Quantum Tomograph for Measurement and Characterization of Quantum States of Biphoton Sources
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QUANTUM TOMOGRAPH FOR MEASUREMENT AND CHARACTERIZATION OF QUANTUM STATES OF BIPHOTON SOURCES D. N. Frolovtsev,1 S. A. Magnitskiy,1 and A. V. Demin2
UDC 53.081.7
The development of methods and devices for measuring the quantum states of photon fluxes is a matter of current interest. In this paper, we propose a prototype device for characterizing biphoton light sources using quantum tomography based on spontaneous parametric down-conversion. This prototype device is an experimental implementation of a specialized quantum tomograph designed to measure the quantum polarization states of radiation generated by biphoton sources. In this article, we present the operational principle of the device for characterizing biphoton light sources and describe our specially developed software that enables determination of the statistical characteristics of the measured quantum state, calculation of the tomographic and most probable estimates of the density matrix, and the measurement errors of the density matrix elements, as well as evaluation of the quality of the quantum state of the biphotons. Keywords: photonics, quantum states, quantum tomography, spontaneous parametric down-conversion.
Introduction. In recent years, interest in the characterization of low-photon light sources has increased dramatically. This is because optical low-photon quantum technologies have moved beyond the laboratories and are under intensive preparation for introduction into industry applications. As such, the creation of methods for measuring the quantum states of photon fluxes and devices for measuring such states has become particularly relevant. The authors in ref. [1] state that quantum tomography can provide the most complete information about the quantum state of a photon flux [2]. To date, various tomographic methods have been developed (see, for example, [3, 4]), which measure the quantum states of the light fields generated by single photon sources. Researchers at the Lomonosov Moscow State University have created a prototype quantum tomograph device for characterizing biphoton light sources as part of the currently developed quantum optical standards [5]. This tomograph comprises a quantum state meter and a personal computer with specialized software, i.e., the QTomograhy program. This program enables full characterization of the quantum state of the tested biphoton fluxes generated by a spontaneous parametric down-conversion (SPD) source. The program calculates the elements of the density matrix, estimates the measurement errors of these density matrix elements, and calculates the values of the measured quantum state characteristics, namely purity (linear entropy and von Neumann entropy) and entanglement (concurrence, tangle, and the formation entanglement). Device and operational principle of quantum tomography. Figure 1 shows an installation diagram of the device that measures the polarization quantum states of biphotons. This device comprises two modules, namely, the test biphoton source (a source of time-correlated photon pairs) and
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