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arXiv:1005.2937 · Optics Express

Detection of multimode spatial correlation in PDC and application to the absolute calibration of a CCD camera

G. Brida, Ivo Pietro Degiovanni, Marco Genovese, Maria Luisa Rastello, Ivano Ruo Berchera

Published Sep 14, 2010• 73 citations
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Abstract

We propose and demonstrate experimentally a new method based on the spatial entanglement for the absolute calibration of analog detectors. The idea consists on measuring the sub-shot-noise intensity correlation between two branches of parametric down conversion, containing many pairwise correlated spatial modes. We calibrate a scientific CCD camera and a preliminary evaluation of the uncertainty indicates the metrological interest of the method.

DOI
10.1364/oe.18.020572

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We propose and demonstrate experimentally a new method based on the spatial entanglement for the absolute calibration of analog detectors. The idea consists on measuring the sub-shot-noise intensity correlation between two branches of parametric down conversion, containing many pairwise correlated spatial modes. We calibrate a scientific CCD camera and a preliminary evaluation of the uncertainty indicates the metrological interest of the method.
We report on the absolute calibration of a Charge-Coupled Device (CCD) camera by exploiting quantum correlation. This method exploits a certain number of spatial pairwise quantum correlated modes produced by spontaneous parametric-down-conversion. We develop a measurement model accounting for all the uncertainty contributions, and we reach the relative uncertainty of 0.3% in low photon flux regime. This represents a significant step forward for the characterization of (scientific) CCDs used in mesoscopic light regime.
Istituto Nazionale di Ricerca Metrologica (INRIM), Strada delle Cacce 91, 10135 Torino, Italy. E-mail:i.ruoberchera@inrim.it Quantum correlations of twin beams represent a resource of great importance for the developments of quantum technologies allowing unprecedented results in quantum information, metrology and quantum imaging. Focusing on the measurement of the photon numbers, spatial multimode correlations beyond the shot noise limit have been experimentally demonstrated by using parametric down conversion (PDC) and CCD camera (both at the single photon level and at higher photon flux) [1], and also with other interesting techniques [2]. One of their natural application is the quantum imaging of weak absorbing objects, showed in Fig.1, with sensitivity beyond the shot-noise-level [3]. The idea consists in measuring the intensity pattern on one branch of PDC, where the object has been inserted, and then subtracting the correlated noise pattern measured in the other branch that does not interact with the object. In our presentation we overview the sub-shot noise imaging protocol mentioned above and other applications where the spatial quantum correlation can give some advantages over the classical ones. As an example, we studied the signalto-noise ratio (SNR) of ghost imaging protocols with quantum and classical light [4]. We realized a ghost imaging experiment in the mesoscopic regime (one thousand of photon per pixel per shot), obtaining substantial quantum enhanced performances (see Fig.1). We also demonstrated that spatial quantum correlations provide the possibility for the absolute calibration of analog and/or photon number resolving detectors, providing the first absolute calibration (without any comparison with reference standards, neither optical source nor detector) of a scientific CCD camera [5]. In this first realization we already reached interesting uncertainty level. All these findings indicate that the control and detection of quantum spatial correlation is a rather mature field and other application to different sensing scenarios can be envisaged.
Absolute calibration can be realized by means of correlation photon which is generated by the parametric down conversion. The main difficulty lies in obtaining correlation information about photon flux when this method is applied to analog detector calibration process. A novel method of processing the photocurrent on the basis of detecting multimode spatial correlation is proposed. By converting the charge quantity contained in the photocurrent detected in a certain time interval into the photon counting, and by using double channels balance detection and measuring mean photon counts of each model to correct the dual channels fluctuations, the high accuracy calibration of quantum efficiency can be achieved. The photon fluxes of two channels are balanced by inserting an adjustable attenuator in one optical path. The cross section of pumping beam is comparable to the detection area to ensure three-wave colinearity, and the coherent area of the correlation photons is obtained by measuring pump beam waist and lens focus length. With the known detection area, coherence time and coherence area, the average photon number of each mode is computed. This process should be performed under the average photon number of each mode as a reference which could be used for the proportional scaling of equivalent photons of two channels. Based on this new approach, the absolute power responsivity of an InSb detector is calibrated at 3390 nm with correlated photon pairs at 631 and 3390 nm. The calibration procedure and experiments are described and the uncertainty of this method is analyzed. The results show a relative combination uncertainty of about 7.785% for this calibration method, which agrees well with the result independently obtained in the national photoelectronic metrology laboratory within a relative difference of about 3.6%. This result verifies that the quantum efficiency of an analog detector can be calibrated by the correlated photon method, which has potential applications in highly accurate radiometric calibration without external standards.
Twin beams generated by Parametric Down Conversion (PDC) exhibit quantum correlations that has been effectively used as a tool for many applications including calibration of single photon detectors. By now, detection of multi-mode spatial correlations is a mature field and in principle, only depends on the transmission and detection efficiency of the devices and the channel. In [2, 4, 5], the authors utilized their know-how on almost perfect selection of modes of pairwise correlated entangled beams and the optimization of the noise reduction to below the shot-noise level, for absolute calibration of Charge Coupled Device (CCD) cameras. The same basic principle is currently being considered by the same authors for possible use in Quantum Key Distribution (QKD) [3, 1]. The main advantage in such an approach would be the ability to work with much higher photon fluxes than that of a single photon regime that is theoretically required for discrete variable QKD applications (in practice, very weak laser pulses with mean photon count below one are used).The natural setup of quantization of CCD detection area and subsequent measurement of the correlation statistic needed to detect the presence of the eavesdropper Eve, leads to a QKD channel model that is a Discrete Memoryless Channel (DMC) with a number of inputs and outputs that can be more than two (i.e., the channel is a multi-level DMC). This paper investigates the use of Low Density Parity Check (LDPC) codes for information reconciliation on the effective parallel channels associated with the multi-level DMC. The performance of such codes are shown to be close to the theoretical limits
Quantum properties of the optical field represent a resource of the utmost relevance for the development of quantum technologies, allowing unprecedented results in disciplines ranging from quantum information and metrology to quantum imaging. Spatial quantum correlations generated by of parametric down conversion (PDC) represents a tool for quantum imaging because they are intrinsically multimode, a requirement for obtaining large degree of correlation over small portions of the beams, allowing to register the spatial structure of an object. In particular a very interesting example is provided by the detection of weak objects, a result that could have important practical applications. The principle of this technique is to take advantage of the correlation in the noise of two conjugated branches of PDC emission: in fact, subtracting the noise measured on one branch from the image of a weak object obtained in the other branch, the image of the object, eventually previously hidden in the noise, could be restored. Here, after a general summary of quantum imaging techniques, firstly we will show how we have reached a sub shot noise regime and then improved this result up to reach a regime where it was possible to achieve the first experimental realisation of sub shot noise imaging of a weak absorbing object.