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The safety of a quantum key distribution system relies on the fact that any eavesdropping attempt on the quantum channel creates errors in the transmission. For a given error rate, the amount of information that may have leaked to the eavesdropper depends on both the particular system and the eavesdropping strategy. In this work, we discuss quantum cryptographic protocols based on the transmission of weak coherent states and present a new system, based on a symbiosis of two existing ones, and for which the information available to the eavesdropper is significantly reduced. This system is therefore safer than the two previous ones. We also suggest a possible

We present a time-multiplexed interferometer based on Faraday mirrors, and apply it to quantum key distribution. The interfering pulses follow exactly the same spatial path, ensuring very high stability and self balancing. Use of Faraday mirrors compensates automatically any birefringence effects and polarization dependent losses in the transmitting fiber. First experimental results show a fringe visibility of 0.9984 for a 23km-long interferometer, based on installed telecom fibers.In so-called private-key cryptographic systems, secure transmission through unprotected channels rely on the exchange of secret keys, known only to the sender, Alice, and the receiver, Bob. Quantum cryptography (QC) relies on the properties of quantum mechanics to obtain a provably secure key distribution [1,2,3,4]. Most exisiting implementations rely on either the polarization [2,5,6] or the phase [7,8,9, 10] of very weak pulses of light as information carrier. To date, the longest transmission spans were obtained in optical fibers, at a wavelength of 1300 nm [6,9,10].The main difficulty with polarization-based systems is the need to keep stable polarizations over distances of tens of kilometers, in standard telecom cables. Indeed, due to the birefringence of the fibers and the effect of the environment, the output polarization fluctuates randomly. Recent experiments [6] have shown that in general the time-scale of these fluctuations is long enough (tens of minutes) to enable polarization tracking to compensate for them. However, while this is no major problem for preliminary experiments, this would be inconvenient for practical applications of QC.Interferometric quantum key distribution systems are usually based on a double Mach-Zehnder interferometer [9, 10], one side for Alice and one for Bob (see Fig. 1). These interferometers already implement time-multiplexing, as both interfering pulses follow the same path between Alice and Bob, with some time delay. However, the pulses do follow different paths within both Alice's and Bob's interferometers. In order to obtain a good interference, both users therefore need to have identical interferometers, with the same coupling ratios in each arm and the same path lengths, and also need to keep them stable within a few tens of nm during a transmission. Moreover, since optical components like phase modulators (PM) are polarization dependent, polarization control is still necessary both in the transmission line and within each interferometer. This again is inconvenient for practical applications.In this letter, we present a new interferometric system implementing phase-encoded quantum key distribution [11]. It is based on time-multiplexing, the interfering pulses now following exactly the same spatial path, albeit with a small time delay. Therefore, in contrast to the usual schemes, it does not require any path length control between the various paths. Moreover, all pulses are reflected back at the end of the fibers. Use of Faraday mirrors instead of regular mirrors makes it possible ...

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We present calculations of the rates of decay of an excited atom embedded in an absorbing dielectric. Decay can occur by spontaneous emission into transverse radiative modes of the electromagnetic field and by Joule heating via longitudinal coupling of the atom to the dielectric. The spontaneous emission (transverse) decay rate is modified in a dielectric, being the free-space rate multiplied by the real part of the refractive index at the transition frequency of the atom. There is a further modification due to the difference between the macroscopic dielectric field and the local field at the position of the atom. In addition there is a longitudinal decay rate which is proportional to the imaginary part of the dielectric constant and therefore vanishes in non-absorbing media. We derive expressions for each of these rates of decay and discuss the physical mechanisms leading to them.