Today’s information technology seamlessly combines on-chip electronics with light signals that transport information in optical fibres or with microwave signals generated by a WiFi router. Similar adapters are required for future quantum technologies, to convert information and to allow distribution between different systems.
But creating adapters for quantum systems is much more challenging than for classical ones. For example, superconducting quantum systems are a leading technology for quantum information processing and computing, but are incompatible in many ways with quantum communication systems based on light in optical fibres. They can be used to create very complex photonic states at microwave frequencies, but cannot couple to long-distance optical networks without an efficient, coherent and low-noise interface between microwave and optical photons.
Hybrid technologies would make it possible to combine quantum resources from the microwave domain (such as superconducting qubits) to those from the optical domain (such as long-distance quantum communication networks).
This combination could be a very powerful basis for large-scale quantum computers. EQUS researchers in the Quantum Integration Laboratory—led by Chief Investigator John Bartholomew—are exploring a particular solution for hybrid quantum technologies that would allow quantum computers based on superconducting electronics at ultralow temperatures to be linked by room-temperature optical fibre networks.
Microwave–optical transduction
To convert a signal from a microwave frequency (5 gigahertz) to an optical frequency (200 terahertz), at least four orders of magnitude in energy must be bridged. Energy input is therefore required, when working at the quantum level, to convert a single microwave photon to a single optical photon. In general, the weaker the conversion strength, the more energy needs to be added to get the single photon out with high probability. However, most quantum systems that use microwave signals need to maintain temperatures of around 10 millikelvin, and the more energy input to mediate the conversion, the more the system starts to heat up.
To link superconducting quantum computers using light signals in room-temperature optical fibres, high-performance adapters that efficiently convert between microwave and optical signals are needed, as are sources that provide ready-to-go entanglement between microwaves and light. In 2023, the team designed converters and sources of entanglement using atoms embedded in crystals with atomic transitions at microwave and optical frequencies.
In collaboration with Chief Investigator Andrew Doherty and Associate Investigator Thomas Smith, the team—led by PhD student Gargi Tyagi— pioneered a time-dependent theory of hybrid optical–microwave entangled photon sources using magneto-optical nonlinearities in rare-earth-ion crystals. This technique, based on rephased amplified spontaneous emission, is inherently temporally multimode, pulsed-operation and contains an in-built long-term quantum memory, potentially providing advantages over existing three-wave-mixing implementations that use electro-optic nonlinearities.
In related work, in a collaboration led by researchers from Caltech, the team have demonstrated an initial step towards using solid-state atomic platforms for transduction. They integrated superconducting electronics, erbium atoms and nanophotonic resonators in a single on-chip platform.
The future of hybrid quantum technologies based on solid-state systems
The conversion efficiency for solid-state rare-earth-ion transducers is currently many orders of magnitude below state-of-the-art implementations in other systems. However, if higher-efficiency designs could be demonstrated, the suite of quantum memory technologies available in rare-earth systems could be leveraged for hybrid quantum computing.
Rare-earth systems provide one avenue to scaling up superconducting quantum computers and allowing them to escape the confines of ultralow-temperature dilution refrigerators. If successful, they may lead to larger, more useful quantum computers, and potentially a route to distributed quantum computing. Hybrid quantum technologies also have application in sensing very weak low-frequency signals and creating photonic resources for optical quantum computing or quantum networks.
In 2024, the team will translate the simple models for hybrid optical–microwave photon sources into more complex theories that more accurately describe the loss and noise in the system. In parallel, they will start to fabricate devices to measure critical device parameters, which will ultimately determine the performance of the entanglement source.
By combining their work on solid-state memory and transducer technologies, the team envisage being able to combine the best parts of microwave and optical systems into future advanced hybrid quantum technologies. EQUS offers the team the chance to explore such integration with Chief Investigators Arkady Fedorov’s and Xanthe Croot’s work on superconducting quantum devices, and Chief Investigator Jacqui Romero’s work on quantum optics.
This story was first published in the 2023 EQUS annual report, and was written by Kristen Harley.