Phonon- and magnon-mediated decoherence of a magnonic qubit

TL;DR

This paper analyzes the decoherence mechanisms of magnonic qubits in ferromagnetic insulators, focusing on magnon-phonon and magnon-magnon interactions, and provides explicit rate expressions using advanced theoretical methods.

Contribution

It introduces a combined Bloch-Redfield and Keldysh approach to explicitly calculate decoherence rates for magnonic qubits, highlighting the effects of size, damping, and material properties.

Findings

Decay into two phonons is suppressed at zero temperature in yttrium-iron-garnet.

Dephasing rate increases as the magnet size decreases and with lower damping.

Magnon-phonon interactions are the dominant relaxation process at zero temperature.

Abstract

We investigate the decoherence of magnonic qubits in small ferromagnetic insulators and compute the relaxation and dephasing rates due to magnon-phonon and magnon-magnon interactions. We combine a Bloch--Redfield description with Keldysh non-equilibrium field theory to find explicit expressions for the rates. For a quadratic dispersion and assuming a uniform mode defines the qubit, we find that decay into two phonons is the only allowed relaxation process at zero temperature. The low resonance frequency and heavy unit cell strongly suppress this process in yttrium-iron-garnet. We also find that the dephasing rate scales with the inverse of size and damping of the magnet, and could become large for small and clean magnets. Our calculation thus provides additional insight into the viability of magnon-based quantum devices.