Generation of multimode quantum states has drawn much attention recently due to its importance for both fundamental science and the future development of quantum technologies. Here, by using a four-wave mixing process with a conical pump beam, we have experimentally observed about −3.8  dB−3.8  dB of intensity-difference squeezing between a single-axial probe beam and a conical conjugate beam. The multi-spatial-mode nature of the generated quantum-correlated beams has been shown by comparing the variation tendencies of the intensity-difference noise of the probe and conjugate beams under global attenuation and local cutting attenuation. Due to its compactness, phase-insensitive nature, and easy scalability, our scheme may find potential applications in quantum imaging, quantum information processing, and quantum metrology.

  Fig. 1. Scheme for generating and detecting quantum correlated beams. (a) Double-Λ energy level diagram of the 85Rb D1 line; (b) experimental layout. M1-3, high reflectivity mirrors; L1-4, lenses; Axicon, metal axicon mirror; GL, Glan–Laser polarizer; GT, Glan–Thompson polarizer; SA, spectrum analyzer. (c) Schematic of the conical-pump-based FWM. (d) Two-dimensional and threedimensional beam pattern captured by a laser beam profiler.

Experimental observation of quantum correlations in four-wave mixing with a conical pump.pdf