量子情報変換に向けた量子マグノニクスアーキテクチャの追求

YIG中磁子在毫凯尔文温度下的寿命目前限制了量子磁振子的发展。以前已经发现，磁振子的寿命受铁磁晶体中所谓的二能级系统(TLSS)的限制。虽然TLS的实验特征是在2014年观察到的，但这些TLS的微观来源仍然未知，这极大地限制了研究人员为基于量子磁振子的量子技术提高磁子寿命的能力。根据该奖学金，对YIG中的TLS进行的一项调查发现，这些系统的寿命可能非常长，最长可达10秒。在毫克尔文温度下，晶体中的自旋寿命如此之长是很常见的，但在磁有序晶体中从未观察到过。这些结果首次提供了量子磁振子中的TLS可能是YIG中杂质的自旋自由度的实验证据。此外，我们还证明了量子体系中磁子弛豫的部分抑制。事实上，由于TLS的寿命比磁子和超导量子比特的寿命长得多，它们对磁子衰变的贡献可以被部分抑制，同时使静磁模式和量子比特都非常接近它们的基态，这是量子磁振子应用所必需的。

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.