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arXiv:2005.01855 · Physical Review Applied

Quantum Optical Description of Phase Conjugation of Vector Vortex Beams in Stimulated Parametric Down-Conversion

A.G. de Oliveira, Nara Rubiano da Silva, R. Medeiros de Araújo, P.H. Souto Ribeiro, Stephen Patrick Walborn

Published Aug 18, 2020• 20 citations
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Abstract

We present a quantum-optics approach for describing stimulated parametric down-conversion in the two-type-I crystal ``sandwich'' configuration, which allows for parametric interaction of vector vortex beams. We analyze the conditions for which phase conjugation of the seed vector beam occurs. We then use two strategies for defining generalized Stokes parameters to describe phase conjugation of vector vortex beams. These allow for geometrical representations, such as higher-order Poincar\'e spheres. Our results are useful for the description and design of stimulated and spontaneous parametric down-conversion experiments with vector vortex beams.

DOI
10.1103/physrevapplied.14.024048

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Journal of the Optical Society of America B•Nov 26, 2025•2 citations
We present a quantum-optics approach for describing stimulated parametric down-conversion in the two-type-I crystal ``sandwich'' configuration, which allows for parametric interaction of vector vortex beams. We analyze the conditions for which phase conjugation of the seed vector beam occurs. We then use two strategies for defining generalized Stokes parameters to describe phase conjugation of vector vortex beams. These allow for geometrical representations, such as higher-order Poincar'e spheres. Our results are useful for the description and design of stimulated and spontaneous parametric down-conversion experiments with vector vortex beams.
A vector vortex beam presents local polarization states that are spatially modulated in the plane transverse to propagation. By employing a two-crystal-sandwich source allowing for nonlinear interactions between input fields of any polarization, we analyze experimentally the process of stimulated parametric down-conversion and observe unique effects when the pump and seed fields are vector vortex beams. We reconstruct the transverse polarization pattern of the generated idler beam by Stokes polarimetry. The experimental results obtained are in good qualitative agreement with theory and exhibit manifold polarization transverse distributions for different combinations of pump and seed vector vortex beams, which may find applications in both classical and quantum optics.
Stimulated parametric down-conversion (StimPDC) is the seeded version of the process that underlies much of photonic quantum information. Replacing the vacuum at one input port with a coherent seed leaves the interaction Hamiltonian, the phase-matching function and the two-photon amplitude unchanged, while raising the output to levels accessible with ordinary classical detectors. This review treats the transverse spatial physics of that process. We show that a single relation, in which the idler amplitude is the pointwise product of the pump field with the complex conjugate of the seed, organizes results usually presented separately: the image and coherence transfer experiments of the 1990s; phase conjugation and its use for wavefront correction; the conservation and conjugation of orbital angular momentum, together with the generation of radial modes and geometric intensity patterns; and the processing of vector vortex beams carrying spatially varying polarization. We then consider StimPDC as an instrument rather than as an object of study, covering stimulated emission tomography, the experimental realization of Klyshko's advanced-wave picture, and the transfer of partial coherence, including the conservation of the twist phase of twisted Gaussian Schell-model beams. We close with the quantum regime, reached when the seed approaches the single-photon level or the idler is used as a herald, and with an assessment of open problems and prospective applications.
Summary form only given. Spontaneous degenerate parametric scattering [down-conversion] is a 3-photon process with one pump photon of frequency 2/spl omega/ and two output photons of frequency /spl omega/. An optical vortex is defined as a beam containing a phase singularity, i.e. the change of phase around the center of the beam equals 2 /spl pi/m; where the integer m is the topological charge. Our experimental results using a BaB/sub 2/O/sub 4/ crystal under type II phase matching conditions and numerical simulations suggest that the vortex in the pump beam is transformed into a speckle-like structure, effectively that structure is composed of randomly occurring vortex and Gaussian beams. The two down-converted output signals (signal and idler) appear as rings of orthogonal polarization and, therefore, can be easily distinguished.
We re-examine the properties of the states produced by nondegenerate coherently stimulated parametric down-conversion wherein the signal and idler modes are seeded with coherent states of light and where the nonlinear crystal is driven by a strong classical field as described by the parametric approximation. The states produced are the two-mode squeezed coherent states defined with a specific ordering of operators, namely, the displacement operators of the two modes acting on the double vacuum state followed by the action of the two-mode squeeze operator representing the down-conversion process. Though mathematically equivalent to the reverse ordering of operators, but with different displacement parameters, the ordering we consider is closely related to what could most easily be implemented in the laboratory. The statistical properties of the state are studied with an emphasis on how they, and its average photon number, are affected by the various controllable phases, namely, those of the classical pump field of the two input coherent states. We then consider the multiphoton interference effects that arise if the two beams are overlapped on a 50:50 beam splitter, investigating the role of the phases in controlling the statistical properties of the output states. Finally, we study the prospects for the application of the states to quantum-optical interferometry to obtain sensitivities in phase-shift measurements beyond the standard quantum limit.
Stimulated parametric down-conversion (StimPDC) produces an idler beam that exhibits a phase conjugate of the spatial structure of the classical seed beam. Extending this effect to the single-photon regime can, in principle, enable the control of the orbital angular momentum of single photons. However, its experimental implementation remains challenging due to the lower photon flux of realistic single-photon sources compared to a classical pump beam, which causes the StimPDC process to occur less frequently compared to its spontaneous counterpart. To overcome this, we propose a cascaded parametric down-conversion scheme, in which the signal photon from a spontaneously down-converted pair seeds the StimPDC process, while the idler photon heralds the seed's presence. This configuration uses coincidence detection to effectively post-select for stimulated events among the background of spontaneous contributions. We show that phase conjugation at the single-photon level manifests as a bias towards equal orbital angular momentum between the heralding idler and the stimulated idler. Our results offer a practical route to manipulate single-photon correlations in the orbital angular momentum degree of freedom and could have implications for applications such as free-space quantum communication and quantum-enhanced sensing.
The ability to manipulate the orbital angular momentum (OAM) of a single photon is of vital importance to photonic quantum technologies. A particularly relevant operation is the phase conjugation of OAM. One can in principle achieve this using stimulated parametric down-conversion (StimPDC), which produces an idler beam that exhibits a phase conjugate of the seed beam’s spatial structure. However, implementing StimPDC for single-photon OAM phase conjugation remains challenging. This is because the photon flux of realistic single-photon sources is low compared to that of a classical pump beam, which causes the StimPDC process to occur less frequently compared to its spontaneous counterpart. To overcome this, we propose a cascaded parametric down-conversion scheme, in which the signal photon from a spontaneously down-converted pair seeds the StimPDC process, while the idler photon heralds the seed’s presence. This configuration uses coincidence detection to effectively post-select for stimulated events among the background of spontaneous contributions. We show that single-photon OAM phase conjugation manifests as a bias toward equal OAM between the heralding idler and the stimulated idler. Our results offer a practical route to manipulate single-photon correlations in the OAM degree of freedom and could have implications for applications such as free-space quantum communication and quantum-enhanced sensing.