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arXiv:1205.6585 · Physical Review A

Generation of correlated photon pairs in different frequency ranges

Fernando Oster, Christoph H. Keitel, Mihai Macovei

Published Jun 12, 2012• 25 citations
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Abstract

The feasibility of generating correlated photon pairs at variable frequencies is investigated. For this purpose we consider the interaction of an off-resonant laser field with a two-level system possessing broken inversion symmetry. We show that the system generates nonclassical photon pairs exhibiting strong intensity-intensity correlations. The intensity of the applied laser tunes the degree of correlation while the detuning controls the frequency of one of the photons, which can be in the terahertz domain. Furthermore, we observe the violation of a Cauchy-Schwarz inequality characterizing these photons.

DOI
10.1103/physreva.85.063814

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The feasibility of generating correlated photon pairs at variable frequencies is investigated. For this purpose we consider the interaction of an off-resonant laser field with a two-level system possessing broken inversion symmetry. We show that the system generates nonclassical photon pairs exhibiting strong intensity-intensity correlations. The intensity of the applied laser tunes the degree of correlation while the detuning controls the frequency of one of the photons, which can be in the terahertz domain. Furthermore, we observe the violation of a Cauchy-Schwarz inequality characterizing these photons.
Non-classical paired photons are generated by a four-wave mixing process in a far-detuning three-level system with cold atoms. A violation of the Cauchy-Schwartz inequality of a factor of 310 is observed. This phenomenon shows that paired photons have a non-classical correlation. The experimental results are compared with theoretical results obtained using perturbation theory. The oscillation frequencies of the two-photon intensity correlation functions are in reasonable agreement with the effective Rabi frequencies of the coupling laser. However, we find that the dephasing rates (or decay rates) observed are far larger than the theoretical values.
A scheme for the generation of nonclassical pairs of photons in atomic vapors is proposed. The scheme exploits the fact that the cross correlation of the emission of photons from the extreme transitions of a four-level cascade system shows antibunching, unlike the case of the three-level cascade emission, which shows bunching. The Cauchy-Schwarz inequality, which is the ratio of the cross-correlation to the autocorrelation function, in this case is estimated to be 103−−106{10}^{3}--{10}^{6}103−−106 for controllable time delay, and is one to four orders of magnitude larger compared to previous experiments. The choice of Doppler-free geometry, in addition to the fact that at three-photon resonance the excitation and deexcitation processes occur in a very narrow frequency band, ensures cleaner signals.
We experimentally generate a non-classical correlated two-color photon pair at 780 and 1529.4 nm in a ladder-type configuration using a hot 85Rb atomic vapor with the production rate of ~10(7)/s. The non-classical correlation between these two photons is demonstrated by strong violation of Cauchy-Schwarz inequality by the factor R = 48 ± 12. Besides, we experimentally investigate the relations between the correlation and some important experimental parameters such as the single-photon detuning, the powers of pumps. We also make a theoretical analysis in detail and the theoretical predictions are in reasonable agreement with our experimental results.
We experimentally prepare the non-classical correlated photon pairs at the wavelengths of 780 and 776 nm via the cascade transition of 5S 1/2 –5P 3/2 –5D 5/2 in a hot 85 Rb atomic ensemble. By measuring the function of cross-correlation and auto-correlation of photons, a violation of Cauchy—Schwarz inequality by a factor of 283 is obtained, which clearly indicates a strong non-classical correlation between the generated photons. We also find that noise photons scattered from pump lasers have a strong effect on the Cauchy—Schwarz inequality factor by changing the intensity of the pump laser, the experimental results are consistent with the theoretical predictions.