Phonon-assisted relaxation and decoherence of singlet-triplet qubits in Si/SiGe quantum dots

TL;DR

This paper theoretically investigates phonon-induced relaxation and decoherence in Si/SiGe singlet-triplet qubits, identifying optimal conditions for long qubit lifetimes and highlighting differences from GaAs systems.

Contribution

It provides a detailed theoretical analysis of phonon effects on Si/SiGe qubits, including the impact of magnetic field gradients and optimal regimes for coherence.

Findings

Magnetic field gradients can significantly reduce relaxation times.

Optimal regimes exist where magnetic field gradients minimally affect qubit coherence.

Si-based quantum dots exhibit longer spin lifetimes than GaAs counterparts.

Abstract

We study theoretically the phonon-induced relaxation and decoherence of spin states of two electrons in a lateral double quantum dot in a SiGe/Si/SiGe heterostructure. We consider two types of singlet-triplet spin qubits and calculate their relaxation and decoherence times, in particular as a function of level hybridization, temperature, magnetic field, spin orbit interaction, and detuning between the quantum dots, using Bloch-Redfield theory. We show that the magnetic field gradient, which is usually applied to operate the spin qubit, may reduce the relaxation time by more than an order of magnitude. Using this insight, we identify an optimal regime where the magnetic field gradient does not affect the relaxation time significantly, and we propose regimes of longest decay times. We take into account the effects of one-phonon and two-phonon processes and suggest how our theory can be tested experimentally. The spin lifetimes we find here for Si-based quantum dots are significantly longer than the ones reported for their GaAs counterparts.