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Optics Letters

Joint spectral intensity of entangled photon pairs from a warm atomic ensemble via stimulated emission beat interferometry

Taek Jeong, Han Seb Moon

Published Apr 3, 2020• 9 citations
Publisher

Abstract

Photonic quantum states generated from atomic systems play prominent roles in long-distance quantum networks and scalable quantum communication, because entangled photon pairs from atomic ensembles possess a universal identity and narrow spectral bandwidth for quantum repeaters. In this study, we propose and demonstrate a novel, to the best of our knowledge, method for the joint spectral intensity measurement of narrowband continuous wave (CW)-mode photon pairs from a warm atomic ensemble using stimulated emission and beat interferometry for the first time. Our approach offers the advantage of sub-megahertz resolution, absolute optical frequency measurements with megahertz-level accuracy, fast collection time, and high signal-to-noise ratio; thus, our method can find important applications in the characterization of narrowband photon pairs generated from sources including atoms and artificially structured material.

DOI
10.1364/ol.391239

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Photonic quantum states generated from atomic systems play prominent roles in long-distance quantum networks and scalable quantum communication, because entangled photon pairs from atomic ensembles possess a universal identity and narrow spectral bandwidth for quantum repeaters. In this study, we propose and demonstrate a novel, to the best of our knowledge, method for the joint spectral intensity measurement of narrowband continuous wave (CW)-mode photon pairs from a warm atomic ensemble using stimulated emission and beat interferometry for the first time. Our approach offers the advantage of sub-megahertz resolution, absolute optical frequency measurements with megahertz-level accuracy, fast collection time, and high signal-to-noise ratio; thus, our method can find important applications in the characterization of narrowband photon pairs generated from sources including atoms and artificially structured material.
We investigate the temporal and spectral properties of narrowband photon pairs from a double-Λ-type atomic system of a warm 87Rb atomic ensemble. The temporal properties of the narrowband photons are investigated by measuring their auto-correlation and cross-correlation functions. The spectral measurement of the photon pair is obtained by applying the stimulated emission method. We show that the biphoton spectral waveform with a spectral width of ∼6 MHz corresponds to the biphoton temporal waveform with a temporal width of ∼26 ns. We believe that our results can contribute to the characterization of narrowband photons generated from atomic ensembles and aid in the development of new photonic quantum states generated from atomic systems.
Photon-pair sources from atomic ensembles are considered essential to the development of quantum science and technology based on atom-photon interfaces. The measurement of spectral-temporal biphoton wave forms is important for the characterization of frequency-time entangled photon pairs. Here, we demonstrate a stimulated measurement of the biphoton spectral wave form (BSW) of narrow-band photon pairs emitted from a Doppler-broadened cascade-type atomic ensemble. A BSW with submegahertz resolution is obtained as a function of the absolute optical frequency determined by atomic transitions, which reveals a spectral shift of approximately 510 MHz from the expected resonance frequency of the $5{P}_{3/2}\ensuremath{-}5{D}_{5/2}$ and $5{S}_{1/2}\ensuremath{-}5{P}_{3/2}$ transitions of 87Rb^{87}\mathrm{Rb}87Rb atoms.
Cold atom based narrowband entangled photon sources are important for efficient atom - photon interaction, which is at the heart of long-distance quantum communication and quantum memory protocols. Complete characterization of the narrowband entangled photons requires acquiring the frequency-time two-photon wavefunction, involving both joint temporal intensity (JTI) and joint temporal phase (JTP) measurements. Here, we demonstrate stimulated emission tomography of the frequency-time two-photon wavefunction of narrowband entangled photons from cold atoms. We show more than six orders of magnitude (x106) improvement in the measurement time for obtaining JTI and JTP compared to the conventional direct photon counting method, thus paving the way toward ultrafast high-resolution quantum tomography of photonic quantum states. (C) 2017 Optical Society of America
Controlling the spatial and spectral–temporal properties of photon pairs produced in artificially structured materials is fundamental to the realization of numerous photonic quantum information applications. Tailoring the joint spectral properties of photon pairs is of particular importance for applications relying on time–energy entanglement, high-visibility interference, and heralding. Yet measuring the joint spectral properties is a time-consuming task requiring coincidence counting, typically resulting in low-resolution spectra with a poor signal-to-noise ratio. In this work we capture the joint spectral correlations of photon pairs that would be produced in optical fibers with unprecedented speed, resolution, and signal-to-noise ratio, using a scheme based on stimulated four-wave mixing. We also illustrate that this technique can be used in engineering joint spectral correlations, making it a powerful tool for studying quantum states.
Abstract The realization of high-quality and stable entangled states is crucial for the advancement of quantum information science and technology. The entangled photons generated from a warm atomic ensemble exhibit high spectral purity and indistinguishability, offering significant potential for atom–photon-based quantum networks. We report high-quality time–frequency entangled continuous-wave (CW) photon pairs via a spontaneous four-wave mixing process in the cascade-type atomic transition of 5S 1/2 –5P 3/2 –5D 5/2 from a warm 87 Rb atomic ensemble. We observed Franson interference with the visibility of 99.6(8) % using two independent Franson interferometers, obtained by post-selecting the temporally indistinguishable two-photon events. The interferometer phases were actively stabilized using the reference laser frequency-locked to the 133 Cs (6S 1/2 -6P 3/2 ) transition. Beyond stabilizing the interferometers, the active control of these phases allowed us to measure the CHSH S -value directly and achieve a near-maximal Bell inequality violation with S = 2.83(7). This result demonstrates that warm atomic ensembles can serve as bright sources of time–frequency entanglement, which combined with active interferometer stabilization enable stable high-visibility operation for practical applications in quantum network.
The ability to manipulate and measure the time-frequency structure of quantum light is useful for information processing and metrology. Measuring this structure is also important when developing quantum light sources with high modal purity that can interfere with other independent sources. Here, we present and experimentally demonstrate a scheme based on intensity interferometry to measure the joint spectral mode of photon pairs produced by spontaneous parametric down-conversion. We observe correlations in the spectral phase of the photons due to chirp in the pump. We show that our scheme can be combined with stimulated emission tomography to quickly measure their mode using bright classical light. Our scheme does not require phase stability, nonlinearities, or spectral shaping and thus is an experimentally simple way of measuring the modal structure of quantum light.