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Single-Photon Microwave Devices: era of quantum optics outside cavities

Final Report Summary - SINGLEOUT (Single-Photon Microwave Devices: era of quantum optics outside cavities)

This ERC starting grant project aims to expand the power of microwave photons witnessed in resonators to propagating photons in waveguides. The cornerstone is the design and implementation of a detector for microwave photons. In the future, the detector will allow for the single-shot measurement of the photon state in the waveguide in a similar fashion as the photon detectors are routinely used in optical quantum computing. Thus together with the single-photon sources and phase sifters, the detector will be a critical step towards quantum information processing with microwave photons.

In addition to opening this novel field in physics, the detector can be utilized in the characterization of microwave components and devices at ultra-high sensitivities. In this project, we implemented a platform for such characterization and built circuit elements to control single microwave photons. Namely, we have realized a detector design which allows for broad-band microwave detection at extreme sensitivities. We improved the energy resolution of thermal detectors by a factor of 14 and we have recently observed that the heat conduction out of our detector element is only one percent of the so-called quantum of thermal conductance. We are still improving the detector noise level and are optimistic in being able to detect single microwave photons in near future. We also implemented a cryogenic thermal photon source to create microwave photons and a tunable phase shifter to realize quantum gates for propagating microwave qubits.

In addition to the photon detector, we have been able to observe photonic heat conduction across macroscopic distances in superconducting waveguides. These observations lay the foundation for the integration of normal-metal components into the framework of superconducting quantum information processors to work in analogy with cooling fins in a normal room-temperature processor. However, the coolers in quantum computers have to be tunable since they should not disturb the quantum system during coherent operation. Fortunately, we found a very efficient and convenient way to implement such tunable coupling by employing photon-assisted electron tunneling. This led to the experimental realization of the first quantum-circuit refrigerator which can directly cool electric quantum devices at will.

Much of the technology we have created when pursuing this project has already been commercialized with a private company Aivon. This technology is openly available for anybody through Aivon web store. In future, we seek for further opportunities in the commercialization of the developed microwave detector and the quantum-circuit refrigerator.