The recent application of the principles of quantum mechanics to cryptography has led to a remarkable new dimension in secret communication. As a result of these new developments, it is now possible to construct cryptographic communication systems which detect unauthorized eavesdropping should it occur, and which give a guarantee of no eavesdropping should it not occur. Contents 1 Cryptographic systems before quantum cryptography 3 2 Preamble to quantum cryptography 7 Partially supported by ARL Contract #DAAL01-95-P-1884, ARO Grant #P-38804PH -QC, and the L-O-O-P Fund. 3 The BB84 quantum cryptographic protocol without noise 10 3.1 Stage 1. Communication over a quantum channel . . . . . . . 12 3.2 Stage 2. Communication in two phases over a public channel . 14 3.2.1 Phase 1 of Stage 2. Extraction of raw key . . . . . . . 14 3.2.2 Phase 2 of Stage 2. Detection of Eve's intrusion via error detection . . . . . . . . . . . . . . . . . . . . . . 15 4 The BB84 quantum cryptographic pr...

The fates of SIGACT News and Quantum Cryptography are inseparably entangled. The exact date of Stephen Wiesner's invention of "conjugate coding" is unknown but it cannot be far from April 1969, when the premier issue of SIGACT News---or rather SICACT News as it was known at the time---came out. Much later, it was in SIGACT News that Wiesner's paper finally appeared [74] in the wake of the first author's early collaboration with Charles H. Bennett [7]. It was also in SIGACT News that the original experimental demonstration for quantum key distribution was announced for the first time [6] and that a thorough bibliography was published [19]. Finally, it was in SIGACT News that Doug Wiedemann chose to publish his discovery when he reinvented quantum key distribution in 1987, unaware of all previous work but Wiesner's [73, 5]. Most of the first decade of the history of quant

A realistic quantum cryptographic system must function in the presence of noise and channel loss inevitable in any practical transmission. We examine the effects of these channel limitations on the security and throughput of a class of quantum cryptographic protocols known as four-state, or BB84. Provable unconditional security against eavesdropping, which is the principal feature of quantum cryptography, can be achieved despite minor channel defects, albeit at a reduced transmission throughput. We present a semi-empirical relation between the fully-secure throughput and the loss and noise levels in the channel. According to this relation, an implementation of BB84 utilizing commercially available detectors can reach throughputs as high as 10 4 -10 5 secure bits per second over a practical channel of reasonable quality. KEYWORDS: quantum cryptography, security, channel capacity, secrecy capacity, optical networks. B. Slutsky, P. C. Sun, Y. Mazurenko, R. Rao, and Y. Fainman Page 3...

A Quantum Key Distribution (QKD) network can allow multi-user communication via secure key. Moreover, by actively switching communication nodes, one can achieve high key transmission rate for the selected nodes. However, the polarization properties of different fiber path are different and these properties also randomly drift over time. Therefore, polarization recovery after the switching and autocompensation during key transmission are critical for the QKD network. In this work, we use programmable polarization controllers to implement polarization recovery and auto-compensation in the QKD network. We will also discuss its time limitation and future improvement.