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Physical Review Letters

Cross Modulation of Two Laser Beams at the Individual-Photon Level

Kristin M. Beck, Wenlan Chen, Qian Lin, Michael J. Gullans, Mikhail D. Lukin, Vladan Vuletić

Published Sep 12, 2014• 9 citations
Publisher

Abstract

Deterministic photon-photon interactions are a long-standing goal in optical science. Using an atomic ensemble inside a cavity, we demonstrate the mutual cross modulation of two continuous light beams at the level of individual photons. The originally uncorrelated beams derived from independent lasers become anticorrelated, as evidenced by an equal-time cross-correlation function g^{(2)}=0.89(1), showing that one photon in one beam extinguishes a photon in the other beam with a probability of 11(1)%. With further technical improvements, our approach should enable the nondestructive continuous detection of traveling optical photons.

DOI
10.1103/physrevlett.113.113603

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Deterministic photon-photon interactions are a long-standing goal in optical science. Using an atomic ensemble inside a cavity, we demonstrate the mutual cross modulation of two continuous light beams at the level of individual photons. The originally uncorrelated beams derived from independent lasers become anticorrelated, as evidenced by an equal-time cross-correlation function g^{(2)}=0.89(1), showing that one photon in one beam extinguishes a photon in the other beam with a probability of 11(1)%. With further technical improvements, our approach should enable the nondestructive continuous detection of traveling optical photons.
We report the continuous and partially nondestructive measurement of optical photons. For a weak light pulse traveling through a slow-light optical medium (signal), the associated atomic-excitation component is detected by another light beam (probe) with the aid of an optical cavity. We observe strong correlations of g_{sp}^{(2)}=4.4(5) between the transmitted signal and probe photons. The observed (intrinsic) conditional nondestructive quantum efficiency ranges between 13% and 1% (65% and 5%) for a signal transmission range of 2% to 35%, at a typical time resolution of 2.5 μs. The maximal observed (intrinsic) device nondestructive quantum efficiency, defined as the product of the conditional nondestructive quantum efficiency and the signal transmission, is 0.5% (2.4%). The normalized cross-correlation function violates the Cauchy-Schwarz inequality, confirming the nonclassical character of the correlations.
It has been shown that light beams from two independent lasers can produce interference effects at light levels corresponding to the presence, with a high degree of probability, of only one photon in the system. However, the statistical accuracy of earlier experiments performed in the spatial domain has been limited by the fact that individual observations were restricted to very short time intervals, during which only a few photons were registered. This paper describes an experiment performed in the time domain that overcomes these problems and demonstrates interference between two independent laser beams, at the single-photon level, over much longer observation times.
Deterministic optical quantum logic requires a nonlinear quantum process that alters the phase of a quantum optical state by π through interaction with only one photon. Here, we demonstrate a large conditional cross-phase modulation between a signal field, stored inside an atomic quantum memory, and a control photon that traverses a high-finesse optical cavity containing the atomic memory. This approach avoids fundamental limitations associated with multimode effects for traveling optical photons. We measure a conditional cross-phase shift of [Formula: see text] (and up to [Formula: see text] by postselection on photons that remain in the system longer than average) between the retrieved signal and control photons, and confirm deterministic entanglement between the signal and control modes by extracting a positive concurrence. By upgrading to a state-of-the-art cavity, our system can reach a coherent phase shift of π at low loss, enabling deterministic and universal photonic quantum logic.
We investigate deflections, spatial entanglement, and localization of an atomic beam due to its interaction with two modes of a standing electromagnetic field in an optical cavity. It is demonstrated that the deflection properties of an atom passing through two crossed standing light waves are crucially modified when photon-number correlated states of light are used instead of independent waves. Depending on the atom-light interaction scheme, various periodic two-dimensional patterns in a subwavelength regime are reported.
The coherence of light from independent ensembles of elementary atomic emitters plays a paramount role in diverse areas of modern optics. We demonstrate the interference of photons scattered from independent ensembles of warm atoms in atomic vapor. It relies on the feasibility of the preservation of coherence of light scattered elastically in the forward and backward directions from Doppler-broadened atomic ensembles, such that photons with chaotic photon statistics from two opposite atomic velocity groups contribute to the same detection mode. While the random phase fluctuations of the scattered light caused by a large thermal motion prevent direct observability of the interference in the detected photon rate, the stable frequency difference between photons collected from scattering off counterpropagating laser beams provides strong periodic modulation of the photon coincidence rate with the period given by the detuning of the excitation laser from the atomic resonance. The presented interferometry represents a sensitive and robust methodology for Doppler-free optical atomic and molecular spectroscopy based on photon correlation measurements on scattered light.