Hybrid Quantum System of Excitons and Superconductors

In the last 10 years, quantum computing has gone mainstream - with experiments expanding out of university research labs and into industrial R&D, led by technological giants like Google, Microsoft and IBM. Under the bonnet, many of these sophisticated machines rely on superconducting circuits to store and manipulate the quantum information. The operating frequency of these devices is similar to the clock speed of modern classical CPUs - around a few GHz. This frequency, or energy, scale is much, much smaller than that associated with room temperature, and so to get rid of thermal noise and operate in the quantum regime these devices must be cooled to a few thousandths of a degree above absolute zero. While this is possible for a single processor, it is much harder to achieve over the kilometre scales required to build a quantum network.

To overcome this problem, the microwave quantum information needs to be up-converted to an optical signal that can be sent down an optical fibre, or via a satellite. The challenge is to do this efficiently without introducing additional decoherence that might destroy the fragile quantum state. Here we propose to build such a converter using Rydberg excitons - a ``quasi-particle'' with an atom-like spectrum of energy levels that exists inside a semiconducting material called cuprous oxide. Rydberg excitons provide strong coupling to optical and microwave fields and are easily prepared at the ultracold temperatures used in superconducting quantum devices. Our consortium recently became the first group to use Rydberg excitons to map a microwave signal onto light, and in this proposal, we will extend this work into the quantum regime. We will develop the methods required to physically integrate Rydberg excitons and superconducting circuits together, and study ways to maximise the coupling between them, as well as tackling the challenge of reducing optical losses in the conversion process.