fig. 1) These tasks are performed by commercial ferrite junction circulators. An ideal replacement technology would be both monolithic and operable without highbandwidth control lines, which are a limited resource in cryogenic microwave experiments It would be low loss, linear at power levels typical for qubit readout and amplification, and flexible, in the sense that it should be either broadband, like a commercial circulator, or tunable over a wide frequency range, like some Josephson parametric amplifiers [31,32]. These measurements are performed over a range of different operation frequencies and with the circulator configured for clockwise and counterclockwise circulation, highlighting the device’s tunability and the capability to dynamically reconfigure its sense of circulation in situ

THEORY OF OPERATION
SUPERCONDUCTING IMPLEMENTATION
Multiplying elements
Delays
Circulator
EXPERIMENTAL RESULTS
CONCLUSION AND OUTLOOK
Transmission calibration
Reflection calibration
Calibration of group delay
Use of normal metal
Bias line design
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