Fundamental limits of electron and nuclear spin qubit lifetimes in an isolated self-assembled quantum dot

George Gillard,Edmund Clarke,Evgeny A Chekhovich,Ian M Griffiths,Ata Ulhaq,Gautham Ragunathan,Callum Mcewan

doi:10.1038/s41534-021-00378-2

George Gillard, Edmund Clarke + Show 5 more

Open Access

https://doi.org/10.1038/s41534-021-00378-2

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Abstract

Combining external control with long spin lifetime and coherence is a key challenge for solid state spin qubits. Tunnel coupling with electron Fermi reservoir provides robust charge state control in semiconductor quantum dots, but results in undesired relaxation of electron and nuclear spins through mechanisms that lack complete understanding. Here, we unravel the contributions of tunnelling-assisted and phonon-assisted spin relaxation mechanisms by systematically adjusting the tunnelling coupling in a wide range, including the limit of an isolated quantum dot. These experiments reveal fundamental limits and trade-offs of quantum dot spin dynamics: while reduced tunnelling can be used to achieve electron spin qubit lifetimes exceeding 1 s, the optical spin initialisation fidelity is reduced below 80%, limited by Auger recombination. Comprehensive understanding of electron-nuclear spin relaxation attained here provides a roadmap for design of the optimal operating conditions in quantum dot spin qubits.

Highlights

Semiconductor quantum dots (QDs) offer excellent quantum optical properties and well-defined quantum states of individual spins—an attractive combination for quantum information processing devices[1]
The lack of translational motion combined with the mismatch in electron and nuclear spin energies suppresses relaxation[8], providing long spin lifetimes required for spin qubits
Phonon-assisted electron spin relaxation enabled by spin–orbit interaction is a dominant mechanism[9,10,18] at high magnetic field Bz ≳ 2 T, but the limit to electron spin lifetime T1,e at low fields remains unexplored

Summary

INTRODUCTION

Semiconductor quantum dots (QDs) offer excellent quantum optical properties and well-defined quantum states of individual spins—an attractive combination for quantum information processing devices[1]. Phonon-assisted electron spin relaxation enabled by spin–orbit interaction is a dominant mechanism[9,10,18] at high magnetic field Bz ≳ 2 T, but the limit to electron spin lifetime T1,e at low fields remains unexplored. We find that at Bz ≳ 2 T and temperatures θ > 4.2 K nuclear spin relaxation is dominated by a higher-order process assisted by phonons[19,20] and noncollinear hyperfine interaction[13], rather than by cotunneling, which is dominant only at low fields Bz ≲ 2 T. Electron spin lifetimes exceeding T1,e > 1 s are found at Bz ≈ 0.4 T, with a fundamental maximum T1,e ≈ 20 s estimated for an isolated dot at θ = 4.2 K, bounded by phonon relaxation and direct hyperfine interaction at high and low magnetic fields, respectively. While coupling to Fermi reservoir degrades T1,e, it is shown to play a crucial role in counteracting Auger recombination[21] and enabling electron spin initialisation with near-unity fidelity[22]

RESULTS

Gillard et al 2

DISCUSSION