a b s t r a c tWe experimentally show that the non-separability of polarization and orbital angular momentum present in a light beam remains preserved under scattering through a random medium like rotating ground glass. We verify this by measuring the degree of polarization and observing the intensity distribution of the beam when projected to different polarization states, before as well as after the scattering. We extend our study to the non-maximally non-separable states also.

In general, the spatial distribution of individual photons (signal or idler) generated by spontaneous parametric down-conversion (SPDC) does not evidently show any particular spatial mode structure because of their randomness in generation and the incoherent nature. Here, we numerically showed that all individual photons generated by the SPDC process carry the transverse amplitude as that of the pump and then confirmed it experimentally. The pump amplitude is revealed in SPDC when individual photons are spatially filtered from the total SPDC distribution. This is observed simply by imaging the photons that are filtered using a minimum-sized aperture. The phase measurements showed that the observed mode distribution does not possess the transverse phase distribution as that of the pump.

Higher order statistical correlations of the optical vector speckle field, formed due to scattering of a vector-vortex beam, are explored. Here, we report on the experimental construction of the Stokes parameters covariance matrix, consisting of all possible spatial Stokes parameters correlation functions. We also propose and experimentally realize a new Stokes correlation functions called Stokes field auto correlation functions. It is observed that the Stokes correlation functions of the vector-vortex beam will be reflected in the respective Stokes correlation functions of the corresponding vector speckle field. The major advantage of proposing Stokes correlation functions is that the Stokes correlation function can be easily tuned by manipulating the polarization of vector-vortex beam used to generate vector speckle field and to get the phase information directly from the intensity measurements. Moreover, this approach leads to a complete experimental Stokes characterization of a broad range of random fields.

We report on controlling the bi-photon orbital angular momentum (OAM) eigen modes in the spontaneous parametric down conversion process by simply adjusting the asymmetry of the pump vortex beam. Adjusting the optic axis of the spiral phase plate (SPP) of phase winding corresponding to OAM mode, l, with respect to the beam propagation axis, we have transformed a Gaussian beam into an asymmetric vortex beam with OAM modes, l, l-1, l-2 …0 with different weightages. Pumping the nonlinear crystal with such asymmetric vortices and controlling their asymmetry we have tailored the spiral spectrum of the bi-photon OAM eigen modes. Calculating the Schmidt number of the bi-photons we observe the increase in the spiral bandwidth of the OAM eigen modes and hence the dimensionality of the system. Although we have restricted our study to show the increase in spiral bandwidth of the bi-photons by simply controlling the asymmetry of the pump vortices, we can, in principle, further enhance the dimensionality of the entangled states by manipulating the pump beam size and crystal length.

Satellites are expected to play a significant role in the future of global quantum networks. Small satellites such as those following CubeSat standards may enable space-based quantum networks. CubeSat type satellites require less resources to develop and deploy and can be used to rapidly qualify technologies in orbit. The constraints of small satellites drive the development of compact quantum light sources, which will be useful even if the final satellite volume is large due to transmitter aperture requirements. In this article, we discuss the state-of-the-art in compact entangled photon-pair sources and suggest challenges that must be met by new source designs.