Building the quantum internet - microwave to topical transduction in an electro-optic platform.

There are significant barriers to increasing the number of qubits on a single cryogenic platform to the number required for robust quantum computing. In particular, the cooling power and space requirements of high frequency wiring rapidly out-scale that available in dilution refrigerators. A promising solution is to network together distributed quantum processor nodes of manageable size. Thermal noise in room temperature microwave channels makes it necessary to transfer gigahertz frequency quantum signals to the telecommunications band, at a wavelength of around 1550 nm. Fibre-based networking can then scale the number of qubits by connecting separate qubit modules to form larger quantum systems, and would also allow the construction of a ‘quantum internet’ over longer distances.

Both functions require a device capable of efficient coherent transfer of quantum states between the microwave and telecoms frequencies. A transducer like this could also be used for optically mediated qubit control and readout. This would allow coaxial cables to be replaced by low thermal conductivity optical fibres, reducing the resulting heat load on the cryogenic system. The objective of the group's QTA Start-up Project was to demonstrate room temperature proof-of-principle of an efficient electro-optic transducer architecture. This Main Tranche project builds on this to realise a bidirectional microwave-to-optical transducer operating at < 100 mK. This is expected to demonstrate transduction with finite fidelity.

The transducer will be validated with the UK’s National Physical Laboratory as they develop their QLAN (Quantum Local Area Network). Dr Nicholas Lambert, Dr Florian Sedlmeir and team will also work with the National Institute for Science and Technology (NIST) in Colorado to perform local qubit-to-qubit state transfer. At NIST and elsewhere there is a drive towards “hot” qubits operating around 100 GHz, with significantly lower cooling requirements. Quantum transduction at and above 100 GHz will be explored with the help of Professor Harald Schwefel and Dr Mallika Suresh.

These three parallel schemes of work – transducer optimisation, integration into a QLAN, and demonstration of qubit-to-qubit state transfer – will allow the group to reach its objective: a cryogenically deployable transducer module capable of transferring quantum states between microwave and optical frequencies in a ‘real world’ environment.