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Physical Review A

Robust universal gates for quantum-dot spin qubits using tunable adiabatic passages

Bo Gong, Li Wang, Tao Tu, Chuan‐Feng Li, Guang‐Can Guo

Published Sep 12, 2016• 7 citations
Publisher

Abstract

A logical qubit based on electron-spin singlet-triplet states in a semiconductor double dot has emerged as one of the promising candidates for quantum information processing. Here we propose a systematic method to implement rapid quantum gates in the nanosecond time scale for both a single qubit and multiqubits, by applying adiabatic driving pulses with tunable velocity. This yields considerable high-fidelity gate operations reaching $99.9%$ which are crucial to scalable quantum computation in a semiconductor-based architecture, even considering the demanding experimental constraints and realistic spin and charge noises.

DOI
10.1103/physreva.94.032311

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Physical Review A•Sep 10, 2021•1 citation
A logical qubit based on electron-spin singlet-triplet states in a semiconductor double dot has emerged as one of the promising candidates for quantum information processing. Here we propose a systematic method to implement rapid quantum gates in the nanosecond time scale for both a single qubit and multiqubits, by applying adiabatic driving pulses with tunable velocity. This yields considerable high-fidelity gate operations reaching 99.999.9%99.9 which are crucial to scalable quantum computation in a semiconductor-based architecture, even considering the demanding experimental constraints and realistic spin and charge noises.
Hole spin qubits in semiconductor quantum dots (QDs) are promising candidates for quantum information processing due to their weak hyperfine coupling to nuclear spins, and to the strong spin-orbit coupling, which allows for rapid operation time. We propose a coherent control on two heavy-hole spin qubits in a double QD by a fast adiabatic driving protocol, which helps to achieve higher fidelities than other experimentally commonly used protocols as linear ramping, $\ensuremath{\pi}$ pulses or Landau-Zener passages. Using fast quasiadiabatic driving via spin-orbit coupling, it is possible to reduce charge noise significantly for qubit manipulation and achieve high robustness for the qubit initialization. We also implement one- and two-qubit gates, in particular, not, cnot, and swaplike gates, of hole spins in a double QD achieving fidelities above $99\mathrm{%}$, exhibiting the capability of hole spins to implement universal gates for quantum computing.
Several logical qubits and quantum gates have been proposed for semiconductor quantum dots controlled by voltages applied to top gates. The different schemes can be difficult to compare meaningfully. Here we develop a theoretical framework to evaluate disparate qubit-gating schemes on an equal footing. We apply the procedure to two types of double-dot qubits: the singlet-triplet and the semiconducting quantum dot hybrid qubit. We investigate three quantum gates that flip the qubit state: a DC pulsed gate, an AC gate based on logical qubit resonance, and a gate-like process known as stimulated Raman adiabatic passage. These gates are all mediated by an exchange interaction that is controlled experimentally using the interdot tunnel coupling g and the detuning [Symbol: see text], which sets the energy difference between the dots. Our procedure has two steps. First, we optimize the gate fidelity (f) for fixed g as a function of the other control parameters; this yields an f(opt)(g) that is universal for different types of gates. Next, we identify physical constraints on the control parameters; this yields an upper bound f(max) that is specific to the qubit-gate combination. We show that similar gate fidelities (~99:5%) should be attainable for singlet-triplet qubits in isotopically purified Si, and for hybrid qubits in natural Si. Considerably lower fidelities are obtained for GaAs devices, due to the fluctuating magnetic fields ΔB produced by nuclear spins.
We show that universal quantum control of a two-electron singlet-triplet spin qubit can be achieved using Landau-Zener-St\uckelberg interferometry. Going beyond normal Landau-Zener dynamics with infinitely long constant velocity sweeps across an energy level anti-crossing, we focus on a physical system consisting of a two-electron double quantum dot, where the spin states can be admixtures of charge states and the level velocity can be tuned in a time-dependent fashion. Our results indicate that charge coherence must be treated on an equal footing with spin coherence. In particular, we predict the presence of finite-time effects, which result in population transfer even in cases of an incomplete sweep through the anti-crossing. The competing requirements of adiabaticity and coherence are reconciled using specially designed pulses with a tunable level velocity. As a relevant example, we demonstrate that a Hadamard gate can be implemented for a realistic set of conditions in a GaAs double quantum dot device.
Single-qubit operations on singlet-triplet qubits in GaAs double quantum dots have not yet reached the fidelities required for fault-tolerant quantum information processing. Considering experimentally important constraints and using measured noise spectra, we numerically minimize the effect of decoherence (including high-frequency 1/f-like noise) and show, theoretically, that quantum gates with fidelities higher than 99.9% are achievable. We also present a self-consistent tuning protocol which should allow the elimination of individual systematic gate errors directly in an experiment.
Semiconductor quantum-dot spin qubits are a promising platform for quantum computation, because they are scalable and possess long coherence times. In order to realize this full potential, however, high-fidelity information transfer mechanisms are required for quantum error correction and efficient algorithms. Here, we present evidence of adiabatic quantum-state transfer in a chain of semiconductor quantum-dot electron spins. By adiabatically modifying exchange couplings, we transfer single- and two-spin states between distant electrons in less than 127 ns. We also show that this method can be cascaded for spin-state transfer in long spin chains. Based on simulations, we estimate that the probability to correctly transfer single-spin eigenstates and two-spin singlet states can exceed 0.95 for the experimental parameters studied here. In the future, state and process tomography will be required to verify the transfer of arbitrary single qubit states with a fidelity exceeding the classical bound. Adiabatic quantum-state transfer is robust to noise and pulse-timing errors. This method will be useful for initialization, state distribution, and readout in large spin-qubit arrays for gate-based quantum computing. It also opens up the possibility of universal adiabatic quantum computing in semiconductor quantum-dot spin qubits.
Electron spins in semiconductor quantum dots are a promising platform for qubits. An essential step toward quantum error correction with spin qubits requires the ability to read out states and identify errors in the two-spin logical basis in a fast and high-fidelity way. Here we propose a scheme for spin singlet-triplet readout in a double quantum dot, with a superconducting resonator serving as the detector. Making use of a spin driving pulse sequence, this procedure can read out the spin states with a fidelity of 99.999.9%99.9 in a time 100 ns. Interestingly, this method makes use of the spin driving pulse to adjust the resonator signal field, leading to significant improvement of the spin readout. In addition, this method can be directly applied to multiplexed readout of qubits and repeated readout to identify logical errors. These results pave the way for the implementation of quantum error correction and scalable quantum processor with spin qubits.