To realise large-scale quantum computers, isolated processors must be connected into distributed systems. This requires new interfaces capable of coherently linking superconducting circuits operating at cryogenic temperatures with optical networks that function at room temperature.

In 2023, EQUS researchers in the Quantum Integration Laboratory, led by Chief Investigator John Bartholomew (University of Sydney), developed theoretical and experimental approaches to achieve precisely this. Their work focuses on converting microwave photons – used in superconducting qubits – into optical photons suitable for transmission in fibre networks.

The team explored solid-state rare-earth-ion crystals as a medium for this conversion. These crystals exhibit transitions at both microwave and optical frequencies, making them strong candidates for hybrid quantum interfaces. PhD student Gargi Tyagi, working with Chief Investigator Andrew Doherty and Associate Investigator Thomas Smith, proposed a source of entangled microwave–optical photon pairs using magneto-optical nonlinearities. This design supports pulsed operation, long-term quantum memory, and multimode entanglement – features valuable for scalable architectures.

In parallel, the team collaborated on an integrated platform combining superconducting circuits, erbium atoms and nanophotonic resonators on a single chip – an important step towards practical quantum transducers.

This work supports the goals of EQUS’s Quantum Engines and Instruments program, which aims to integrate disparate components into functioning quantum machines. By developing tools that connect superconducting processors to optical networks, the team is addressing a key challenge in scaling quantum computing. These hybrid interfaces also have broader applications in quantum communication and sensing, and will be essential for real-world deployment of quantum technologies.