Entanglement in a Solid State Spin Ensemble

TL;DR

This paper demonstrates the on-demand creation and verification of entanglement between electron and nuclear spins in silicon, a crucial step for silicon-based quantum computing, using high-fidelity operations on a massive scale.

Contribution

It reports the first experimental generation of high-fidelity entanglement in a solid-state spin ensemble, enabling scalable quantum information processing in silicon.

Findings

Achieved 98% fidelity in entanglement verification.

Generated entanglement simultaneously in 10^10 spin pairs.

Used hyperpolarisation and density matrix tomography for state verification.

Abstract

Entanglement is the quintessential quantum phenomenon and a necessary ingredient in most emerging quantum technologies, including quantum repeaters, quantum information processing (QIP) and the strongest forms of quantum cryptography. Spin ensembles, such as those in liquid state nuclear magnetic resonance, have been powerful in the development of quantum control methods, however, these demonstrations contained no entanglement and ultimately constitute classical simulations of quantum algorithms. Here we report the on-demand generation of entanglement between an ensemble of electron and nuclear spins in isotopically engineered phosphorus-doped silicon. We combined high field/low temperature electron spin resonance (3.4 T, 2.9 K) with hyperpolarisation of the 31P nuclear spin to obtain an initial state of sufficient purity to create a non-classical, inseparable state. The state was verified using density matrix tomography based on geometric phase gates, and had a fidelity of 98% compared with the ideal state at this field and temperature. The entanglement operation was performed simultaneously, with high fidelity, to 10^10 spin pairs, and represents an essential requirement of a silicon-based quantum information processor.