Four-wave mixing in photonic crystal fibers for generation of tailored quantum states of light

We demonstrate a versatile fiber-optic platform for the generation of tailored quantum states of light, designed to enhance the performance of quantum sensing and imaging. The developed platform is based on optical fibers of special design, including photonic-crystal fibers (PCFs).
The design of optical fibers is optimized to provide an ultrahigh-contrast generation of entangled photon pairs confined to entanglement volumes many orders of magnitude smaller than the entanglement volumes attainable through spontaneous parametric down-conversion. Four-wave mixing (FWM) with cross-polarized pump and cross-polarized sidebands enables efficient generation of entangled photon pairs. Generation of cross-polarized photon pairs is achieved by coupling the laser pump into orthogonal polarization modes of a highly birefringent, highly nonlinear, anomalously dispersive PCF. The contrast of photon pairs against the Raman noise is enhanced by setting the pump wavelength to lie deeply within the anomalous dispersion region of the fiber. Broadband quantum states of light generated via vectorial FWM are tailored, by varying the pump wavelength and polarization geometry, from a high-purity, low-dimensional entangled state to a high-entropy entangled state in a space of a very high dimensionality.