arXiv subjects

Deividas Sabonis

Publications and source records attributed to Deividas Sabonis.

16 recordsLinked to original sources

arXiv2025

Bidirectional microwave-optical conversion with an integrated soft-ferroelectric barium titanate transducer

Efficient, low-noise, and high-bandwidth transduction between optical and microwave photons is key to long-range quantum communication between distant superconducting quantum processors. Recent demonstrations of microwave-optical transduction using the broadband direct electro-optic (Pockels) effect in optical thin films made of AlN or LiNbO$_3$ have shown promise. To improve efficiency and added noise, materials with larger Pockels coefficients, such as the soft ferroelectrics BaTiO$_3$ or SrTiO$_3$, are required. However, these materials require adapted designs and fabrication approaches due to their nonlinear and, in some cases, hysteretic electro-optic response. Here, we engineer an on-chip, triply resonant transducer comprising low-loss BaTiO$_3$-on-SiO$_2$ waveguides monolithically integrated with a superconducting microwave resonator made of Nb. We demonstrate bidirectional microwave-optical transduction and reach total off-chip efficiencies of $1\times10^{-6}$ using pulsed pumping. Our novel device concept permits in-situ poling of the ferroelectric material without introducing excess microwave loss, using a fully subtractive fabrication process with superconducting air bridges. In addition, we investigate optically induced heating, revealing fast thermalization and quasiparticle resilience of the microwave resonator. Our transducer concept and fabrication process are applicable to other materials with a large bias-induced Pockels effect and pave the way for efficient, low-power quantum interconnects.

physics.optics↗

arXiv2025

Exploring the energy spectrum of a four-terminal Josephson junction: Towards topological Andreev band structures

Hybrid multiterminal Josephson junctions (JJs) are expected to harbor a novel class of Andreev bound states (ABSs), including topologically nontrivial states in four-terminal devices. In these systems, topological phases emerge when ABSs depend on at least three superconducting phase differences, resulting in a three-dimensional (3D) energy spectrum characterized by Weyl nodes at zero energy. Here, we realize a four-terminal JJ in a hybrid Al/InAs heterostructure, where ABSs form a synthetic 3D band structure. We probe the energy spectrum using tunneling spectroscopy and identify spectral features associated with the formation of a tri-Andreev molecule, a bound state whose energy depends on three superconducting phases and, therefore, is able to host topological ABSs. The experimental observations are well described by a numerical model. The calculations predict the appearance of four Weyl nodes at zero energy within a gap smaller than the experimental resolution. These topological states are theoretically predicted to remain stable within an extended region of the parameter space, well accessible by our device. These findings establish an experimental foundation to study high-dimensional synthetic band structures in multiterminal JJs, and to realize topological Andreev bands.

cond-mat.mes-hall↗

arXiv2024

Development of a Nb-based semiconductor-superconductor hybrid platform

Semiconductor-superconductor hybrid materials are used as a platform to realise Andreev bound states, which hold great promise for quantum applications. These states require transparent interfaces between the semiconductor and superconductor, which are typically realised by in-situ deposition of an Al superconducting layer. Here we present a hybrid material based on an InAs two-dimensional electron gas (2DEG) combined with in-situ deposited Nb and NbTi superconductors, which offer a larger operating range in temperature and magnetic field due to their larger superconducting gap. We overcome the inherent difficulty associated with the formation of an amorphous interface between III-V semiconductors and Nb-based superconductors by introducing a 7 nm Al interlayer. The Al interlayer provides an epitaxial connection between an in-situ magnetron sputtered Nb or NbTi thin film and a shallow InAs 2DEG. This metal-to-metal epitaxy is achieved by optimization of the material stack and results in an induced superconducting gap of approximately 1 meV, determined from transport measurements of superconductor-semiconductor Josephson junctions. This induced gap is approximately five times larger than the values reported for Al-based hybrid materials and indicates the formation of highly-transparent interfaces that are required in high-quality hybrid material platforms.

cond-mat.supr-con↗

arXiv2023

Quartet Tomography in Multiterminal Josephson Junctions

We investigate the detection of quartets in hybrid multiterminal Josephson junctions. Using simple models of quantum dots coupled to superconducting leads, we find that quartets are ubiquitous and show how to rigorously extract their contribution to the current-phase relation (CPR). We also demonstrate that quartets are closely related to the hybridization of Andreev bound states (ABSs) in these systems and propose a method to identify quartets directly in ABS spectra. We illustrate our method by analyzing the spectroscopic measurements of the ABS spectrum in a three-terminal Josephson junction realized in an InAs/Al heterostructure. Our analysis strongly suggests the existence of quartets in the studied hybrid system.

