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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Engineering of spectral properties of two-photon states, preliminary results

Alessio Avella, G. Brida, Maria V. Chekhova, Marco Genovese, Marco Gramegna, Mélissa Vieille Grosjean, Alexander P. Shurupov

Published Oct 15, 2012• 0 citations
Publisher

Abstract

We present an experimental implementation of spectral properties engineering on biphoton light, emitted via ultrafast type II spontaneous parametric down conversion (SPDC), based on the shaping of the pump pulse spectrum and propagation of the emitted correlated photons through dispersive media. Spectral properties of a biphoton state are fully characterized by the two-photon spectral amplitude (TPSA). Exploiting the group velocity dispersion (GVD) induced by the passage of optical fields through dispersive media, an energy to time two dimensional Fourier transform of the TPSA is operated: this returns a technique to reconstruct TPSA by means of a temporal measurement among the delay between the laser pulse emission (trigger) and the detection times of the two correlated photons. Exploiting this kind of measurement it is possible to deeply resolve the interference pattern in the shape of TPSA. In this research we report on the conditions under which subtle structure on TPSA spectra can be deliberately engineered via modulation of the pump beam spectrum.

DOI
10.1117/12.981993

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We present an experimental implementation of spectral properties engineering on biphoton light, emitted via ultrafast type II spontaneous parametric down conversion (SPDC), based on the shaping of the pump pulse spectrum and propagation of the emitted correlated photons through dispersive media. Spectral properties of a biphoton state are fully characterized by the two-photon spectral amplitude (TPSA). Exploiting the group velocity dispersion (GVD) induced by the passage of optical fields through dispersive media, an energy to time two dimensional Fourier transform of the TPSA is operated: this returns a technique to reconstruct TPSA by means of a temporal measurement among the delay between the laser pulse emission (trigger) and the detection times of the two correlated photons. Exploiting this kind of measurement it is possible to deeply resolve the interference pattern in the shape of TPSA. In this research we report on the conditions under which subtle structure on TPSA spectra can be deliberately engineered via modulation of the pump beam spectrum.
The ability to access high dimensionality in Hilbert spaces represents a demanding key-stone for state-of-the-art quantum information. The manipulation of entangled states in continuous variables, wavevector as well frequency, represents a powerful resource in this sense. The number of dimensions of the Hilbert space that can be used in practical information protocols can be determined by the number of Schmidt modes that it is possible to address one by one. In the case of wavevector variables, the Schmidt modes can be losslessly selected using single-mode fibre and a spatial light modulator, but no similar procedure exists for the frequency space. The aim of this work is to present a technique to engineer the spectral properties of biphoton light, emitted via ultrafast spontaneous parametric down conversion, in such a way that the two-photon spectral amplitude (TPSA) contains several non-overlapping Schmidt modes, each of which can be filtered losslessly in frequency variables. Such TPSA manipulation is operated by a fine balancing of parameters like the pump frequency, the shaping of pump pulse spectrum, the dispersion dependence of spontaneous parametric down-conversion crystals as well as their length. Measurements have been performed exploiting the group velocity dispersion induced by the passage of optical fields through dispersive media, operating a frequency-to-time two-dimensional Fourier transform of the TPSA. Exploiting this kind of measurement we experimentally demonstrate the ability to control the Schmidt modes structure in TPSA through the pump spectrum manipulation.
In spontaneous parametric down-conversion, the spectral correlations between the signal and the idler are expressed by the joint spectral amplitude (JSA) function. However, in the standard coincidence measurements, the phase information of the JSA is lost, and only the square of the absolute value of the JSA is recorded, thus preventing full characterization of the biphoton state. Here, we present an experimental technique to investigate the interference of biphoton joint spectral amplitudes, unlocking new avenues in quantum photonics research. Our method explores phase-dependent phenomena within entangled biphoton spectra. This is achieved by simultaneously pumping two structured nonlinear photonic crystals and observing their interference, which reveals previously inaccessible effects with direct intensity measurements. We demonstrate the versatility of our technique by analyzing two types of joint spectra: one exhibiting a two-lobe shape and the other a three-lobe shape. Additionally, we reconstruct the joint spectral amplitudes for both scenarios and observe good agreement with theoretical predictions. These results pave the way for developing advanced quantum communication and information processing technologies using biphoton spectra.
This work is focused on the engineering and characterization of the frequency-time properties of photon pairs generated with a semiconductor source. This device emits photons propagating in opposite directions at telecom wavelengths using spontaneous parametric down-conversion at room temperature. A microcavity integrated in the source allows the resonance of the transverse pump beam, enhancing the efficiency of the nonlinear interaction. We give a theoretical description of the down-conversion process in the source and we show how the spatial and spectral properties of the pump beam impact the properties of the photons, in particular their degree of entanglement in frequency allowing a versatile engineering. The variety of states that can be produced is illustrated using the Joint Spectral Amplitude. Two techniques allowing the characterization of the Joint Spectral Intensity have been implemented. The first one is a single photon spectrograph. The second one, based on the stimulation of the down-conversion process showed a dramatic improvement in terms of resolution and integration time. Both techniques allowed the demonstration of the frequency correlation engineering by varying the waist and curvature radius of the pump beam. Examples of more exotic states and a technique to characterize their chronocyclic Wigner function are theoretically investigated. The relationship between time-frequency properties and degree of entanglement in polarization of the biphoton is also explored.
Spontaneous parametric down-conversion (SPDC) is the most often used source of entangled photon states in quantum optics. Recently, it has been suggested in theory that the reduction of polarization entanglement between the photon pair generated via the type-II SPDC process pumped by a ultrafast laser is due to the asymmetry in the joint spectrum function, which is the indicative of the frequency entanglement between the two photons. In this paper, we report the measurement of the joint spectrum function (i.e., direct observation of the degree of frequency entanglement) of the two-photon state of type-II SPDC in both cw- and ultrafast-laser pumping conditions. We observed that, in agreement with the theory, the degree of frequency entanglement (the shape of the joint spectrum function) is indeed different for cw- and ultrafast-pumped type-II SPDC. Our results are of importance in "engineering" the two-photon state for specific quantum applications, such as quantum cryptography.