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arXiv:2303.04784 · PRX Quantum

Quasiparticle Dynamics in Epitaxial Al - InAs Planar Josephson Junctions

Bassel Heiba Elfeky, William M. Strickland, Jaewoo Lee, James T. Farmer, Sadman Shanto, Azarin Zarassi, Dylan Langone, Maxim Vavilov, Eli M. Levenson-Falk, Javad Shabani

Published Sep 22, 2023• 15 citations
arXivHTMLPDF

Abstract

Quasiparticle (QP) effects play a significant role in the coherence and fidelity of superconducting quantum circuits.The Andreev bound states of high-transparency Josephson junctions can act as low-energy traps for QPs, providing a mechanism for studying the dynamics and properties of both the QPs and the junction.Using locally injected and thermal QPs, we study QP loss and QP poisoning in epitaxial Al-InAs Josephson junctions incorporated in a superconducting quantum interference device (SQUID) galvanically shorting a superconducting resonator to ground.We observe changes in the resonance line shape and frequency shifts consistent with QP trapping into and clearing out of the ABSs of the junctions when the junctions are phase biased.By monitoring the QP trapping and clearing mechanisms in time, we find a time scale of O(1 µs) for these QP dynamics, consistent with the presence of phonon-mediated QP-QP interactions.Our measurements suggest that electron-phonon interactions play a significant role in the relaxation mechanisms of our system, while electron-photon interactions and electron-phonon interactions govern the clearing mechanisms.Our results highlight the QP-induced dissipation and complex QP dynamics in superconducting quantum circuits fabricated on superconductor-semiconductor heterostructures.

