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Physical Review Letters

Hanbury Brown–Twiss Correlations for a Driven Superatom

Jacob Lampen, Alisher Duspayev, Huy Minh Nguyen, Hikaru Tamura, P. R. Berman, A. Kuzmich

Published Nov 15, 2019• 6 citations
Publisher

Abstract

Hanbury Brown-Twiss interference and stimulated emission, two fundamental processes in atomic physics, have been studied in a wide range of applications in science and technology. We study interference effects that occur when a weak probe is sent through a gas of two-level atoms that are prepared in a singly excited collective (Dicke or "superatom") state and for atoms prepared in a factorized state. We measure the time-integrated second-order correlation function g^{(2)} of the output field as a function of the delay τ between the input probe field and radiation emitted by the atoms and find that, for the Dicke state, g^{(2)} is twice as large for τ=0 as it is for γ_{e}τ≫1 (γ_{e} is an excited state decay rate), while for the product state, this ratio is equal to 3/2. The results agree with those of a theoretical model in which any effects related to stimulated emission are totally neglected-the coincidence counts measured in our experiment arise from Hanbury Brown-Twiss interference between the input field and the field radiated by the atoms.

DOI
10.1103/physrevlett.123.203603

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•Jan 1, 2020•0 citations
Hanbury Brown-Twiss interference and stimulated emission, two fundamental processes in atomic physics, have been studied in a wide range of applications in science and technology. We study interference effects that occur when a weak probe is sent through a gas of two-level atoms that are prepared in a singly excited collective (Dicke or "superatom") state and for atoms prepared in a factorized state. We measure the time-integrated second-order correlation function g^{(2)} of the output field as a function of the delay τ between the input probe field and radiation emitted by the atoms and find that, for the Dicke state, g^{(2)} is twice as large for τ=0 as it is for γ_{e}τ≫1 (γ_{e} is an excited state decay rate), while for the product state, this ratio is equal to 3/2. The results agree with those of a theoretical model in which any effects related to stimulated emission are totally neglected-the coincidence counts measured in our experiment arise from Hanbury Brown-Twiss interference between the input field and the field radiated by the atoms.
The interaction of a single-photon pulse of radiation with an atom or an ensemble of atoms is studied using a source-field approach. The atom-field interaction is weak insofar as it can be treated in lowest, nonvanishing order of perturbation theory. The output field intensity and second-order correlation function of the field are calculated. It is shown that, even when any modification of the atomic dynamics produced by the incident field is neglected, as a function of the time delay $\ensuremath{\tau}$ between the incident field pulse and the field radiated by the atom(s), there is a ``bump'' in the time-integrated second-order correlation at $\ensuremath{\tau}=0$. The increase in coincidence counts for $\ensuremath{\tau}=0$ can be interpreted in terms of Hanbury Brown and Twiss--type interference. By looking at the field intensity, we show that the bump has no direct relation with stimulated emission---it occurs even when the input field is attenuated. In recent down conversion experiments, an increase in coincidence counts has been attributed to stimulated emission---we comment on the validity of such an interpretation in light of our results.
Superradiance typically requires preparation of atoms in highly entangled multiparticle states, the so-called Dicke states. In this paper we discuss an alternative route where we prepare such states from initially uncorrelated atoms by a measurement process. By measuring higher-order intensity-intensity correlations we demonstrate that we can simulate the emission characteristics of Dicke superradiance by starting with atoms in the fully excited state. We describe the essence of the scheme by first investigating two excited atoms. Here we demonstrate how via Hanbury Brown and Twiss type of measurements we can produce Dicke superradiance and subradiance displayed commonly with two atoms in the single excited symmetric and antisymmetric Dicke states, respectively. We thereafter generalize the scheme to arbitrary numbers of atoms and detectors, and explain in detail the mechanism which leads to this result. The approach shows that the Hanbury Brown and Twiss type of intensity interference and the phenomenon of Dicke superradiance can be regarded as two sides of the same coin. We also present a compact result for the characteristic functional which generates all order intensity-intensity correlations.
Superradiance has been an outstanding problem in quantum optics since Dicke introduced the concept of enhanced directional spontaneous emission by an ensemble of identical two-level atoms. The effect is based on the correlated collective Dicke states which turn out to be highly entangled. Here we show that enhanced directional emission of spontaneous radiation can be produced also with statistically independent incoherent sources, via the measurement of higher-order correlation functions of the emitted radiation. Our analysis is applicable to a wide variety of quantum emitters, like trapped atoms, ions, quantum dots, or nitrogen-vacancy centers, and is also valid for incoherent classical emitters. This is experimentally confirmed with up to eight statistically independent thermal light sources. The arrangement to measure the higher-order correlation functions corresponds to a generalized Hanbury Brown-Twiss setup, demonstrating that the two phenomena, superradiance and the Hanbury Brown-Twiss effect, stem from the same interference phenomenon.
Rydberg ensembles have many diverse and important applications for quantum information and quantum optics. For example, Rydberg ensembles can generate non-classical states of light which would be useful in a quantum network, and they could be used as qubits in a future quantum computer. For this reason, this thesis will present investigations of light-matter interactions in Rydberg ensembles. Non-classical states of light, such as single-photon states, are important for many quantum communication protocols. We have performed an experiment that demonstrates the generation of single-photon states from a Rydberg ensemble. We then studied the second-order correlations between this single-photon state of light and an incident coherent field. Under these conditions, we observed Hanbury Brown Twiss interference between the emission from a driven super-atom and a coherent field in the absence of stimulated emission. For a universal quantum computing architecture, coherence times must be much longer than gate operations. We observe ground-Rydberg coherence times in excess of 20 microseconds by using a "magic-wavelength" optical lattice to confine the atoms. Using this coherence time, we measured the differential nuclear-spin-dependent light shifts for principal quantum numbers, n, between n = 30 and n = 65, which is relevant for future high-fidelity Rydberg qubits in optical potentials. Also, we measured the hyperfine constant for atomic Rb to be 35.71 GHz with an uncertainty of 0.18 GHz.