Abstract The implementation of high fidelity two-qubit gates is a bottleneck in the progress toward universal quantum computation in semiconductor quantum dot qubits. We study capacitive coupling between two triple quantum dot spin qubits encoded in the S = 1/2, Sz = −1/2 decoherence-free subspace—the exchange-only (EO) spin qubits. We report exact gate sequences for CPHASE and CNOT gates, and demonstrate theoretically, the existence of multiple two-qubit sweet spots (2QSS) in the parameter space of capacitively coupled EO qubits. Gate operations have the advantage of being all-electrical, but charge noise that couple to electrical parameters of the qubits cause decoherence. Assuming noise with a 1/f spectrum, two-qubit gate fidelities and times are calculated, which provide useful information on the noise threshold necessary for fault-tolerance. We study two-qubit gates at single and multiple parameter 2QSS. In particular, for two existing EO implementations—the resonant exchange (RX) and the always-on exchange-only (AEON) qubits—we compare two-qubit gate fidelities and times at positions in parameter space where the 2QSS are simultaneously single-qubit sweet spots (1QSS) for the RX and AEON. These results provide a potential route to the realization of high fidelity quantum computation.Abstract The implementation of high fidelity two-qubit gates is a bottleneck in the progress toward universal quantum computation in semiconductor quantum dot qubits. We study capacitive coupling between two triple quantum dot spin qubits encoded in the S = 1/2, Sz = −1/2 decoherence-free subspace—the exchange-only (EO) spin qubits. We report exact gate sequences for CPHASE and CNOT gates, and demonstrate theoretically, the existence of multiple two-qubit sweet spots (2QSS) in the parameter space of capacitively coupled EO qubits. Gate operations have the advantage of being all-electrical, but charge noise that couple to electrical parameters of the qubits cause decoherence. Assuming noise with a 1/f spectrum, two-qubit gate fidelities and times are calculated, which provide useful information on the noise threshold necessary for fault-tolerance. We study two-qubit gates at single and multiple parameter 2QSS. In particular, for two existing EO implementations—the resonant exchange (RX) and the always-on exchange-only (AEON) qubits—we compare two-qubit gate fidelities and times at positions in parameter space where the 2QSS are simultaneously single-qubit sweet spots (1QSS) for the RX and AEON. These results provide a potential route to the realization of high fidelity quantum computation.

arXiv:2103.15681npj Quantum InformationJul 16, 20214 citations

High-fidelity Two and Three Spin Operations in Si with Triple Quantum Dots

Seigo Tarucha

We use Si/SiGe triple quantum dots to operate two and three spin qubits with high fidelities. For the two qubit we speed up the single qubit and exchange control to achieve the fault tolerance threshold fidelity of $> 99$ %. For the three qubits we control two exchange couplings to generate a three-spin entangled state.We use Si/SiGe triple quantum dots to operate two and three spin qubits with high fidelities. For the two qubit we speed up the single qubit and exchange control to achieve the fault tolerance threshold fidelity of$> 99$%. For the three qubits we control two exchange couplings to generate a three-spin entangled state.

Technical digest - International Electron Devices Meeting/Technical digestDec 11, 20211 citation

Fast and High-Fidelity State Preparation and Measurement in Triple-Quantum-Dot Spin Qubits

Jacob Blumoff, Andrew Pan, Tyler Keating +19 more

We demonstrate rapid high-fidelity state preparation and measurement in exchange-only Si/SiGe triplequantum-dot qubits.Fast measurement integration (980-ns) and initialization (approximately 300-ns) operations are performed with all-electrical baseband control.We emphasize a leakage-sensitive joint initialization and measurement metric, developed in the context of exchange-only qubits but applicable more broadly, and report an infidelity of 2.5 ± 0.5×10 -3 .This result is enabled by a high-valley-splitting heterostructure, initialization at the two-to three-electron charge boundary, and careful assessment and mitigation of T 1 during spin-to-charge conversion.The ultimate fidelity is limited by a number of comparably important factors and we identify clear paths toward further improved fidelity and speed.Along with an observed single-qubit randomized benchmarking error rate of 1.7 × 10 -3 , this work demonstrates initialization, control, and measurement of Si/SiGe triple-dot qubits at fidelities and durations that are promising for scalable quantum information processing.We demonstrate rapid high-fidelity state preparation and measurement in exchange-only Si/SiGe triplequantum-dot qubits.Fast measurement integration (980-ns) and initialization (approximately 300-ns) operations are performed with all-electrical baseband control.We emphasize a leakage-sensitive joint initialization and measurement metric, developed in the context of exchange-only qubits but applicable more broadly, and report an infidelity of 2.5 ± 0.5×10 -3 .This result is enabled by a high-valley-splitting heterostructure, initialization at the two-to three-electron charge boundary, and careful assessment and mitigation of T 1 during spin-to-charge conversion.The ultimate fidelity is limited by a number of comparably important factors and we identify clear paths toward further improved fidelity and speed.Along with an observed single-qubit randomized benchmarking error rate of 1.7 × 10 -3 , this work demonstrates initialization, control, and measurement of Si/SiGe triple-dot qubits at fidelities and durations that are promising for scalable quantum information processing.

arXiv:2112.09801PRX QuantumMar 29, 202259 citations

> 99

%. For the three qubits we control two exchange couplings to generate a three-spin entangled state.