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Journal of Modern Optics

Manipulation of quantum spatial properties of light

P. H. Souto Ribeiro, D. P. Caetano, Carlos Henrique Monken, J. T. C. Pardal, Antonio Zelaquett Khoury

Published Apr 1, 2004• 1 citation
Publisher

Abstract

We discuss how it is possible to manipulate quantum spatial properties of light beams by using the transfer of the angular spectrum. Twin beams from the parametric down-conversion can be prepared in a state that carries two different quantum images. At the same time, the twin photons can be entangled in polarization. This preparation allows the selection between the detection of each one of the images at a time or a mixture by performing proper projections onto polarization sub-spaces. As another example, we show how to produce a light beam presenting spatial anti-bunching through the control of the entangled angular spectrum and reflections.

DOI
10.1080/09500340408233613

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We discuss how it is possible to manipulate quantum spatial properties of light beams by using the transfer of the angular spectrum. Twin beams from the parametric down-conversion can be prepared in a state that carries two different quantum images. At the same time, the twin photons can be entangled in polarization. This preparation allows the selection between the detection of each one of the images at a time or a mixture by performing proper projections onto polarization sub-spaces. As another example, we show how to produce a light beam presenting spatial anti-bunching through the control of the entangled angular spectrum and reflections.
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We demonstrate both theoretically and experimentally the manipulation of quantum entangled images, using the coupling between polarization entanglement and the entanglement for the transverse degrees of freedom. Starting from a two-crystal bright source of polarization-entangled photons, we arrive at a configuration in which we are able to prepare superposition states for the down-converted angular spectrum while the polarization superposition state is preserved. This scheme allows us to select the detected quantum image by performing projections onto polarization subspaces. We also demonstrate the potential of the system for applications to quantum cryptography of images.
The two-photon state generated by spontaneous parametric down-conversion (SPDC) exhibit spatial entanglement embedded in the corresponding mode function. The control of the spatial characteristics of the generated two-photon state is an issue of paramount importance. For example, the spatial entanglement of the two down converted photons forms the basis of quantum imaging, and entanglement in orbital angular momentum has opened a new scenario for implementing multidimensional Hilbert spaces. We put forward several techniques to engineer the spatial structure of entangled two-photon states generated in SPDC. The first strategy we consider for spatial control of the quantum state makes use of the direct manipulation of the pump beam. This technique makes feasible to prepare arbitrary engineered entangled states in any d-dimensional Hilbert space. The second strategy is based on the proper preparation of the down-converting crystal itself, namely quantum state manipulation by quasi-phase-matching (QPM) engineering. We use properly designed transversely varying QPM gratings in nonlinear crystals.
A recent work [H. H. Arnaut and G. A. Barbosa, Phys. Rev. Lett. 85, 286 (2000)] has shown eigenstates describing pairs of photons from spontaneous parametric down-conversion, which are eigenstates of orbital, intrinsic, and total momentum. Experimental demonstration of transfer of angular momentum to parametric down-converted photons has also been achieved [A. Mair, A. Vaziri, G. Weihs, and A. Zeilinger, Nature 412, 313 (2001)]. This work shows conditions for phase matching and examples are given for a Laguerre-mode pump beam. Polar and azimuthal angles are connected by phase matching giving a peculiar divergence pattern for the down-converted light. Full entanglement of polarization and angular-momentum states of photons is discussed.
We develop the formal approach to the angular spectrum transfer in parametric down-conversion that allows pumping with a structured beam. The scheme is based on an entangled photon source pumped by a laser beam structured with a vector-vortex polarization profile. This creates a two-photon quantum state exhibiting polarization-dependent transverse correlations that can be accessed through coincidence measurements on the spatially separated photons. The calculated correlations are shown to present a spin-orbit profile typical of vector beams, however, distributed on separate measurement regions. Our approach allows the design of the pump beam vector spatial structure and measurement strategies for potential applications of these entangled states, such as in quantum communication.
The ability to use the temporal and spatial degrees of freedom of quantum states of light to encode and transmit information is crucial for a robust and efficient quantum network. In particular, the potential offered by the large dimensionality of the spatial degree of freedom remains unfulfilled, as the necessary level of control required to encode information remains elusive. We encode information in the distribution of the spatial correlations of entangled twin beams by taking advantage of their dependence on the angular spectrum of the pump needed for four-wave mixing. We show that the encoded information can only be extracted through joint spatial measurements of the twin beams and not through individual beam measurements and that the temporal quantum correlations are not modified. The ability to engineer the spatial properties of twin beams will enable high-capacity quantum networks and quantum-enhanced spatially resolved sensing and imaging.