We propose to use multiphoton interferences from statistically independent light sources in combination with linear optical detection techniques to enhance the resolution in imaging. Experimental results with up to five independent thermal light sources confirm this approach to improve the spatial resolution. Since no involved quantum state preparation or detection is required, the experiment can be considered an extension of the Hanbury Brown-Twiss experiment for spatial intensity correlations of order N>2.

Superradiance is one of the outstanding problems 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 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 systems like trapped atoms, ions, quantum dots or NV-centers, and is also valid for statistically independent incoherent classical emitters. This is experimentally confirmed with up to eight independent thermal light sources.PACS numbers: 42.50. Gy, 42.50.Nn, 42.50.Dv, 03.67.Bg Dicke superradiance [1][2][3][4][5] remains an important problem in quantum optics primarily due to ones inability to generate entangled states of a modest number of atoms. Using single photon excitation one can produce Dicke states where only one atom out of the ensemble is excited. For this case several ground breaking experiments have been recently reported, including observation of collective Lamb shifts in regular arrays of nuclei [6,7] or directed forward scattering from atomic ensembles in collective first excited [8][9][10][11] or Rydberg states [12][13][14]. Beyond single-photon excited Dicke states the production of Dicke states with higher number of excitations remains a challenge. One option is the repeated measurements of photons at particular positions starting from the fully excited system. This amounts to measuring the m-th order photon correlation function for N > m emitters. In this case, if the detection is unable to identify the individual photon source, the collective system cascades down the ladder of symmetric Dicke states each time a photon is recorded via projective measurements. This is another example of measurement induced entanglement among parties which do not directly interact with each other [15][16][17][18][19][20][21][22].The inability to distinguish the emitters is fulfilled in case of atoms confined to a region smaller than the wavelength λ of the emitted radiation. However, if the dipole-dipole interaction between the atoms is taken into account the collective system quickly leaves the symmetric subspace populating different super-and subradiant states so that the superradiant phenomena are obscured [3,5].The condition of indistinguishability can also be met in case of widely separated emitters as long as the detection occurs in the far field [1][2][3][4][5]23]. This is fulfilled for example for atomic clouds involving many particles, relevant for most experiments in the optical domain. However, in this regime the superradiant characteristics depend critically on the geometry of the sample due to diffraction and propagation effects [3,5] so that the superradiant behavior is concealed by geometrical considerations.To observe the effects o...

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.

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.

We investigate Dicke subradiance of N≥2 distant quantum sources in free space, i.e., the spatial emission patterns of spontaneously radiating noninteracting multilevel atoms or multiphoton sources, prepared in totally antisymmetric states. We find that the radiated intensity is marked by a full suppression of spontaneous emission in particular directions. In resemblance to the analogous, yet inverted, superradiant emission profiles of N distant two-level atoms prepared in symmetric Dicke states, we call the corresponding emission patterns directional Dicke subradiance. We further derive that higher-order intensity correlations of the light emitted by statistically independent thermal light sources display the same directional Dicke subradiant behavior and show that it stems from the same interference phenomenon as in the case of quantum sources. We finally present measurements of directional Dicke subradiance for N=2,…,5 distant thermal light sources corroborating the theoretical findings.