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Journal of Optics B Quantum and Semiclassical Optics

Entanglement of the transverse degrees of freedom of the photon

P. H. Souto Ribeiro, Marcelo França Santos, P. Milman

Published Aug 1, 2002• 4 citations
Publisher

Abstract

Using two parametric down-converters, it is possible to prepare twin photons in position-entangled states, where the photons are labelled by their birth position. We have measured the interference between the two probability amplitudes for a twin-photon pair: of being born in the first crystal or in the second one. The oscillation frequency of the coincidence pattern can be varied through independent phase differences in the signal and idler beams. Entangled states in position and also in the polarization sub-space can be prepared with this source. In this case, measurement of the position interference can give us information about the degree of polarization entanglement.

DOI
10.1088/1464-4266/4/4/335

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Using two parametric down-converters, it is possible to prepare twin photons in position-entangled states, where the photons are labelled by their birth position. We have measured the interference between the two probability amplitudes for a twin-photon pair: of being born in the first crystal or in the second one. The oscillation frequency of the coincidence pattern can be varied through independent phase differences in the signal and idler beams. Entangled states in position and also in the polarization sub-space can be prepared with this source. In this case, measurement of the position interference can give us information about the degree of polarization entanglement.
We produce polarization entangled states with variable degree of entanglement for twin photons. Entanglement in polarization is coupled to entanglement in position that produces transverse coincidence interference fringes. We show both theoretically and experimentally that we can use the position interference pattern to measure the polarization degree of entanglement. The coupling between polarization and transverse degrees of freedom are demonstrated to be useful in the manipulation of the entanglement of the twin photons.
Summary form only given. The vast majority of experimental studies of entanglement is based on the use of a polarization entangled photon pair or multiples thereof. Typically, such pairs are produced by the process of spontaneous parametric down conversion in a crystal that exhibits a nonzero second-order nonlinear susceptibility and is cut for type II phase matching. These photons are orthogonally polarized along directions imposed by the symmetry, cut and orientation of the nonlinear crystal. Polarization-entangled pairs are then selected by spatial, and possibly spectral, filtering of the down-converted light. When the crystal is pumped with linearly polarized light a rather intuitive picture arises. If one selects the polarization of the signal photon to be linear, the idler photon will also be linearly polarized in such a way that /spl alpha//sub p/ = -(/spl alpha//sub s/ + /spl alpha//sub i/). Here /spl alpha//sub p,s,i/ refer to the polarization directions of pump, signal and idler, respectively. Their polarization state can be characterized by points on the equator of the Poincare sphere. If, for linear polarized pump, the signal is selected to be elliptically polarized, the idler will also be elliptically polarized. On the Poincare sphere signal and idler have equal latitude (on the same hemisphere), while their longitude is determined by the same relation as above. In all cases signal and idler are maximally entangled.
We characterize entanglement in two-qubit pure states encoded in transverse momenta of twin photons obtained from spontaneous parametric down-conversion. Two alternate methods are employed: (i) measurement of conditional interference patterns and (ii) measurement of the marginal probability that yields the single-photon interference pattern. Both methods are local with classical communication and rely on Schmidt decomposition of the quantum state, which is generated by letting the photons propagate through an appropriate lens system. In both cases we can obtain the concurrence either through the Schmidt coefficients in the first method or directly relating the concurrence to the visibility of the interference pattern in the second one.
In the non-linear process called parametric down-conversion, it is possible to perform frequency conversion in the optical domain. For instance, when one laser beam shines a non-linear crystal, some of its photons are converted into two new photons with frequencies that are different from the frequency of the incident laser. This process is also called parametric down-conversion, because it is based in spontaneous emmission. The pair of photons obtained, are produced basically in the same instant of time, then they are called twin photons. Moreover, besides simultaneity, they also share a non-factorable quantum state. This property makes these photons a very special tool, because they are in an entangled state for some degrees of freedom and they can be prepared in an entangled state for other ones. In this talk, I will discuss the motivation of Quantum Computation and Quantum Communication, present the concept of Quantum Entanglement, and discuss the application of photoic entanglement for the understanding of the Foundations of Quantum Mechanics and the research in Quantum Information.
We review recent theoretical and experimental works where are proposed and demonstrated how to use photon pairs created by spontaneous parametric down-conversion to generate entangled states of D-dimensional quantum systems, or qudits. This is the first demonstration of high-dimensional entanglement based on the intrinsic transverse momentum entanglement of the type-II down-converted photons. The qudit space is defined by an aperture made up of an opaque screen with D slits (paths), placed in the arms of the twin photons. By manipulating the pump beam profile we can prepare different entangled states of these possible paths. We focus our attention on an important case for applications in quantum information: the maximally entangled states. Experimental results for qudits with D=4 and D=8 are shown and measuring a two-photon conditional interference, we also demonstrate the nonclassical character of the correlations.