Entanglement in Photonic Crystal Cavities

Quantum entanglement is an essential resource in quantum information science, such as in quantum computation and quantum cryptography. It has been realised in various environments, even at ambient conditions, but often complex experimental setups make them undesirable for use in distributed networks. Instead, optical cavities have been proposed as a key to harness both local and global entanglement across a network. Entanglement can be generated, processed, and stored locally in quantum nodes via light-matter strong coupling, although the integration of many proficient local nodes into a widespread network, connected by photons, remains an outstanding challenge.

Here, we design photonic crystal waveguides that overcome these challenges by commanding unprecedented optical confinement (Q = 1 x10⁷ and V < 0.6 λ³) while operating at 780 nm for strong coupling with cold atoms (e.g. ⁸⁷Rb). Our designs exist deep into the strong coupling regime, unlike most other cavities, and via the entropy dynamics of two emitters we demonstrate local multipartite entanglement. We further show, due to the extreme light confinement, that entangled states are very robust to fluctuations and displacement of the trapped atoms. Most importantly though, the waveguide nature of our designs allows us to scale up to an unprecedented number of qubits and are ideal for constructing large quantum networks, where both local and remote entanglement can be realized.