From heat capacity to coherence in ultranarrow-linewidth solid-state optical emitters at subkelvin temperatures

The coherence properties of optical emitters in crystals are crucial for quantum technologies and optical frequency metrology. Cooling to subkelvin temperatures can markedly enhance coherence, making it useful to identify the parameters governing emitter and host crystal behavior in this regime. We investigate a Czochralski-grown europium-doped yttrium orthosilicate crystal, reporting measurements of its heat capacity and optical coherence. Heat capacity not only informs thermal noise limits in metrology schemes but can also reveal two-level systems (TLS) arising from crystal imperfections via a linear-in-temperature term. Below 1 K, where phonon contributions are suppressed, TLS can drive decoherence, leading to a linear broadening of the homogeneous linewidth. From our data, we place an upper bound on the TLS contribution. This, together with constant optical linewidths between 300 mK and 2 K measured via photon-echo lifetimes, is consistent with minimal TLS effects in our sample. A low level of TLS is particularly useful for the performance of optical quantum devices based on doped crystals, since their presence could otherwise limit further improvements in coherence at subkelvin temperatures.