arXiv subjects

L. A. Krivitsky

Publications and source records attributed to L. A. Krivitsky.

7 recordsLinked to original sources

arXiv2007

Generation of different Bell states within the SPDC phase-matching bandwidth

We study the frequency-angular lineshape for a phase-matched nonlinear process producing entangled states and show that there is a continuous variety of maximally-entangled states generated for different mismatch values within the natural bandwidth. Detailed considerations are made for two specific methods of polarization entanglement preparation, based on type-II spontaneous parametric down-conversion (SPDC) and on SPDC in two subsequent type-I crystals producing orthogonally polarized photon pairs. It turns out that different Bell states are produced at the center of the SPDC line and on its slopes, corresponding to about half-maximum intensity level. These Bell states can be filtered out by either frequency selection or angular selection, or both. Our theoretical calculations are confirmed by a series of experiments, performed for the two above-mentioned schemes of producing polarization-entangled photon pairs and with two kinds of measurements: frequency-selective and angular-selective.

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arXiv2007

Entanglement generation via phase-matched processes: different Bell states within the linewidth

It is shown, theoretically and experimentally, that at any type-II spontaneous parametric down-conversion (SPDC) phase matching, the decoherence-free singlet Bell state is always present within the natural bandwidth and can be filtered out by a proper spectral selection. Instead of the frequency selection, one can perform time selection of the two-photon time amplitude at the output of a dispersive fibre. Applications to quantum communication are outlined.

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arXiv2006

Go and return propagation of biphotons in fiber and polarization entanglement

Propagation of entangled photons in optical fiber is one of the fundamental issues for realizing quantum communication protocols. When entanglement in polarization is considered, arises the problem of compensating for the fiber effect on photons polarization. In this paper we demonstrate an effective solution where a Faraday mirror allows to cancel undesired effects of polarization drift in fiber. This technique is applied to a protocol for generating Bell states by a narrow temporal selection of the second-order intensity correlation function.

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arXiv2006

Dispersion spreading of polarization-entangled states of light and two-photon interference

We study the interference structure of the second-order intensity correlation function for polarization-entangled two-photon light obtained from type-II collinear frequency-degenerate spontaneous parametric down-conversion (SPDC). The structure is visualised due to the spreading of the two-photon amplitude as two-photon light propagates through optical fibre with group-velocity dispersion (GVD). Because of the spreading, polarization-entangled Bell states can be obtained without any birefringence compensation at the output of the nonlinear crystal; instead, proper time selection of the intensity correlation function is required. A birefringent material inserted at the output of the nonlinear crystal (either reducing the initial o-e delay between the oppositely polarized twin photons or increasing this delay) leads to a more complicated interference structure of the correlation function.

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arXiv2006

Dispersion spreading of biphotons in optical fibres and two-photon interference

We present the first observation of two-photon polarization interference structure in the second-order Glauber's correlation function of two-photon light generated via type-II spontaneous parametric down-conversion. In order to obtain this result, two-photon light is transmitted through an optical fibre and the coincidence distribution is analyzed by means of the START-STOP method. Beyond the experimental demonstration of an interesting effect in quantum optics, these results also have considerable relevance for quantum communications.

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arXiv2004

Statistical Reconstruction of Qutrits

We discuss a procedure of measurement followed by the reproduction of the quantum state of a three-level optical system - a frequency- and spatially degenerate two-photon field. The method of statistical estimation of the quantum state based on solving the likelihood equation and analyzing the statistical properties of the obtained estimates is developed. Using the root approach of estimating quantum states, the initial two-photon state vector is reproduced from the measured fourth moments in the field . The developed approach applied to quantum states reconstruction is based on the amplitudes of mutually complementary processes. Classical algorithm of statistical estimation based on the Fisher information matrix is generalized to the case of quantum systems obeying Bohr's complementarity principle. It has been experimentally proved that biphoton-qutrit states can be reconstructed with the fidelity of 0.995-0.999 and higher.

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arXiv2003

Orthogonality of Biphoton Polarization States

Orthogonality of two-photon polarization states belonging to a single frequency and spatial mode is demonstrated experimentally, in a generalization of the well-known anti-correlation 'dip' experiment.

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