# Deterministic entanglement between a propagating photon and a singlet--triplet qubit in an optically active quantum dot molecule

## TL;DR

This paper demonstrates deterministic entanglement between a photon and a spin qubit in a quantum dot molecule, showcasing a high overlap with a fully entangled state and introducing a novel heterodyne detection method for energy-encoded photonic qubits.

## Contribution

It reports the first deterministic entanglement between a propagating photon and a spin qubit in a quantum dot molecule, with a new heterodyne detection technique for energy-encoded photonic states.

## Key findings

- Achieved 69.5% overlap with a fully entangled state.
- Demonstrated entanglement between a photon and a spin qubit.
- Developed a heterodyne detection method for color qubits.

## Abstract

Two-electron charged self-assembled quantum dot molecules exhibit a decoherence-avoiding singlet-triplet qubit subspace and an efficient spin-photon interface. We demonstrate quantum entanglement between emitted photons and the spin-qubit after the emission event. We measure the overlap with a fully entangled state to be $69.5\pm2.7\,\%$, exceeding the threshold of $50\,\%$ required to prove the non-separability of the density matrix of the system. The photonic qubit is encoded in two photon states with an energy difference larger than the timing resolution of existing detectors. We devise a novel heterodyne detection method, enabling projective measurements of such photonic color qubits along any direction on the Bloch sphere.