Shashi Prabhakar

Experimental generation of ring-shaped beams with random sources

We have experimentally reproduced ring-shaped beams from the scattered Laguerre-Gaussian and Bessel-Gaussian beams. A rotating ground glass plate is used as a scattering medium, and a plano-convex lens collects the scattered light to generate ring-shaped beams at the Fourier plane. The obtained experimental results are supported with the numerical results and are in good agreement with the theoretical results proposed by Wang et al. [Opt. Express17, 22366 (2009)].

Information content of optical vortex fields.

We study, experimentally as well as theoretically, the spatial coherence function and the Wigner distribution function for one-dimensional projections of optical vortices of different orders. The information entropy derived from the spatial coherence functions has been used to quantify the information content of the vortices and compared with those obtained for the Gaussian beam. The experimental results verify the theoretical findings of Agarwal and Banerji [Opt. Lett. 27, 800 (2002)].

Higher order optical vortices and formation of speckles

We have experimentally generated higher order optical vortices and scattered them through a ground glass plate that results in speckle formation. Intensity autocorrelation measurements of speckles show that their size decreases with an increase in the order of the vortex. It implies an increase in the angular diameter of the vortices with their order. The characterization of vortices in terms of their annular bright ring also helps us to understand these observations. The results may find applications in stellar intensity interferometry and thermal ghost imaging.

Entanglement of a quantum optical elliptic vortex

Abstract We calculate the entanglement of a generalized elliptical vortex formed by quantized radiation field, using Wigner quasiprobability distribution function for such states. We find a critical squeezing parameter above which the entanglement is less for higher vorticity, which is counter intuitive.

Divergence of optical vortex beams.

We show, both theoretically and experimentally, that the propagation of optical vortices in free space can be analyzed by using the width [w(z)] of the host Gaussian beam and the inner and outer radii of the vortex beam at the source plane (z=0) as defined in [Opt. Lett.39, 4364 (2014)10.1364/OL.39.004364OPLEDP0146-9592]. We also studied the divergence of vortex beams, considered as the rate of change of inner or outer radius with the propagation distance (z), and found that it varies with the order in the same way as that of the inner and outer radii at z=0. These results may be useful in designing optical fibers for orbital angular momentum modes that play a crucial role in quantum communication.

Recovering the vorticity of a light beam after scattering

We generate optical vortices and scatter them through a rough surface. However, the scattered light passing through a lens shows the same vorticity when probed at the Fourier plane. The vorticity is measured using a nonseparable state of polarization and orbital angular momentum of light as it cannot be confirmed by the standard interferometric technique. The observed vorticity is found to be independent of the amount of scattered light collected. Therefore, vortices can be used as information carriers even in the presence of scattering media. The experimental results are well supported by the theoretical results.

Measuring the Mueller matrix of an arbitrary optical element with a universal SU(2) polarization gadget

We propose a new method for determining the Mueller matrix of an arbitrary optical element and verify it with three known optical elements. This method makes use of two universal SU(2) polarization gadgets to obtain the projection matrix directly from the experiment. It allows us to determine the Mueller matrix without precalibration of the setup, since the generated polarization states are fully determined by the azimuths of the wave plates. We calculate errors in determining the Mueller matrix and compare with other techniques.

Non-diffracting speckles of a perfect vortex beam

We generate perfect optical vortex (POV) beams, whose intensity distribution is independent of the order, and scatter them through a rough surface. We show that the size of produced speckles is independent of the order of the POV and their Fourier transform gives the random non-diffracting fields. The invariant size of speckles over the free space propagation verifies their non-diffracting or non-diverging nature. The size of speckles can be easily controlled by changing the axicon parameter, used to generate the Bessel–Gauss beams whose Fourier transform provides the POV. These results may be useful in applications of POV for authentication in cryptography.

Three-particle hyper-entanglement: teleportation and quantum key distribution

We present a scheme to generate three-particle hyper-entanglement utilizing polarization and orbital angular momentum (OAM) of photons. We show that the generated state can be used to teleport a two-qubit state described by the polarization and the OAM. The proposed quantum system has also been used to describe a new efficient quantum key distribution (QKD) protocol. We give a sketch of the experimental arrangement to realize the proposed teleportation and the QKD.

Violation of Bell's inequality for phase singular beams

We have considered optical beams with phase singularity and experimentally verified that these beams, although being classical, have properties of two mode entanglement in quantum states. We have observed the violation of Bells inequality for continuous variables using the Wigner distribution function (WDF) proposed by Chowdhury et al. [Phys. Rev. A \textbf{88}, 013830 (2013)]. Our experiment establishes a new form of Bells inequality in terms of the WDF which can be used for classical as well as quantum systems.