Coherent Raman Beat Analysis of the Hyperfine Sublevel Coherence Properties of 167Er3+ Ions Doped in an Y2SiO5 Crystal

Kaoru Shimizu and Daisuke Hashimoto
Optical Science Laboratory

 A metastable Λ-type three-level system of atoms and ions provides us with a medium with which to demonstrate electromagnetically induced transparency (EIT) and coherent population trapping (CPT). EIT and CPT media have attracted a lot of attention in the field of quantum optics, where they are expected to be used for devices capable of converting quantum information between light and matter. An ensemble of Pr3+ ions doped sparsely in a host crystal Y2SiO5 cooled to a cryogenic temperature (< 4 K) has been studied intensively for demonstrating EIT and CPT. Recently, a Λ-type three-level system was also identified for 167Er3+ ions doped in an Y2SiO5 crystal, which had an optical transition at a telecom wavelength and a set of hyperfine sublevels in ground and excited states [1]. Although the measured population life time t1 of the sublevels is approximately 100 ms at 1.5 K, the phase dissipation time t2 of the sublevel coherence has not yet been examined. To examine the feasibility of an Er3+:Y2SiO5 crystal as a solid state medium for EIT and CPT, we must clarify the t2 value.
 At first glance, Raman echoes or optically detected spin echoes appear suitable for measuring the t2 value provided that the optical coherence time T2 or the sublevel population life time t1 is long enough. However, an Er3+:Y2SiO5 crystal cannot satisfy the conditions for the above echo techniques. This is because an Er3+ ion is a Kramers ion and has a half-integer electronic spin, which results in a large average magnetic moment and makes the quantum state being sensitive to the fluctuating local magnetic field. In contrast, coherent Raman beat (CRB) spectroscopy is insensitive to the T2 and t1 values and worth employing if we can cope with inhomogeneous dephasing in the signal.
 By comparing carefully with the effects of sublevel inhomogeneous broadening sub on the various Raman signals (Fig. 1) that are obtained with different Er3+ ion dopant concentrations, we can estimate the t2 value even when t2 > 1/sub [2]. The ground-state t2 values of the Λ-type three-level system identified in the 167Er3+ ions are approximately 12 and 50 µs at 2.3 K when the dopant concentrations are 0.005 (Fig. 2) and 0.001 at%, respectively. These t2 values are long enough to realize EIT and CPT with an Er3+:Y2SiO5 crystal at a wavelength of 1.5 µm.

Fig. 1. Experimental configuration of the CRB spectroscopy. Fig. 2. Sublevel phase relaxation rate 1/t2.