Keeping a spin qubit alive in natural silicon: Comparing optimal working points and dynamical decoupling

TL;DR

This study compares the effectiveness of dynamical decoupling and optimal working points in extending spin qubit coherence in silicon, revealing that near OWPs, many pulses are needed for significant improvement, unlike far from OWPs.

Contribution

It provides a detailed quantum many-body analysis of decoherence suppression techniques near and far from OWPs, highlighting the need for multi-spin clusters in simulations.

Findings

Near OWPs, hundreds of pulses are needed for T2 enhancement.

Independent spin pairs do not suffice to explain decoherence at OWPs.

Far from OWPs, fewer pulses (~10) achieve similar T2 improvements.

Abstract

There are two distinct techniques of proven effectiveness for extending the coherence lifetime of spin qubits in environments of other spins. One is dynamical decoupling, whereby the qubit is subjected to a carefully timed sequence of control pulses; the other is tuning the qubit towards 'optimal working points' (OWPs), which are sweet-spots for reduced decoherence in magnetic fields. By means of quantum many-body calculations, we investigate the effects of dynamical decoupling pulse sequences far from and near OWPs for a central donor qubit subject to decoherence from a nuclear spin bath. Key to understanding the behavior is to analyse the degree of suppression of the usually dominant contribution from independent pairs of flip-flopping spins within the many-body quantum bath. We find that to simulate recently measured Hahn echo decays at OWPs (lowest-order dynamical decoupling), one must consider clusters of three interacting spins, since independent pairs do not even give finite $T_2$ decay times. We show that while operating near OWPs, dynamical decoupling sequences require hundreds of pulses for a single order of magnitude enhancement of $T_2$, in contrast to regimes far from OWPs, where only about ten pulses are required.