Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

arXiv:1412.1503 · Physical Review B

Characterizing gate operations near the sweet spot of an exchange-only qubit

Jianjia Fei, Jo-Tzu Hung, Teck Seng Koh, Yun-Pil Shim, S. N. Coppersmith, Xuedong Hu, Mark G. Friesen

Published May 26, 2015• 31 citations
arXivHTMLPDF

Abstract

Optimal working points or ``sweet spots'' have arisen as an important tool for mitigating charge noise in quantum dot logical spin qubits. The exchange-only qubit provides an ideal system for studying this effect because $Z$ rotations are performed directly at the sweet spot, while $X$ rotations are not. Here, we quantify the ability of the sweet spot to mitigate charge noise by treating $X$ and $Z$ rotations on an equal footing. Specifically, we optimize $X$ rotations and determine an upper bound on their fidelity. We find that sweet spots offer a fidelity improvement factor of at least 20 for typical GaAs devices, and more for Si devices.

DOI
10.1103/physrevb.91.205434

Related papers

Leakage and sweet spots in triple-quantum-dot spin qubits: A molecular-orbital study

Chengxian Zhang, Xu-Chen Yang, Xin Wang

A triple-quantum-dot system can be operated as either an exchange-only qubit or a resonant-exchange qubit. While it is generally believed that the decisive advantage of the resonant-exchange qubit is the suppression of charge noise because it is operated at a sweet spot, we show that the leakage is also an important factor. Through molecular-orbital-theoretic calculations, we show that when the system is operated in the exchange-only scheme, the leakage to states with double electron occupancy in quantum dots is severe when rotations around the axis ${120}^{\ensuremath{\circ}}$ from $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{z}$ is performed. While this leakage can be reduced by either shrinking the dots or separating them further, the exchange interactions are also suppressed at the same time, making the gate operations unfavorably slow. When the system is operated as a resonant-exchange qubit, the leakage is three to five orders of magnitude smaller. We have also calculated the optimal detuning point which minimizes the leakage for the resonant-exchange qubit, and have found that although it does not coincide with the double sweet spot for the charge noise, they are rather close. Our results suggest that the resonant-exchange qubit has another advantage, that leakage can be greatly suppressed compared to the exchange-only qubit, and operating at the double sweet spot point should be optimal both for reducing charge noise and suppressing leakage.

arXiv:1711.06418•Physical Review A•Apr 16, 2018•15 citations

Robust operating point for capacitively coupled singlet-triplet qubits

M. A. Wolfe, F. A. Calderon-Vargas, J. P. Kestner

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 $1/f$ noise. We report theoretical two-qubit gate fidelities above 99% in GaAs and Si systems.

arXiv:1709.09165•Physical review. B./Physical review. B•Nov 30, 2017•5 citations

Double sweet-spot operation of the resonant exchange qubit in three-electron quantum dots

Guido Burkard

The resonant exchange (RX) qubit is a promising variant of the exchange-only spin qubit in a triple quantum dot which responds to a narrow-band resonant frequency. But the advantage of a permanently applied exchange splitting for spin control generally entails an increased susceptibility to charge noise. We have investigated the influence of electrical charge noise on a resonant exchange (RX) qubit by taking into account uncorrelated noise in each quantum dot, giving rise to two independent noisy bias parameters e and ∆ [1]. Calculating the energy splitting of the two qubit states as a function of these two bias detuning parameters, we have identified sweet spots, where the qubit is least susceptible to noise. Our investigation shows that the sweet spots exist within the low-bias regime, in which the bias detuning parameters have the same magnitude as the hopping parameters between the dots. By calculating and comparing the charge dephasing rates at the various operating points of the RX qubit, we identify a new favorable operating regime for the RX qubit in the case of weak noise, based on these double sweet spots. In contrast, spin noise can be mitigated using exchange-based dynamical decoupling sequences that have been optimized using two different strategies, Uhrig dynamical decoupling (UDD) and optimized filter function dynamical decoupling (OFDD) [2]. Finally, we give a brief outlook towards the possibility of long-distance coupling between resonant exchange qubits mediated by a microwave cavity [3]. [1] M. Russ and G. Burkard, Phys. Rev. B 91, 235411 (2015). [2] N. Rohling and G. Burkard, arXiv:1510.04098. [3] M. Russ and G. Burkard, Phys. Rev. B (accepted) [arXiv:1508.07122].

Bulletin of the American Physical Society•Mar 15, 2016•0 citations

Optimal Working Points of a Singlet-Triplet S-T Qubit

Clement H. Wong, M. A. Eriksson, S. N. Coppersmith +1 more

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.

