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 (T2) 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 Er3+:Y2SiO5 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 μs 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 T2 can be reached at lower temperatures for the excited-state spins, whereas ground-state spin states could be limited to a few μs due to resonant interactions with the other Er3+ 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.
A protocol is discussed which allows one to realize a transducer for single photons between the optical and the microwave frequency range. The transducer is a spin ensemble, where the
individual emitters possess both an optical and a magnetic-dipole transition. Reversible frequency conversion is realized by combining optical photon storage, by means of EIT, with the controlled switching of the coupling between the magnetic-dipole transition and a superconducting qubit, which is realized by means of a microwave cavity. The efficiency is quantified by the global fidelity for transferring coherently a qubit excitation between a single optical photon and the superconducting qubit. We test various strategies and show that the total efficiency is essentially limited by the optical quantum memory: It can exceed 80% for ensembles of NV centers and approaches 99% for cold atomic ensembles, assuming state-of-the-art experimental parameters. This protocol allows one to bridge the gap between the optical and the microwave regime so to efficiently combine superconducting and optical components in quantum networks.
We propose a scheme to couple short single photon pulses to superconducting qubits. An optical photon is first absorbed into an inhomogeneously broadened rare-earth doped crystal using
controlled reversible inhomogeneous broadening. The optical excitation is then mapped into a spin state using a series of π-pulses and subsequently transferred to a superconducting qubit via a microwave cavity. To overcome the intrinsic and engineered inhomogeneous broadening of the optical and spin transitions in rare earth doped crystals, we make use of a special transfer protocol using staggered π-pulses. We predict total transfer efficiencies on the order of 90%.