Spectroscopy of Single Rare Earth Solid-State Qubits

Rare-earth-doped crystals are excellent hardware for quantum storage of optical information. In these quantum memories the quantum state of a photon is stored in an electron or nuclear spin created in an ensemble of spins. This type of memory is an essential ingredient of quantum repeaters and quantum computing protocols based on linear optics. In spite of progress made with ensembles of rare earth ions, the detection and manipulation of individual ions is still challenging. If the rare earth ions can be detected in single-ion level, it offers a new approach to investigate the fundamental spectroscopic properties of solid-state rare earth dopants.
In this talk, I will introduce the recent progresses of optical detection, coherent manipulation and optical readout of single rare earth ions in solids. The first solid proof of optical detection of single rare earth ions, namely single Pr ions in YAG crystal will be present [1]. Followed after this result, the identification of single Ce ions in crystals will be introduced. Not only the optical detection, but the coherent manipulation of single Ce ions will be shown including: high fidelity optical initialization, efficient coherent manipulation, and optical readout of a single electron spin of Ce3+ ion in a YAG crystal. Under dynamic decoupling, spin coherence lifetime reaches T2 = 2 ms and is almost limited by the measured spin-lattice relaxation time T1 = 4 ms. Strong hyperfine coupling to aluminum nuclear spins suggests that cerium electron spins can be exploited as an interface between photons and long-lived nuclear spin memory [2]. In the talk I will also present the all-optical generation of coherent dark state of a single Ce3+ ion in YAG. The dark state was formed under the condition of coherent population trapping. In addition, high-resolution spectroscopic studies of single Ce ions have been performed. They revealed narrow and spectrally stable optical transitions between the spin sublevels of the ground and excited optical states, indicating the feasibility of interfacing single photons with a single electron spin of a cerium ion. Combined with high brightness of Ce3+ emission and a possibility of creating photonic circuits out of the host material, this makes cerium spins an interesting option for integrated quantum photonics [3].