We characterize spectroscopic and coherence properties of the 1451.37 nm excited-state zero-phonon line (ZPL) between the ³F₄ and the ³F₄ manifolds of a thulium-doped yttrium aluminum perovskite (Tm³⁺:YAlO₃) crystal at temperatures around 1.5 K. We measure the absorption spectrum between ³H₆ - ³F₄ and ³F₄ - ³H₄ manifolds, the inhomogeneous broadening of the ³F₄ - ³H₄ (excited-state) ZPL, and the lifetimes of the higher-lying and lower-lying excited states. We also investigate level shifts caused by the quadratic Zeeman interaction as well as spectral hole-burning spectra with varying magnetic fields, providing insights into hyperfine interactions. Using spectral holes again but also optical free induction decays, we assess optical coherence times, finding a maximum of 4.75± 0.07 s at B=2 T and low ion concentration in the ³F₄ level. Our results—the first to demonstrate coherence of an excited-state transition in a rare-earth crystal—suggest the possibility of exploiting such transitions for quantum technology. ... Read more

We report on a quantum interface linking a diamond NV center quantum network node and 795nm photonic time-bin qubits compatible with Thulium and Rubidium quantum memories. The interface makes use of two-stage low-noise quantum frequency conversion and waveform shaping to match temporal and spectral photon profiles. Two-photon quantum interference shows high indistinguishability between converted 795nm photons and the native NV center photons. We use the interface to demonstrate quantum teleportation including real-time feedforward from an unbiased set of 795nm photonic qubit input states to the NV center spin qubit, achieving a teleportation fidelity well above the classical bound. This proof-of-concept experiment shows the feasibility of interconnecting different quantum network hardware. ... Read more