Di Palma, Luigi (2023) The Josephson Digital Phase Detector: principle, design and operation. [Tesi di dottorato]

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Abstract

Quantum computing platforms based on superconducting qubits have emerged as one of the most promising candidates in the race to build a large-scale quantum computer. However, while the performance of small superconducting quantum processors has advanced the threshold necessary for fault tolerance, the current technique to control and readout the qubit state imposes severe system scaling challenges. Within this framework, digital control based on energy-efficient superconducting Single Flux Quantum (eSFQ) logic, is being adapted to perform qubit control and readout for scalable quantum architectures, thus leading to the development of innovative concepts for control and benchmarking in this linked digital-quantum hybrid system. I present a new SFQ-compatible readout technique based on a flux-switchable Quantum Flux Parametron (QFP), which is capable of discriminating between two phase values of a coherent input tone. In the proposed detection protocol, the QFP is at first flux-biased into harmonic configuration to sense the input coherent tone, then quickly flux-switched to a bistable configuration to store the information on the input tone phase. We name our device Josephson Digital Phase Detector (JDPD), since the result of the detection, is naturally encoded in the occupation probability of a phase particle in either of the wells in the bistable configuration. In this work, the JDPD approach has been completely investigated from a theoretical and experimental perspective. Numerical simulations demonstrate that detection can be accomplished in a time scale of few nanoseconds with fidelity approaching 1. During the operations, the JDPD is not required to be in resonance with the input signal frequency. Thus, the device can be designed to have precise energy transitions, which reduce the backaction on the surrounding circuitry. Theoretical predictions are supported by experimental outcomes obtained on several devices. Digital phase detection has been demonstrated in a wide range of operation regimes and device configurations up to a frequency of 1 GHz. As a future perspective, I will discuss a possible implementation of this device to readout the state of a superconducting qubit, which can be accomplished by properly adjusting the JDPD design parameters. Therefore, we envision the JDPD as part of a more complex architecture, in which the classical qubit control, measurement, and data processing are performed by a classical SFQ processor. This approach would have tremendous advantages to superconducting quantum systems’ scalability, which remains a big engineering challenge to realize practical error-corrected quantum computers.

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