optics logo

Spatial correlation functions and their measurement with an array single-photon detector

Fig.1. Biphoton correlation radius versus the distance from the crystal. Insets: shapes of the spatial correlation function at different points of the dependence.

Preparation of entangled photons with small correlation times and transverse lengths is a key problem in the realization of interaction between two-photon light and material quantum objects, such as atoms or ions. We have proposed a method for generating biphotons with extremely narrow spatial intensity correlation functions. The method is based on the spatial variation of linear or nonlinear properties of the crystal where biphotons are generated via spontaneous parametric down-conversion. As a result, the crystal acts as a `nonlinear lens’, creating two-photon light with a spatially converging intensity correlation function. Figure 1 shows the dependence of the transverse correlation length (correlation radius) of the biphoton versus the distance from the crystal (red line). The insets show the shapes of the spatial correlation functions at different distances from the crystal. This ‘transverse compression’ effect is similar to the time compression of chirped pulses due to their propagation through dispersive media.

Fig. 2. A 'ring' of parametric down-conversion obtained by averaging over 2000 frames of an ICCD camera.

It is planned to observe this effect in experiment using a single-photon CCD camera (ICCD) for the measurement of spatial intensity correlation functions. As the first step, it is necessary to develop the technique of measuring spatial intensity correlation functions with an array single-photon detector. Towards this, we have performed a test measurement of the spatial intensity correlation function of a pseudo-thermal light source based on a rotating ground-glass disc, as well as of a parametric down-conversion source. The measurement is in the single-photon regime, i.e., each pixel of the ICCD camera either fires, registering a photon, or does not fire. Our first results demonstrate the feasibility of such a measurement. Figure 2 shows the far-field distribution of the parametric down-conversion intensity at the degenerate wavelength 709 nm, recorded by summing over 2000 frames of an ICCD camera. Note that each frame contains only single-photon events, the probability of a single pixel firing being below 0.02. By processing the frames, we can measure the spatial correlations.

Photon-number resolution with an array single-photon detector

Multi-photon states of light (MPL) are, in principle, available via their conditional preparation from squeezed vacuum generated via parametric down-conversion. However, a huge problem consists of the lack of reliable photon-number resolving detectors. As a substitute, one uses time-multiplexing detectors but here we choose a different strategy, which is space multiplexing.

It is planned to produce three-, four-, and higher-photon states via conditional preparation, using spontaneous parametric down-conversion with intermediate gain values. Characterization of the produced state will be through the measurement of a higher-order Glauber’s intensity correlation function depending on the intensity, similar to the way it was done in [1]. The correlation functions of different orders will be measured by illuminating many pixels of the ICCD by a single mode of parametric down-conversion radiation. Different pixels will then play the role of different detectors, and their correlated counts will provide the same information as a multi-port Hanbury Brown-Twiss interferometer.

1. M. Avenhaus, K. Laiho, M.V. Chekhova, and C. Silberhorn, Accessing Higher Order Correlations in Quantum Optical States by Time Multiplexing, PRL 104, 063602 (2010).