Measurement induced focussing of radiation from independent classical and non-classical sources
We present a technique based on the measurement of intensity correlations of higher order in a generalized Hanbury Brown and Twiss setup from quantum as well as classical sources which leads to a strong focusing of the recorded intensity correlation and thereby of the last detected photon. By studying the state evolution of the fields produced by the independent sources, 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. From this it can be seen that the focusing effect results from the well-known phenomenon of Dicke superradiance, commonly observed with atoms in symmetric Dicke states. As a consequence, we demonstrate that the Hanbury Brown and Twiss effect, originally established in astronomy to determine the dimensions or distances of stars, and Dicke superradiance, are two sides of the same coin.
 D. Bhatti, S. Oppel, R. Wiegner, G. S. Agarwal, J. von Zanthier, Phys. Rev. A. 94, 013810 (2016)
 R. Wiegner, S. Oppel, D. Bhatti, J. von Zanthier, G. S. Agarwal, Phys. Rev. A 92, 033832 (2015)
 S. Oppel, R. Wiegner, G. S. Agarwal, J. von Zanthier, Phys. Rev. Lett. 113, 263606 (2014)
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 Dicke’s 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.
 R. Wiegner, J. von Zanthier, G. S. Agarwal, Phys. Rev. A 84, 023805 (2011)
Measurement-induced spatial modulation of spontaneous decay and photon
We report on the manipulation of the spontaneous emission process leading to a spatial modulation of spontaneous decay . The effect is observed in case of coherently driven atoms separated by less than a transition wavelength. It is quantified by Glauber's photon-photon second order correlation function. We show that the photon arrival time, usually regarded as an entirely random process, depends not only on where a photon is detected but also on where a former photon had been recorded previously. Our results shed light on the unexpected consequences of state reduction and entanglement for the fundamental process of spontaneous emission.
 J. von Zanthier, T. Bastin, G. S. Agarwal, Phys. Rev. A 74 (Rapid Comm.), 061802 (2006)
Intensity-intensity correlations as a probe of interferences under
conditions of noninterference in the intensity
The different behavior of first-order interferences and second-order correlations are investigated for the case of two coherently excited atoms. For intensity measurements this problem is in many respects equivalent to Young’s double-slit experiment and was investigated in an experiment by Eichmann et al. [Phys. Rev. Lett. 70, 2359 (1993)] and later analyzed in detail by Itano et al. [Phys. Rev. A 57, 4176 (1998)]. Our results show that in cases where the intensity interferences disappear the intensity-intensity correlations can display an interference pattern with a visibility of up to 100%. The contrast depends on the polarization selected for the detection and is independent of the strength of the driving field. The nonclassical nature of the calculated intensity-intensity correlations is also discussed .
 G. S. Agarwal, J. von Zanthier, C. Skornia, H. Walther, Phys. Rev. A 65, 053826 (2002)
Nonclassical interference effects in the radiation from coherently
driven uncorrelated atoms
We demonstrate the existence of nonclassical correlations in the radiation of two atoms, that are coherently driven by a continuous laser source. The photon-photon correlations of the fluorescence light show a spatial interference pattern not present in a classical treatment. A feature of this phenomenon is that bunched and antibunched light is emitted in different spatial directions. The calculations are performed analytically. It is pointed out that the correlations are induced by state reduction due to the measurement process when the detection of the photons does not distinguish between the atoms. It is interesting to note that the phenomena show up even without any interatomic interaction .
 C. Skornia, J. von Zanthier, G. S. Agarwal, E. Werner, H. Walther, Phys. Rev. A 64, 063801 (2001)