Solid state quantum memory using the 31P nuclear spin

TL;DR

This paper demonstrates a solid-state quantum memory using the nuclear spin of 31P donors in silicon, achieving long coherence times and high fidelity transfer between electron and nuclear spins for quantum information storage.

Contribution

It presents the first coherent transfer of quantum states between electron and nuclear spins in silicon, establishing 31P nuclear spins as effective quantum memories with long coherence times.

Findings

Quantum state transfer between electron and nuclear spins achieved.

Nuclear spin coherence lifetime exceeds one second at 5.5K.

Overall storage and readout fidelity around 90%.

Abstract

The transfer of information between different physical forms is a central theme in communication and computation, for example between processing entities and memory. Nowhere is this more crucial than in quantum computation, where great effort must be taken to protect the integrity of a fragile quantum bit. Nuclear spins are known to benefit from long coherence times compared to electron spins, but are slow to manipulate and suffer from weak thermal polarisation. A powerful model for quantum computation is thus one in which electron spins are used for processing and readout while nuclear spins are used for storage. Here we demonstrate the coherent transfer of a superposition state in an electron spin 'processing' qubit to a nuclear spin 'memory' qubit, using a combination of microwave and radiofrequency pulses applied to 31P donors in an isotopically pure 28Si crystal. The electron spin state can be stored in the nuclear spin on a timescale that is long compared with the electron decoherence time and then coherently transferred back to the electron spin, thus demonstrating the 31P nuclear spin as a solid-state quantum memory. The overall store/readout fidelity is about 90%, attributed to systematic imperfections in radiofrequency pulses which can be improved through the use of composite pulses. We apply dynamic decoupling to protect the nuclear spin quantum memory element from sources of decoherence. The coherence lifetime of the quantum memory element is found to exceed one second at 5.5K.