Secure and efficient decoy-state quantum key distribution with inexact pulse intensities

We present a general theorem for the efficient verification of the lower bound of single-photon transmittance. We show how to do decoy-state quantum key distribution efficiently with large random errors in the intensity control. In our protocol, the linear terms of fluctuation disappear and only the quadratic terms take effect. We then show the unconditional security of decoy-state method with whatever error pattern in intensities of decoy pulses and signal pulses provided that the intensity of each decoy pulse is less than $\mu$ and the intensity of each signal pulse is larger than $\mu'$

Quantum key distribution with realistic states: photon-number statistics in the photon-number splitting attack

Quantum key distribution can be performed with practical signal sources such as weak coherent pulses. One example of such a scheme is the Bennett-Brassard protocol that can be implemented via polarization of the signals, or equivalent signals. It turns out that the most powerful tool at the disposition of an eavesdropper is the photon-number splitting attack. We show that this attack can be extended in the relevant parameter regime such as to preserve the Poissonian photon number distribution of the combination of the signal source and the lossy channel.Comment: 4 page

Experimental demonstration of four-party quantum secret sharing

Secret sharing is a multiparty cryptographic task in which some secret information is splitted into several pieces which are distributed among the participants such that only an authorized set of participants can reconstruct the original secret. Similar to quantum key distribution, in quantum secret sharing, the secrecy of the shared information relies not on computational assumptions, but on laws of quantum physics. Here, we present an experimental demonstration of four-party quantum secret sharing via the resource of four-photon entanglement

Unconditional security at a low cost

By simulating four quantum key distribution (QKD) experiments and analyzing one decoy-state QKD experiment, we compare two data post-processing schemes based on security against individual attack by L\"{u}tkenhaus, and unconditional security analysis by Gottesman-Lo-L\"{u}tkenhaus-Preskill. Our results show that these two schemes yield close performances. Since the Holy Grail of QKD is its unconditional security, we conclude that one is better off considering unconditional security, rather than restricting to individual attacks.Comment: Accepted by International Conference on Quantum Foundation and Technology: Frontier and Future 2006 (ICQFT'06

Decoy State Quantum Key Distribution With Modified Coherent State

To beat PNS attack, decoy state quantum key distribution (QKD) based on coherent state has been studied widely. We present a decoy state QKD protocol with modified coherent state (MCS). By destruction quantum interference, MCS with fewer multi-photon events can be get, which may improve key bit rate and security distance of QKD. Through numerical simulation, we show about 2-dB increment on security distance for BB84 protocol.Comment: 4 pages, 4 figure

Practical Decoy State for Quantum Key Distribution

Decoy states have recently been proposed as a useful method for substantially improving the performance of quantum key distribution. Here, we present a general theory of the decoy state protocol based on only two decoy states and one signal state. We perform optimization on the choice of intensities of the two decoy states and the signal state. Our result shows that a decoy state protocol with only two types of decoy states--the vacuum and a weak decoy state--asymptotically approaches the theoretical limit of the most general type of decoy state protocols (with an infinite number of decoy states). We also present a one-decoy-state protocol. Moreover, we provide estimations on the effects of statistical fluctuations and suggest that, even for long distance (larger than 100km) QKD, our two-decoy-state protocol can be implemented with only a few hours of experimental data. In conclusion, decoy state quantum key distribution is highly practical.Comment: 31 pages. 6 figures. Preprint forma

Interference contrast in multi-source few photon optics

Many recent experiments employ several parametric down conversion (PDC) sources to get multiphoton interference. Such interference has applications in quantum information. We study here how effects due to photon statistics, misalignment, and partial distinguishability of the PDC pairs originating from different sources may lower the interference contrast in the multiphoton experiments.Comment: 23 pages, 9 figures, journal versio

Decoherence-Free Quantum Information Processing with Four-Photon Entangled States

Decoherence-free states protect quantum information from collective noise, the predominant cause of decoherence in current implementations of quantum communication and computation. Here we demonstrate that spontaneous parametric down-conversion can be used to generate four-photon states which enable the encoding of one qubit in a decoherence-free subspace. The immunity against noise is verified by quantum state tomography of the encoded qubit. We show that particular states of the encoded qubit can be distinguished by local measurements on the four photons only.Comment: 4 pages, 4 eps figures, revtex

Performance of various quantum key distribution systems using 1.55 um up-conversion single-photon detectors

We compare the performance of various quantum key distribution (QKD) systems using a novel single-photon detector, which combines frequency up-conversion in a periodically poled lithium niobate (PPLN) waveguide and a silicon avalanche photodiode (APD). The comparison is based on the secure communication rate as a function of distance for three QKD protocols: the Bennett-Brassard 1984 (BB84), the Bennett, Brassard, and Mermin 1992 (BBM92), and the coherent differential phase shift keying (DPSK). We show that the up-conversion detector allows for higher communication rates and longer communication distances than the commonly used InGaAs/InP APD for all the three QKD protocols.Comment: 9 pages, 9 figure

Quantum Key Distribution using Multilevel Encoding: Security Analysis

We present security proofs for a protocol for Quantum Key Distribution (QKD) based on encoding in finite high-dimensional Hilbert spaces. This protocol is an extension of Bennett's and Brassard's basic protocol from two bases, two state encoding to a multi bases, multi state encoding. We analyze the mutual information between the legitimate parties and the eavesdropper, and the error rate, as function of the dimension of the Hilbert space, while considering optimal incoherent and coherent eavesdropping attacks. We obtain the upper limit for the legitimate party error rate to ensure unconditional security when the eavesdropper uses incoherent and coherent eavesdropping strategies. We have also consider realistic noise caused by detector's noise.Comment: 8 pages, 3 figures, REVTe