A capacitively shunted S-QD-S junction embedded in a general circuit environment which does not restrict phase dynamics.
Going to the March Meeting? Check out these great presentations from members of the group. Look below for some more detail on the research (collaborations between LPS, MIT/Lincoln Lab, U.Texas-El Paso, Brandeis):
- A new algorithm for efficiently probing the entanglement dynamics of a continuously monitored quantum circuit: https://arxiv.org/abs/2401.17367
- The potential for Andreev spin qubits in super-germanium junctions
- A new theory for decoherence in Andreev spin qubits in super-semi weak links (is it the quantum dot story again, or not? tldr: yes, but there are more knobs): arxiv imminent
- A new, more complete theory for the physics of super-semi junctions [big deal]: https://arxiv.org/abs/2402.10330
- Understanding the performance limitations of hole spin qubits in germanium dots
- Using today’s quantum computers to simulate novel measurement-based gates (unpublished, but previous work: https://arxiv.org/abs/2305.09727)
- Using longitudinal readout to entangle quantum dot qubits via measurement (based on a complete and general theory for how solid-state qubits couple to wires: https://arxiv.org/abs/2312.03118)
TALKS
B31.00012 Observing measurement induced entanglement transitions via a tensor network based hybrid quantum algorithm https://meetings.aps.org/Meeting/MAR24/Session/B31.12
D46.00001 Andreev Spin Qubits in Germanium-Based Josephson Junctions https://meetings.aps.org/Meeting/MAR24/Session/D46.1
D46.00002 Charge and spin control of Andreev states in semiconducting Josephson junctions https://meetings.aps.org/Meeting/MAR24/Session/D46.2
D46.00007 Quantum dynamics of superconductor-quantum dot-superconductor (super-semi) Josephson junctions https://meetings.aps.org/Meeting/MAR24/Session/D46.7
G49.00006 Characterizing the effective g-tensor of hole qubits in planar germanium quantum wells https://meetings.aps.org/Meeting/MAR24/Session/G49.6
G51.00008 Measurement Based Simulation of Geometric Gates in Topological Qubits on NISQ Devices https://meetings.aps.org/Meeting/MAR24/Session/G51.8
S46.00011 Dephasing mechanisms and fidelity of entangled spin qubits preparation under joint continuous measurement via longitudinal couplings to a superconducting resonator https://meetings.aps.org/Meeting/MAR24/Session/S46.11
Relevant Preprints
Decoherence in Andreev spin qubits
https://arxiv.org/abs/2403.00710
Silas Hoffman, Max Hays, Kyle Serniak, Thomas Hazard, Charles Tahan
We theoretically study the dephasing of an Andreev spin qubit (ASQ) due to electric and magnetic noise. Using a tight-binding model, we calculate the Andreev states formed in a Josephson junction where the link is a semiconductor with strong spin-orbit interaction. As a result of both the spin-orbit interaction and induced superconductivity, the local charge and spin of these states varies as a function of externally controllable parameters: the phase difference between the superconducting leads, an applied magnetic field, and filling of the underlying semiconductor. Concomitantly, coupling to fluctuations of the electric or magnetic environment will vary, which informs the rate of dephasing. We qualitatively predict the dependence of dephasing on the nature of the environment, magnetic field, phase difference between the junction, and filling of the semiconductor. Comparing the simulated electric- and magnetic-noise-induced dephasing rate to experiment suggests that the dominant source of noise is magnetic. Moreover, by appropriately tuning these external parameters, we find sweet-spots at which we predict an enhancement in ASQ coherence times.
