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arXiv:2306.11364 · Physical Review Applied

Discriminating the Phase of a Coherent Tone with a Flux-Switchable Superconducting Circuit

Luigi Di Palma, Alessandro Miano, P. Mastrovito, D. Massarotti, M. Arzeo, Giovanni Piero Pepe, Francesco Tafuri, Oleg A. Mukhanov

Published Jun 7, 2023• 16 citations
arXivHTMLPDF

Abstract

We propose a new phase detection technique based on a flux-switchable superconducting circuit, the Josephson digital phase detector (JDPD), which is capable of discriminating between two phase values of a coherent input tone. When properly excited by an external flux, the JDPD is able to switch from a single-minimum to a double-minima potential and, consequently, relax in one of the two stable configurations depending on the phase sign of the input tone. The result of this operation is digitally encoded in the occupation probability of a phase particle in either of the two JDPD wells. In this work, we demonstrate the working principle of the JDPD up to a frequency of 400 MHz with a remarkable agreement with theoretical expectations. As a future scenario, we discuss the implementation of this technique to superconducting qubit readout. We also examine the JDPD compatibility with the single-flux-quantum architecture, employed to fast-drive and measure the device state.

DOI
10.1103/physrevapplied.19.064025

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We propose a new phase detection technique based on a flux-switchable superconducting circuit, the Josephson digital phase detector (JDPD), which is capable of discriminating between two phase values of a coherent input tone. When properly excited by an external flux, the JDPD is able to switch from a single-minimum to a double-minima potential and, consequently, relax in one of the two stable configurations depending on the phase sign of the input tone. The result of this operation is digitally encoded in the occupation probability of a phase particle in either of the two JDPD wells. In this work, we demonstrate the working principle of the JDPD up to a frequency of 400 MHz with a remarkable agreement with theoretical expectations. As a future scenario, we discuss the implementation of this technique to superconducting qubit readout. We also examine the JDPD compatibility with the single-flux-quantum architecture, employed to fast-drive and measure the device state.
We present a detection technique based on a flux-switchable Josephson Digital Phase Detector (JDPD), which is capable of discriminating between two phase values of a coherent input tone. When suitably flux stimulated, the JDPD switches from a single-minimum to a double-minima potential and, as a result, relaxes in one of two stable configurations depending on the phase sign of the input stimulus. The outcome of this procedure is digitally encoded in the occupation probability of the phase particle in either of the two JDPD wells. In this manuscript, we demonstrate the working principle of the JDPD by discriminating the phase of a 5 GHz tone with a protocol of 300 ns of time duration. This result supports the future implementation of the JDPD to readout the state of a qubit.
Quantum computation requires high-fidelity qubit readout, preserving the quantum state. In the case of superconducting (SC) qubits, readout is typically performed using a complex analog experimental setup operated at room temperature, which poses significant technological and economic barriers to large system scalability. An alternative approach is to perform a cryogenic on-chip qubit readout based on a Josephson Digital Phase Detector (JDPD): a flux switchable device capable of digitizing the phase sign of a coherent input. The readout operation includes the flux excitation of the JDPD to evolve from a single to a double-minima potential. In this work, the effect of the flux bias characteristics on the JDPD performances is studied numerically. To meet the identified requirements that maximize detection fidelity and tackle the engineering challenges, a cryogenic on-chip Single Flux Quantum based flux bias driver is proposed and discussed.
