In most cases, any amplifier, whether boosting sound in a high-tech recording studio or enhancing images from a spy satellite, degrades the input signal. This is especially true in optical information processing and quantum computation, where propagating errors can be devastating. Several tricks involving nonlinear optics are in play for noiselessly amplifying a single stream of photons, but applying these to systems that handle D optical images has been more elusive. Writing in Physical Review Letters, Neil Corzo, at the Joint Quantum Institute and NIST in Maryland, and co-workers report how they used rubidium vapor to amplify hundreds of spatial modes, similar to the pixels in an image, while minimizing noise.
The authors use a nonlinear optical process called four-wave mixing to achieve their goal. A weak input signal beam interacts with two strong pump beams in a rubidium vapor cell. By adjusting the phases of the various optical fields, the authors could dial-in maximum gain for the input signal. In this situation, a photon from each of the pumps is added to the signal but in such a way that all of the unavoidable quantum noise is added to the phase rather than the intensity of the signal. Thus the experimenters achieve noiseless amplification of the image without violating Heisenberg’s uncertainty principle.
Corzo et al. attain noiseless amplification but also find that their system can amplify hundreds of D patterns or modes simultaneously with only small degradation. With such systems, high-fidelity amplification of complex optical patterns for quantum computing and image enhancement can be improved. – David Voss