DOI
10.1103/prxquantum.4.030339

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Quasiparticle (QP) effects play a significant role in the coherence and fidelity of superconducting quantum circuits.The Andreev bound states of high-transparency Josephson junctions can act as low-energy traps for QPs, providing a mechanism for studying the dynamics and properties of both the QPs and the junction.Using locally injected and thermal QPs, we study QP loss and QP poisoning in epitaxial Al-InAs Josephson junctions incorporated in a superconducting quantum interference device (SQUID) galvanically shorting a superconducting resonator to ground.We observe changes in the resonance line shape and frequency shifts consistent with QP trapping into and clearing out of the ABSs of the junctions when the junctions are phase biased.By monitoring the QP trapping and clearing mechanisms in time, we find a time scale of O(1 µs) for these QP dynamics, consistent with the presence of phonon-mediated QP-QP interactions.Our measurements suggest that electron-phonon interactions play a significant role in the relaxation mechanisms of our system, while electron-photon interactions and electron-phonon interactions govern the clearing mechanisms.Our results highlight the QP-induced dissipation and complex QP dynamics in superconducting quantum circuits fabricated on superconductor-semiconductor heterostructures.
Single flux quantum (SFQ) circuitry is a promising candidate for a scalable and integratable cryogenic quantum control system. However, the operation of SFQ circuits introduces nonequilibrium quasiparticles (QPs), which are a significant source of qubit decoherence. In this study, we investigate QP behavior in a superconducting quantum-classical hybrid chip that comprises an SFQ circuit and a qubit circuit. By monitoring qubit relaxation time, we explore the dynamics of SFQ-circuit-induced QPs. Our findings reveal that the QP density near the qubit reaches its peak after several microseconds of SFQ circuit operation, which corresponds to the phonon-mediated propagation time of QPs in the hybrid circuits. This suggests that phonon-mediated propagation dominates the spreading of QPs in the hybrid circuits. Our results lay the foundation to suppress QP poisoning in quantum-classical hybrid systems.
We report continuous measurements of quasiparticles trapping and clearing from Andreev bound states in aluminum nanobridge Josephson junctions integrated into a superconducting-qubit-like device. We find that trapping is well modeled by independent spontaneous emission events. Above 80 mK the clearing process is well described by the absorption of thermal phonons, but other temperature-independent mechanisms dominate at low temperature. We find a complex structure in the dependence of the low-temperature clearing rate on the Andreev bound state energy. Our results shed light on quasiparticle behavior in qubitlike circuits.
Superconducting qubits based on Josephson junctions have emerged as a leading hardware platform in the quest to realize large-scale quantum computers. However, qubit decoherence remains a major hurdle on the path toward fault-tolerant quantum computing. In particular, excess quasiparticle (QP) excitations constitute a significant source of dissipation in superconducting quantum devices. The empirically observed QP density in these systems is orders of magnitude larger than that predicted by Bardeen-Cooper-Schrieffer (BCS) theory. Moreover, QP poisoning due to high-energy particle impacts can cause correlated errors that simultaneously affect multiple qubits, thereby impeding quantum error correction. Although QP-induced decoherence has been extensively studied in widely implemented transmon qubits, its impact on fluxonium qubits, a leading alternative architecture, remains far less explored. In this thesis, we investigate energy relaxation in fluxonium qubits induced by controlled QP poisoning using an on-chip Josephson injector junction that generates Cooper-pair-breaking phonons and photons. We separately measure the relaxation and excitation rates, instead of the conventional energy relaxation time. For specific injection parameters, we observe an apparent reversal of the rates, with the excitation rate exceeding the relaxation rate. We further measure the rates as a function of external flux to disentangle the effects of QP tunneling across the small junction and the junction array superinductor. Across all measured samples, differing in qubit spectra and injection conditions, we infer roughly an order of magnitude higher QP density at the small junction than in the array junctions. Our findings advance the understanding of QP-induced decoherence in fluxonium qubits and provide insight for reconciling discrepancies in reported bounds on QP densities, potentially enabling the development of QP mitigation strategies tailored to fluxonium qubits.
Nonequilibrium quasiparticles represent a significant source of decoherence in superconducting quantum circuits. Here we investigate the mechanism of quasiparticle poisoning in devices subjected to local quasiparticle injection. We find that quasiparticle poisoning is dominated by the propagation of pair-breaking phonons across the chip. We characterize the energy dependence of the time scale for quasiparticle poisoning. Finally, we observe that incorporation of extensive normal metal quasiparticle traps leads to a more than order-of-magnitude reduction in quasiparticle loss for a given injected quasiparticle power.
Nonequilibrium quasiparticles are ubiquitous in superconducting electronics. These quasiparticles can trap in the internal Andreev bound states of a phase-biased Josephson junction, providing a mechanism for studying their presence and behavior. We characterize a quasiparticle trapping detector device based on a two junction aluminum nanobridge superconducting quantum interference device incorporated into a transmission line resonator. When the device is flux-biased, distinct resonant frequencies develop depending on the trapped quasiparticle number. We demonstrate continuous detection of up to 3 trapped quasiparticles, with detection of a trapped quasiparticle with a signal-to-noise ratio of 27 in 5 μs. We describe initial measurements of quasiparticle behavior and discuss the possible optimization and application of such detector devices.
Conventional tunnel Josephson junctions form the basic building blocks of superconducting electronics. Their operation is based on comparatively simple dynamics where the superconducting phases across the junctions is the only dynamical degree of freedom. The goal of the project on which this thesis is based, is to understand the physics and develop a consistent theoretical description of the quantum dynamics of unconventional Josephson structures: spin active junctions, junctions containing low energy quasiparticles, and junctions composed of ddd-wave superconductors. Spin active Josephson junctions involve the additional degree of freedom of quasiparticle spin. Here we theoretically investigate the spin-structure of the current carrying interface bound states - Andreev Bound States (ABS), and induced ground state spin polarization of a short (shorter than the superconducting coherence length) nanowire junction containing a localized magnetic moment. We show that temporal variation of the superconducting phase difference and magnetic moment induce rotations in spin polarized and unpolarized subspaces respectively, which is manifested by temporal oscillation of the Josephson current.The interlevel transitions obey a selection rule that forbids manipulations in a certain region ofexternal parameters and results from specific properties of Andreev bound states in magnetic contacts where, in a certain regime of phase bias and strength of magnetic scatterer, ABS belong to different eigenstates of the combined charge and parity inversion symmetry operator.A general framework is developed to include low energy quasiparticle states into the dynamics of Josephson junctions. Such states exist in e.g. constriction type junctions with high transparency channels or resonant states, as well as in junctions of unconventional superconductors. The existence of such states prevent the separation of energy scales which in conventional tunnel junctions freezes out the quasiparticle degrees of freedom. We show that the resonant interaction with such low energy states rather than the Josephson potential defines the nonlinear Josephson dynamics at small amplitudes.The framework is applied to high temperature Josephson junctions, whose unconventional order parameter admits ABS with energies in the middle of the superconducting gap - Mid Gap States (MGS). In addition to the non-linear dynamics induced by the MGS, we identify a reentrance effect in the transition between thermal activation and macroscopic quantum tunneling associated with the MGS, and connect this phenomenon to experimental observations.