Spin-photon interface for single rare earth ions

Rare earth ions provide exceptional optical and hyperfine coherence, which makes them promising candidates for quantum information applications, ranging from quantum memories and quantum computing nodes to quantum-nonlinear optics. However, the dipole-forbidden nature of the narrow transitions results in ultra-low emission rates, limiting most experiments to macroscopic ensembles. We want to gain efficient access to individual ions and small ensembles by coupling them to a high finesse optical microcavity. Purcell enhancement can boost the emission rate by several orders of magnitude, thereby making the weak transitions bright and enabling efficient state readout. We use microcavities based on laser-machined optical fibers as mirror substrates, which combine high finesse with small mode cross sections and full tunability. 

The fiber-opposing planar mirror incorporates the rare-earth ion doped nanocrystals in the cavity (Fig.1). Addressing of individual ions is achieved in the spectral domain via a stabilized, narrowband laser. This scheme makes use of the large ratio between the broad inhomogeneous crystal linewidth versus a narrow homogeneous single ion linewidth (Fig.2) opening up the possibility to have efficient access to possibly thousands of qubits contained in one cavity mode.