Experimental observation of spatial antibunching of photons

Direct transfer of pump amplitude to parametric down-converted photons

In general, the spatial distribution of individual photons (signal or idler) generated by spontaneous parametric down-conversion (SPDC) does not evidently show any particular spatial mode structure because of their randomness in generation and the incoherent nature. Here, we numerically showed that all individual photons generated by the SPDC process carry the transverse amplitude as that of the pump and then confirmed it experimentally. The pump amplitude is revealed in SPDC when individual photons are spatially filtered from the total SPDC distribution. This is observed simply by imaging the photons that are filtered using a minimum-sized aperture. The phase measurements showed that the observed mode distribution does not possess the transverse phase distribution as that of the pump.

Young's double-slit interference with two-color biphotons

In classical optics, Young's double-slit experiment with colored coherent light gives rise to individual interference fringes for each light frequency, referring to single-photon interference. However, two-photon double-slit interference has been widely studied only for wavelength-degenerate biphoton, known as subwavelength quantum lithography. In this work, we report double-slit interference experiments with two-color biphoton. Different from the degenerate case, the experimental results depend on the measurement methods. From a two-axis coincidence measurement pattern we can extract complete interference information about two colors. The conceptual model provides an intuitional picture of the in-phase and out-of-phase photon correlations and a complete quantum understanding about the which-path information of two colored photons.

Interference effects induced by non-local spatial filtering

The spatial correlation between down-converted photons allows for non-local spatial filtering when two-photon coincidences are registered. This allows one to non-locally control the visibility of interference fringes, to observe ghost images and interference patterns, and to "retrieve" a coherent quantum image from an incoherent field distribution. We show theoretically that non-local spatial filtering can lead to counter-intuitive effects when the pump beam is no longer given by a Gaussian profile. Namely, increased non-local filtering can actually decrease the visibility of interference fringes, contrary to what has been observed so far. We explain this behavior through the transverse spatial parity entanglement of the down-converted photons.

Broadband frequency mode entanglement in waveguided parametric downconversion

We report the observation of beatings of the coincidence event rate in a Hong-Ou-Mandel interference (HOMI) between signal and idler photons from a parametric downconversion (PDC) process inside a multimode KTP waveguide. As an explanation we introduce biphotonic states entangled in their broadband frequency modes generated by waveguide mode triples and propose a suitable entanglement detection scheme.

Measuring photon antibunching from continuous variable sideband squeezing

We present a technique for measuring the second-order coherence function g(2)(tau) of light using a Hanbury Brown-Twiss intensity interferometer modified for homodyne detection. The experiment was performed entirely in the continuous-variable regime at the sideband frequency of a bright carrier field. We used the setup to characterize g(2)(tau) for thermal and coherent states and investigated its immunity to optical loss. We measured g(2)(tau) of a displaced-squeezed state and found a best antibunching statistic of g(2)(0)=0.11+/-0.18.

Generation of a two-photon singlet beam

Controlling the pump beam transverse profile in multimode Hong-Ou-Mandel interference, we generate a "localized" two-photon singlet state, in which both photons propagate in the same beam. This type of multiphoton singlet beam may be useful in quantum communication to avoid decoherence. We show that although the photons are part of the same beam, they are never in the same plane-wave mode, which is characterized by spatial antibunching behavior in the plane normal to the propagation direction.