State Tomography of Itinerant Magnons

State tomography is a tool that can fully identify a state, such as the state of a qubit, and has been extensively used as a major technique in quantum information. Magnon state tomography has thus far been demonstrated only for stationary (non-propagating) Kittel magnons in a superconducting qubit–magnon coupled system [1] (quantum regime) and a heavy-metal/ferromagnet heterostructure [2] (classical regime), both via the Wigner function. Here, we present state tomography of itinerant (propagating) magnons via the Husimi Q function. We set up a microwave heterodyne detection circuit, which is commonly used in circuit quantum electrodynamics (cQED), including a magnonic waveguide made of yttrium iron garnet (YIG). We collected coherently driven, microwave signals and constructed the Husimi Q function for over-the-air transmission of microwaves, magnon-mediated microwaves, and thermal noises. We observe that the distribution of the magnon part exhibits elongated phase fluctuations, which might be attributed to a squeezed state or Kerr-type evolution of itinerant magnons. State tomography of itinerant magnons will be an essential tool for advancing both quantum magnonic applications toward quantum information [3] and applications of magnon-based logic devices, such as magnon-based neuromorphic computer or Ising machine.

[1] D. Xu et al., Phys Rev Lett, 130 193603 (2023).

[2] T. Hioki et al., Phys. Rev. B, 104 L100419 (2021).

[3] Z.H. Jiang et al., Appl. Phys. Lett., 123 130501 (2023).