Experimental Demonstration of Quantum Stationary Light Pulses in an Atomic Ensemble

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Experimental Demonstration of Quantum Stationary Light Pulses in an Atomic Ensemble

Kwang-Kyoon Park, Young-Wook Cho, Young-Tak Chough, and Yoon-Ho Kim

Phys. Rev. X 8, 021016 – Published 13 April 2018

Abstract

We report an experimental demonstration of the nonclassical stationary light pulse (SLP) in a cold atomic ensemble. A single collective atomic excitation is created and heralded by detecting a Stokes photon in the spontaneous Raman scattering process. The heralded single atomic excitation is converted into a single stationary optical excitation or the single-photon SLP, whose effective group velocity is zero, effectively forming a trapped single-photon pulse within the cold atomic ensemble. The single-photon SLP is then released from the atomic ensemble as an anti-Stokes photon after a specified trapping time. The second-order correlation measurement between the Stokes and anti-Stokes photons reveals the nonclassical nature of the single-photon SLP. Our work paves the way toward quantum nonlinear optics without a cavity.

Received 14 December 2017
Revised 6 March 2018

DOI:https://doi.org/10.1103/PhysRevX.8.021016

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Effects of atomic coherence on light propagation Quantum information with atoms & light Quantum states of light

Atomic, Molecular & OpticalQuantum Information, Science & Technology

Authors & Affiliations

Kwang-Kyoon Park¹, Young-Wook Cho², Young-Tak Chough³, and Yoon-Ho Kim^1,*

¹Department of Physics, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea
²Center for Quantum Information, Korea Institute of Science and Technology (KIST), Seoul 02792, Korea
³Department of Automotive Engineering, Gwangju University, Gwangju 61743, Korea

^*yoonho72@gmail.com

Popular Summary

Light normally travels at roughly 300,000 km/s, but it can be fully stored in a finely tuned gas of ultracold atoms. When light is stored in an atomic gas, no optical energy remains. An alternate approach known as a stationary light pulse (SLP), however, can retain optical energy even while the light stands still. A SLP could greatly enhance interactions between photons, thus enabling new approaches to quantum computing and quantum optics. The SLP effect could even lead to new and exotic light-based quantum materials, such as a photonic crystal. Despite the fact that applications of SLPs require quantum states of light, all experimental demonstrations to date have been limited to classical light pulses only. Here, we report the first experimental demonstration of a quantum SLP.

In a quantum SLP, a single-photon state is trapped in a cold ensemble of atoms without the help of an optical cavity, such as those used in classical approaches to this problem. Our atomic ensemble consists of 87 rubidium atoms all prepared in the ground state. First, a single collective atomic excitation is prepared in the atomic medium with a laser beam. The single collective atomic excitation is then converted into a single-photon state trapped inside the atomic medium by a pair of counterpropagating trapping laser beams. After a specified trapping time (ranging from 0.6 to $1.8 μ s$ ), the trapped single photon is released from the atomic medium by turning off one of the two trapping laser beams.

Our work paves the way toward quantum nonlinear optics without a cavity, as well as novel quantum devices and materials.

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Vol. 8, Iss. 2 — April - June 2018

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Figure 1
Experimental sequence and schematic. (a)–(c) The energy level diagram and the experimental sequence. (a) Initially, all atoms are prepared in the ground state $| g ⟩$ . When the write pulse $Ω_{w}$ is applied with detuning $Δ_{1}$ , creation of a single collective atomic excitation is heralded by the detection event of the Stokes photon ${\hat{E}}_{s}$ at detector $D_{1}$ . (b) From the single collective atomic excitation, the single-photon SLP (denoted by ${\hat{E}}_{a s}^{\pm}$ ) is formed by applying two counterpropagating read lasers $Ω_{r}^{\pm}$ . Detuning $Δ_{2}$ is necessary to suppress higher order atomic coherence in cold atoms. (c) After a specified trapping time $τ$ , the single-photon SLP is released from the atomic ensemble as the anti-Stokes photon ${\hat{E}}_{a s}^{+}$ by turning off the read lasers $Ω_{r}^{-}$ and is detected at detector $D_{2}$ . (d) Experimental schematic. The optical depth of the Rb MOT is approximately 120. All the optical fields have the same circular polarization in the atomic frame. Because of the phase-matching condition, the read/write lasers and the Stokes/anti-Stokes modes are tilted by approximately 0.35°. The bandpass filter $F_{1}$ ( $F_{2}$ ) consists of a 780-nm (795-nm) interference filter and a solid etalon filter. The arrival time difference between the heralding photon at $D_{1}$ and the released single-photon SLP at $D_{2}$ is recorded with a time-correlated single-photon counting (TCSPC) module. PBS stands for polarizing beam splitter; QWP stands for quarter wave plate.
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Figure 2
Heralded generation of a single-photon state. The read laser $Ω_{r}^{+}$ is turned on $0.8 μ s$ after the write pulse. The TCSPC histogram triggered by the Stokes photon at $t_{1}$ displays the waveform of the anti-Stokes single photon. The TCSPC histogram is accumulated for 3000 s.
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Figure 3
Formation of the single-photon SLP. (a) The two counter-propagating read lasers $Ω_{r}^{+}$ and $Ω_{r}^{-}$ are simultaneously turned on $0.8 μ s$ after the write pulse. After a specified trapping time $τ$ , $Ω_{r}^{-}$ is turned off and released is the heralded single-photon from the atomic cloud as the anti-Stokes photon. The trapping times are, (i) $τ = 0.6 μ s$ , (ii) $τ = 1.2 μ s$ , (iii) $τ = 1.8 μ s$ . (b) The TCSPC histogram shows the waveform of the released single-photon. During the formation of the single-photon SLP, emission of the anti-Stokes photon is greatly suppressed, albeit showing some leakage. When the read laser $Ω_{r}^{-}$ is turned off, the remaining single-photon SLP is released. Each TCSPC histogram is accumulated for 3000 s. Inset shows the extended region for case (i), in which uncorrelated counts are also presented. Since the write/read pulses are applied at every $5 μ s$ , the coincidence count peaks appear at every $5 μ s$ in the TCSPC histogram. The pronounced peak at $2 μ s$ is due to the quantum-correlated photon pair generated by the corresponding write/read pulse sequence. The other low-lying periodic peaks represent uncorrelated random coincidence counts due to a Stokes photon by a write pulse and an anti-Stokes photon by subsequent read pulses.
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Figure 4
Nonclassical nature of the single-photon SLP. (a) The second-order correlation $g_{s, a s}^{(2)} (0)$ between Stokes and anti-Stokes photons as a function of the single-photon trapping time $τ$ . The raw $g_{s, a s}^{(2)} (0)$ was calculated without subtracting any noise counts. Unwanted noise counts due to unfiltered strong read lasers were independently measured and subtracted for the calculation of the noise-subtracted $g_{s, a s}^{(2)} (0)$ . The bound for nonclassical correlation, calculated from the Cauchy-Schwarz inequality, is $g_{s, a s}^{(2)} (0) = 2$ , and the bound for antibunching of the heralded anti-Stokes photon is $g_{s, a s}^{(2)} (0) = 2 + \sqrt{2}$ . (b) The relative intensity of the anti-Stokes photon as a function of $τ$ . The SLP decay rate of $γ_{τ} = 2 π \times 154 kHz$ was obtained from the exponential decay fit (solid line).
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