Sensing magnetization oscillation in quantum regime*

Quanta of magnetization oscillations, i.e., magnons, are essential ingredients in spintronics technology. Although their characteristics have been investigated for a long time, the behavior in the quantum regime, where the number of thermal excited magnons is nearly zero, is still unknown. Here we demonstrate ultra-sensitive sensing of magnons using a superconducting qubit. Transmon-type qubits, which are formed by two electrodes shunted by Josephson junctions, have dipole antennas in their structures and thus they couple to surrounding electromagnetic fields. Owing to their huge dipole moments, the qubits can detect a change in microwave signal to a single photon level. We exploit such feature for sensing the magnetization oscillation in a magnet. We use a microwave cavity to induce an effective coupling between them; both the qubit and the magnetization couple with the same microwave field mode but through different components [1]. With an appropriate frequency detuning between the qubit and a magnetization oscillation mode, the qubit frequency shifts depending on the number of magnons in the oscillation mode. The peak shift is discretized in the limit where the qubit linewidth is narrower than the shift for a single magnon, so that we can count magnons in the oscillation mode to a single magnon level. We experimentally show that the coherently excited magnetization oscillation obeys the Poissonian magnon number distribution [2]. Our ultra-sensitive sensing method provides a powerful tool for magnetization oscillation sensing as well as quantum information processing.
[1] Y. Tabuchi et al., Science 349, 405-408 (2015).
[2] D. Lachance-Quirion et al., Science Advances 3, e1603150 (2017).