Spatial quantum correlations in entangled twin beams

Abstract

The study of the spatial quantum correlations in entangled beams of light has become an active research area due to the role that these quantum correlations play in the field of quantum imaging, which promises to improve optical resolution and image detection and to enhance quantum communications through parallel quantum information encoding. The presence of spatial quantum correlations is a result of momentum conservation in the process. It is also an indication that the entangled beams are composed of multiple spatial modes and that the entanglement is contained in corresponding subregions of the twin beams. The smallest size of these independently correlated subregions, known as the coherence area, places a limit on quantum imaging applications and is ultimately linked to the number of spatial modes that make up the entangled beams. We have shown that non-degenerate four-wave mixing (FWM) in a double-lambda configuration in a rubidium vapor cell is an excellent source of continuous-variable (CV) entangled twin beams, with an intensity-difference noise of less than 13% of the corresponding classical shot-noise level. Unlike other systems that rely on the use of a cavity, this system can support a large number of spatial modes, which makes it possible to generate CV entangled images. I will describe some of the spatial quantum properties of the twin beams generated by the FWM process. In particular, I will show how the size of the spatial correlations, or coherence area, can be controlled through a change in the size and profile of the pump required for the FWM and present results of a direct measurement of the spatial quantum correlations with a high quantum efficiency CCD camera.