In the quest to build a fault-tolerant quantum computer, superconducting circuits based on Josephson junctions have emerged as a leading architecture. Coherence times have increased significantly over the last two decades, and processors with ∼ 50 qubits have been experimentally demonstrated. These systems traditionally utilize microwave frequency control signals, and heterodyne based detection schemes for measurement. Both of these techniques rely heavily on room temperature microwave generators, high-bandwidth lines from room temperature to millikelvin temperatures, and bulky non-reciprocal elements such as cryogenic microwave isolators. Reliance on these elements makes it impractical to scale existing devices up a single order of magnitude, let alone the 5-6 orders of magnitude needed for performing fault-tolerant quantum algorithms. Here, I present results that suggesting superconducting digital logic, namely Single Flux Quantum (SFQ) logic, can replace analog control and measurement techniques, avoiding the significant overhead involved. I describe a scheme for measuring qubits with a device known as a Josephson Photomultiplier (JPM), which crucially stores the result of a qubit measurement in a classical circulating supercurrent within the device and allows for integration with SFQ detection circuitry. This technique is experimentally demonstrated, with single-shot measurement fidelity of 92%. Two methods for accessing this measurement result are presented, one utilizing ballistic fluxons, and another utilizing flux comparison. Initial experimental results of the latter are presented. In addition, I describe a scheme for controlling qubits with sequences of digital SFQ pulses. This method is then used to control a qubit without a microwave signal generator, with results of an average single-qubit gate fidelity of around 95%. When combined, these techniques form a nearly fully digital interface to superconducting qubits, which could allow these systems to scale much more easily.
An experiment demonstrating a link between classical single-flux quantum digital logic and a superconducting quantum circuit is reported. We implement coupling between a moving Josephson vortex (fluxon) and a flux qubit by reading out of a state of the flux qubit through a frequency shift of the fluxon oscillations in an annular Josephson junction. The energy spectrum of the flux qubit is measured using this technique. The implemented hybrid scheme opens an opportunity to readout quantum states of superconducting qubits with the classical fluxon logic circuits.
Abstract Scalable superconducting quantum computing systems require ultrafast and low-latency readout architectures. A Josephson phase digital detector (JDPD) converts the phase of an incoming analog microwave signal into a constant digital current state, enabling ultrafast, fully cryogenic phase discrimination. Thus, a JDPD is a critical component for scalable single-flux quantum (SFQ)-based qubit readout architectures and quantum computation. The flux bias driver (FBD) is a key element of the JDPD system, as it dynamically adjusts the detector’s operating point and enables the combined FBD-JDPD setup to function as a scalable SFQ-based qubit readout solution. Previous work has focused on detector operational and bias control, while information leakage through the shared bias infrastructure has received limited attention. Accordingly, this work investigates side-channel leakage due to the fluctuations in the bias current of an FBD-JDPD system. Extensive simulations demonstrate that phase-dependent switching events in a JDPD may cause bias current fluctuations that propagate backward through the inductively coupled bias network. When temporally aligned with the FBD set and reset operations, these fluctuations manifest as measurable perturbations on the shared bias line. The results show that the peak probabilities of 43.8 μ A for phase θ r = 0 and of 46.7 μ A for phase θ r = π appear due to the interface between the room temperature electronics and the device’s bias network. This leakage presents a serious security and privacy concern, as the shared bias network, which interfaces with electronics operating at room temperature, can serve as an easy access point for a malicious adversary.
We present and theoretically analyze the performance of a nonlocal superconducting single-photon detector. The device operates due to the energy-to-phase conversion mechanism, where the energy of the absorbed single photon is transformed in a variation of the superconducting phase. Within this scope, the detector is designed in the form of a double-loop superconductor--normal-metal--superconductor (SSS-NNN-SSS) Josephson interferometer, where the detection occurs in a long SSS-NNN-SSS junction and the readout is operated by a short SSS-NNN-SSS junction. The variation of the superconducting phase across the readout junction is measured by recording the quasiparticle current flowing through a tunnel-coupled superconducting probe. By exploiting realistic geometry and materials, the detector is able to reveal single photons of frequency down to 10 GHz when operated at 10 mK. Furthermore, the device provides a value of the signal-to-noise ratio of up 104{10}^{4}104 in the range from 10 GHz to 10 THz by selection of the magnetic flux and the bias voltage. This device will hopefully find direct application as a single-photon detector in both basic science and quantum technology, while the energy-to-phase conversion mechanism could be the basis of nonlocal readout and memory architectures for superconducting qubits.