R. Wiegner

Quantum-interference-initiated superradiant and subradiant emission from entangled atoms

We calculate the radiative characteristics of emission from a system of entangled atoms which can have a relative distance larger than the emission wavelength. We develop a quantum multipath interference approach which explains both super- and subradiance though the entangled states have zero dipole moment. We derive a formula for the radiated intensity in terms of different interfering pathways. We further show how the interferences lead to directional emission from atoms prepared in symmetric W states. As a byproduct of our work we show how Dickes classic result can be understood in terms of interfering pathways. In contrast to the previous works on ensembles of atoms, we focus on finite numbers of atoms prepared in well characterized states.

Directional superradiant emission from statistically independent incoherent nonclassical and classical sources.

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.

Simulating superradiance from higher-order-intensity-correlation measurements: Single atoms

Superradiance typically requires preparation of atoms in highly entangled multi-particle 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 Hanbury Brown and Twiss type 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.

Hong–Ou–Mandel interference without beam splitters

We propose a new interferometric setup which displays a completely destructive generalized N-photon Hong–Ou–Mandel interference. The key property of this scheme is that it does not require any optical elements like beam splitters or integrated waveguide structures. The interference is intrinsically produced by the evolution of N photons in free space when emitted by N identical single photon sources and measured by N detectors in the far field. In this sense, the setup is a most simple and natural implementation of the Hong–Ou–Mandel interference effect, i.e. of a completely destructive multi-photon interference produced by independent incoherent sources.

Simulating Dicke-like superradiance with classical light sources

In this paper we investigate the close relationship between Dicke superradiance, originally predicted for an ensemble of two-level atoms in entangled states, and the Hanbury Brown and Twiss effect, initially established in astronomy to determine the dimensions of classical light sources like stars. By studying the state evolution of the fields produced by classical sources -- defined by a positive Glauber-Sudarshan P function -- when recording intensity correlations of higher order in a generalized Hanbury Brown and Twiss setup we find that the angular distribution of the last detected photon, apart from an offset, is identical to the superradiant emission pattern generated by an ensemble of two-level atoms in entangled symmetric Dicke states. We show that the phenomenon derives from projective measurements induced by the measurement of photons in the far field of the sources and the permutative superposition of quantum paths identical to those leading to superradiance in the case of single photon emitters. We thus point out an important similarity between classical sources and quantum emitters upon detection of photons if the particular photon source remains unknown. We finally present a compact result for the characteristic functional which generates intensity correlations of arbitrary order for any kind of light sources.

Quantum interference and entanglement of photons that do not overlap in time.

We discuss the possibility of quantum interferences and entanglement of photons that exist at different intervals of time, i.e., one photon being recorded before the other has been created. The corresponding two-photon correlation function is shown to violate Bells inequalities.

Quantum interference and non-locality of independent photons from disparate sources

We quantitatively investigate the non-classicality and non-locality of a whole new class of mixed disparate quantum and semiquantum photon sources at the quantum–classical boundary. The latter include photon-added thermal and photon-added coherent sources, experimentally investigated recently by Zavatta et al (2009 Phys. Rev. Lett. 103 140406). The key quantity in our investigations is the visibility of the corresponding photon–photon correlation function. We present explicit results on the violations of the Cauchy–Schwarz inequality—which is a measure of non-classicality—as well as of Bell-type inequalities.

Creating path entanglement and violating Bell inequalities by independent photon sources

We demonstrate a novel approach of violating position-dependent Bell inequalities by photons emitted via independent single photon sources in free space. We trace this violation back to path entanglement created a posteriori by the selection of modes due to the process of detection.

Dicke Superradiance and Hanbury Brown and Twiss Intensity Interference: Two Sides of the Same Coin

We show that Hanbury Brown and Twiss intensity interference and Dicke superradiance can be considered as two sides of the same coin, resulting from multi-photon interferences appearing in higher order photon correlations.

Superradiance from Entangled Atoms

We discuss the radiation properties of entangled atomic sources in comparison to sources in a separable state. We explain superradiance and subradiance of entangled sources in terms of interference among different photon quantum path ways.