1,721,032 research outputs found
Numerical model for spontaneous down-conversion in atravelling-wave parametric amplifier
No full textRecently there is a rise of interest in optical systems displaying non classical features in the spatial domain. For example the possibility of amplifying a faint coherent image preserving the signal to noise ratio by means of a phase-sensitive parametric amplifier has been recently demonstrated (see e.g. [1]). The physical process we consider, parametric down conversion in a crystal with a (2) nonlinearity, is a good candidate to display such a kind of phenomena since its large amplification bandwidth leads to quantum entanglement between a large number of spatial modes of the optical field.
The observation of such a spatial quantum correlation effects is the aim of an experiment presently performed at the University of Como under the direction of Prof. P. Ditrapani in the framework of the E.C. project QUANTIM (quantum images), in which we are involved.
We consider here the spontaneously down-converted field that is emitted by the amplifier even when no signal field is injected. In this case, phase-matching conditions determine the angular distribution and the frequency spectrum of the emitted field. The signal and idler photons generated in this process are highly correlated one to each other and close to the degenerate frequency they are emitted almost symmetrically with respect to the system axis because of momentum conservation. As a consequence, intensity correlations are observable in the far field when one considers symmetrical detection areas; more precisely, it has been possible to demonstrate analytically that almost complete noise reduction in the subtracted signals measured by two identical detectors can be achieved [2].
The theoretical model that we have developed includes both diffraction and linear dispersion of the signal field. However, all analytical results have been obtained within the perfectly monochromatic plane wave pump approximation. In order to obtain a more realistic description of the system we have also developed a numerical model which includes the effects of the finite size of the pump beam profile and its finite frequency bandwidth. It is important to investigate how these features affects the spatial quantum correlation phenomena predicted by the theory within the plane wave pump approximation, also in view to identify the better conditions under which these correlations can be observed in the experiment. Moreover, a stochatic method has been implemented in order to evaluate the intensity correlation function and the amount of intensity squeezing which can be achieved.
[1] Quantum aspects of imaging, A.Gatti, E.Brambilla, L.A.Lugiato and M.Kolobov J. Opt. B, Quant. and Semiclass. Opt. 2, 196 (2000)
[2] Quantum Entangled Images,
A. Gatti, E. Brambilla, L.A. Lugiato and M.I. Kolobov,
Phys. Rev. Lett. 83, 1763 (1999
Temporal properties of counter-propagating twin beams close to the threshold for mirrorless optical parametric oscillation
Full text linkWe investigate the coherence and correlation properties of counter-propagating twin beams generated in a Mirrorless Optical Parametric Oscillator (MOPO) with distributed feedback in the neighborhood of the device emission threshold
Quantum Imaging
No full textWith respect to the classical area of imaging, the field of Quantum Imaging (QI) aims to devise novel techniques for optical imaging and parallel information processing at the quantum level, by exploiting the quantum nature of light and
the intrinsic parallelism of optical signals. As a matter of fact, the cross-section of a broad-area radiation beam offers the possibility for multichannel configuration, i.e. the availability of several “quantum” channels, each corresponding to a small
area in the section. For example, in each of these independent regions the light may be squeezed, or there may be quantum entanglement between pair of channels. Essentially this approach paves the way to parallel quantum information processing, basically different from the sequential configuration which characterizes, for instance, most schemes of quantum cryptography, in which one detects one photon (or one photon pair) at a time. Needless to say, such a parallel configuration allows for notably increasing the information capacity of the system
Critical behavior of coherence and correlation of counterpropagating twin beams
No full textThis work analyzes the temporal coherence and correlation of twin beams generated in a quasi-phase-matched nonlinear crystal in a counterpropagating configuration, ranging from the low-gain regime, where counterpropagating photon pairs are generated spontaneously, to the regime of stimulated pair production, close to the mirrorless optical parametric oscillator threshold. Here, we show a critical divergence of the correlation time and slowing down of quantum fluctuations originating from the feedback mechanism responsible for the MOPO threshold
Heralding pure single photons: A comparison between counterpropagating and copropagating twin photons
No full textWe investigate different strategies suitable to generate pure heralded single photons through spontaneous parametric down-conversion, comparing the counterpropagating geometry with more conventional copropagating configurations which enhance the purity of the heralded photon state through the technique of group-velocity matching. We derive general results for the Schmidt number associated to the temporal modes, which provide a quantitative estimate of the purity. An analysis of the correlation of twin photons in the temporal domain provides a more physical view of the mechanisms that permit to eliminate the temporal entanglement of the state and to generate high-purity heralded photons. The efficiency of the various strategies and the individual properties of the heralded photons thereby generated are then compared
An introduction to Quantum Imaging
No full textIn the field of quantum imaging one takes advantage of the quantum aspects of light and of the intrinsic parallelism of optical signals to develops new techniques in image processing and parallel processing at the quantum leve
Ghost imaging using homodyne detection
No full textWe present a theoretical study of ghost imaging based on correlated beams arising from parametric down-conversion, and which uses balanced homodyne detection to measure both the signal and idler fields. We analytically show that the signal-idler correlations contain the full amplitude and phase information about an object located in the signal path, both in the near-field and the far-field case. To this end we discuss how to optimize the optical setups in the two imaging paths, including the crucial point regarding how to engineer the phase of the idler local oscillator as to observe the desired orthogonal quadrature components of the image. As is well known, the near-field image resolution is inherently linked to the far-field bandwidth of the image, determined by the bandwidth of the source of the correlated beams. We show how to circumvent this limitation by using a spatial averaging technique which dramatically improves the imaging bandwidth of the far-field correlations as well as speeds up the convergence rate. The results are backed up by numerical simulations taking into account the finite size and duration of the pump pulse
Dimensionality of the spatiotemporal entanglement of parametric down-conversion photon pairs
No full textIn this work, the Schmidt number of the two-photon state generated by parametric down-conversion (PDC) is
evaluated in the framework of a fully spatiotemporal model for PDC. A comparison with the results obtained in
either purely spatial or purely temporal models shows that the degree of entanglement of the PDC state cannot be
trivially reduced to the product of the Schmidt numbers obtained in models with lower dimensionality, unless the
detected bandwidth is very narrow. This result is a consequence of the nonfactorability of the state in the spatial
and temporal degrees of freedoms of twin photons. In the limit of a broad pump beam, we provide a geometrical
interpretation of the Schmidt number as the ratio between the volume of the phase-matching region and of a
correlation volume
Correlated imaging with macroscopic fields
No full textWe demonstrate analytically that correlated imaging in the regime of large photon number can be implemented using both a classical and an entangled source. The classically correlated beams are obtained by dividing thermal radiation by a beam splitter while entangled beams are obtained through parametric down-conversion. By treating the two system in parallel, a formal analogy is outlined between the two cases showing that classical beams can mimic qualitatively all the imaging features of the entangled beams. We also propose a method to improve substantially the imaging performances of correlated imaging which is based on homodyne detection combined with spatial averages
Entanglement of PDC photon pairs
No full textWe investigate the spatio-temporal structure of the biphoton entanglement in parametric down-conversion (PDC) and we demonstrate its non-factorable X-shaped geometry. Such a structure gives access to the ultrabroad bandwidth of PDC, and can be exploited to achieve a biphoton temporal localization in the femtosecond range. This extreme localization is conditioned by our ability to resolve the photon positions in the source near field. The nonfactorability opens the possibility of tailoring the temporal entanglement by acting on the spatial degrees of freedom of twin photons. We also put in evidence distinctive features that characterize type II with respect to type I PD
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