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arXiv:2004.07658 · Physical Review Research

Exchange interaction of hole-spin qubits in double quantum dots in highly anisotropic semiconductors

Bence Hetényi, Christoph Kloeffel, Daniel Loss

Published Jul 8, 2020• 18 citations
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Abstract

This paper presents a definition of a hole-spin qubit based on a twofold symmetry that is applicable for a variety of materials and device geometries. If such symmetry is preserved, exchange interaction between two hole-spin qubits greatly simplifies increasing the fidelity of two-qubit gates. Exchange anisotropy in a silicon nanowire double quantum dot is studied in detail for both conserved and broken mirror symmetry.

DOI
10.1103/physrevresearch.2.033036

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This paper presents a definition of a hole-spin qubit based on a twofold symmetry that is applicable for a variety of materials and device geometries. If such symmetry is preserved, exchange interaction between two hole-spin qubits greatly simplifies increasing the fidelity of two-qubit gates. Exchange anisotropy in a silicon nanowire double quantum dot is studied in detail for both conserved and broken mirror symmetry.
A spin-orbit qubit is a hybrid qubit that contains both orbital and spin degrees of freedom of an electron in a quantum dot. Here we study the exchange coupling between two spin-orbit qubits in a nanowire double quantum dot (DQD) with strong spin-orbit coupling (SOC). We find that while the total tunneling in the DQD is irrelevant to the SOC, both the spin-conserved and spin-flipped tunnelings are SOC dependent and can compete with each other in the strong SOC regime. Moreover, the Coulomb repulsion between electrons can combine with the SOC-dependent tunnelings to yield an anisotropic exchange coupling between the two spin-orbit qubits. Also, we give an explicit physical mechanism for this anisotropic exchange coupling.
The exchange coupling between quantum dot spin qubits is isotropic, which restricts the types of quantum gates that can be formed. Here, we propose a method for controlling anisotropic interactions between spins arranged in a bus geometry. The symmetry is broken by an external magnetic field, resulting in XXZ-type interactions that can efficiently generate maximally entangled Greenberger-Horne-Zeilinger states or universal gate sets for exchange-only quantum computing. We exploit the XXZ couplings to propose a qubit scheme, based on double dots.
Hole spin qubits hosted in Germanium quantum dots are promising candidates for scalable quantum computing. The strong spin-orbit interaction can enable fast and all-electrical quantum control. Furthermore, the platform can implement universal quantum control using only baseband signals, which may mitigate the impact of crosstalk and microwave-induced heating. At the same time, spin-orbit interaction gives rise to an anisotropic exchange interaction, whose potential for implementing two-qubit gates has remained largely unexplored. However, the current performance of operating a hole-based quantum computer is mostly limited by dephasing due to low-frequency charge noise. In this work, we propose a novel two-qubit gate protocol for Germanium hole spin qubits operated in the gapless regime. This gate protocol exploits the anisotropic exchange interaction between neighboring spins and utilizes a composite pulse scheme implemented solely through electrical baseband signals. Using this approach, we predict high-fidelity two-qubit controlled-Z operations that can suppress exchange-energy fluctuations, offering a pathway toward fault-tolerant semiconductor quantum processors.
We analyzed the fidelity of the quantum state transfer (QST) from a photon-polarization qubit to an electron-spin-polarization qubit in a semiconductor quantum dot, with special attention to the exchange interaction between the electron and the simultaneously created hole. In order to realize a high-fidelity QST we had to separate the electron and hole as soon as possible, since the electron-hole exchange interaction modifies the orientation of the electron spin. Thus, we propose a double-dot structure to separate the electron and hole quickly, and show that the fidelity of the QST can reach as high as 0.996 if the resonant tunneling condition is satisfied.
Hole spin qubits hosted in Germanium quantum dots are promising candidates for scalable quantum computing. The strong spin-orbit interaction can enable fast and all-electrical quantum control. Furthermore, the platform can implement universal quantum control using only baseband signals, which may mitigate the impact of crosstalk and microwave-induced heating. At the same time, spin-orbit interaction gives rise to an anisotropic exchange interaction, whose potential for implementing two-qubit gates has remained largely unexplored. However, the current performance of operating a hole-based quantum computer is mostly limited by dephasing due to low-frequency charge noise. In this work, we propose a novel two-qubit gate protocol for Germanium hole spin qubits operated in the gapless regime. This gate protocol exploits the anisotropic exchange interaction between neighboring spins and utilizes a composite pulse scheme implemented solely through electrical baseband signals. Using this approach, we predict high-fidelity two-qubit controlled-Z operations that can suppress exchange-energy fluctuations, offering a pathway toward fault-tolerant semiconductor quantum processors.
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.