Nonlinear optics provides a way to generate light with different frequencies and different quantum statistical properties. We have developed a nonlinear 4-wave mixing technology in atomic vapors for generating non-classical light. It allows us to fairly easily generate squeezed light (that is, light with noise properties below the "standard quantum limit" achievable with a coherent state; the coherent state being the closest approximation to a classical state that we have). This light has properties that, in turn, can be used to enhance optical trace-detection capabilities, as well as to enhance interferometry. Our technique allows the squeezed light to be generated in multiple spatial modes (images) and this allows us to directly apply such sub-shot-noise advantages to image processing applications. In addition the light is narrow in frequency and can interact easily with laser-cooled atoms and can thus be used in quantum information processing and quantum memory applications.
We have generated "twin beams" of light using four-wave mixing (4WM) that are correlated at a level better than can be displayed by classical radiators. One particularly useful feature of the 4WM technique is that the light can easily be made in multiple spatial modes. That is, images with quantum correlations can be produced. Pixel-by-pixel, the light in these pairs of images is correlated to levels better than the shot noise of the photon numbers involved. Light in the corresponding pixels is not just correlated in intensity, but also in phase. The intensity-difference and the phase-sum are quadrature variables displaying quantum entanglement at levels that violate the inequality expressing the Einstein-Podolsky-Rosen paradox. Measurements of the phase involve generating "local oscillator" phase references with the 4-wave mixing as well. Quantum-correlated and entangled images can be used for faint-object detection (a small absorption or scattering from one of the beams, even at a level below the shot noise, can be detected in the differencing between the beams). Another use of such images is in information storage. The parallel storage of quantum information in images has not yet been demonstrated, however we have taken the first steps in that direction with the "slowing" of quantum images. A vapor cell with a dispersion (change of index with frequency) can display a large group-velocity index, which corresponds to a very small pulse velocity in the medium. We have demonstrated the slowing and delay, for times of 20 -30 ns, of continuous beams carrying quantum-correlated images, while preserving the quantum correlations. Future work will pursue stopping, holding, and releasing pulses of light that carry images in a configuration suitable for a quantum memory. The correlated and quantum-entangled beams that we generate are entangled in continuous variables (phase and amplitude rather than discrete variables like polarization, which has only two possibilities). The fields of quantum communications and quantum information processing using continuous variables are relatively less-developed than the corresponding discrete-variable studies. Our ability to generate entangled beams of narrowband light near atomic resonance frequencies will allow us to pursue such studies. Not only can this light interact readily with hot atoms in a vapor cell, but also with laser-cooled atoms, and the generation and transfer of quantum information between sources and memory devices can be studied.