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Observations of unusual diffraction and interference by two-photon correlation measurements are reported. The signal and idler beams produced by spontaneous parametric down-conversion are sent in different directions, and detected by two distant pointlike photon counting detectors. A double slit or a single slit is inserted into the signal beam. Interference-diffraction patterns are observed in coincidences by scanning the detector in the idler beam. PACS numbers: 42.50.Dv, 03.65.Bz Spontaneous parametric down-conversion (SPDC) [1]is the most effective source of two-photon light, consisting of pairs of correlated photons. The essentially quantum nature of the corresponding two-photon state [2,3] has been confirmed in a number of two-photon correlation experiments [4]. This quantum feature allows us to demonstrate an unusual two-photon effect [5], which looks very strange from the classical point of view. The SPDC light beam, which consists of two orthogonal polarization components (usually called signal and idler), is split by a polarization beam splitter into two beams, and detected by two distant pointlike photon counting detectors for coincidences (see Fig. 1). A Young's double-slit

Quantum theory predicts that Bose-Einstein condensation of a spatially homogeneous gas with attractive interactions is precluded by a conventional phase transition into either a liquid or solid. When confined to a trap, however, such a condensate can form, provided that its occupation number does not exceed a limiting value. The stability limit is determined by a balance between the self-attractive forces and a repulsion that arises from position-momentum uncertainty under conditions of spatial confinement. Near the stability limit, self-attraction can overwhelm the repulsion, causing the condensate to collapse. Growth of the condensate is therefore punctuated by intermittent collapses that are triggered by either macroscopic quantum tunnelling or thermal fluctuation. Previous observations of growth and collapse dynamics have been hampered by the stochastic nature of these mechanisms. Here we report direct observations of the growth and subsequent collapse of a 7Li condensate with attractive interactions, using phase-contrast imaging. The success of the measurement lies in our ability to reduce the stochasticity in the dynamics by controlling the initial number of condensate atoms using a two-photon transition to a diatomic molecular state.

The quantum state of a single photon stands amongst the most fundamental and intriguing manifestations of quantum physics [1]. At the same time single photons and pairs of single photons are important building blocks in the fields of linear optical based quantum computation [2] and quantum repeater infrastructure [3] . These fields possess enormous potential [4] and much scientific and technological progress has been made in developing individual components, like quantum memories and photon sources using various physical implementations [5][6][7][8][9][10][11]. However, further progress suffers from the lack of compatibility between these different components. Ultimately, one aims for a versatile source of single photons and photon pairs in order to overcome this hurdle of incompatibility. Such a photon source should allow for tuning of the spectral properties (wide wavelength range and narrow bandwidth) to address different implementations while retaining high efficiency. In addition, it should be able to bridge different wavelength regimes to make implementations compatible. Here we introduce and experimentally demonstrate such a versatile single photon and photon pair source based on the physics of whispering gallery resonators. A diskshaped, monolithic and intrinsically stable resonator is made of lithium niobate and supports a cavity-assisted triply-resonant spontaneous parametric down-conversion process. Measurements show that photon pairs are efficiently generated in two highly tunable resonator modes. We verify wavelength tuning over 100 nm between both modes with a controllable bandwidth between 7.2 and 13 MHz. Heralding of single photons yields anti-bunching with g (2) (0) < 0.2. This compact source provides unprecedented possibilities to couple to different physical quantum systems and makes it ideal for the implementation of quantum repeaters and optical quantum information processing.It is known that in a nonlinear medium a photon can spontaneously decay into a pair of photons, usually called signal and idler. This process, referred to as spontaneous parametric down-conversion (SPDC), preserves the energy and momentum of the parent photon. The resulting pair of photons posses the ability to bridge different wavelength ranges. At the same time detecting one photon of this pair unambiguously heralds the presence of a single photon. In principle, the process of SPDC has a very high bandwidth. By assisting it with a high quality factor (high-Q) resonator, the desired narrow bandwidth of a few MHz for the individual photons can be ensured [12]. A thorough description of this resonator-assisted SPDC leads to a two-mode EPR entangled state [13] and has successfully been used to generate heralded single photons [14]. Recently, resonator-assisted SPDC has led to a substantial progress towards an efficient narrow-band source [15]. However, the wavelength and bandwidth tunability remained a major challenge.We overcome this problem by using an optical whispering gallery mode resonator (WGMR). These resonators a...