Quantum Key Distribution (QKD) systems usually exploit the polarization of light to encode bit values, thus limiting to 1 bit the amount of information carried by each photon and placing serious limits on the error rates tolerated. Here we propose the use of two mutually unbiased bases for high-dimensional QKD that exploit the transverse spatial structure and coherence properties of the light field, allowing for the transfer of more than 1 bit per photon. Our proposed method employs coherence modulation with an orthonormal basis of time delays (TD) and the corresponding mutually unbiased basis of wave trains (WT). We construct the mutually unbiased basis set WT using a linear combination of orbital angular momentum OAM modes. Through the use of a high-dimensional alphabet encoded in the TD and WT bases, we achieve a high channel capacity of bits per inspected photon. In addition to exhibiting increased channel capacity, multidimensional QKD systems based on spatiotemporal encoding may be more resilient against intercept-resend eavesdropping attacks. Numerical simulations are presented as tests of the proposed QKD system. Experiments remain to be conducted to verify the concept.
The measurement of the topological charge of laser beams with orbital angular momentum (OAM) is key to many
applications like deciphering information encoded in several channels. Current techniques useful for that purpose are
interferometry, diffraction through different poligonal apertures like triangular or pentagonal and, azimuthal and radial
decomposition. A less explored issue is the diffraction of OAM beams through circular sectors. Jack et al. studied the
angular diffraction of Gaussian beams (whose OAM is null) through a circular sector.
By means of a Fourier transform of the truncated Gaussian beam they showed that the orbital angular momentum
spectrum of the transmitted beam has a sinc-shaped envelope centered at zero orbital angular momentum, the width of
which increases as the central angle of the circular sector decreases.
We analyze here the spectrum of a laser beam with integer OAM that has been diffracted by a circular sector. We
present results for circular sectors of different central angles. For circular π-sector, we also study the influence of the
transmittance in the OAM spectra of the transmitted beam, using straight borders of nanometric thin films of titanium
oxide with different thicknesses.
We use a spatial light modulator with a fork hologram placed on to generate the incoming OAM beam and measure the
evolution of the intensity profile of the diffracted beam as it propagates away from the circular sector. The spectra of the
diffracted OAM beams are shown numerically and experimentally to have a sinc shaped envelope centered at the OAM
value of the incoming OAM wave.