We show that the group velocity of a probe pulse in an ensemble of Λ-type atoms driven by a quantized cavity mode depends on the quantum state-of-the input probe pulse. In the strong-coupling regime of the atom-cavity system the probe group delay is photon-number selective. This can be used to spatially separate the single photon from higher photon-number components of a few-photon probe pulse and thus create a deterministic single-photon source.
We examine electromagnetically induced transparency (EIT), the optical preparation of persistent nuclear spin coherences and the retrieval of light pulses both in a Λ-type and a V-type coupling scheme in a Pr3+:Y2SiO5 crystal, cooled to cryogenic temperatures. The medium is prepared by optical pumping and spectral hole burning, creating a spectrally isolated Λ-type and a V-type system within the inhomogeneous bandwidth of the 3H4 ↔ 1D2 transition of the Pr3+ ions. By EIT, in the Λ-type scheme we drive a nuclear spin coherence between the ground-state hyperfine levels, while in the V-type scheme we drive a coherence between the excited-state hyperfine levels. We observe the cancellation of absorption due to EIT and the retrieval of light pulses in both level schemes. This also permits the determination of dephasing times of the nuclear spin coherence, either in the ground state or the optically excited state.
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