# Entanglement between a diamond spin qubit and a photonic time-bin qubit at telecom wavelength

## TL;DR

This paper demonstrates quantum entanglement between a diamond NV center spin qubit and a telecom-wavelength photon, enabling integration of solid-state qubits with fiber-optic quantum networks.

## Contribution

It reports the first entanglement transfer from a visible-wavelength spin qubit to a telecom-band photon using quantum frequency conversion.

## Key findings

- Bell state fidelity of 0.77 +/- 0.03 achieved
- Entanglement verified after frequency conversion
- Progress towards fiber-based quantum networks

## Abstract

We report on the realization and verification of quantum entanglement between an NV electron spin qubit and a telecom-band photonic qubit. First we generate entanglement between the spin qubit and a 637 nm photonic time-bin qubit, followed by photonic quantum frequency conversion that transfers the entanglement to a 1588 nm photon. We characterize the resulting state by correlation measurements in different bases and find a lower bound to the Bell state fidelity of F = 0.77 +/- 0.03. This result presents an important step towards extending quantum networks via optical fiber infrastructure.