Abstract

In a nondegenerate parametric amplifier (NOPA) with vacuum-state inputs, the quantum noise at. the input is amplified to generate signal and idler beams at the output, each of which is indistinguishable from a thermal source with large However, the relative fluctuations of these two beams arc correlated such that in the limit of infinite gain and in the absence of loss, the quadrature-phase amplitudes of signal and idler fields become quantum copies of each other. This type of nonclassical correlation has been used to demonstrate the EPR paradox¹ and to perform a back-action evasion experiment,² In this paper we suggest a new scheme and present an experiment that exploits the large but correlated fluctuations of the signal and idler for quantum communication. The basic idea is to use the large quantum fluctuations of signal and idler beams to shield coherent information from an umauthorized receiver by encoding this information with a signal-to-noise ratio (SNR) much smaller than unity on either signal or idler taken alone. Indeed, the encoded information can be at a level below the vacuum- state limit for either of these two beams. However, because the fluctuations arc correlated, the coherent information can be recovered with large SNR if both signal and idler beams are available to an authorized receiver.⁵

© 1993 Optical Society of America