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Scientific Reports

Spectral–temporal biphoton waveform of photon pairs from cascade-type warm atoms

Jiho Park, Taek Jeong, Han Seb Moon

Published Oct 2, 2020• 2 citations
Publisher

Abstract

Abstract We investigate the spectral–temporal biphoton waveforms of the photon pairs emitted from cascade-type two-photon-coherent warm 87Rb atoms via the spontaneous four-wave mixing process in the 5S1/2–5P3/2–5D5/2 transition, under the condition of the different detuning frequencies (symmetric detuning conditions of ± 1 GHz) of the pump and coupling lasers relative to the 5P3/2 state. In both detuning cases corresponding to ± 1 GHz, the biphoton temporal waveforms and biphoton spectral waveforms of the photon pairs are measured by means of time-resolved coincidence photon counting and stimulated measurements, respectively. Although photon-pairs were generated using opposite detunings, we confirm that the spectral–temporal biphoton waveforms of the photon pairs are very similar. Furthermore, we observe Hong–Ou–Mandel interference with 82% visibility with the two independent heralded single photons.

DOI
10.1038/s41598-020-73610-2

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Physical Review Letters•Jul 1, 2016•41 citations
Abstract We investigate the spectral–temporal biphoton waveforms of the photon pairs emitted from cascade-type two-photon-coherent warm 87Rb atoms via the spontaneous four-wave mixing process in the 5S1/2–5P3/2–5D5/2 transition, under the condition of the different detuning frequencies (symmetric detuning conditions of ± 1 GHz) of the pump and coupling lasers relative to the 5P3/2 state. In both detuning cases corresponding to ± 1 GHz, the biphoton temporal waveforms and biphoton spectral waveforms of the photon pairs are measured by means of time-resolved coincidence photon counting and stimulated measurements, respectively. Although photon-pairs were generated using opposite detunings, we confirm that the spectral–temporal biphoton waveforms of the photon pairs are very similar. Furthermore, we observe Hong–Ou–Mandel interference with 82% visibility with the two independent heralded single photons.
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.
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.
Telecom band photonic quantum states generated from atomic systems are critical quantum resources for long‐distance and scalable quantum communication networks. Here, the beat phenomenon in the autocorrelation function of the telecom‐wavelength photon due to the velocity‐selective classes of the atoms in a Doppler‐broadened atomic ensemble of the cascade‐type 5S1/2–5P3/2–4D5/2 transition of 87Rb is reported. Although the signal (1529 nm) and idler (780 nm) photons are emitted from a single‐atomic ensemble, the autocorrelation functions of the signal and idler photons exhibit significantly different features, with temporal widths of 7 and 0.13 ns, respectively. Furthermore, the biphoton spectral waveform of the telecom band photon pair is measured using a stimulated emission method. The biphoton waveforms of the telecom‐wavelength biphoton source demonstrate good agreement with the magnitude of the Fourier components of the measured biphoton temporal and spectral waveforms.
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].
Abstract Photonic quantum states generated from atomic ensembles will play important roles in future quantum networks and long-distance quantum communication because their advantages, such as universal identity and narrow spectral bandwidth, are essential for quantum nodes and quantum repeaters based on atomic ensembles. In this study, we report the collectively coherent superposition of biphoton wavefunction emitted from different velocity classes in a Doppler-broadened cascade-type atomic ensemble. We experimentally demonstrate that the three times difference of temporal width of both biphoton temporal waveforms varies dependent on the wavelengths of the signal and idler photons from both 6S1/2–6P3/2–6D5/2 and –8S1/2 transitions of 133Cs, corresponding to the idler and signal wavelengths of 852 nm–917 nm and 852 nm–795 nm, respectively. Our results help understand the characteristics of biphoton sources from a warm atomic ensemble and can be applied to long-distance quantum networks and practical quantum repeaters based on atom–photon interactions.
The temporal purity of single photons is crucial to the indistinguishability of independent photon sources for the fundamental study of the quantum nature of light and the development of photonic technologies. Currently, the technique for single photons heralded from time-frequency entangled biphotons created in nonlinear crystals does not guarantee the temporal-quantum purity, except using spectral filtering. Nevertheless, an entirely different situation is anticipated for narrow-band biphotons with a coherence time far longer than the time resolution of a single-photon detector. Here we demonstrate temporally pure single photons with a coherence time of 100 ns, directly heralded from the time-frequency entangled biphotons generated by spontaneous four-wave mixing in cold atomic ensembles, without any supplemented filters or cavities. A near-perfect purity and indistinguishability are both verified through Hong-Ou-Mandel quantum interference using single photons from two independent cold atomic ensembles. The time-frequency entanglement provides a route to manipulate the pure temporal state of the single-photon source.