Charge-noise-limited coherence and three-nines fidelity of an enriched Si/SiGe spin qubit

Probing Residual Noise at a Decoherence Sweet Spot in a $^{28}$ Si/SiGe Spin QubitProbing Residual Noise at a Decoherence Sweet Spot in a $^{28}$Si/SiGe Spin Qubit

Shinwoo Lee, Hanseo Sohn, Jaemin Park +7 more

In $^{28}$ Si/SiGe spin qubits with micromagnets, the longitudinal stray field gradient transduces charge noise into qubit frequency noise and limits coherence. We compare two neighboring qubits in the same device with 800 ppm residual $^{29}$ Si, one near a decoherence sweet spot where the gradient is locally minimized and the other at a position with a larger gradient. At the sweet spot, $T_2^*$ reaches 67 $μ\mathrm{s}$ and the Carr-Purcell-Meiboom-Gill coherence time reaches 4.6 ms, whereas $T_2^*$ is 5.2 $μ\mathrm{s}$ at the neighboring qubit. Near 1 Hz, the frequency noise power spectral density at the sweet spot is nearly two orders of magnitude lower than at the neighboring qubit. The magnitude and low-frequency decay of the residual spectrum are compatible with the prediction for $^{29}$ Si nuclear spin noise, and the weak interqubit correlation indicates that local noise becomes important for dephasing at the sweet spot. Despite a finite correlation with the charge sensor, sweet-spot operation strongly reduced the transduction of charge noise, bringing the residual frequency noise close to the level predicted for $^{29}$ Si nuclear spin fluctuations.In $^{28}$Si/SiGe spin qubits with micromagnets, the longitudinal stray field gradient transduces charge noise into qubit frequency noise and limits coherence. We compare two neighboring qubits in the same device with 800 ppm residual $^{29}$Si, one near a decoherence sweet spot where the gradient is locally minimized and the other at a position with a larger gradient. At the sweet spot, $T_2^*$ reaches 67 $μ\mathrm{s}$ and the Carr-Purcell-Meiboom-Gill coherence time reaches 4.6 ms, whereas $T_2^*$ is 5.2 $μ\mathrm{s}$ at the neighboring qubit. Near 1 Hz, the frequency noise power spectral density at the sweet spot is nearly two orders of magnitude lower than at the neighboring qubit. The magnitude and low-frequency decay of the residual spectrum are compatible with the prediction for $^{29}$Si nuclear spin noise, and the weak interqubit correlation indicates that local noise becomes important for dephasing at the sweet spot. Despite a finite correlation with the charge sensor, sweet-spot operation strongly reduced the transduction of charge noise, bringing the residual frequency noise close to the level predicted for $^{29}$Si nuclear spin fluctuations.

quant-pharXiv:2609.15860Sep 15, 2026

Noise correlations in a two-qubit Si/SiGe quantum dot device

Jelmer M. Boter, Xiao Xue, Thomas F. Watson +10 more

Bulletin of the American Physical SocietyMar 5, 20190 citations

Enhanced Coherence and High Figure of Merit in a Silicon Charge qubit

Zhan Shi, C. B. Simmons, Daniel R. Ward +9 more

Bulletin of the American Physical SocietyMar 21, 20130 citations

Probing Residual Noise at a Decoherence Sweet Spot in a

^{28}

Si/SiGe Spin Qubit

^{28}

Si/SiGe spin qubits with micromagnets, the longitudinal stray field gradient transduces charge noise into qubit frequency noise and limits coherence. We compare two neighboring qubits in the same device with 800 ppm residual

^{29}

Si, one near a decoherence sweet spot where the gradient is locally minimized and the other at a position with a larger gradient. At the sweet spot,

T_2^*

reaches 67

μ\mathrm{s}

and the Carr-Purcell-Meiboom-Gill coherence time reaches 4.6 ms, whereas

T_2^*

is 5.2

μ\mathrm{s}

at the neighboring qubit. Near 1 Hz, the frequency noise power spectral density at the sweet spot is nearly two orders of magnitude lower than at the neighboring qubit. The magnitude and low-frequency decay of the residual spectrum are compatible with the prediction for

^{29}

Si nuclear spin noise, and the weak interqubit correlation indicates that local noise becomes important for dephasing at the sweet spot. Despite a finite correlation with the charge sensor, sweet-spot operation strongly reduced the transduction of charge noise, bringing the residual frequency noise close to the level predicted for

^{29}

Si nuclear spin fluctuations.