Entanglement in a solid-state spin ensemble

Ensemble entanglement Spin ensembles, such as those used in liquid-state nuclear magnetic resonance, have been important for the development of quantum control methods. But these demonstrations contained no entanglement, an essential element for a quantum information processor. Simmons et al . now report the on-demand generation of entanglement between an ensemble of electron and nuclear spins in phosphorus-doped silicon, simultaneously creating ten billion spin pairs with high fidelity. This fulfils one of the essential requirements for a silicon-based quantum information processor. Spin ensembles, such as those used in liquid-state nuclear magnetic resonance, have been important for the development of quantum control methods. However, these demonstrations contained no entanglement, which is essential for a quantum information processor. This study reports the on-demand generation of entanglement between an ensemble of electron and nuclear spins in phosphorus-doped silicon, simultaneously creating 10 10 spin pairs. Entanglement is the quintessential quantum phenomenon. It is a necessary ingredient in most emerging quantum technologies, including quantum repeaters 1 , quantum information processing 2 and the strongest forms of quantum cryptography 3 . Spin ensembles, such as those used in liquid-state nuclear magnetic resonance 4 , 5 , have been important for the development of quantum control methods. However, these demonstrations contain 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 (3.4 T), low-temperature (2.9 K) electron spin resonance with hyperpolarization of the 31 P 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% relative to the ideal state at this field and temperature. The entanglement operation was performed simultaneously, with high fidelity, on 10 10 spin pairs; this fulfils one of the essential requirements for a silicon-based quantum information processor.