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Physical Review A

Effects of dispersion on squeezing and photon statistics of down-converted light

Yuta Takahashi, Jonas Söderholm, Keiichi Hirano, Naoto Namekata, Susumu Machida, Shigehiko Mori, Sunao Kurimura, Shinichi Komatsu, Shuichiro Inoue

Published Apr 1, 2008• 5 citations

Abstract

The high peak intensity achieved with ultrashort pump pulses drastically increases the conversion efficiency in spontaneous parametric down-conversion. However, when ultrashort pulses are used, dispersion limits the degree of squeezing. Here, we investigate the effects of dispersion on both quadrature squeezing and photon statistics by varying the interaction length when pumping a periodically poled lithium niobate crystal with femtosecond pulses. The down-converted light in the 1550 nm telecom band attains its maximum observed squeezing level of 3.1 dB for the shortest interaction length. Our photon-statistics measurements agree with the photon-pair distribution going from thermal to Poissonian as the interaction length is increased. This is explained by considering the dispersion-induced distinguishability of the generated photon pairs.

DOI: 10.1103/physreva.77.043801

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The high peak intensity achieved with ultrashort pump pulses drastically increases the conversion efficiency in spontaneous parametric down-conversion. However, when ultrashort pulses are used, dispersion limits the degree of squeezing. Here, we investigate the effects of dispersion on both quadrature squeezing and photon statistics by varying the interaction length when pumping a periodically poled lithium niobate crystal with femtosecond pulses. The down-converted light in the 1550 nm telecom band attains its maximum observed squeezing level of 3.1 dB for the shortest interaction length. Our photon-statistics measurements agree with the photon-pair distribution going from thermal to Poissonian as the interaction length is increased. This is explained by considering the dispersion-induced distinguishability of the generated photon pairs.

The process of spontaneous parametric down-conversion (SPDC) in nonlinear crystals makes it fairly easy to generate entangled photon states. It has been known for some time that the conversion efficiency can be improved by employing quasi-phase-matching in periodically poled crystals. Using two single-photon detectors, we have analyzed the photon pairs generated by SPDC in a periodically poled lithium niobate crystal pumped by a femtosecond laser. Several parameters could be varied in our setup, allowing us to obtain data in close agreement with both thermal and Poissonian photon-pair distributions.

We present a theoretical and experimental study on the bandwidth of parametric down-converted photons generated from periodically poled lithium niobate (PPLN) crystal pumped by a pulsed laser. By comparison of crystals with different lengths and pump beams of different bandwidths, we demonstrate that the bandwidth of down-converted photons will increase for a broader bandwidth of pump pulse and decrease for a longer crystal, but the influence of crystal length will become weaker along with increase of both crystal length and pump pulse bandwidth. Especially under the conditions of our experiment, the bandwidth almost remains unchanged for longer crystals when pumped by a femtosecond laser. This may be helpful for schemes in which pulsed lasers are used to pump PPLN crystals.

We report the efficient generation of high-gain parametric down-conversion, including pump depletion, with pump powers as low as 100 µW (energies 0.1 µJ/pulse) and conversion efficiencies up to 33%. In our simple configuration, the pump beam is tightly focused into a bulk periodically poled lithium niobate crystal placed in free space. We also observe a change in the photon number statistics for both the pump and down-converted beams as the pump power increases to reach the depleted pump regime. The experimental results are a clear signature of the interplay between the pump and the down-converted beams in highly efficient parametric down-conversion sources.