We report the first experimental demonstration of two-photon imaging with a pseudothermal source. Similarly to the case of entangled states, a two-photon Gaussian thin lens equation is observed, indicating EPR type correlation in position. We introduce the concepts of two-photon coherent and two-photon incoherent imaging. The differences between the entangled and the thermal cases are explained in terms of these concepts.

We report the measurement of the entangled two-photon absorption (ETPA) cross section, σ, at 808 nm on organic chromophores in solution in a low photon flux regime. We performed measurements on zinc tetraphenylporphyrin (ZnTPP) in toluene and rhodamine B (RhB) in methanol. This is, to the best of our knowledge, the first time that σ is measured for RhB. Additionally, we report a study of the dependence of σ on the molecular concentration for both molecular systems. In contrast to previous experiments, our measurements are based on detecting the pairs of photons that are transmitted by the molecular system. By using a coincidence count circuit it was possible to improve the signal-to-noise ratio. This type of work is important for the development of spectroscopic and microscopic techniques using entangled photons.

We report a proof-of-principle experiment on distant clock synchronization.Besides the achievement of picosecond resolution at 3 kilometer distance, this experiment demonstrated a novel concept for high accuracy non-local timing and positioning based on the quantum feature of entangled states.

We report an experimental study of group-velocity dispersion effect on an entangled two-photon wavepacket, generated via spontaneous parametric downconversion and propagating through a dispersive medium. Even in the case of using CW laser beam for pump, the biphoton wavepacket and the secondorder correlation function spread significantly. The study and understanding of this phenomenon is of great importance for quantum information applications, such as quantum communication and distant clock synchronization.

We demonstrate experimentally the tunable control of the joint spectrum, i.e. waveform and degree of frequency correlations, of paired photons generated in spontaneous parametric downconversion. This control is mediated by the spatial shape of the pump beam in a type-I noncollinear configuration. We discuss the applicability of this technique to other sources of frequency entangled photons, such as electromagnetically induced Raman transitions. PACS numbers:The quantum description of paired photons includes the spatial shape, the polarization state and the joint spectrum. The later contains all the information about bandwidth, type of frequency correlations and waveform of the two-photon state. Quantum light has been proved to be useful in many quantum information applications and the most appropriate form of the joint spectrum depends on the specific realization under consideration. For example, uncorrelated pairs of photons can be used as a source of heralded single photons with a high degree of quantum purity [1,2]; the tolerance against the effects of mode mismatch in linear optical circuits can be enhanced by using photons with appropriately tailored waveform shape [3]; the use of frequency-correlated or anticorrelated photons allows erasing the distinguishing information coming from the spectra when considering polarization entanglement [4,5]; some protocols for quantum enhanced clock synchronization and positioning measurements rely on the use of frequency anticorrelated [6] or correlated photons [7]. Moreover, the entanglement in the frequency domain offers by itself a new physical resource where to explore quantum physics in a high-dimensional Hilbert space [8]. This requires the development of new techniques for the control of the joint spectrum that will allow the generation of multidimensional waveform alphabets.The most widely used method for the generation of pairs of entangled photons is spontaneous parametric down conversion (SPDC). Notwithstanding, paired photons with the desired joint spectrum may not be harvested directly at the output of the downconverting crystal. The question that arises is how to control independently different aspects of the joint spectrum of entangled paired photons generated in SPDC; importantly, the sought-after techniques should work for any frequency band of interest and any nonlinear crystal.Various methods have been proposed and developed to control the type of frequency correlations and the bandwidth of downconverted photons. Some of these methods rely on an appropriate selection of the nonlinear crystal length and its dispersive properties [4,9]. Others are based on SPDC pumped by pulses with angular dispersion [10] or the design of nonlinear crystal superlattices [11]. Noncollinear SPDC has also been propose as a way to tailor the waveform of the downconverted photons [12,13,14,15,16,17]. Contrary to the case of collinear SPDC, where the transverse spatial shape of the pump beam translate into specific features of the spatial waveform of the two-photon state; in...