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arXiv:2102.00406 · Annalen der Physik

Universal Singlet‐Triplet Qubits Implemented Near the Transverse Sweet Spot

Wen‐Xin Xie, Chengxian Zhang, Zheng‐Yuan Xue

Published Jun 16, 2021• 2 citations
arXivHTMLPDF

Abstract

Abstract The key to realizing fault‐tolerant quantum computation for singlet‐triplet (ST) qubits in semiconductor double quantum dot (DQD) is to operate both the single‐ and two‐qubit gates with high fidelity. The feasible way includes operating the qubit near the transverse sweet spot (TSS) to reduce the leading order of the noise, as well as adopting the proper pulse sequences which are immune to noise. The single‐qubit gates can be achieved by introducing an AC drive on the detuning near the TSS. The large dipole moment of the DQDs at the TSS has enabled strong coupling between the qubits and the cavity resonator, which leads to two‐qubit entangling gates. When operating in the proper region and applying modest pulse sequences, both single‐ and two‐qubit gates have fidelity higher than 99%. The results in this paper suggest that taking advantage of the appropriate pulse sequences near the TSS can be effective to obtain high‐fidelity ST qubits.

DOI
10.1002/andp.202100054

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Abstract The key to realizing fault‐tolerant quantum computation for singlet‐triplet (ST) qubits in semiconductor double quantum dot (DQD) is to operate both the single‐ and two‐qubit gates with high fidelity. The feasible way includes operating the qubit near the transverse sweet spot (TSS) to reduce the leading order of the noise, as well as adopting the proper pulse sequences which are immune to noise. The single‐qubit gates can be achieved by introducing an AC drive on the detuning near the TSS. The large dipole moment of the DQDs at the TSS has enabled strong coupling between the qubits and the cavity resonator, which leads to two‐qubit entangling gates. When operating in the proper region and applying modest pulse sequences, both single‐ and two‐qubit gates have fidelity higher than 99%. The results in this paper suggest that taking advantage of the appropriate pulse sequences near the TSS can be effective to obtain high‐fidelity ST qubits.
The key to realizing fault-tolerant quantum computation for semiconductor quantum-dot singlet-triplet (ST) qubits is to operate both the single- and two-qubit gates with high fidelity. The feasible way includes operating the qubit near the transverse sweet spot (TSS) to reduce the leading order of the noise, as well as adopting the merit of the global property of the geometric phase. Here, we propose the implementation of the universal geometric gates for the ST qubits in the double quantum dot (DQD). The single-qubit non-cyclic geometric gate which is using the non-cyclic evolution path of the state to obtain the geometric phase can be achieved via introducing the AC-driven detuning operating near the TSS. We perform numerical simulation under the realistic 1/f1/f1/f noise environment in DQD. We find that the non-cyclic geometric gate can surpass its dynamical counterpart with a relatively high fidelity of 99.81\%. Meanwhile, we have used the dipole coupling between the DQDs and the cavity resonator which is maximized at the TSS to construct the geometric (holonomic) entangling geometric gate. We have shown that the fidelity for this two-qubit gate can be as high as 99.63\%. Our results suggests that taking advantage of the non-cyclic geometric gates together with using the TSS can be effective to obtain high-fidelity ST qubits.
The search for a sweet spot, the locus in qubit parameters where quantum control is first-order insensitive to noises, is key to achieve high-fidelity quantum gates. Efforts to search for such a sweet spot in conventional double-quantum-dot singlet-triplet qubits where each dot hosts one electron (``two-electron singlet-triplet qubit''), especially for two-qubit operations, have been unsuccessful. Here we consider singlet-triplet qubits allowing each dot to host more than one electron, with a total of four electrons in the double quantum dots (``four-electron singlet-triplet qubit''). We theoretically demonstrate, using configuration interaction calculations, that sweet spots appear in this coupled qubit system. We further demonstrate that, under realistic charge noise and hyperfine noise, a two-qubit operation at the proposed sweet spot could offer gate fidelities ($\ensuremath{\sim}99%$) that are higher than the conventional two-electron singlet-triplet qubit system ($\ensuremath{\sim}90%$). Our results should facilitate realization of high-fidelity two-qubit gates in singlet-triplet qubit systems.
Abstract A fundamental challenge for quantum dot spin qubits is to extend the strength and range of qubit interactions while suppressing their coupling to the environment, since both effects have electrical origins. Key tools include the ability to take advantage of physical resources in different regimes, and to access optimal working points, sweet spots, where dephasing is minimized. Here, we explore an important resource for singlet-triplet qubits: a transverse sweet spot (TSS) that enables transitions between qubit states, a strong dipolar coupling, and leading-order protection from electrical fluctuations. Of particular interest is the possibility of transitioning between the TSS and symmetric operating points while remaining continuously protected. This arrangement is ideal for coupling qubits to a microwave cavity, because it combines tunability of the coupling with noise insensitivity. We perform simulations with 1/f1/f1/f1∕f-type electrical noise, demonstrating that two-qubit gates mediated by a resonator can achieve fidelities >99% under realistic conditions.
A fundamental challenge for quantum dot spin qubits is to extend the strength and range of qubit interactions while suppressing their coupling to the environment, since both effects have electrical origins. Key tools include the ability to take advantage of physical resources in different regimes, and to access optimal working points, sweet spots, where dephasing is minimized. Here, we explore an important resource for singlet-triplet qubits: a transverse sweet spot (TSS) that enables transitions between qubit states, a strong dipolar coupling, and leading-order protection from electrical fluctuations. Of particular interest is the possibility of transitioning between the TSS and symmetric operating points while remaining continuously protected. This arrangement is ideal for coupling qubits to a microwave cavity, because it combines tunability of the coupling with noise insensitivity. We perform simulations with [Formula: see text]-type electrical noise, demonstrating that two-qubit gates mediated by a resonator can achieve fidelities >99% under realistic conditions.
Singlet-triplet qubits in lateral quantum dots in semiconductor heterostructures exhibit high-fidelity single-qubit gates via exchange interactions and magnetic field gradients. High-fidelity two-qubit entangling gates are challenging to generate since weak interqubit interactions result in slow gates that accumulate error in the presence of noise. However, the interqubit electrostatic interaction also produces a shift in the local double well detunings, effectively changing the dependence of exchange on the gate voltages. We consider an operating point where the effective exchange is first-order insensitive to charge fluctuations while maintaining nonzero interactions. This ``sweet spot'' exists only in the presence of interactions. We show that working at the interacting sweet spot can directly produce maximally entangling gates, and we simulate the gate evolution under realistic 1/f1/f1/f noise. We report theoretical two-qubit gate fidelities above 99% in GaAs and Si systems.
We propose an optimal set of quantum gates for a singlet-triplet qubit in a double quantum dot with two electrons utilizing the S-T− subspace. Qubit rotations are driven by the applied magnetic field and an orthogonal field gradient provided by a micromagnet. We optimize the fidelity of this qubit as a function of magnetic fields, taking advantage of “sweet spots” where the rotation frequencies are independent of the energy level detuning, providing protection against charge noise. We simulate gate operations and qubit rotations in the presence of quasistatic noise from charge and nuclear spins as well as leakage to nonqubit states, and predict that in silicon quantum dots gate fidelities greater than 99% can be achieved for two nearly-orthogonal rotation axes.