We analyze the quantitative improvement in performance provided by a novel quantum key distribution (qkd) system that employs a correlated photon source (cps) and a photon-number resolving detector (pnr). Our calculations suggest that given current technology, the cps/pnr implementation offers an improvement of several orders of magnitude in secure bit rate over previously described implementations.

A new method for generating entangled photons with controllable frequency correlation via spontaneous parametric down-conversion (SPDC) is presented. The method entails initiating counter-propagating SPDC in a single-mode nonlinear waveguide by pumping with a pulsed beam perpendicular to the waveguide. 02002 Optical Society of America OCIS codes: (270.0270) Quantum opticsIn a typical spontaneous parametric down-conversion (SPDC) experiment, a photon from a monochromatic pump beam decays into two photons (often referred to as signal and idler) via interaction with a nonlinear optical crystal. While the signal and idler may be broadband individually, conservation of energy requires that the sum of their respective frequencies equals the single fRquency of the monochromatic pump. This engenders frequency anticorrelation in the down-converted beams. Aside from the frequency-anti-correlated case, the frequency-correlated and frequency-uncorrelated cases were also investigated theoretically by Campos et al. in 1990 [l]. At that time, neither a method of creating these novel states nor a practical application of the states was known.Two developments in quantum mformation theory have renewed interest in these generalized states of frequency correlation. First, quantum information processes requiring the synchronized creation of multiple photon pairs have been devised, such as quanmin teleportation [2]. The requisite temporal control can be achieved by pumping the crystal with a brief pulse. The availability of pump photons of differing frequencies relaxes the strict frequency anticorrelation in the down-converted beams 131. Second, applications such as entanglement-enhanced clock synchronization [4] and one-way auto-compensating quantum cryptography [5] have been introduced that specifically require frequency correlation, as opposed to the usual frequency anti-correlation. Fig. 1. A schematic of auto-phase-matched SPDC, a new method for geiierating entangled-photon pairs with controllable frequency correlation. The z-polarized pulsed pump beam initiates counter-propagating y-polarized SPDC in the singlemode nonlinear waveguide. The joint spectrum of the down-converted beams is controlled by the spatial and temporal characteristics of the pump beam, as described in the text.Our method for obtaining controllable frequency entanglement entails initiating type-I SPDC (signal and idler identically polarized) in a single-mode nonlinear waveguide by pumping with a pulsed beam perpendicular to the waveguide (see Fig. 1). The down-converted photons emerge from opposite ends of the waveguide with a joint spectrum that can be varied from frequency anti-correlated to frequency correlated by adjusting the temporal and spatial characteristics of the pump beam. The primary advantage of this method is that the limiting cases of perfect frequency correlation and perfect frequency anti-correlation can be obtained regardless of the dispersion relation of the waveguide. Thus, we refer to the method as auto-phase-matched. It is well known t...