Eur. Phys. J. D 18, 141–146 (2002) DOI: 10.1140/epjd/e20020017 T HE EUROPEAN P HYSICAL JOURNAL D c  EDP Sciences Societ` a Italiana di Fisica Springer-Verlag 2002 Integrated quantum key distribution system using single sideband detection J.-M. Merolla 1, a , L. Duraffourg 1 , J.-P. Goedgebuer 2 , A. Soujaeff 1 , F. Patois 1 , and W.T. Rhodes 1 1 GTL-CNRS Telecom b , Georgia Tech Lorraine, 2-3 rue Marconi, 57070 Metz, France 2 Laboratoire d’Optique P.M. Duffieux b , Universit´ e de Franche-Comt´ e, 25030 Besan¸ con Cedex, France Received 13 July 2001 and Received in final form 30 November 2001 Abstract. We report a new quantum cryptographic system involving single sideband detection and allowing an implementation of the BB84 protocol. The transmitted bits are reliably coded by the phase of a high frequency modulating signal. The principle of operation is described in terms of both classical and quantum optics. The method has been demonstrated experimentally at 1 550 nm using compact and conventional device technology. Single photon interference has been obtained with a fringe visibility greater than 98%, indicating that the system can be used in view of quantum key distribution potentially beyond 50-km-long standard single-mode fiber. PACS. 03.67.Dd Quantum cryptography – 42.79.-e Optical elements, devices, and systems 1 Introduction The objective of quantum key distribution is to permit two parties, Alice and Bob, to exploit fundamental properties of quantum optics in order to share in secret a random bit sequence – the key –. The general procedure in quan- tum key distribution includes the following steps. First, Alice sends a sequence of individual photons, choosing at random the quantum state in which each photon is pre- pared. The state of the photons serves to encode bits of information. Upon receiving the photons, Bob performs measurements on their states. Alice and Bob retain data only from photons that have been measured in the cor- rect basis. Should Eve tap the transmission line, inter- cept some of photons, and retransmit them after perform- ing her own measurements, transmission errors occur due to the quantum-mechanical nature of photons. To detect these errors, Bob and Alice verify statistically a set of shared bits. If too many errors are detected in the ver- ification process, the bit in that set are discarded. The security of transmission is guaranteed by a protocol: the BB84 protocol [1] if four quantum states are used, the B92 protocol if two non-orthogonal states are used [2] or one of variety of other protocols for other schemes [3,4]. Two principal methods have been used to encode infor- mation. The first is based on polarization coding [1,5,6]. The problem with this method is that it is difficult to preserve polarization over long transmission distances in standard telecommunication optical fibers. The second a e-mail: merolla@georgiatech-metz.fr b UMR CNRS 6603 method is based on delay-coded quantum states [7–9]. In this latter case, each bit is encoded into an optical path difference. Two interferometers, with matched path im- balances greater than the pulse length, form the trans- mitter and receiver. The difficulty with delay coding is to maintain the optical delay in the interferometers con- stant despite inevitable mechanical vibrations and ther- mal drifts [9]. Systems based on Faraday mirrors have been proposed to overcome this drawback of polarization coding [10]. Other solutions have also, been proposed in- volving acousto-optic deflectors or multicolored photons, with wavelength now serving as the basis for encoding information [11]. Recently, we reported a new encoding method based on single-photon phase modulation. In that system, Alice encodes each bit of the transmitted key into an optical frequency by randomly selecting a modulation phase from two possible values. Bob modulates light at the same frequency carrier frequency, again selecting ran- domly between two phases. By means of single-photon in- terference experiments and an additional (possibly public) exchange of information with Alice, Bob is then able to determine the states of the photons sent by Alice [12,13]. This method of quantum key distribution can only be car- ried out under the B92 protocol. In what follows, we describe an improvement of the modulation transmission scheme, based on single- sideband (SSB) detection, which allows the BB84 proto- col to be used with a view to increase the transmission rate and the distance limit for secret bits distribution (the reader is referred to references [14–19] for a detailed dis- cussion on the security aspects related with the various protocols) in a robust scheme for quantum cryptography.