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arXiv:2212.11164 · Physical Review Applied

Flip-Chip-Based Microwave Spectroscopy of Andreev Bound States in a Planar Josephson Junction

Manuel Hinderling, Deividas Sabonis, Stephan Paredes, D. Z. Haxell, Marco Coraiola, Sofieke C. ten Kate, Erik Cheah, Filip Křížek, Rüdiger Schott, W. Wegscheider, Fabrizio Nichele

Published May 8, 2023• 19 citations
arXivHTMLPDF

Abstract

Measurement and coherent manipulation of Andreev bound states (ABSs) in planar superconductor-semiconductor heterostructures could enable gate-controllable, small-footprint superconducting spin qubits with long coherence times. So far, readout capabilities and device coherence have been limited by lossy $I\phantom{\rule{0}{0ex}}I\phantom{\rule{0}{0ex}}I\ensuremath{-}V$ substrates, but here researchers overcome this limitation with a flip-chip approach to microwave experiments on ABSs. Superconductor-semiconductor devices and microwave resonators are placed on different chips and inductively coupled via a vacuum gap, enabling high-quality readout without sacrificing device properties.

DOI
10.1103/physrevapplied.19.054026

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Measurement and coherent manipulation of Andreev bound states (ABSs) in planar superconductor-semiconductor heterostructures could enable gate-controllable, small-footprint superconducting spin qubits with long coherence times. So far, readout capabilities and device coherence have been limited by lossy I\phantom{\rule{0}{0ex}}I\phantom{\rule{0}{0ex}}I\ensuremath{-}V substrates, but here researchers overcome this limitation with a flip-chip approach to microwave experiments on ABSs. Superconductor-semiconductor devices and microwave resonators are placed on different chips and inductively coupled via a vacuum gap, enabling high-quality readout without sacrificing device properties.
We demonstrate microwave measurements of the Andreev-bound-state (ABS) spectrum in planar Josephson junctions (JJs) defined in Ge high-mobility two-dimensional hole gases contacted by superconducting platinum germanosilicide ( Pt Si Ge ). The JJs and readout circuitry are located on separate chips and inductively coupled via flip-chip bonding. For a device with 350-nm junction length, the spectroscopic signatures were consistent with the short-junction limit, with an induced superconducting gap Δ ∗ ≈ 48 μ eV and transmission τ ≈ 0.94 . The interaction between the highest-transmission ABS and the resonator was well described by a Jaynes-Cummings model with a vacuum Rabi splitting of approximately 6 MHz. A device with a junction length of 1 μ m showed an ABS spectrum consistent with a long-junction model. Time-resolved monitoring of the readout resonator in the dispersive regime revealed gate-voltage tunable-junction parity fluctuations on the time scale of seconds. Our work indicates a viable path toward hybrid quantum devices based on planar Ge . Published by the American Physical Society 2024
The properties of superconducting devices depend sensitively on the parity (even or odd) of the quasiparticles that they contain. Encoding quantum information in the parity degree of freedom is central in several emerging solid-state qubit architectures, including in hybrid superconductor-semiconductor devices. In the latter case, accurate, nondestructive, and time-resolved parity measurements are a challenging issue. Here, we report on control and real-time parity measurement in a superconducting island embedded in a superconducting loop and realized in a hybrid two-dimensional heterostructure using a microwave resonator. To avoid microwave losses impeding time-resolved measurements, the device and readout resonator are located on separate chips, connected via flip-chip bonding, and couple inductively through vacuum. The superconducting resonator detects the parity-dependent circuit inductance, allowing for fast parity readout. We have resolved even- and odd-parity states with a signal-to-noise ratio of SNR ≈ 3 for an integration time of 20 μ s and a detection fidelity exceeding 98 % . The real-time parity measurement shows a state lifetime extending into the millisecond range. Our approach will lead to a better understanding of coherence-limiting mechanisms in superconducting quantum hardware and help to advance inductive-readout schemes for hybrid qubits. Published by the American Physical Society 2024
Abstract Josephson junctions based on semiconductor nanowires are potential building blocks for electrically tunable qubit structures, e.g., the gatemon or the Andreev qubit. However, an actual realization requires the thorough investigation of the intrinsic excitation spectrum. Here, we demonstrate the fabrication of low-loss superconducting microwave circuits that combine high quality factors with a well-controlled gate architecture by utilizing a flip-chip approach. This platform is then used to perform single-tone and two-tone experiments on Andreev states in in-situ grown InAs/Al core/half-shell nanowires with shadow mask defined Josephson junctions. In gate-controlled and flux-biased spectroscopic measurements we find clear signatures of single quasiparticle as well as quasiparticle pair transitions between discrete Andreev bound states mediated by photon-absorption. Our experimental findings are supported by simulations that show that the junction resides in the intermediate channel length regime.
Superconducting weak-link junctions host electron--hole hybridized excitations called Andreev bound states, central to mesoscopic transport and emerging quantum information platforms. Andreev physics has so far been synonymous with the microwave range. However, advancements in superconductor--semiconductor hybrid junctions open the door to the characterization, and manipulation, of Andreev states by light. Here we introduce a model for light--Andreev interaction, with distinct features: Electrons transitioning into Andreev levels can sidestep Pauli exclusion, resulting in two optical absorption resonances separated by twice the bound state energy. One resonance populates the Andreev state and the other empties it, enabling optical control of Andreev qubit parity; pumping both resets the junction and prevents saturation. Given their strong microwave coupling, we show how Andreev bound states can operate as optical--to--microwave transducers with MHz-scale intermode coupling, a key ingredient for heat load management and quantum networking. We illustrate these effects with realistic device parameters. Our results highlight the possibilities in the new field of Andreev optoelectronics.
The modern understanding of the Josephson effect in mesosopic devices derives from the physics of Andreev bound states, fermionic modes that are localized in a superconducting weak link. Recently, Josephson junctions constructed using semiconducting nanowires have led to the realization of superconducting qubits with gate-tunable Josephson energies. We have used a microwave circuit QED architecture to detect Andreev bound states in such a gate-tunable junction based on an aluminum-proximitized indium arsenide nanowire. We demonstrate coherent manipulation of these bound states, and track the bound-state fermion parity in real time. Individual parity-switching events due to nonequilibrium quasiparticles are observed with a characteristic timescale T_{parity}=160±10 μs. The T_{parity} of a topological nanowire junction sets a lower bound on the bandwidth required for control of Majorana bound states.
We propose a variant of Andreev spin qubits (ASQs) defined in planar Josephson junctions based on spin-orbit coupled two-dimensional electron gases (2DEGs) in a weak out-of-plane magnetic field. The magnetic field induces a linear phase gradient across the junction, generating Josephson vortices that can host low-energy Andreev bound states (ABSs). We show that, in certain parameter regimes, the combined effect of the phase gradient and spin-orbit coupling stabilizes an odd-fermion parity ground state, where a single Josephson vortex binds a spinful low-energy degree of freedom that is energetically separated from the other ABSs. This low-energy degree of freedom can be exploited to define a special type of ASQ, which we dub the vortex spin qubit (VSQ). We show that single-qubit gates for VSQs can be performed via flux driving, while readout can be achieved by adapting standard circuit quantum electrodynamics (cQED) techniques developed for conventional ASQs. We further outline how an entangling two-qubit gate can be performed using an ac current drive. We argue that VSQs offer prospects for a substantial reduction in device complexity and hardware overhead compared to conventional ASQ implementations, while preserving key advantages such as supercurrent-based readout, single-qubit gates, and long-range two-qubit gates.