Axel Kuhn1, Jason Smith2, Matthias Keller3, and David M. Lucas1
1 University of Oxford, Department of Physics, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU
2 University of Oxford, Department of Materials, Parks Road, Oxford, OX1 3PH
3 University of Sussex, Department of Physics and Astronomy, Falmer, BN1 9QH
The coherent interconnection of small quantum units to larger networks is a key procedure enabling numerous applications of quantum technology, such as distributed quantum processing, quantum simulation, quantum-enhanced sensing, and quantum communication. Faithful state mapping or remote entanglement of qubits are the basic requirements to be met. In turn, these call for the efficient and reliable interfacing of matter and light to couple the states of stationary and flying qubits. Whilst the direct approach of collecting a fraction of the scattered photons has been good for a proof of principle, high photon losses diminish its success rate. This makes scaling difficult if not impossible, and underpins the need for a refined strategy.
One popular remedy is cavity-enhanced light-matter coupling and single-photon emission, used to eventually demonstrate, realize or implement quantum network links. For a successful implementation of such a quantum interface, an indicative series of steps has to be taken which are largely based on one another:
(1a) Cavity mirror manufacturing and high-finesse dielectric coating.
(1b) Cavity mounting, alignment and vibrational isolation.
(1c) High-bandwidth cavity frequency locking far off resonance with the emitter.
(1d) Efficient coupling of photons from the emitter via the cavity into the output field mode.
(1e) Preparation and sufficient localisation of the emitter within the cavity mode.
(2a) Demonstration of the Purcell effect with a single emitter coupled to a cavity.
(2b) Demonstration of the singleness of emitted photons (Hanbury-Brown Twiss).
(2c) Coherent quantum control of amplitude and phase in triggered photon emissions.
(2d) Efficient and fast initialization of the emitter.
(2e) Proof of photon indistinguishability (Hong-Ou-Mandel type two-photon interference).
(3a) Efficient emission scheme, taking the entire level structure into account.
(3b) Full polarisation, amplitude and phase control of the photons.
(3c) Optimum birefringence or polarisation-mode splitting, mode volume and cooperativity.
(4a) Deterministic emitter-photon entanglement (spin and polarization).
(4b) Quantum networking and photonic quantum processing with multiple cavity photons.
(4c) Entanglement swapping by projective measurements to entangle remote emitters.
(4d) Going beyond pairwise entanglement to prepare non-local cluster states.
(5a) Non-demolition measurement of the intra-cavity emitter state.
(5b) Cavity-mediated quantum gates and entanglement distillation acting on the emitters.
(5c) Cavity-mediated quantum gates acting on reflected photons.
(5d) Hybrid gate operations combining (5b) and (5c).
(6) Cavity-based quantum memory or EIT for photon storage and quantum repeaters.
(7) Intra-cavity one and two qubit quantum gates.
(8a) Reproducible cavities for multiple systems in a quantum network.
(8b) Multiple operational cavities working simultaneously on the same chip.
(9) Real-world compliance (wavelength, distance, scale)
Technical progress has reached different stages, depending on the chosen pathway. Groups implementing new cavities are often at level (1), whereas established ion-cavity systems reach level (2). This is different for neutral atoms, with the state-of-the-art at level (3-7). Solid-state implementations are equally successful, but suffer from additional challenges related to inhomogeneity and yield. Typically it takes four years to move by one level, and past experience suggests any new development needs 12 years to reach a mature stage.
This might seem long at first glance but is well worth the effort. Light-matter interfacing in cavities is the best known technique to seriously tackle most limitations to scalability in any networked implementation of a quantum-technological application. For the most advanced forerunners in the field, very few steps remain to be taken to demonstrate and eventually implement faithful quantum-network links on a larger scale.