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arXiv:1505.05606 · New Journal of Physics

Polarization entanglement and quantum beats of photon pairs from four-wave mixing in a cold 87 Rb ensemble

Gurpreet Kaur Gulati, Bharath Srivathsan, Brenda Chng, Alessandro Ceré, Christian Kurtsiefer

Published Sep 18, 2015• 29 citations
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Abstract

We characterize correlations in polarization and time of photon pairs generated from a cold cloud of 87 Rb atoms via a four-wave mixing process in a cascade level scheme. Quantum state tomography reveals entangled polarization states of high purity for each of the decay paths through two different intermediate hyperfine levels. When allowing both decay paths, we observe quantum beats in time-resolved correlation measurements.

DOI
10.1088/1367-2630/17/9/093034

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We characterize correlations in polarization and time of photon pairs generated from a cold cloud of 87 Rb atoms via a four-wave mixing process in a cascade level scheme. Quantum state tomography reveals entangled polarization states of high purity for each of the decay paths through two different intermediate hyperfine levels. When allowing both decay paths, we observe quantum beats in time-resolved correlation measurements.
"We describe the measurements of the cross-correlation function of photon pairs at 780 nm and 1367 nm showing quantum beats from Four- Wave Mixing (FWM) generated in Rubidium 87. The quantum beats come from the interference between two-photon decay paths through different intermediate hyperfine states in the process. We observe that it is possible to select different decay paths, and modify the biphoton spectra, by coupling different hyperfine levels of the atomic configuration in Rubidium with the pump fields and using selection rules."
Two coupled parametric four-wave-mixing processes in Rb atoms are studied using perturbation theory, which reveals clear evidence of the appearance of quantum beat at 608 cm −1 , corresponding to the energy difference of the 7 s −5 d states of Rb atoms, in the parametric four-wave-mixing signals. A pump–probe technique is utilized to observe the quantum beat. Time-varying characteristics of the quantum beat are investigated using time-dependent Fourier transform. The results show that the time-varying characteristics of the quantum beat not only offers a sensitive detecting method for observing the decay of atomic wave packets, but also provides a potential tool for monitoring the dissociation of molecules.
We observe time-correlated four photons within a correlation window of 20ns from spontaneous four-wave mixing via a double-Lambda scheme in a cold cloud of Rb-87 atoms. In contrast to high-power pulsed pumping of chi^(2) nonlinear processes in crystals, our scheme generates correlated four-photon states by direct continuous-wave pumping at nominal powers. We verify the presence of genuinely correlated four-photon states over accidentals by higher-order intensity cross-correlation measurements and accidental subtraction. We infer a time-correlated four-photon generation rate of 2.5(4)x10^6 counts per second close to saturation. The photons produced are near-resonant with atomic transitions, and have a bandwidth in the order of MHz, making them readily compatible with quantum networking applications involving atoms.
We demonstrate a technique of quantum-state tomography for measuring the complex temporal wave function of narrowband biphotons with polarization-dependent and time-resolved two-photon interference. While the amplitude function of the biphoton waveform is directly related to the second-order correlation function which is determined by the two-photon coincidence measurement, the phase function is retrieved from six sets of time-resolved two-photon interference measurements projected onto different polarization subspaces. We apply this technique to experimentally reconstruct the temporal quantum states of the narrow-band biphotons generated from the spontaneous four-wave mixing in cold atoms. As compared with the homodyne detection, our method doesn't require any external phase reference.
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.