•Jan 1, 2014•0 citations

Charge-noise-insensitive gate operations for always-on, exchange-only qubits

Yun-Pil Shim, Charles Tahan

We introduce an always-on, exchange-only qubit made up of three localized semiconductor spins that offers a true ``sweet spot'' to fluctuations of the quantum dot energy levels. Both single- and two-qubit gate operations can be performed using only exchange pulses while maintaining this sweet spot. We show how to interconvert this qubit to other three-spin encoded qubits as a resource for quantum computation and communication.

arXiv:1602.00320•Physical review. B./Physical review. B•Mar 23, 2016•61 citations

Asymmetric resonant exchange qubit under the influence of electrical noise

Maximilian Rimbach-Russ, Guido Burkard

We investigate the influence of electrical charge noise on a resonant exchange (RX) qubit in a triple quantum dot. This RX qubit is a variation of the exchange-only spin qubit which responds to a narrow-band resonant frequency. Our noise model includes uncorrelated charge noise in each quantum dot giving rise to two independent (noisy) bias parameters $\ensuremath{\varepsilon}$ and $\mathrm{\ensuremath{\Delta}}$. We calculate the energy splitting of the two qubit states as a function of these two bias detuning parameters to find ``sweet spots,'' where the qubit is least susceptible to noise. Our investigation shows that such sweet spots exist within the low-bias regime, in which the bias detuning parameters have the same magnitude as the hopping parameters. The location of the sweet spots in the $(\ensuremath{\varepsilon},\mathrm{\ensuremath{\Delta}})$ plane depends on the hopping strength and asymmetry between the quantum dots. In the regime of weak charge noise, we identify a new favorable operating regime for the RX qubit based on these sweet spots.

arXiv:1502.06109•Physical Review B•Jun 9, 2015•40 citations
Optimal working points or ``sweet spots'' have arisen as an important tool for mitigating charge noise in quantum dot logical spin qubits. The exchange-only qubit provides an ideal system for studying this effect because ZZZ rotations are performed directly at the sweet spot, while XXX rotations are not. Here, we quantify the ability of the sweet spot to mitigate charge noise by treating XXX and ZZZ rotations on an equal footing. Specifically, we optimize XXX rotations and determine an upper bound on their fidelity. We find that sweet spots offer a fidelity improvement factor of at least 20 for typical GaAs devices, and more for Si devices.
A triple-quantum-dot system can be operated as either an exchange-only qubit or a resonant-exchange qubit. While it is generally believed that the decisive advantage of the resonant-exchange qubit is the suppression of charge noise because it is operated at a sweet spot, we show that the leakage is also an important factor. Through molecular-orbital-theoretic calculations, we show that when the system is operated in the exchange-only scheme, the leakage to states with double electron occupancy in quantum dots is severe when rotations around the axis ${120}^{\ensuremath{\circ}}$ from $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{z}$ is performed. While this leakage can be reduced by either shrinking the dots or separating them further, the exchange interactions are also suppressed at the same time, making the gate operations unfavorably slow. When the system is operated as a resonant-exchange qubit, the leakage is three to five orders of magnitude smaller. We have also calculated the optimal detuning point which minimizes the leakage for the resonant-exchange qubit, and have found that although it does not coincide with the double sweet spot for the charge noise, they are rather close. Our results suggest that the resonant-exchange qubit has another advantage, that leakage can be greatly suppressed compared to the exchange-only qubit, and operating at the double sweet spot point should be optimal both for reducing charge noise and suppressing leakage.
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.
The resonant exchange (RX) qubit is a promising variant of the exchange-only spin qubit in a triple quantum dot which responds to a narrow-band resonant frequency. But the advantage of a permanently applied exchange splitting for spin control generally entails an increased susceptibility to charge noise. We have investigated the influence of electrical charge noise on a resonant exchange (RX) qubit by taking into account uncorrelated noise in each quantum dot, giving rise to two independent noisy bias parameters e and ∆ [1]. Calculating the energy splitting of the two qubit states as a function of these two bias detuning parameters, we have identified sweet spots, where the qubit is least susceptible to noise. Our investigation shows that the sweet spots exist within the low-bias regime, in which the bias detuning parameters have the same magnitude as the hopping parameters between the dots. By calculating and comparing the charge dephasing rates at the various operating points of the RX qubit, we identify a new favorable operating regime for the RX qubit in the case of weak noise, based on these double sweet spots. In contrast, spin noise can be mitigated using exchange-based dynamical decoupling sequences that have been optimized using two different strategies, Uhrig dynamical decoupling (UDD) and optimized filter function dynamical decoupling (OFDD) [2]. Finally, we give a brief outlook towards the possibility of long-distance coupling between resonant exchange qubits mediated by a microwave cavity [3]. [1] M. Russ and G. Burkard, Phys. Rev. B 91, 235411 (2015). [2] N. Rohling and G. Burkard, arXiv:1510.04098. [3] M. Russ and G. Burkard, Phys. Rev. B (accepted) [arXiv:1508.07122].
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.
We investigate the influence of electrical charge noise on a resonant exchange (RX) qubit in a triple quantum dot. This RX qubit is a variation of the exchange-only spin qubit which responds to a narrow-band resonant frequency. Our noise model includes uncorrelated charge noise in each quantum dot giving rise to two independent (noisy) bias parameters $\ensuremath{\varepsilon}$ and $\mathrm{\ensuremath{\Delta}}$. We calculate the energy splitting of the two qubit states as a function of these two bias detuning parameters to find ``sweet spots,'' where the qubit is least susceptible to noise. Our investigation shows that such sweet spots exist within the low-bias regime, in which the bias detuning parameters have the same magnitude as the hopping parameters. The location of the sweet spots in the $(\ensuremath{\varepsilon},\mathrm{\ensuremath{\Delta}})$ plane depends on the hopping strength and asymmetry between the quantum dots. In the regime of weak charge noise, we identify a new favorable operating regime for the RX qubit based on these sweet spots.