Detailed site-selective spectroscopy has been performed as a function of temperature on the 7 F 0 ↔ 5 D 0 transition of Eu 3ϩ :Y 2 SiO 5 for Eu 3ϩ concentrations of 0.02%, 0.1%, 0.5%, and 1%. Time-domain optical dephasing, spectral hole lifetimes, anisotropic absorption coefficients, inhomogeneous linewidths, and fluorescence lifetimes for Eu 3ϩ ions at both crystallographic sites were measured. The temperature dependence of the optical dephasing, transition energy, and linewidth of the 7 F 0 → 5 D 0 absorption was measured and interpreted in terms of Raman scattering of phonons. Photon echo measurements of optical dephasing gave T 2 values as long as 2.6 ms, approaching the limit set by the fluorescence decay time. Spectral hole lifetimes were measured for temperatures from 2 K to 18 K, with observed lifetimes varying from 1 s at 18 K to an estimated value of greater than 20 days at 2 K. Anisotropic absorption coefficients were measured, and an increase in Eu 3ϩ concentration from 0.02% to 7% produced an increase in the inhomogeneous linewidth ⌫ inh from 0.5 GHz to ϳ150 GHz, indicating that Eu 3ϩ doping induces significant strain in the crystal. New determinations of many energy levels of 7 F J multiplets have been made for Jϭ0 to 6.

The influence of the anisotropic Zeeman effect on optical decoherence was studied for the 1.54 m telecom transition in Er 3+ :Y 2 SiO 5 using photon echo spectroscopy as a function of applied magnetic field orientation and strength. The decoherence strongly correlates with the Zeeman energy splittings described by the groundand excited-state g factor variations for all inequivalent Er 3+ sites, with the observed decoherence times arising from the combined effects of the magnetic dipole-dipole coupling strength and the ground-and excited-state spin-flip rates, along with the natural lifetime of the upper level. The decoherence time was maximized along a preferred magnetic field orientation that minimized the effects of spectral diffusion and that enabled the measurement of an exceptionally narrow optical resonance in a solid-demonstrating a homogeneous linewidth as narrow as 73 Hz.

We present the complete Zeeman g tensors for the lowest-energy 4 I 15/2 and 4 I 13/2 states of Er 3+ doped into Y 2 SiO 5 for both crystallographic sites deduced from orientation-dependent optical Zeeman spectroscopy over three orthogonal crystal planes. From these data, principal axes of the g tensors were determined for each crystallographic site. Along axes with maximum values, the effective g factors are 14.65 ͑site 1͒ and 15.46 ͑site 2͒ for the ground state, and 12.97 ͑site 1͒ and 13.77 ͑site 2͒ for the excited state. To minimize optical decoherence and spectral diffusion in device applications and high resolution spectroscopy, special directions for applying an external magnetic field have been found for each site, for which the ground-and excited-state g factors are equal. Among those directions, choices are presented that also maximize the ground-state splittings for all four magnetically inequivalent sites, thus optimizing the prospects for freezing out electron spin fluctuations and reducing decoherence and spectral diffusion significantly.

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Rare-earth-ion-doped solids are promising materials as light-matter interfaces for quantum applications. Europium doped into an yttrium orthosilicate crystal in particular has interesting coherence properties and a suitable ground-state energy-level structure for a quantum memory for light. In this paper we report on spectroscopic investigations of this material from the perspective of implementing an atomic frequency comb (AFC)-type quantum memory with spin-wave storage. For this goal we determine the order of the hyperfine levels in the 7 F 0 ground state and 5 D 0 excited state, and we measure the relative strengths of the optical transitions between these levels. We also apply spectral hole burning techniques in order to prepare the system as a well-defined system, as required for further quantum memory experiments. Furthermore, we measure the optical Rabi frequency on one of the strongest hyperfine transitions, a crucial experimental parameter for the AFC protocol. From this we also obtain a value for the transition dipole moment which is consistent with that obtained from absorption measurements.

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