PHASE-CONJUGATE QUANTUM COMMUNICATION WITH ZERO ERROR-PROBABILITY AT FINITE AVERAGE PHOTON NUMBER

The Paley-Wiener restriction on the statistics of single-mode quantum phase measurements implies that single-mode, phase-modulated quantum c ommunication always has nonzero error probability at finite average ph oton number. A two-mode formulation is demonstrated which circumvents the Paley-Wiener constraint, leading to a scheme for zero-error probab ility phase-conjugate quantum communication at finite average photon n umber. The minimum root-mean-square (RMS) total photon number for erro r-free K-ary phase-conjugate communication turns out to be K/2, and th e state achieving this optimum performance is exhibited. Application o f the construct to precision measurements is briefly discussed. Here, the optimum state with RMS photon number K/2 can be used to guarantee that the phase estimate is within +/- pi/K radians of the true value.