Abstract:
The amount of information and the bit error rate caused by eavesdropper are analyzed while the BB84 quantum key distribution protocol is attacked with intercept/resend attack using different measurement bases. The bit error rate threshold that legitimate user used to determine intercept/resend eavesdropping attack is under 15%, and a new standard is provided for secure quantum communications and the detection of eavesdropper.

Abstract:
Motivated by a recent generalization of the Balian-Low theorem and by new research in wireless communications we analyze the construction of Wilson bases for general time-frequency lattices. We show that orthonormal Wilson bases for $\LtR$ can be constructed for any time-frequency lattice whose volume is $\tfrac12$. We then focus on the spaces $\ell^2(\ZZ)$ and $\CC^L$ which are the preferred settings for numerical and practical purposes. We demonstrate that with a properly adapted definition of Wilson bases the construction of orthonormal Wilson bases for general time-frequency lattices also holds true in these discrete settings. In our analysis we make use of certain metaplectic transforms. Finally we discuss some practical consequences of our theoretical findings.

Abstract:
Based on the hypothesis that the velocity of light in vacuum is the maximum velocity possible for any signal, we present a dual velocity protocol for a hybrid quantum key distribution (QKD) system, and prove that its security is the same as that for the BB84 protocol. We show that this protocol can improve the efficiency of quantum key generation from 50% to 100%, and, at the same time, reduce Eve's information. Because it breaks the symmetry between Bob and Eve before open discussion, the dual velocity protocol extends the concept of QKD and increases our choice of protocol bases. We present three application examples and analyze in detail their security under intercept/resend attacks.

Abstract:
We consider a modified version of the BB84 quantum key distribution protocol in which the angle between two different bases are less than $\pi/4$. We show that the channel parameter estimate becomes the same as the original protocol with sufficiently many transmitted qubits. On the other hand, the statistical correlation between bits transmitted in one basis and those received in the other basis becomes stronger as the angle between two bases becomes narrower. If the angle is very small, the statistical correlation between bits transmitted in one basis and those received in the other basis is as strong as those received in the same basis as transmitting basis, which means that the modified protocol can generate almost twice as long secret key as the original protocol, provided that Alice and Bob choose two different bases with almost the same probability. We also point out that the reverse reconciliation often gives different amount of secret key to the direct reconciliation over Pauli channels with our modified protocol.

Abstract:
We prove the security of the 1984 protocol of Bennett and Brassard (BB84) for quantum key distribution. We first give a key distribution protocol based on entanglement purification, which can be proven secure using methods from Lo and Chau's proof of security for a similar protocol. We then show that the security of this protocol implies the security of BB84. The entanglement-purification based protocol uses Calderbank-Shor-Steane (CSS) codes, and properties of these codes are used to remove the use of quantum computation from the Lo-Chau protocol.

Abstract:
We consider the mismatched measurements in the BB84 quantum key distribution protocol, in which measuring bases are different from transmitting bases. We give a lower bound on the amount of a secret key that can be extracted from the mismatched measurements. Our lower bound shows that we can extract a secret key from the mismatched measurements with certain quantum channels, such as the channel over which the Hadamard matrix is applied to each qubit with high probability. Moreover, the entropic uncertainty principle implies that one cannot extract the secret key from both matched measurements and mismatched ones simultaneously, when we use the standard information reconciliation and privacy amplification procedure.

Abstract:
In all papers on the BB84 protocol, the transmission probability of each bit value is usually set to be equal. In this paper, we show that by assigning different transmission probability to each transmitted qubit within a single polarization basis, we can generally improve the key generation rate of the BB84 protocol and achieve a higher key rate.

Abstract:
A simplified eavesdropping-strategy for BB84 protocol in quantum cryptography (refer to quant-ph/9812022) is proposed. This scheme implements by the `indirect copying' technology. Under this scheme, eavesdropper can exactly obtain the exchanged information between the legitimate users without being detected.

Abstract:
This paper presents the principles and experimental results of an optical fiber QKD system operating at 1550 nm, and using the BB84 protocol with QPSK signals. Our experimental setup consists of a time-multiplexed super-homodyne configuration using P.I.N detectors in a differential scheme as an alternative to avalanche photon counting. Transmission over 11km of optical fiber has been done using this detection scheme and major relevant characteristics such as noise, quantum efficiency and bit error rate (BER) are reported.

Abstract:
We prove that BB84 protocol with random privacy amplification is secure with a higher key rate than Mayers' estimate with the same error rate. Consequently, the tolerable error rate of this protocol is increased from 7.5 % to 11 %. We also extend this method to the case of estimating error rates separately in each basis, which enables us to securely share a longer key.