cond-mat.supr-con↗

arXiv2022

Spin-resolved spectroscopy using a quantum dot defined in InAs 2DEG

We realize a device, based on InAs two-dimensional electron gas proximitized by superconducting Al, which allows a single quantum dot level to be used as a spectrometer of the density of states in a nanowire. Applying a magnetic field parallel to the plane of the device causes the levels of the dot to split, enabling spin resolved spectroscopy. Using this method, we are able to study the spin and charge polarization of the allowed transport through sub-gap states which form in the nanowire and evolve with varying magnetic field and gate voltage.

cond-mat.mes-hall↗

arXiv2022

Deterministic and stochastic sampling of two coupled Kerr parametric oscillators

The vision of building computational hardware for problem optimization has spurred large efforts in the physics community. In particular, networks of Kerr parametric oscillators (KPOs) are envisioned as simulators for finding the ground states of Ising Hamiltonians. It was shown, however, that KPO networks can feature large numbers of unexpected solutions that are difficult to sample with the existing deterministic (i.e., adiabatic) protocols. In this work, we experimentally realize a system of two classical coupled KPOs, and we find good agreement with the predicted mapping to Ising states. We then introduce a protocol based on stochastic sampling of the system, and we show how the resulting probability distribution can be used to identify the ground state of the corresponding Ising Hamiltonian. This method is akin to a Monte Carlo sampling of multiple out-of-equilibrium stationary states and is less prone to become trapped in local minima than deterministic protocols.

cond-mat.mes-hall↗

arXiv2022

Few-mode to mesoscopic junctions in gatemon qubits

We investigate a semiconductor nanowire-based gatemon qubit with epitaxial Al on two facets of the nanowire, allowing gate control of wire density. Two segments have the Al removed, one forming a Josephson junction and the other operating as a transistor, providing in-situ switching between dc transport and qubit operation. Gating the NW changes the bulk wire potential distribution, while gating the Josephson junction changes the number of junction modes. Both effects are revealed by the dependence of qubit frequency on parallel magnetic field. A detailed model of the wire and junction yields behavior consistent with experiment. In the multi-mode regime, fluctuations in qubit frequency are considerably smaller than the theoretical "universal" value, also smaller than numerics, and consistent with previous measurements of fluctuating critical current.

cond-mat.mes-hall↗

arXiv2022

Nonlocal conductance spectroscopy of Andreev bound states in gate-defined InAs/Al nanowires

The charge character of Andreev bound states (ABSs) in a three-terminal semiconductor-superconductor hybrid nanowire was measured using local and nonlocal tunneling spectroscopy. The device is fabricated using an epitaxial InAs/Al two-dimensional heterostructure with several gate-defined side probes. ABSs are found to oscillate around zero as a function of gate voltage, with modifications of their charge consistent with theoretical expectations for the total Bardeen-Cooper- Schrieffer (BCS) charge of ABSs.

cond-mat.mes-hall↗

arXiv2022

Nonlocal signatures of hybridization between quantum dot and Andreev bound states

We investigate local and nonlocal signatures of hybridization between a quantum dot state and an extended Andreev bound state (ABS) in a gate-defined InAs nanowire with multiple side probes. When a quantum dot in one of the side probes was hybridized with an ABS in the nanowire, a characteristic spectroscopic pattern was observed both locally, i.e., in the probe with the quantum dot, and nonlocally, in the tunnel conductance of a remote probe. Nonlocal signatures of hybridization reveal the extended nature of the ABS.

cond-mat.mes-hall↗

arXiv2021

Soft-clamped silicon nitride string resonators at millikelvin temperatures

We demonstrate that soft-clamped silicon nitride strings with large aspect ratio can be operated at \si{\milli\kelvin} temperatures. The quality factors ($Q$) of two measured devices show consistent dependency on the cryostat temperature, with soft-clamped mechanical modes reaching $Q > 10^9$ at $46~\mathrm{mK}$. For low optical readout power, $Q$ is found to saturate, indicating good thermalization between the sample and the stage it is mounted on. Our best device exhibits a force sensitivity of $9.6~\mathrm{zN}/\sqrt{\mathrm{Hz}}$ and a thermal decoherence time of $0.38~\mathrm{s}$ which bode well for future applications such as nanomechanical force sensing and beyond.

cond-mat.mes-hall↗

arXiv2021

Extracting the lifetime of a synthetic two-level system

The Kerr Parametric Oscillator (KPO) is a nonlinear resonator system that is often described as a synthetic two-level system. In the presence of noise, the system switches between two states via a fluctuating trajectory in phase space, instead of following a straight path. The presence of such fluctuating trajectories makes it hard to establish a precise count or even a useful definition, of the "lifetime" of the state. Addressing this issue, we compare several rate counting methods that allow to estimate a lifetime for the levels. In particular, we establish that a peak in the Allan variance of fluctuations can also be used to determine the levels' lifetime. Our work provides a basis for characterizing KPO networks for simulated annealing where an accurate determination of the state lifetime is of fundamental importance.

