We demonstrate a substantial improvement in the spin-exchange gate using symmetric control instead of conventional detuning in GaAs spin qubits, up to a factor of six increase in the quality factor of the gate. For symmetric operation, nanosecond voltage pulses are applied to the barrier that controls the interdot potential between quantum dots, modulating the exchange interaction while maintaining symmetry between the dots. Excellent agreement is found with a model that separately includes electrical and nuclear noise sources for both detuning and symmetric gating schemes. Unlike exchange control via detuning, the decoherence of symmetric exchange rotations is dominated by rotation-axis fluctuations due to nuclear field noise rather than direct exchange noise.
Conditional-phase (CZ) gates in transmons can be realized by flux pulsing computational states towards resonance with noncomputational ones. We present a 40 ns CZ gate based on a bipolar flux pulse suppressing leakage (0.1%) by interference and approaching the speed limit set by exchange coupling. This pulse harnesses a built-in echo to enhance fidelity (99.1%) and is robust to long-timescale distortion in the flux-control line, ensuring repeatability. Numerical simulations matching experiment show that fidelity is limited by high-frequency dephasing and leakage by short-timescale distortion.
We present conductance-matrix measurements of a three-terminal superconductor-semiconductor hybrid device consisting of two normal leads and one superconducting lead. Using a symmetry decomposition of the conductance, we find that the antisymmetric components of pairs of local and nonlocal conductances match at energies below the superconducting gap, consistent with expectations based on a non-interacting scattering matrix approach. Further, the local charge character of Andreev bound states is extracted from the symmetry-decomposed conductance data and is found to be similar at both ends of the device and tunable with gate voltage. Finally, we measure the conductance matrix as a function of magnetic field and identify correlated splittings in low-energy features, demonstrating how conductance-matrix measurements can complement traditional tunneling-probe measurements in the search for Majorana zero modes.PACS numbers: 03.67. Lx, 81.07.Gf, 85.25.Cp Symmetry relations for quantum transport are often connected to deep physical principles, and make strong predictions for comparison with experiment. For instance, the Onsager-Casimir relations [1-3] arise from microscopic reversibility, and were central in early studies of quantum-coherent transport [4][5][6]. Later, predicted departures from these relations due to interaction effects [7-9], which include bias-dependence of the effective potentials, were observed in nonlinear transport [10,11]. The introduction of superconducting terminals results in additional symmetries, as conductance occurs via Andreevreflection from electrons to holes, and is invariant under particle-hole conjugation [12]. For a two-terminal normal-superconducting device, the conductance, g(V ), is a symmetric function of bias voltage, V , neglecting interaction effects. As shown in a partner theoretical paper, for multi-terminal superconducting devices g(V ) need not be symmetric, although a curious relation exists between the antisymmetric components of the local and nonlocal conductances [13]. These predictions have, to our knowledge, not been tested.Hybrid superconductor-semiconductor nanowire structures have recently become a topic of intense interest [14][15][16][17][18][19], motivated in part by proposals for achieving topological superconductivity and Majorana zero modes (MZM) [20,21]. In two-terminal superconductor-semiconductor devices, observed asymmetries in the subgap conductance [22] have been suggested to arise from a dissipative fermionic reservoir, effectively acting as a third lead [23], although, as in the normal-conducting case [3], biasdependence of the self-consistent potential can also cause a deviation from symmetry [24]. Multi-terminal super-conducting devices are a topic of particular interest, as they can be used for MZM [25][26][27][28][29][30][31], Cooper-pair splitter [32,33], and multi-terminal Josephson studies [34][35][36][37][38]. In multi-terminal superconducting quantum dot devices, bias asymmetries have been observed [39], and a relationship between nonloca...
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