Coherence Protection for Mobile Spin Qubits in Silicon

TL;DR

This paper demonstrates methods to preserve spin coherence during transport in silicon quantum dots, significantly enhancing coherence times and enabling scalable quantum computing with mobile spin qubits.

Contribution

It introduces systematic noise mitigation techniques, including magnetic gradient reduction, motional narrowing, and dynamical decoupling, to improve coherence during spin shuttling in silicon quantum dots.

Findings

Dephasing time doubled from 4.4 to 8.5 μs by reducing magnetic field gradients.

Coherence time increased to 11.5 μs through motional narrowing during shuttling.

Dynamical decoupling extended coherence to 32 μs during transport.

Abstract

Mobile spin qubit architectures promise flexible connectivity for efficient quantum error correction and relaxed device layout constraints, but their viability rests on preserving spin coherence during transport. While shuttling transforms spatial disorder into time-dependent noise, its net impact on spin coherence remains an open question. Here we demonstrate systematic noise mitigation during spin shuttling in a linear $^{28}$Si/SiGe quantum dot device. First, by passively reducing magnetic field gradients, we minimize charge-noise coupling to the spin and double the spatially averaged dephasing time $T_2^*(x_n)$ from $4.4$ to $8.5\,\mu\text{s}$. Next, we exploit motional narrowing by periodically shuttling the qubit, achieving a further enhancement in coherence time up to $T_{2}^{*,sh} = 11.5\,\mu\text{s}$. Finally, we incorporate dynamical decoupling techniques while periodically shuttling over distances exceeding $200\,\text{nm}$, reaching $T_\text{2}^{H,sh}= 32\,\mu\text{s}$. For the same setup, we demonstrate that dressed-state shuttling provides robust protection against low-frequency noise with a decay time $T_R^{\text{sh}} = 21\,\mu\text{s}$, without the overhead of pulsed control and allowing protection during one-way spin transport. By preserving coherence over timescales exceeding typical gate and readout operations, the demonstrated strategies establish mobile spin qubits as a viable solution for scalable silicon quantum processors.