# Bipartite entanglement in a nuclear spin register mediated by a quasi-free electron spin

## TL;DR

Researchers created a three-qubit nuclear spin register in diamond and demonstrated bipartite entanglement between two 13C nuclei using a silicon-vacancy electron spin.

## Contribution

This work demonstrates nuclear spin entanglement in a diamond-based quantum register using a silicon-vacancy center as a mediator.

## Key findings

- A three-qubit 13C nuclear spin register was entangled via a silicon-vacancy electron spin.
- The electron spin’s long lifetime enabled sensing of nuclear-nuclear couplings at low frequencies.
- A nuclear spin conditional phase-gate mediated bipartite entanglement without dynamic decoupling.

## Abstract

Quantum networks will rely on photons entangled to robust, local quantum registers for computation and error correction. We demonstrate control of and entanglement in a fully connected three-qubit 13C nuclear spin register in diamond. The register is coupled to a quasi-free electron spin-1/2 of a silicon-vacancy center (SiV). High strain decouples the SiVs electron spin from spin-orbit interaction reducing the susceptibility to phonons at liquid helium temperature. As a result, the electron spin lifetime of hundreds of milli seconds enables sensing of nuclear-nuclear couplings down to few hertz. To detect and control the register we leverage continuous decoupling using shaped, low-power microwave and direct radio frequency driving. Furthermore, we implement a nuclear spin conditional phase-gate on the electron spin to mediate bipartite entanglement. This approach presents an alternative to dynamically decoupled nuclear spin entanglement, not limited by the electron spin-1/2’s nature, opening up new avenues to an optically-accessible, solid-state quantum register.

Group-IV color centers in diamond are promising for quantum networks, but despite progress on electron-nuclear spin registers in these systems, nuclear spin entanglement has remained unexplored. Here the authors demonstrate entanglement of two 13C nuclear spins in a three-qubit register coupled to a silicon-vacancy electron spin.