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Photonics Research

Applying a mixed light field generated from a two-level atomic ensemble to two-photon interference

Shuyu Zhou, Shanchao Zhang, Ying Wang, Yuzhu Wang

Published Mar 20, 2020• 2 citations
Publisher

Abstract

A mixed light field generated from a two-level atomic ensemble can be used for two-photon interference. In this mixed light field, correlated paired photons generated from a four-wave mixing process provide a signal of two-photon interference, while Rayleigh scattered photons of the pump laser provide a stable reference to calibrate the normalized second-order correlation function. We demonstrate two-photon interference using the Hong–Ou–Mandel and Hanbury Brown–Twiss interferometers. A direct quantitative comparison between theoretical predictions and experimental data is performed under perturbed experimental conditions, which reveal this kind of light source has potential application for quantum metrology.

DOI
10.1364/prj.386557

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A mixed light field generated from a two-level atomic ensemble can be used for two-photon interference. In this mixed light field, correlated paired photons generated from a four-wave mixing process provide a signal of two-photon interference, while Rayleigh scattered photons of the pump laser provide a stable reference to calibrate the normalized second-order correlation function. We demonstrate two-photon interference using the Hong–Ou–Mandel and Hanbury Brown–Twiss interferometers. A direct quantitative comparison between theoretical predictions and experimental data is performed under perturbed experimental conditions, which reveal this kind of light source has potential application for quantum metrology.
We find that in a two-level system there are two kinds of four-wave mixing processes which destructively contribute to the third-order nonlinear susceptibility. In a paired-photon generation scheme by using a single retroreflected pump beam, these two processes occur with definite time orders. The biphoton temporal correlation, which is measured with a back-to-back geometry in a laser-cooled two-level atomic ensemble, shows a damped Rabi oscillation and photon antibunching. Without suppressing the background of Rayleigh scattering, the upper limit of Cauchy criteria is estimated.
Quantum entanglement first attracted attention as the heart of the Einstein-Podolsky-Rosen (EPR) paradox and one of the most counterintuitive phenomena in the view of classical physics [1]. Hong-Ou-Mandel (HOM) [2] and Franson interferences [3] are regarded as important evidence of the non-classical nature of photon sources. Since a two-photon interference experiment with time-energy entangled photons was proposed by J. D. Franson [2], the fourth-order interferences beyond the one-photon coherence length have attracted considerable attention as important and interesting quantum phenomena in quantum optics. Surprisingly, Franson interference has been not experimentally demonstrated by use of two-photon pairs emitted from a cascade-type atomic system with continuous wave (CW) pumping, as originally proposed by Franson.
The limits of the well-established single-atom model for describing photon-pair generation via four-wave mixing in a diamond configuration in atomic ensembles are experimentally tested. Using a cold-atom source, biphotons are generated and detected through polarization analyzers that resolve the emitted light into horizontal and vertical components in the laboratory frame. The pump lasers driving the first and second excitation transitions are horizontally and vertically polarized, respectively. To predict single-photon counts and coincidence rates in all polarization channels, the first- and second-order correlation functions are calculated directly using a comprehensive model that accounts for all Zeeman sublevels of the relevant hyperfine states. Furthermore, the population dynamics with and without the re-pump laser of the magneto-optical trap are compared, revealing substantial differences in the populations of the Zeeman sublevels. This motivates the inclusion of two additional hyperfine levels and their corresponding Zeeman sublevels in the final model. The resulting density matrix is used to calculate expectation values of the far-field electric-field operators and compare them with experimental measurements over a range of pump-laser powers. Excellent agreement with the measured photon counts is obtained over most of this range. For the coincidence measurements, good agreement is found when the polarization of the photon generated by the first (second) decay is parallel to that of the first (second) pump beam. In contrast, the model consistently underestimates the experimental coincidence rates for the opposite polarization configuration. These findings indicate that effects beyond the internal level dynamics of individual atoms, most notably collective phenomena, are required to fully account for photon coincidences generated by four-wave mixing in atomic ensembles.
Hong-Ou-Mandel interference between independent sources is a fundamental primitive of many quantum communication and computation protocols. We present a study of the Hong-Ou-Mandel interference of single photons generated via two different physical processes by two independent atomic systems: scattering by a single atom, and parametric generation via four-wave mixing in a cloud of cold atoms. By controlling the coherence time and central frequency of the heralded single photons generated by four-wave mixing we observe quantum beat and a varying degree of interference.
We report the experimental verification of nonclassical correlations for an unfiltered spontaneous four-wave mixing process in an ensemble of cold two-level atoms, confirming theoretical predictions by Du et al. in 2007 for the violation of a Cauchy-Schwarz inequality in the system, and obtaining R=(1.98±0.03)≰1. Quantum correlations are observed in a nanoseconds timescale in the interference between the central exciting frequency and sidebands dislocated by the detuning to the atomic resonance. They prevail over the noise background coming from Rayleigh scattering from the same optical transition. These correlations are fragile with respect to processes that disturb the phase of the atomic excitation, but are robust to variations in number of atoms and to increasing light intensities.
Superposition and indistinguishablility between probability amplitudes have played an essential role in observing quantum interference effects of correlated photons. The Hong-Ou-Mandel interference and interferences of the path-entangled photon number state are of special interest in the field of quantum information technologies. However, a fully generalized two-photon quantum interferometric scheme accounting for the Hong-Ou-Mandel scheme and path-entangled photon number states has not yet been proposed. Here we report the experimental demonstrations of the generalized two-photon interferometry with both the interferometric properties of the Hong-Ou-Mandel effect and the fully unfolded version of the path-entangled photon number state using photon-pair sources, which are independently generated by spontaneous parametric down-conversion. Our experimental scheme explains two-photon interference fringes revealing single- and two-photon coherence properties in a single interferometer setup. Using the proposed interferometric measurement, it is possible to directly estimate the joint spectral intensity of a photon pair source.