# Single-spin qubits in isotopically enriched silicon at low magnetic field

## TL;DR

This paper demonstrates low-field operation of silicon quantum dot spin qubits with high-fidelity readout, revealing faster decoherence at low magnetic fields due to residual nuclear spins, impacting scalability.

## Contribution

It introduces low-field silicon quantum dot qubits with Pauli-spin-blockade readout and analyzes their decoherence mechanisms at reduced magnetic fields.

## Key findings

- Decoherence times are shorter at low magnetic fields.
- Residual $^{29}$Si nuclear spins limit qubit coherence.
- Isotopic enrichment is crucial for scalable silicon qubits.

## Abstract

Single-electron spin qubits employ magnetic fields on the order of 1 Tesla or above to enable quantum state readout via spin-dependent-tunnelling. This requires demanding microwave engineering for coherent spin resonance control and significant on-chip real estate for electron reservoirs, both of which limit the prospects for large scale multi-qubit systems. Alternatively, singlet-triplet (ST) readout enables high-fidelity spin-state measurements in much lower magnetic fields, without the need for reservoirs. Here, we demonstrate low-field operation of metal-oxide-silicon (MOS) quantum dot qubits by combining coherent single-spin control with high-fidelity, single-shot, Pauli-spin-blockade-based ST readout. We discover that the qubits decohere faster at low magnetic fields with $T_{2}^{Rabi}=18.6$~$\mu$s and $T_2^*=1.4$~$\mu$s at 150~mT. Their coherence is limited by spin flips of residual $^{29}$Si nuclei in the isotopically enriched $^{28}$Si host material, which occur more frequently at lower fields. Our finding indicates that new trade-offs will be required to ensure the frequency stabilization of spin qubits and highlights the importance of isotopic enrichment of device substrates for the realization of a scalable silicon-based quantum processor.