Martin Hamar

Direct measurement and reconstruction of nonclassical features of twin beams generated in spontaneous parametric down-conversion

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 quasidistributions determined from experimental data have nonclassical features.

Transverse coherence of photon pairs generated in spontaneous parametric down-conversion

Coherence properties of the down-converted beams generated in spontaneous parametric down-conversion are investigated in detail using an iCCD camera. Experimental results are compared with those from a theoretical model developed for pulsed pumping with a Gaussian transverse profile. The results allow us to tailor the shape of correlation area of the signal and idler photons using pump-field and crystal parameters. As an example, splitting of a correlation area caused by a two-peak pump-field spectrum is experimentally studied.

Photon-number distributions of twin beams generated in spontaneous parametric down-conversion and measured by an intensified CCD camera

The measurement of photon-number statistics of fields composed of photon pairs, generated in spontaneous parametric down-conversion and detected by an intensified CCD camera is described. Final quantum detection efficiencies, electronic noises, finite numbers of detector pixels, transverse intensity spatial profiles of the detected beams as well as losses of single photons from a pair are taken into account in a developed general theory of photon-number detection. The measured data provided by an iCCD camera with single-photon detection sensitivity are analyzed along the developed theory. Joint signal-idler photon-number distributions are recovered using the reconstruction method based on the principle of maximum likelihood. The range of applicability of the method is discussed. The reconstructed joint signal-idler photon-number distribution is compared with that obtained by a method that uses superposition of signal and noise and minimizes photoelectron entropy. Statistics of the reconstructed fields are identified to be multi-mode Gaussian. Elements of the measured as well as the reconstructed joint signal-idler photon-number distributions violate classical inequalities. Sub-shot-noise correlations in the difference of the signal and idler photon numbers as well as partial suppression of odd elements in the distribution of the sum of signal and idler photon numbers are observed.

State reconstruction of a multimode twin beam using photodetection

Robust and reliable method for reconstructing quasi-distributions of integrated intensities of twin beams generated in spontaneous parametric down-conversion and entangled in photon numbers is suggested. It utilizes the first and second photocount moments and minimizes the declination from experimental photocount histograms. Qualitatively different forms of quasi-distributions for quantum and classical states are suggested in the method. The transition from quantum to classical states caused by an increased detection noise is discussed. Momentum criterion for non-classicality of twin beams is suggested.

Absolute detector calibration using twin beams

A method for the determination of absolute quantum detection efficiency is suggested based on the measurement of photocount statistics of twin beams. The measured histograms of joint signal-idler photocount statistics allow us to eliminate an additional noise superimposed on an ideal calibration field composed of only photon pairs. This makes the method superior above other approaches presently used. Twin beams are described using a paired variant of quantum superposition of signal and noise.

Simple direct measurement of nonclassical joint signal?idler photon-number statistics and the correlation area of twin photon beams

The measurement of the joint signal–idler photon-number distribution of a field obtained from spontaneous parametric downconversion using an intensified CCD camera is presented. It is shown that a classicality criterion is violated directly by the measured data. Characteristic dimensions of the area of correlation are determined in the same experimental set-up.

Absolute spectral calibration of an intensified CCD camera using twin beams

Twin beams composed of photon pairs are used for absolute determination of quantum detection efficiency of an intensified CCD camera. A method of fitting the experimental photocount statistics with the paired superposition of signal and noise, combined with additional determination of losses in the beams before the detector, is used to reveal the detection efficiencies. The experiment has been performed in several spectral regions defined by the used interference filters. Also, the used twin beams have been reconstructed.

Correlations in far field of photons emitted by parametric fluorescence

The fundamental research of the parametrical fluorescence is discussed. We start with the mathematical model. We have included the final size of nonlinear crystal, the chirped pump pulse of Gaussian elliptical beam in the model. We have verified the numerical results experimentally. We have used two different methods. First one used ICCD camera, and second one, scan by Hong-Ou-Mandel interferometer.

Angular uncertainty of momentum correlations in parametric fluorescence

Uncertainty in determining the emission angles of the signal and idler photons is investigated. The role of the pump beam divergence, the pump-spectrum width, and the crystal size is elucidated. Experimental data obtained using an intensified CCD camera are in good agreement with a numerical model that provides angular and spectral characteristics of the signal and idler fields.

Photocount measurements as a tool for investigation of non-classical properties of twin beams

Joint signal-idler photoelectron distributions of twin beams have been measured recently in two distinct regimes: either the mean number of photon pairs per one pump pulse is lower that the number of independent modes or vice versa. Exploiting a microscopic quantum theory for joint quasi-distributions in parametric down-conversion based on the model of superposition of signal and noise we characterize properties of twin beams in terms of quasidistributions using experimental data. In parallel to the microscopic model, joint signal-idler photon-number distribution is reconstructed using the method of maximum likelihood. Negative values as well as oscillating behavior in quantum region are characteristic for the joint signal-idler quasi-distributions of integrated intensities. The larger the mean number of photon pairs per mode the weaker the quantum features are. However, they survive even in the mesoscopic regime, i.e. when tens of photon pairs per mode are present on average. Also the conditional and difference photon-number distributions are shown to be sub-Poissonian and sub-shot-noise, respectively. Violation of classical inequalities for photon-number distributions is discussed.