Quantum cryptography with coherent states

Abstract

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

Unambiguous modification of nonorthogonal single- and two-photon polarization states

“…We want to remark that this demonstrate that our protocol is optimal for the class of states considered. This setup is well suited for an experimental implementation of the so called "4+2" [28] quantum key distribution protocols using onephoton states, where two families of non-orthogonal quantum states are used for establishing a common secret key between two legitimate users of a quantum channel, Alice and Bob.…”

Quantum key distribution using two coherent states of light and their superposition

“…Such measurement yields no information about the state most of the time, but it sometimes identifies the state unambiguously. Another strategy, called the generalized beamsplitter attack, has also been reported on by several authors [13,48,49,57]. Since the polarization and photon number are independent observables, there is no problem in principle in selecting a few pulses with two or more photons and separating them into two one-photon pulses without changing the polarization, for example, by means of quantum nondemolition measurement [58].…”

Single-photon quantum key distribution in the presence of loss

“…This scheme can be seen as a combination of two two-state QKD protocols [51,62]. More precisely, Alice selects, at random and independently each time, one of the following four signal states, {|ϕ k = α|0 + (−1) k β|1 , |ϕk = α|0 + i(−1) k β|1 } with k = 0, 1, and sends it to Bob.…”