Resolving magnon number states in quantum magnonics

TL;DR

This paper demonstrates the detection of individual magnon number states in a ferromagnet coupled to a superconducting qubit, advancing quantum magnonics by enabling precise magnon state probing and potential quantum information applications.

Contribution

It reports the first observation of magnon number states in a solid-state system using a superconducting qubit in the strong dispersive regime, a significant step forward in quantum magnonics.

Findings

Resolved magnon number states spectroscopically

Detected magnetic dipole changes equivalent to a single spin flip among over 10^19 spins

Achieved strong dispersive coupling enabling quantum magnonic control

Abstract

Collective excitation modes in solid state systems play a central role in circuit quantum electrodynamics, cavity optomechanics, and quantum magnonics. In the latter, quanta of collective excitation modes in a ferromagnet, called magnons, interact with qubits to provide the nonlinearity necessary to access quantum phenomena in magnonics. A key ingredient for future quantum magnonics systems is the ability to probe magnon states. Here we observe individual magnons in a millimeter-sized ferromagnet coherently coupled to a superconducting qubit. Specifically, we resolve magnon number states in spectroscopic measurements of a transmon qubit with the hybrid system in the strong dispersive regime. This enables us to detect a change in the magnetic dipole of the ferromagnet equivalent to a single spin flipped among more than $10^{19}$ spins. The strong dispersive regime of quantum magnonics opens up the possibility of encoding superconducting qubits into non-classical magnon states, potentially providing a coherent interface between a superconducting quantum processor and optical photons.