Quantum sensing of magnons with a superconducting qubit

Opportunities for quantum sensing of quanta of collective spin excitations in a ferromagnet, called magnons, are now possible thanks to the demonstrations of both strong resonant and dispersive couplings between a superconducting qubit and the uniform precession mode, or Kittel mode, of a ferromagnetic sphere [1,2]. Based on operations on the qubit conditional on the state of the Kittel mode, single-shot detection of a single magnon with an efficiency reaching about 50% is demonstrated using the protocol of Ref. [3]. The detection efficiency is mainly limited by the qubit readout fidelity. Furthermore, a magnon detection sensitivity of about 10^-3 magnons/√Hz is demonstrated using a standard Ramsey interferometry technique. These two complementary quantum sensing methods could find applications in quantum technologies based on magnonics and the detection of axions in dark matter searches.

[1] Y. Tabuchi et al., Science 349, 405-408 (2015).
[2] D. Lachance-Quirion et al., Science Advances 3, e1603150 (2017).
[3] A. Narla et al., Physical Review X 6, 031036 (2016).