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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Time-Energy Entangled Photon Pairs from Doppler-Broadened Atomic Ensemble Via Collective Two-Photon Coherence

Jiho Park, Taek Jeong, Heonoh Kim, Han Seb Moon

Published Jun 1, 2019• 2 citations
Publisher

Abstract

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.

DOI
10.1109/cleoe-eqec.2019.8872405

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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.
We experimentally demonstrate two-photon interference of a time-energy entangled photon pair generated via collective two-photon coherence in Doppler-broadened cascade-type ^{87}Rb atoms. The two photons originally proposed by J. D. Franson are realized as a photon pair due to collective effects, which are generated from the cascade atomic system with a relatively long lifetime of the initial state and a considerably shorter lifetime of the intermediate state. The achievement of two-photon interference with photon-pair sources generated from inhomogeneous atomic ensembles constitutes an important result for time-energy entanglement based on an atom-photon interaction.
We experimentally demonstrate Franson interference of a time–energy entangled photon pair generated via collective two-photon coherence in the 5S1/2−5P3/2−5D5/2 transition of warm Rb87 atoms. The two unbalanced Michelson interferometers used in our setup are spatially separated in order to understand entanglement as a nonlocal property of the photon pairs from the warm atomic ensemble. We observe a Franson interference fringe with a high visibility of 99.1±1.3% with continuous-wave-mode photon pairs from the cascade-type atomic ensemble. To the best of our knowledge, this work is the first demonstration of the nonlocal two-photon interference experiment in separated photon channels by use of two-photon pairs emitted from a cascade-type atomic system as originally proposed by Franson [Phys. Rev. Lett.62, 2205 (1989)PRLTAO0031-900710.1103/PhysRevLett.62.2205].
We experimentally demonstrate nonlocal two-photon interference of energy-time entangled photon pairs via spontaneous four-wave mixing (SFWM) process in Doppler-broadened ladder-type 87Rb^{87}\mathrm{Rb}87Rb atoms. The Doppler-broadened ladder-type atomic system offers collective two-photon coherence which results in strong temporal correlation between the entangled photons. The spacelike separated photon pairs clearly show nonlocal two-photon interference with high visibility of $97.1\ifmmode\pm\else\textpm\fi{}2.5%$, which violates the Bell-CHSH (Clauser-Horne-Shimony-Holt) inequality by 10.5 standard deviations, in which the travel time of a single-photon towards the interferometer is much longer than the coincidence window. Our work will provide an atom-based entangled photon source which is applicable for long distance atom-memory-based quantum communication.
We have performed an EPR-type experiment first proposed by Franson,1 resulting in two-particle interference that is both nonclassical and nonlocal. We used photon pairs produced in an entangled state of energy via spontaneous down-conversion. Each photon enters an unbalanced interferometer, and we detect singles and coincidence rates at the output ports. If the long- and short-arm path-length difference is much greater than the photons’ coherence lengths, no fringes are seen in singles as the arm lengths are varied. Yet when the difference between the path-length differences of the two interferometers is less than these coherence lengths, fringes are observed in coincidence, resulting from an interference of the (indistinguishable) “long–long” and “short–short" coincidence possibilities. If the electronic resolution is sufficient to eliminate the background of "short–long" and "long–short” coincidences, the visibility may be as high as 100%.2 For V > 71%, with certain reasonable auxiliary assumptions, Bell’s inequalities can be violated. The interpretation is that no local, realistic model can yield the observed results. The use of this type of entangled state is significant because it is much closer to the original EPR paradox than the spin-singlet case commonly considered.
To realize a quantum network based on quantum entanglement swapping, bright and completely autonomous sources are essentially required. Here, we experimentally demonstrate Hong-Ou-Mandel (HOM) quantum interference between two independent bright photon pairs generated via the spontaneous four-wave mixing in Doppler-broadened ladder-type 87Rb atoms. Bright autonomous heralded single photons are operated in a continuous-wave (CW) mode with no synchronization or supplemental filters. The four-fold photon coincidence counts per hour correspond to 5000 events for a temporal range of 10 ns, one order higher than those of previously reported for autonomous single photons. We observe an HOM dip with 83% visibility with the two autonomous heralded single photons for an effective measurement time of 900 s. The achievement of the HOM interference between the two bright autonomous photon-pair sources has made an important step towards a practical scalable quantum network.