Course 8 Quantum cryptography: From one to many photons

Abstract

This chapter discusses the theory of quantum cryptography. It addresses two issues: (1) The practical realization of light sources emitting single photons “on demand,” in order to eliminate the possibility for the eavesdropper (Eve) to break the secret key transmission by using so called “photon number splitting” (PNS) attacks. (2) The question whether quantum continuous variables (QCV) may provide a valid alternative to the usual QKD schemes based on single photon counting. The initial proposals to use QCV for QKD were based on the use of “non-classical” light beams, namely squeezed light or entangled light beams. It was recently shown, however, that there is actually no need for squeezed light in this context: an equivalent level of security may be obtained simply by generating and transmitting random distributions of quasi-classical (coherent) states. The basic ideas of these techniques is reviewed in this chapter. In photon-counting QKD, the key rate is limited by the single-photon detectors, in which the avalanche processes are difficult to control reliably at very high counting rates. In contrast, homodyne detection may run at frequencies up to tens of MHz. In addition, a specific advantage of the high dimensionality of the QCV phase space is that the field quadratures can be modulated with a large dynamics, allowing the encoding of several key bits per pulse. Very high secret bit rates are therefore attainable with the coherent-state protocol on low-loss lines.