Detecting Measurement-Induced Entanglement Transitions With Unitary Mirror Circuits
https://arxiv.org/abs/2401.17367
Yariv Yanay, Brian Swingle, Charles Tahan
Monitored random circuits, consisting of alternating layers of entangling two-qubit gates and projective single-qubit measurements applied to some fraction p of the qubits, have been a topic of recent interest. In particular, the resulting steady state exhibits a phase transition from highly correlated states with “volume-law” entanglement at p<pc to localized states with “area-law” entanglement at p>pc. It is hard to access this transition experimentally, as it cannot be seen at the ensemble level. Naively, to observe it one must repeat the experiment until the set of measurement results repeats itself, with likelihood that is exponentially small in the number of measurements. To overcome this issue, we present a hybrid quantum-classical algorithm which creates a matrix product state (MPS) based “unitary mirror” of the projected circuit. Polynomial-sized tensor networks can represent quantum states with area-law entanglement, and so the unitary mirror can well-approximate the experimental state above pc but fails exponentially below it. The breaking of this mirror can thus pinpoint the critical point. We outline the algorithm and how such results would be obtained. We present a bound on the maximum entanglement entropy of any given state that is well-represented by an MPS, and from the bound suggest how the volume-law phase can be bounded. We consider whether the entanglement could similarly be bounded from below where the MPS fails. Finally, we present numerical results for small qubit numbers and for monitored circuits with random Clifford gates.
Quantum dynamics of superconductor-quantum dot-superconductor Josephson junctions
https://arxiv.org/abs/2402.10330
Utkan Güngördü, Rusko Ruskov, Silas Hoffman, Kyle Serniak, Andrew J. Kerman, Charles Tahan
Josephson junctions constructed from superconductor-quantum dot-superconductor (S-QD-S) heterostructures have been used to realize a variety of voltage-tunable superconducting quantum devices, including qubits and parametric amplifiers. In such devices, the interplay between the charge degree of freedom associated with the quantum dot and its environment must be considered for faithful modeling of circuit dynamics. Here we describe the self-consistent quantization of a capacitively-shunted S-QD-S junction via path-integral formulation. In the effective Hamiltonian, the Josephson potential for the Andreev bound states reproduces earlier results for static phase bias, whereas the charging energy term has new features: (i) the system’s capacitance is renormalized by the junction gate voltage, an effect which depends on the strength of the tunneling rates between the dot and its superconducting leads as well, and (ii) an additional charge offset appears for asymmetric junctions. These results are important to understand future experiments and quantum devices incorporating S-QD-S junctions in arbitrary impedance environments.
Longitudinal (curvature) couplings of an N-level qudit to a superconducting resonator at the adiabatic limit and beyond
https://arxiv.org/abs/2312.03118
Understanding how and to what magnitude solid-state qubits couple to metallic wires is crucial to the design of quantum systems such as quantum computers. Here, we investigate the coupling between a multi-level system, or qudit, and a superconducting (SC) resonator’s electromagnetic field, focusing on the interaction involving both the transition and diagonal dipole moments of the qudit. Specifically, we explore the effective dynamical (time-dependent) longitudinal coupling that arises when a solid-state qudit is adiabatically modulated at small gate frequencies and amplitudes, in addition to a static dispersive interaction with the SC resonator. For the first time, we derive Hamiltonians describing the longitudinal multi-level interactions in a general dispersive regime, encompassing both dynamical longitudinal and dispersive interactions. These Hamiltonians smoothly transition between their adiabatic values, where the couplings of the n-th level are proportional to the level’s energy curvature concerning a qudit gate voltage, and the substantially larger dispersive values, which occur due to a resonant form factor. We provide several examples illustrating the transition from adiabatic to dispersive coupling in different qubit systems, including the charge (1e DQD) qubit, the transmon, the double quantum dot singlet-triplet qubit, and the triple quantum dot exchange-only qubit. In some of these qubits, higher energy levels play a critical role, particularly when their qubit’s dipole moment is minimal or zero. For an experimentally relevant scenario involving a spin-charge qubit with magnetic field gradient coupled capacitively to a SC resonator, we showcase the potential of these interactions. They enable close-to-quantum-limited quantum non-demolition (QND) measurements and remote geometric phase gates, demonstrating their practical utility in quantum information processing.