Directional Superradiant Emission from Statistically Independent Incoherent Nonclassical and Classical Sources

Abstract

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...

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“…, m (see Fig. 1) [20]. A linear polarizer was placed in front of the camera to ensure that light of equal polarization was used.…”