| 研究実績の概要 |
The lifetime of magnons in YIG at millikelvins temperatures is currently limiting the development of quantum magnonics. It had been previously identified that the magnon lifetime is limited by so-called two-levels systems (TLSs) in the ferromagnetic crystal. While the experimental signature of TLSs was observed in 2014, the microscopic origin of these TLSs was still unknown, greatly limiting the ability of researchers to enhance the lifetime of magnons for quantum technologies based on quantum magnonics. An investigation of TLSs in YIG started under the fellowship has led to the discovery that these systems can have very long lifetimes, up to about ten seconds. Such long lifetimes are common for spins in crystals at millikelvin temperatures but had never been observed in a magnetically-ordered crystal. These results provide the first experimental evidence that the TLSs in quantum magnonics are probably spin degrees of freedom of impurities in YIG. Furthermore, we have demonstrated the partial suppression of the relaxation of magnons in the quantum regime. Indeed, thanks to the lifetime of the TLSs being much longer than the lifetime of magnons and superconducting qubits, their contribution to the decay of magnons can be partially suppressed while having both the magnetostatic mode and the qubit very close to their ground state, as required for applications of quantum magnonics.
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