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Journal of Experimental and Theoretical Physics Letters

Controlling the spectrum of a two-photon field: Inhomogeneous broadening due to a temperature gradient

Dmitry A. Kalashnikov, Konstantin G. Katamadze, S. P. Kulik

Published May 1, 2009• 14 citations
Publisher

Abstract

A method is proposed and implemented for controlling the spectrum of spontaneous parametric down-conversion. The method is based on generating a temperature gradient along a nonlinear crystal in which biphotons are generated. In this case, the phase-synchronism conditions are modified due to the temperature dependence of the refractive indices. As a result, the spectral composition of the emitted two-photon field is changed.

DOI
10.1134/s0021364009050026

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A method is proposed and implemented for controlling the spectrum of spontaneous parametric down-conversion. The method is based on generating a temperature gradient along a nonlinear crystal in which biphotons are generated. In this case, the phase-synchronism conditions are modified due to the temperature dependence of the refractive indices. As a result, the spectral composition of the emitted two-photon field is changed.
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Generation of two-photon light with given spectral and temporal properties is of great interest for quantum communication and quantum metrology applications. In particular, preparation of biphotons with ultra-narrow correlation time is a very important task. In a recent series of papers, our group analyzed the generation of twophoton wavepackets, produced by Spontaneous Parametric Down Conversion, in crystals with linearly chirped quasi-phase matching grating. Wavepackets present very broad spectra but a broad spectrum does not necessarily imply small correlation times, although the inverse is true. Indeed, the spectrum broadening induced by the grating is inhomogeneous; for this reason, the two-photon spectral amplitude present a phase (a frequency chirp) that depend nonlinearly on the frequency. Hence, the two-photon wavepackets are not Fourier transform-limited. As suggested in, the ideal way to make the wavepacket perfectly transform limited is to insert in the path of the biphotons a proper optical medium that compensates the non-linear part of the phase factor present in the spectral amplitude. In our work, we investigate the non-local temporal compression of the photons induced by the insertion of a standard optical fibre in the path of one of the two photons. We present and discuss a systematic study of this phenomenon and some optimal situation where the full numerical calculation shows an effect that can be clearly observed with a realistic set-up. The study has open the way to the practical realization of this idea.
A method for generating two-photon wave packets (biphotons) with a small width of the spatial intensity correlation function (correlation radius) has been considered. We propose to prepare a two-photon wave packet inhomogeneously broadened in the angle by means of spontaneous parametric down-conversion in an inhomogeneous crystal. Such biphotons are not diffraction limited; i.e., their correlation radius is much larger than the inverse width of the spatial spectrum. However, the correlation radius of such a biphoton decreases as the biphoton propagates in free space, and transverse compression occurs at a certain distance from the crystal; i.e., the biphoton becomes diffraction limited.