"The field of quantum information arises from a desire to overcome the challenges of solving complex or intractable problems on classical computers by harnessing quantum mechanics to provide efficient and scalable algorithms. Whilst there has been tremendous recent progress in the realisation of small-scale quantum circuits comprising several quantum bits ("qubits''), research indicates that a fault-tolerant quantum computer capable of harnessing the power of quantum mechanics will require a network of thousands of qubits. This goal is presently beyond the reach of any existing implementation based on a single physical qubit type.
Hybrid quantum information processing is an alternative approach that exploits the unique strengths of disparate quantum technologies, and offers a route to overcome the drawbacks associated with of a single-qubit architecture in direct analogy to the design of classical computing hardware. This proposal aims to combine three different technologies:
i) Superconducting circuits, with very fast (10 ns) gate times for fast processing,
ii) Neutral atoms, with long (10 s) coherence times for long lived quantum memory,
iii) Optical photons, for long distance fibre communication,
to create a novel hybrid quantum interface capable of storing, processing and generating highly entangled states of photons for quantum networking and cryptography applications, overcoming the short coherence time associated with the scalable superconducting circuit systems. This also offers applications in quantum metrology for conversion from optical to microwave domain quantum information, making it possible to extend the interface to incorporate a wide range of alternative solid-state based qubits.
The interface relies on use of highly excited Rydberg states, which have incredibly large dipole moments and transitions in the microwave regime, which can resonantly couple to superconducting qubits embedded in planar microwave waveguide cavities. The large Rydberg dipole also leads to strong, controllable interactions between atoms to provide a collective enhancement in the coupling to single photons for efficient storage and retrieval of light.
The first stage of the experiment is to trap spatially addressable atomic ensembles above a superconducting microwave resonator operating at 4 K to demonstrate strong coupling to the waveguide mode, a key milestone for implementing the hybrid interface. The ensembles will then be utilised to perform coherent storage and retrieval of optical photons, as well as generation of single photons using four-wave mixing.
The second stage is to exploit the off-resonant interaction with the cavity to achieve controllable long distance (~1 cm) entanglement between a pair of ensembles trapped within a single microwave resonator. This will then be used to generate entangled photon pairs, exploring the benefits of collective encoding within the ensembles for achieving entanglement in the polarisation degrees of freedom for long-distance cryptographic quantum key distribution. The resulting hybrid quantum interface provides an ideal building block for establishing quantum networks. Long term this can be integrated with existing superconducting qubit technologies, making a significant step towards the realisation of scalable quantum computing."