Deterministic Bell state measurement with a single quantum memory

Abstract

Any quantum information system operates with entanglement as a resource, which should be deterministically generated by a joint measurement known as complete Bell state measurement (BSM). The determinism arises from a quantum nondemolition measurement of two coupled qubits with the help of readout ancilla, which inevitably requires extra physical qubits. We here demonstrate a deterministic and complete BSM with only a nitrogen atom in a nitrogen-vacancy (NV) center in diamond as a quantum memory without reliance on any carbon isotopes by exploiting electron‒nitrogen (14N) double qutrits at a zero magnetic field. The degenerate logical qubits within the subspace of qutrits on the electron and nitrogen spins are holonomically controlled by arbitrarily polarized microwave and radiofrequency pulses via zero-field-split states as the ancilla, enabling the complete BSM deterministically. Since the system works under an isotope-free and field-free environment, the demonstration paves the way for realizing high-yield, high-fidelity, and high-speed quantum repeaters for long-haul quantum networks and quantum interfaces for large-scale distributed quantum computers.