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Physical Review Letters

Engineering Frequency-Time Quantum Correlation of Narrow-Band Biphotons from Cold Atoms

Young‐Wook Cho, Kwang-Kyoon Park, Jong Chan Lee, Yoon-Ho Kim

Published Aug 5, 2014• 54 citations
Publisher

Abstract

The nonclassical photon pair, generated via a parametric process, is naturally endowed with a specific form of frequency-time quantum correlations. Here, we report complete control of frequency-time quantum correlations of narrow-band biphotons generated via spontaneous four-wave mixing in a cold atomic ensemble. We have experimentally confirmed the generation of frequency-anticorrelated, frequency-correlated, and frequency-uncorrelated narrow-band biphoton states, as well as verifying the strong nonclassicality of the correlations. Our work opens up new possibilities for engineering narrow-band entangled photons for various quantum optical and quantum information applications.

DOI
10.1103/physrevlett.113.063602

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The nonclassical photon pair, generated via a parametric process, is naturally endowed with a specific form of frequency-time quantum correlations. Here, we report complete control of frequency-time quantum correlations of narrow-band biphotons generated via spontaneous four-wave mixing in a cold atomic ensemble. We have experimentally confirmed the generation of frequency-anticorrelated, frequency-correlated, and frequency-uncorrelated narrow-band biphoton states, as well as verifying the strong nonclassicality of the correlations. Our work opens up new possibilities for engineering narrow-band entangled photons for various quantum optical and quantum information applications.
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In this short review some essentials concerning creation and testing of nonclassically correlated photons (biphotons) are given. In the introduction we remind the role which the experimentally produced entangled states have been playing for the foundations of the quantum physics, by witnessing against the model of local hidden variables. The well established sources of biphotons are based on spontaneous parametric down conversion in nonlinear crystals. A popular source with two BBO crystals is described, which generates pairs of photons nearly maximally entangled in polarization. Crystalbased sources rely on intrinsically broadband transitions, therefore thus produced biphotons are also broadband. Additional efforts (like applying optical cavities) are needed to reach narrowband biphotons which would comply with the requirements of some implementations in the quantum communication science. The topical issue of our article is a review of another, more recent approaches based on narrowband transitions between levels in cold atoms. Such method provides naturally narrowband biphotons. First, the principles are given of an atomic source of nonclassically paired photons, which is operated in a pulsed write-read mode. Such source is based on two separated in time Raman transitions triggered successively in two Λ-schemes. Next, cw-mode sources based (mainly) on spontaneous four wave mixing process (SFWM) are presented in a generic four-level scheme. Some underlying physics is sketched and profiles of biphoton correlation functions in the time domain are explained. Among other presented SFWM sources, one proves in testing high degree entanglement of generated biphotons, both in time-frequency and polarization (hyperentanglement).
Generation of frequency-correlated narrowband biphotons from the four-wave mixing process is theoretically analyzed through manipulating optical properties of light propagation inside a cold atomic ensemble. In contrast with the spontaneous parametric down-conversion case, we find a much looser condition, which allows producing such frequency-correlated paired photons. We propose a two-photon temporal conditional correlation measurement to detect these photons. In addition, our results provide other evidence for observing backward pulse propagation from the viewpoint of two-photon correlation measurement.