Improved Quantum Communication

Abstract

PHYSICS At a cocktail party, two people speaking softly, to avoid being overheard, often have to listen attentively in order to follow the conversational thread. A similar, but more complex, principle is involved in optical coherent communication: Weak pulses of light encode information in the phases of coherent pulses, but the small number of photons per pulse makes it hard to measure the phases. Quantum mechanics places a fundamental limitation on how precisely observable quantities such as position, momentum, and phase can be determined, and so introduces intrinsic errors in the ability to decode information accurately. Armen et al. describe an adaptive feedback technique in which the leading one or two photons in a single 50-microsecond pulse (with unknown phase and containing merely 10 to 300 photons in toto) are used to adjust the phase of a local oscillator. This procedure allows the determination of the entire phase of the packet of coherent photons and may prove useful for secure communication in which only several quanta are used as the information carriers. — ISO Phys. Rev. Lett . 89 , 133602 (2002).