Recent advances in quantum magnonics 1-3, e.g., strong magnon-photon coupling 2 and single magnon sources 3, enable study of fundamental magnon properties. It is interesting to use these effects to answer fundamental questions of single magnon decoherence (SMD) mechanisms, understanding of which is necessary to use finite magnon number states. Here, we propose a simple measurement scheme for SMD. It is based on entangling a magnon and a qubit (with a much longer lifetime) with resonant frequencies ω_m and ω_q , respectively, and coupling rate κ. When the detuning Δω=ω_m–ω_q is large (│Δω│»κ), the magnon and qubit states evolve independently. A magnetic field pulse can temporary couple (│Δω│«κ) the system, thereby entangling or unentangling it. The scheme (Fig. 1) is a temporal-domain Mach-Zehnder interferometer and works as follows. Initially, the qubit is excited, and the system is unentangled. A magnetic pulse entangles the system and then it evolves freely for some time τ before another “unentangling” pulse is applied. When τ is comparable to the magnon lifetime, the final qubit population P(τ) depends on the SMD mechanisms. Using the Lindblad quantum master equation 4, we numerically simulated the proposed scheme (Fig. 2). We included two possible SMD mechanisms, namely, damping (Fig. 2a) and dephasing (Fig. 2b) with decoherence rates Γ and Γ′, respectively. The fast oscillations in Fig. 2 are caused by usual quantum interference, while decay of the interference pattern is due to SMD. Note, that the two mechanisms lead to different dependences of P(τ); in particular, dephasing alone does not change the average qubit population. We derived analytical expressions that determine both rates Γ and Γ′ from the measurements of P(τ), even when both processes are present simultaneously (Fig. 2c). In summary, we proposed a simple measurement scheme for SMD, which can be realized using currently available technology and which can be used to study fundamental questions of quasi-particle decoherence at a single quantum level.
References
1 D. D. Awschalom et al., IEEE Trans. Quantum Eng. 2, 5500836 (2021). 2 P. G. Baity et al., Appl. Phys. Lett. 119, 033502 (2021). 3 A. V. Chumak et al., IEEE Trans. Magn. 58, 6, 0800172 (2022). 4 H.-P. Breuer and F. Petruccione, Oxford University Press (2002).

Fig. 1 Proposed SMD measurement scheme.

Fig. 2 Numerical simulation of SMD measurement scheme. (a): damping Γ≈38 μs^-1. (b): dephasing Γ′≈50 μs^-1 . (c): mixture of (a) and (b). Dashed lines: average P