We describe experiments and theory showing the generation of counterpropagating paired photons with coherence times of about 50 ns and waveforms that are controllable at a rudimentary level. Using cw lasers, electromagnetically induced transparency and cold 87Rb atoms we generate paired photons into opposing single-mode optical fibers at a rate of approximately 12 000 pairs per second.

We describe the temporal and spatial dynamics of propagating electromagnetically induced transparency pulses in an optically thick medium. Results include pulse velocities as slow as c͞165 with 55% transmission, strong-probe-field effects, and the observation of near diffraction-limited transmitted beam quality.

This Letter describes the generation of biphotons with a temporal length that can be varied over the range of 50-900 ns, with an estimated subnatural linewidth as small as 0.75 MHz. We make use of electromagnetically induced transparency and slow light in a two-dimensional magneto-optical trap with an optical depth as high as 62. We report a sharp leading edge spike that is a Sommerfeld-Brillouin precursor, as observed at the biphoton level.

We use the Stokes photon of a biphoton pair to set the time origin for electro-optic modulation of the wave function of the anti-Stokes photon thereby allowing arbitrary phase and amplitude modulation. We demonstrate conditional single-photon wave functions composed of several pulses, or instead, having gaussian or exponential shapes.PACS numbers: 42.50. Gy, 32.80.Qk, 42.50.Ex, 42.65.Lm This letter demonstrates how single photons may be modulated so as to produce photon wave functions whose amplitude and phase are functions of time. The essential feature of this work is the use of one photon of a biphoton pair that is generated by spontaneous parametric down-conversion to establish the time origin for the modulation of the second photon. This is done by using electromagnetically induced transparency and slow light to produce time-energy entangled biphotons with pulse lengths of several hundred ns, and therefore, very long as compared to the temporal resolution of single photon counting modules (about 40 ps). Once the time origin is established, the photon waveform may be modulated in the same manner as one modulates a classical pulse of light. For example, the single-photon waveform may be phase, frequency, amplitude, or even digitally modulated, with the maximum modulation frequency limited by the resolution of the detection of the first photon.As shown in Fig. 1, we use cw pump and coupling lasers to generate time-energy entangled pairs of Stokes and anti-Stokes photons that propagate in opposite directions and are collected in single mode fibers. The detection of a Stokes photon at D 1 sets the time origin for firing the function generator that drives the electro-optic modulator (EOM) that modulates the wave function of the anti-Stokes photon. This latter photon is incident on the beam splitter where it is detected by D 2 or D 3 . As shown in the following, we generate single photons whose modulated waveform is two rectangular pulses, is Gaussian or is a time reversed exponential.The method demonstrated in this letter might be used to optimally load a single photon into an optical cavity [1], or instead, to study the transient response of atoms to different single photon waveforms. In the context of light-matter interfaces, it may improve the efficiency of storage and retrieval of single photons in atomic ensembles [2]. For quantum information applications, both amplitude and phase modulators could be used to allow full control over the single photon waveforms. For example, one could a construct a single photon waveform that is a train of identical pulses with information encoded into the relative phase difference between consecutive pulses [3].The generation of single photons with controlled wave- forms has been demonstrated earlier by using the techniques of cavity-QED, i.e., by coupling a single trapped ion to a high Q cavity and using an acousto-optic modulator to shape a pumping laser that is tuned close to the resonant transition [4]. In related work Rempe and colleagues use a single Rb atom, again i...

We demonstrate efficient nonlinear frequency conversion in atomic Pb vapor under conditions where the linear susceptibility and the effective nonlinear susceptibility are of the same order. This is accomplished by using electromagnetically induced transparency to prepare a near-maximal atomic coherence on a Raman transition. This strongly driven transition is used to convert an intense laser beam from 425 to 293 nm with an efficiency of ϳ40%. [S0031-9007(96)01644-4]