Electron Spin Coherences in Rare-Earth Optically Excited States for Microwave to Optical Quantum Transducers (1802.03354v2)

Abstract: Efficient and reversible optical to microwave coherent transducers are required to enable entanglement transfer between superconducting qubits and light for quantum networks. Rare-earth-doped crystals that possess narrow optical and spin transitions are a promising way to implement these devices. Current approaches use ground-state electron spin transitions that have coherence lifetimes ($T_2$) often limited by spin flip-flop processes and/or spectral diffusion, even at very low temperatures. Here, we investigate spin coherence in an optically excited state of an Er$^{3+}$:Y$_2$SiO$_5$ crystal at temperatures from 1.6 to 3.5 K and under a weak 8.7 mT magnetic field. Spin coherence and population lifetimes of up to 1.6 $\mu$s and 1.2 ms, respectively, are measured by 2- and 3-pulse optically-detected spin echo experiments. Analysis of the dephasing processes suggest that ms $T_2$ can be reached at lower temperatures for the excited-state spins, whereas ground-state spin states could be limited to a few $\mu$s due to resonant interactions with the other Er$^{3+}$ spins in the lattice (spin diffusion). We propose a quantum transducer scheme with the potential for close to unit efficiency that exploits the specific advantages offered by the spin states of optically excited electronic energy levels.