A spinless spin qubit

Abstract

All-electrical baseband control of qubits facilitates scaling up quantum processors by removing issues of crosstalk and heat generation. In semiconductor quantum dots, this is enabled by multi-spin qubit encodings, such as the exchange-only qubit, where high-fidelity readout and both single- and two-qubit operations have been demonstrated. However, their performance is limited by unavoidable leakage states that are energetically close to the computational subspace. In this work, we introduce an alternative, scalable spin qubit architecture that leverages strong spin-orbit interactions of hole nanostructures for baseband qubit operations while completely eliminating leakage channels and reducing the overall gate overhead. This encoding is intrinsically robust to local variability in hole spin properties and operates with two degenerate states, removing the need for precise calibration and mitigating heat generation from fast signal sources. Finally, our architecture is fully compatible with current technology, utilizing the same initialization, readout, and multi-qubit protocols of state-of-the-art spin-1/2 systems. By addressing critical scalability challenges, our design offers a robust and scalable pathway for semiconductor spin qubit technologies.

Figures (4)

• Figure 1: a) Schematics of an architecture for our novel spinless spin qubit (S2) qubit. Each qubit is controlled using 5 gate electrodes. Single-qubit gates are operated by electrically compressing and stretching the qubit. Readout, initialization, and two-qubit gates can be implemented along $x$ and $y$ direction. Readout and initialization uses Pauli-spin-blockade and either requires a close by charge sensor or readout resonator. During idling, the qubit states are not subject to any dynamics. Optimal magnetic field direction is $\boldsymbol{B}\parallel [110]$ or $\boldsymbol{B}\parallel [1\overline{1}0]$ b) Effective g-tensor $g^\star=||\mathcal{G}\boldsymbol{B}||/||\boldsymbol{B}||$ in the optimal direction as a function of characteristic confinement lengths of the groundstate wavefunction in $X,Y$-direction, assuming biaxial strain. The S2 qubit conditions are $\approx\unit[14]{nm}$. The inset shows the conditions assuming non-biaxial strain. c) The S2 qubit conditions for different heavy-hole light-hole coupling strengths as a function of strain asymmetry $(\epsilon_{xx}-\epsilon_{yy})/(\epsilon_{xx}+\epsilon_{yy})$.
• Figure 2: a) Single qubit gate fidelity fidelity of a $X$-gate as a function of miscalibration $b_{y,z}\neq 0$ from the S2 condition without decoherence effects. Three pulse techniques are shown, using a (blue) single pulse, (red) composite pulse sequence short-CORPSE, and (green) composite pulse sequence CORPSE. b) Fidelity of a (black) $X$-operation and (blue and red) idling $I$-operation under the influence of low frequency noise coupling in via the g-tensor as a function of operation time. Our simulation assumes fluctuations in $b_x$ and $b_y$, uses the quasistatic approximation, neglects transversally coupled noise for the $X$-operation, and in the case of idling, (blue) correlated and (red) uncorrelated fluctuations between $b_x$ and $b_y$.
• Figure 3: a) Energy diagram of the anti-crossing used for Pauli-Spin Blockade (PSB) readout as a function of detuning $\varepsilon$ for stabilized protocol $b_y^{(A)}=-b_y^{(B)}=\unit[0.1]{GHz}$ (dashed lines showing $b_y^{(A)}=b_y^{(B)}=0$) considering $b_x^{(A)}=\unit[0.3]{GHz}$, $b_x^{(B)}=\unit[0.4]{GHz}$, $t_c=\unit[3]{GHz}$, and $\theta_\text{SOI}= 0$. Our protocol induces an anti-crossing between $\ket{00}$ and $\ket{S_{02}}$ identical as in the presence of finite spin-orbit interaction. b) Simulated readout infidelity of the stabilized protocol for a linear ramp from $\varepsilon(t=0)=\unit[0.5]{meV}$ to $\varepsilon(t=T)=-\unit[0.5]{meV}$ with duration $T=\unit[52.7]{ns}$ as a function of the spin-flip angle $\theta_\text{SOI}$ and SOI vector $\boldsymbol{n}=(\cos(\Phi_\text{SOI}),\sin(\Phi_\text{SOI}),0)^T$ and using the same parameters as before. c) Simulated average gate infidelity of an adiabatic CX two-qubit gate up to single-qubit X gates with fixed duration $T=\unit[36]{ns}$ using a Tukey pulse rimbach-russSimpleFrameworkSystematic2023 as a function of the spin-flip angle $\theta_\text{SOI}$ and SOI vector $\boldsymbol{n}=(\cos(\Phi_\text{SOI}),\sin(\Phi_\text{SOI}),0)^T$. The dashed black lines highlight the problematic regime in which SWAP oscillations can be hardly suppressed supplemental_material. Simulation parameters are identical to a) and b). Tunneling and exchange are modelled assuming Gaussian overlaps burkardCoupledQuantumDots1999.
• Figure S1: Expectation value $\braket{k_y^2}$ using a gate layout similar to Fig. 1a) in the main text as a function of voltage applied to the center gate of radius $\unit[80]{nm}$. Blue dots are extracted simulation, the star ($\star$) highlights the S2 operation regime, and the solid line is linear regression with $\braket{k_y^2}=\unit[1.2\times 10^{15}]{m^{-2}}+\unit[4.1\times 10^{15}]{m^{-2}V^{-1}}\,V_g$ around the operation point. Heterostructure and device layout are taken from Ref. wangOperatingSemiconductorQuantum2024a and material parameters Ref. terrazosTheoryHolespinQubits2021. Schrödinger-Poisson simulations are performed using QTCAD philippopoulosAnalysis3DTCAD2024.