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Optics Express

Photon statistics and polarization correlations at telecommunications wavelengths from a warm atomic ensemble

R. T. Willis, F. E. Becerra, Luis A. Orozco, S. L. Rolston

Published Jul 14, 2011• 64 citations
Publisher

Abstract

We present measurements of the polarization correlation and photon statistics of photon pairs that emerge from a laser-pumped warm rubidium vapor cell. The photon pairs occur at 780 nm and 1367 nm and are polarization entangled. We measure the autocorrelation of each of the generated fields as well as the cross-correlation function, and observe a strong violation of the two-beam Cauchy-Schwartz inequality. We evaluate the performance of the system as source of heralded single photons at a telecommunication wavelength. We measure the heralded autocorrelation and see that coincidences are suppressed by a factor of ≈ 20 from a Poissonian source at a generation rate of 1500 s(-1), a heralding efficiency of 10%, and a narrow spectral width.

DOI
10.1364/oe.19.014632

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We present measurements of the polarization correlation and photon statistics of photon pairs that emerge from a laser-pumped warm rubidium vapor cell. The photon pairs occur at 780 nm and 1367 nm and are polarization entangled. We measure the autocorrelation of each of the generated fields as well as the cross-correlation function, and observe a strong violation of the two-beam Cauchy-Schwartz inequality. We evaluate the performance of the system as source of heralded single photons at a telecommunication wavelength. We measure the heralded autocorrelation and see that coincidences are suppressed by a factor of ≈ 20 from a Poissonian source at a generation rate of 1500 s(-1), a heralding efficiency of 10%, and a narrow spectral width.
This thesis discusses four-wave mixing (4WM) in a warm ensemble of rubidium using the diamond configuration level structure. Both classical 4WM and non- classical photon-pair production are investigated. Quantum information science has spawned a great amount of experimental work on the interaction of light with collective modes of excitation in atomic ensem- bles. Plans to build quantum networks and quantum repeaters with atom ensembles take advantage of nonlinear interactions to produce and store non-classical states of light. These technologies will require photon sources that not only generate non- classical light, but also resonant, narrow band light. Here we investigate a system which could be used as such a source. We take advantage of the 4WM interaction in a warm ensemble of Rubidium atoms. Our scheme utilizes the diamond energy level configuration which, in ru- bidium, allows for correlated pairs at telecommunications wavelengths. We start by examining the properties of classical 4WM in the system. We measure the reso- nance structure and see that it can be understood in terms of velocity class selective resonant enhancement and power splitting effects. The efficiency of the process is low and limited by linear absorption of the pumps. Our observations agree with a semi-classical Maxwell-Bloch theoretical treatment. Next we observe pair generation by spontaneous 4WM from the warm ensem- ble. The temporal profile of the cross-correlation function (CCF) for the photons depends on pump-laser power and detuning. This allows us to produce biphotons with controllable spectra. A simple quantum optical theoretical treatment based on linear filtering gives qualitative agreement with the data. We show that the photon pairs are polarization entangled, clearly violating Bell's Inequality. A perturbative quantum optical treatment predicts the polariza- tion state of the pairs and agrees with our measurements. We analyze the photon statistics of the source and find the largest violation of the two beam Cauchy-Schwarz inequality from a warm atomic source yet. We cast the system as a heralded sin- gle photon source at telecommunications wavelengths and see that it is competitive with other systems in terms of spectral brightness.
We present measurements of the cross-correlation function of photon pairs at 780780780 and 136713671367 nm, generated in a hot rubidium vapor cell. The temporal character of the biphoton is determined by the dispersive properties of the medium where the pair generation takes place. We show that short correlation times occur for optically thick samples, which can be understood in terms of off-resonant pair generation. By modifying the linear response of the sample, we produce near-resonant photon pairs, which could in principle be used for entanglement distribution.
We observe narrow band pairs of time-correlated photons of wavelengths 776 and 795 nm from nondegenerate four-wave mixing in a laser-cooled atomic ensemble of ^{87}Rb using a cascade decay scheme. Coupling the photon pairs into single mode fibers, we observe an instantaneous rate of 7700 pairs per second with silicon avalanche photodetectors, and an optical bandwidth below 30 MHz. Detection events exhibit a strong correlation in time [g((2))(τ = 0) ≈ 5800] and a high coupling efficiency indicated by a pair-to-single ratio of 23%. The violation of the Cauchy-Schwarz inequality by a factor of 8.4 × 10(6) indicates a strong nonclassical correlation between the generated fields, while a Hanbury Brown-Twiss experiment in the individual photons reveals their thermal nature. The comparison between the measured frequency bandwidth and 1/e decay time of g((2)) indicates a transform-limited spectrum of the photon pairs. The narrow bandwidth and brightness of our source makes it ideal for interacting with atomic ensembles in quantum communication protocols.
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.
We report on a source of narrowband pairs of time-correlated photons generated via four-wave mixing in a laser-cooled atomic ensemble of87Rb atoms. We employ a cascade decay level scheme in which two copropagating pump beams of 780nm and 776nm excite the atoms to the 5D3/2, F = 3 level and the photon pairs of wavelength 762nm and 795nm from the cascade decay are coupled into single mode fibers and sent to silicon avalanche photodetectors. The photodetection events exhibit a strong correlation in time (g(2)(τ = 0) ≈ 5800), and a high coupling efficiency, as indicated by a pair-to-single ratio of 23%. An instantaneous pair generation rate of 7700 pairs per second (uncompensated for optical losses and detector efficiency) is observed with an increased 776nm pump power of 14mW. The violation of the Cauchy-Schwarz inequality by a factor of 8.4 × 106indicates a strong non-classical correlation between the generated fields, while a Hanbury-Brown-Twiss experiment in the individual photons reveals their thermal nature.