Abstract:
By realizing a quantum cryptography system based on polarization entangled photon pairs we establish highly secure keys, because a single photon source is approximated and the inherent randomness of quantum measurements is exploited. We implement a novel key distribution scheme using Wigner's inequality to test the security of the quantum channel, and, alternatively, realize a variant of the BB84 protocol. Our system has two completely independent users separated by 360 m, and generates raw keys at rates of 400 - 800 bits/second with bit error rates arround 3 percent.

Abstract:
Using polarization-entangled photons from spontaneous parametric downconversion, we have implemented Ekert's quantum cryptography protocol. The near-perfect correlations of the photons allow the sharing of a secret key between two parties. The presence of an eavesdropper is continually checked by measuring Bell's inequalities. We investigated several possible eavesdropper strategies, including pseudo-quantum non-demolition measurements. In all cases, the eavesdropper's presence was readily apparent. We discuss a procedure to increase her detectability.

Abstract:
We present a setup for quantum cryptography based on photon pairs in energy-time Bell states and show its feasability in a laboratory experiment. Our scheme combines the advantages of using photon pairs instead of faint laser pulses and the possibility to preserve energy-time entanglement over long distances. Moreover, using 4-dimensional energy-time states, no fast random change of bases is required in our setup : Nature itself decides whether to measure in the energy or in the time base.

Abstract:
We present an entangled-state quantum cryptography system that operated for the first time in a real world application scenario. The full key generation protocol was performed in real time between two distributed embedded hardware devices, which were connected by 1.45 km of optical fiber, installed for this experiment in the Vienna sewage system. The generated quantum key was immediately handed over and used by a secure communication application.

Abstract:
Real sources of entangled photon pairs (like parametric down conversion) are not perfect. They produce quantum states that contain more than only one photon pair with some probability. In this paper it is discussed what happens if such states are used for the purpose of quantum key distribution. It is shown that the presence of "multi-pair" signals (together with low detection efficiencies) causes errors in transmission even if there is no eavesdropper. Moreover, it is shown that even the eavesdropping, that draws information only from these "multi-pair" signals, increases the error rate. Information, that can be obtained by an eavesdropper from these signals, is calculated.

Abstract:
Recently, a quantum key exchange protocol has been described, which served as basis for securing an actual bank transaction by means of quantum cryptography [quant-ph/0404115]. Here we show, that the authentication scheme applied is insecure in the sense that an attacker can provoke a situation where initiator and responder of a key exchange end up with different keys. Moreover, it may happen that an attacker can decrypt a part of the plaintext protected with the derived encryption key.

Abstract:
Entanglement between two quantum elements is a phenomenon which presents a broad application spectrum, being used largely in quantum cryptography schemes and in physical characterisation of the universe. Commonly known entangled states have been obtained with photons and electrons, but other quantum elements such as quarks, leptons, and neutrinos have shown their informational potential. In this paper, we present the perspective of exploiting the phenomenon of entanglement that appears in nuclear particle interactions as a resource for quantum key distribution protocols. 1. Introduction Quantum entanglement has long been proven to be the most resourceful method of achieving feasible, high fidelity quantum key distribution over growing distances [1–6]. Exploiting entanglement can be done using multiple observables, giving experiments a practical versatility. Polarization entangled states [7], while being very easily obtainable, are very hard to manipulate as they propagate through different media such as optical fibers. On the other hand, time-bin entanglement is harder to obtain, but manipulation requirements, such as temperature, polarization, and propagation velocity, are easier to fulfill. Quantum key distribution [8, 9] schemes that have been applied to regular qubits may be applied to qubits that result from nuclear interactions. The conventional method of achieving quantum key distribution is generating quantum elements photons and manipulating them as such as to obtain the desired states on which quantum key distribution operates. To this extent, quantum key distribution can be obtained using either single or entangled states, and due to the nature of the two, there have been devised two different protocols that extract the information from the generated states. The first protocol [8], created by Charles Bennett and Gilles Brassard—BB84 employed singular photonic states, with no coherence between their informational values. The information-carrying observable was the polarization of the photon, on which the two participants carry out local operations, that is, rotations in the analysis vector bases and classical communication in which the two participants communicate a part of their informational operators either the values obtained or the analysis bases in which the single states are detected. The second protocol [9], elaborated by Artur Ekert—Ek91, is derived from the BB84 protocol but differs from it in an essential manner: it employs bidimensional, maximally entangled states, also called Bell states. The usage of entangled states has major

Abstract:
Entangled photons are a crucial resource for quantum communication and linear optical quantum computation. Unfortunately, the applicability of many photon-based schemes is limited due to the stochastic character of the photon sources. Therefore, a worldwide effort has focused in overcoming the limitation of probabilistic emission by generating two-photon entangled states conditioned on the detection of auxiliary photons. Here we present the first heralded generation of photon states that are maximally entangled in polarization with linear optics and standard photon detection from spontaneous parametric down-conversion. We utilize the down-conversion state corresponding to the generation of three photon pairs, where the coincident detection of four auxiliary photons unambiguously heralds the successful preparation of the entangled state. This controlled generation of entangled photon states is a significant step towards the applicability of a linear optics quantum network, in particular for entanglement swapping, quantum teleportation, quantum cryptography and scalable approaches towards photonics-based quantum computing.

Abstract:
We report the generation of polarization-entangled photons by femtosecond-pulse-pumped spontaneous parametric down-conversion in a cascade of two type-I crystals. Highly entangled pulsed states were obtained by introducing a temporal delay between the two orthogonal polarization components of the pump field. They exhibited high-visibility quantum interference and a large concurrence value, without the need of post-selection using narrow-bandwidth-spectral filters. The results are well explained by the theory which incorporates the space-time dependence of interfering two-photon amplitudes if dispersion and birefringence in the crystals are appropriately taken into account. Such a pulsed entangled photon well localized in time domain is useful for various quantum communication experiments, such as quantum cryptography and quantum teleportation.

Abstract:
Quantum cryptography has attracted much recent attention due to its potential for providing secret communications that cannot be decrypted by any amount of computational effort. This is the first analysis of the secrecy of a practical implementation of the BB84 protocol that simultaneously takes into account and presents the {\it full} set of complete analytical expressions for effects due to the presence of pulses containing multiple photons in the attenuated output of the laser, the finite length of individual blocks of key material, losses due to error correction, privacy amplification, continuous authentication, errors in polarization detection, the efficiency of the detectors, and attenuation processes in the transmission medium. The analysis addresses eavesdropping attacks on individual photons rather than collective attacks in general. Of particular importance is the first derivation of the necessary and sufficient amount of privacy amplification compression to ensure secrecy against the loss of key material which occurs when an eavesdropper makes optimized individual attacks on pulses containing multiple photons. It is shown that only a fraction of the information in the multiple photon pulses is actually lost to the eavesdropper.