cond-mat.mes-hall↗

arXiv2021

Comparing tunneling spectroscopy and charge sensing of Andreev bound states in a semiconductor-superconductor hybrid nanowire structure

Transport studies of Andreev bound states (ABSs) are complicated by the interplay of charging effects and superconductivity. Here, we compare transport approaches to ABS spectroscopy in a semiconductor-superconductor island to a charge-sensing approach based on an integrated radio-frequency single-electron transistor. Consistency of the methods demonstrates that fast, non-invasive charge sensing allows accurate quantitative measurement of ABSs while eluding some complexities of transport.

cond-mat.mes-hall↗

arXiv2020

Destructive Little-Parks Effect in a Full-Shell Nanowire-based Transmon

A semiconductor transmon with an epitaxial Al shell fully surrounding an InAs nanowire core is investigated in the low $E_J/E_C$ regime. Little-Parks oscillations as a function of flux along the hybrid wire axis are destructive, creating lobes of reentrant superconductivity separated by a metallic state at a half-quantum of applied flux. In the first lobe, phase winding around the shell can induce topological superconductivity in the core. Coherent qubit operation is observed in both the zeroth and first lobes. Splitting of parity bands by coherent single-electron coupling across the junction is not resolved beyond line broadening, placing a bound on Majorana coupling, $E_M/h$ < 10 MHz, much smaller than the Josephson coupling $E_J/h$ ~ 4.7 GHz.

cond-mat.mes-hall↗

arXiv2019

Dispersive sensing in hybrid InAs/Al nanowires

Dispersive charge sensing is realized in hybrid semiconductor-superconductor nanowires in gate-defined single- and double-island device geometries. Signal-to-noise ratios (SNRs) were measured both in the frequency and time domain. Frequency-domain measurements were carried out as a function of frequency and power and yield a charge sensitivity of $1 \times 10^{-3} e/\sqrt{\rm Hz}$ for an 11 MHz measurement bandwidth. Time-domain measurements yield SNR > 1 for 20 $μ$s integration time. At zero magnetic field, photon-assisted tunneling was detected dispersively in a double-island geometry, indicating coherent hybridization of the two superconducting islands. At an axial magnetic field of 0.6 T, subgap states are detected dispersively, demonstrating the suitability of the method for sensing in the topological regime.

cond-mat.mes-hall↗

arXiv2019

Photon Assisted Tunneling of Zero Modes in a Majorana Wire

Hybrid nanowires with proximity-induced superconductivity in the topological regime host Majorana zero modes (MZMs) at their ends, and networks of such structures can produce topologically protected qubits. In a double-island geometry where each segment hosts a pair of MZMs, inter-pair coupling mixes the charge parity of the islands and opens an energy gap between the even and odd charge states at the inter-island charge degeneracy. Here, we report on the spectroscopic measurement of such an energy gap in an InAs/Al double-island device by tracking the position of the microwave-induced quasiparticle (qp) transitions using a radio-frequency (rf) charge sensor. In zero magnetic field, photon assisted tunneling (PAT) of Cooper pairs gives rise to resonant lines in the 2e-2e periodic charge stability diagram. In the presence of a magnetic field aligned along the nanowire, resonance lines are observed parallel to the inter-island charge degeneracy of the 1e-1e periodic charge stability diagram, where the 1e periodicity results from a zero-energy sub-gap state that emerges in magnetic field. Resonant lines in the charge stability diagram indicate coherent photon assisted tunneling of single-electron states, changing the parity of the two islands. The dependence of resonant frequency on detuning indicates a sizable (GHz-scale) hybridization of zero modes across the junction separating islands.

cond-mat.mes-hall↗

arXiv2019

Radio-frequency methods for Majorana-based quantum devices: fast charge sensing and phase diagram mapping

Radio-frequency (RF) reflectometry is implemented in hybrid semiconductor-superconductor nanowire systems designed to probe Majorana zero modes. Two approaches are presented. In the first, hybrid nanowire-based devices are part of a resonant circuit, allowing conductance to be measured as a function of several gate voltages ~40 times faster than using conventional low-frequency lock-in methods. In the second, nanowire devices are capacitively coupled to a nearby RF single-electron transistor made from a separate nanowire, allowing RF detection of charge, including charge-only measurement of the crossover from 2e inter-island charge transitions at zero magnetic field to 1e transitions at axial magnetic fields above 0.6 T, where a topological state is expected. Single-electron sensing yields signal-to-noise exceeding 3 and visibility 99.8% for a measurement time of 1 μs.

cond-mat.mes-hall↗