Correlations in twin beams composed of many photon pairs are studied using an intensified CCD camera. Joint signal-idler photon-number distribution and quantum phase-space quasi-distributions determined from experimental data have nonclassical features.Quantum mechanics interprets nonlinear optical processes as physical effects composed of many elementary 'quantum' events in which one (several) photon is annihilated and several (one) photon emerge [1]. This behavior of nonlinearly interacting physical systems is completely unusual in classical physics and lies in heart of nonclassical-light generation. Generation of correlated photon pairs in the process of spontaneous parametric frequency down-conversion is probably the most common example [2]. Fundamental experiments performed for the first time with photon pairs have shown that these nonlinear elementary 'quantum' events have even an 'internal structure'. Measurement using photon pairs in Hong-Ou-Mandel interferometer has revealed this structure demonstrating that photons in one photon pair (photon twins) are generated within a sharp time window typically of several tens of fs [3]. Later experiments have even shown that entangled photon pairs are the same fundamental entities as single photons. Similarly as a single photon can interfere only with itself, an entangled photon pair interferes only with itself [4]. These completely unusual properties of photon pairs have been experimentally tested in numerous experiments with the same qualitative conclusion -fundamental laws of quantum mechanics are a solid basis for the explanation of obtained experimental results. Among others, experimental confirmation of violation of Bell inequalities using photon pairs has excluded neoclassical theories with local hidden variables as a right tool for the description of Nature. Photon pairs are also an indispensable tool in quantum teleportation [5], quantum cryptography or dense coding.The use of intense femtosecond pump fields together with availability of new materials with larger nonlinearities have opened a new area in investigation of twin beams. Nowadays even beams containing many photon pairs generated in a sharp time window can be obtained. The first experiments confirm in agreement with quantum theory that photon pairs inside such beams behave as independent entities. Coherence properties of twin beams reflect those of the pump beam. Moreover, it has been shown that photon pairs can have their origin also in stimulated emission [6] (laser-like generation of twin beams is sometimes mentioned).These fundamental properties of photon pairs then determine statistical properties of 'more intense' twin beams. In this letter, we report on experimental determination of photocount statistics of twin beams [7] using an intensified CCD camera (iCCD). An alternative approach to determine photocount statistics is based on homodyne detection and has revealed correlations in photon numbers of two fields comprising a two-mode squeezed state [8]. In our case, even raw experimental da...

We show first reconstructions of the photon-number distribution obtained with a multi-channel fiber-loop detector. Apart from analyzing the statistics of light pulses this device can serve as a sophisticated post-selection device for experiments in quantum optics and quantum information. We quantify its efficiency by means of the Fisher information and compare it to the efficiency of the ideal photodetector.A major drawback of common detectors of weak light fields is their lack of photon-number resolution. Due to the nonlinear character of amplifying process the response of such detectors is not sensitive to the strength of the input signal. The only two detection events in such a case are "click" and "no click" that correspond to the presence or absence of the signal. A device capable of photon-number resolution would contribute both to fundamental research in quantum optics and to implementation of quantum communication and information protocols. While such devices were recently indeed constructed [1,2,3,4,5,6], they require operation under extreme conditions at present and therefore did not become a common laboratory tool yet. Also, their photonnumber resolution is still limited to only few photons.Another way of circumventing this problem is splitting the input pulse using a multiport device followed by an array of conventional binary detectors as was proposed in [7,8]. In the ideal case of many output ports the input pulse gets perfectly split and each photon is detected separately. However reasonable performance of such device would require a very large number of beamsplitters and detectors which would results in a bulky and costly detection device. It has been suggested [9] to replace the complicated multiport device by a fiber loop and a single photodetector, see Fig. 1. After each round-trip part of the incoming pulse gets transmitted to a conventional binary detector. This results in a time resolved series of detections, each of them corresponding to a different out-put port of the multiport device. Such a multiple photon resolving device has recently been built in our laboratory [10]. The variable ratio coupler inserted at the entrance to the fiber-loop delay line, see Fig. 1, is used to adjust the transmission probabilities of the output channels to a certain extent and thus tweak the overall resolving power of our instrument.The purpose of this communication is twofold. First, we will analyze the performance of the fiber-loop detector and compare it to the efficiency of the ideal photodetection device. Second, we will show how to reconstruct the photon statistics of the input pulse from the data measured at the fiber-loop detector via the maximumlikelihood principle and apply this technique to experimental data.The fiber-loop detector is used to count photons contained in the input pulse in an indirect way. Therefore it is natural to relate its performance to that of the "textbook photon counter"-the ideal photodetector of quantum efficiencyη. The concept of the equivalent efficiency is extremely us...