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Advanced Quantum Technologies

High‐Performance Telecom‐Wavelength Biphoton Source from a Hot Atomic Vapor Cell

Hansol Jeong, Heewoo Kim, Han Seb Moon

Published Nov 28, 2023• 4 citations
Publisher

Abstract

Abstract Telecom‐band quantum light sources are critical to the development of long‐distance quantum communication technologies. A high‐performance telecom‐wavelength biphoton source from a hot 87 Rb atomic vapor cell is reported. Time‐correlated biphotons are generated from the cascade‐type 5S 1/2 –5P 3/2 –4D 5/2 transition of 87 Rb via a spontaneous four‐wave mixing process. The maximum value of biphoton cross‐correlation to be 44(3) is achieved, under the condition of a high optical depth of 112(3), including two‐photon absorption, with a spectral width of approximately 300 MHz. The coincidence count rate of biphoton is estimated to be of the order of 38 000 cps mW −1 . It is believed that the telecom‐wavelength biphoton source from an atomic vapor cell can be applied in long‐distance quantum networks and practical quantum repeaters based on atom–photon interactions.

DOI
10.1002/qute.202300108

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Abstract Telecom‐band quantum light sources are critical to the development of long‐distance quantum communication technologies. A high‐performance telecom‐wavelength biphoton source from a hot 87 Rb atomic vapor cell is reported. Time‐correlated biphotons are generated from the cascade‐type 5S 1/2 –5P 3/2 –4D 5/2 transition of 87 Rb via a spontaneous four‐wave mixing process. The maximum value of biphoton cross‐correlation to be 44(3) is achieved, under the condition of a high optical depth of 112(3), including two‐photon absorption, with a spectral width of approximately 300 MHz. The coincidence count rate of biphoton is estimated to be of the order of 38 000 cps mW −1 . It is believed that the telecom‐wavelength biphoton source from an atomic vapor cell can be applied in long‐distance quantum networks and practical quantum repeaters based on atom–photon interactions.
Biphotons, generated from a hot atomic vapor via the process of spontaneous four-wave mixing (SFWM), have the following merits: Stable and tunable frequencies as well as linewidth. Such merits are very useful in the application of long-distance quantum communication. However, the hot-atom SFWM biphoton sources previously had far lower values of generation rate per linewidth, i.e., spectral brightness, as compared with the sources of biphotons generated by the spontaneous parametric down-conversion process. Here we report a hot-atom SFWM source of biphotons with a linewidth of 960 kHz and a generation rate of $3.7\ifmmode\times\else\texttimes\fi{}{10}^{5}$ pairs/s. The high generation rate, together with the narrow linewidth, results in a spectral brightness of $3.8\ifmmode\times\else\texttimes\fi{}{10}^{5}$ pairs/s/MHz, which is 17 times the previous best result with atomic vapors and also better than all known results with all kinds of media. The all-co-propagating scheme together with a large optical depth (OD) of the atomic vapor is the key improvement, enabling the achieved spectral brightness to be about one quarter of the ultimate limit. Furthermore, this biphoton source had a signal-to-background ratio (SBR) of 2.7, which violated the Cauchy-Schwartz inequality for classical light by about 3.6-fold. Although an increasing spectral brightness usually leads to a decreasing SBR, our systematic study indicates that both of the present spectral brightness and SBR can be enhanced by further increasing the OD. This work demonstrates a significant advancement and provides useful knowledge in quantum technology using photons.
Hybrid photonic quantum networks require photonic quantum states generated from different systems, such as atoms and quantum dots. Photonic quantum sources based on atomic ensembles are excellent candidates due to their brightness, low noise, and narrowband characteristics. Herein, a new platform for a highly efficient biphoton source is presented using a thin, dense atomic medium from a hot 1 mm‐long chip‐scale Cs atomic vapor cell. Strongly correlated bright biphotons are generated via spontaneous four‐wave mixing from a dense atomic ensemble based on the 6S1/2–6P3/2–6D5/2 transition of 133Cs. Biphoton source achieves a detected biphoton count rate of 100 kilo‐counts per second, a heralding efficiency of 15%, and a maximum normalized crosscorrelation function value of 100 between the signal and idler photons, despite the low detector efficiency of a silicon avalanche photodetector being less than 25% at 917 nm. Herein, the maximal violation of the Cauchy–Schwarz inequality by a factor greater than 106 at a pump power of 1 μW is obtained. The scheme for a highly efficient photon source is believed to be useful for scalable quantum networks.
We report the generation of narrowband entangled photon pairs (biphotons) from a hot atomic vapor cell. Making use of backward spontaneous four-wave mixing with electromagnetically induced transparency (EIT), we produced subnatural-linewidth (1.9 MHz < 6 MHz) biphotons from a Doppler-broadened (0.5 GHz) hot (63 C) paraffin-coated rubidium 87 vapor cell. The biphoton coherence time is controable and can be tuned up to 100 ns by EIT. The uncorrelated photons from resonance Raman scattering are suppressed by a spatially separated and tailored optical pumping beam. The spectral brightness is as high as 14,000 s−1MHz−1. As compared with the cold-atom experiment , the hot atomic vapour cell configuration is much simpler for operation and maintenance, and it is a continuous biphoton source. Our demonstration may lead to miniature narrowband biphoton sources based on atomic vapour cells for practical quantum applications and engineering.
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.
Review of Biphoton Sources Based on the Double‐ΛΛ\LambdaΛ Spontaneous Four‐Wave Mixing Process
Abstract This review article focuses on biphoton sources based on the double‐ spontaneous four‐wave mixing (SFWM) process in laser‐cooled as well as room‐temperature or hot atomic ensembles. These biphoton sources have the advantage of providing stable frequencies, ultranarrow linewidths, and a tunability of the temporal biphoton width of more than one order of magnitude for high‐bandwidth applications. Therefore, the generated photons can be efficiently interfaced to, e.g., atomic quantum memories. In contrast, solid‐state biphoton sources typically require assistance by an optical cavity to operate at narrow linewidth that limits the tunability of the temporal width of the biphotons. Present state‐of‐the‐art double‐ SFWM biphoton sources can achieve one of the following results: a spectral linewidth of 50 kHz (290 kHz) or a temporal width of 13 (580 ns) with cold (hot) atoms, a detection rate of about 7 cps, and a generation rate of cps at a duty cycle of 0.4% or of cps in the steady state. The theoretical background of these biphoton sources, experimental implementations with cold and hot atoms, and progress over the years, will be illustrated.