20,854 research outputs found
Reduced randomness in quantum cryptography with sequences of qubits encoded in the same basis
We consider the cloning of sequences of qubits prepared in the states used in
the BB84 or 6-state quantum cryptography protocol, and show that the
single-qubit fidelity is unaffected even if entire sequences of qubits are
prepared in the same basis. This result is of great importance for practical
quantum cryptosystems because it reduces the need for high-speed random number
generation without impairing on the security against finite-size attacks.Comment: 8 pages, submitted to PR
Implementation of two-party protocols in the noisy-storage model
The noisy-storage model allows the implementation of secure two-party
protocols under the sole assumption that no large-scale reliable quantum
storage is available to the cheating party. No quantum storage is thereby
required for the honest parties. Examples of such protocols include bit
commitment, oblivious transfer and secure identification. Here, we provide a
guideline for the practical implementation of such protocols. In particular, we
analyze security in a practical setting where the honest parties themselves are
unable to perform perfect operations and need to deal with practical problems
such as errors during transmission and detector inefficiencies. We provide
explicit security parameters for two different experimental setups using weak
coherent, and parametric down conversion sources. In addition, we analyze a
modification of the protocols based on decoy states.Comment: 41 pages, 33 figures, this is a companion paper to arXiv:0906.1030
considering practical aspects, v2: published version, title changed in
accordance with PRA guideline
Key distillation from quantum channels using two-way communication protocols
We provide a general formalism to characterize the cryptographic properties
of quantum channels in the realistic scenario where the two honest parties
employ prepare and measure protocols and the known two-way communication
reconciliation techniques. We obtain a necessary and sufficient condition to
distill a secret key using this type of schemes for Pauli qubit channels and
generalized Pauli channels in higher dimension. Our results can be applied to
standard protocols such as BB84 or six-state, giving a critical error rate of
20% and 27.6%, respectively. We explore several possibilities to enlarge these
bounds, without any improvement. These results suggest that there may exist
weakly entangling channels useless for key distribution using prepare and
measure schemes.Comment: 21 page
Decoy state quantum key distribution with two-way classical post-processing
Decoy states have recently been proposed as a useful method for substantially
improving the performance of quantum key distribution protocols when a coherent
state source is used. Previously, data post-processing schemes based on one-way
classical communications were considered for use with decoy states. In this
paper, we develop two data post-processing schemes for the decoy-state method
using two-way classical communications. Our numerical simulation (using
parameters from a specific QKD experiment as an example) results show that our
scheme is able to extend the maximal secure distance from 142km (using only
one-way classical communications with decoy states) to 181km. The second scheme
is able to achieve a 10% greater key generation rate in the whole regime of
distances
A simple proof of the unconditional security of quantum key distribution
Quantum key distribution is the most well-known application of quantum
cryptography. Previous proposed proofs of security of quantum key distribution
contain various technical subtleties. Here, a conceptually simpler proof of
security of quantum key distribution is presented. The new insight is the
invariance of the error rate of a teleportation channel: We show that the error
rate of a teleportation channel is independent of the signals being
transmitted. This is because the non-trivial error patterns are permuted under
teleportation. This new insight is combined with the recently proposed quantum
to classical reduction theorem. Our result shows that assuming that Alice and
Bob have fault-tolerant quantum computers, quantum key distribution can be made
unconditionally secure over arbitrarily long distances even against the most
general type of eavesdropping attacks and in the presence of all types of
noises.Comment: 13 pages, extended abstract. Comments will be appreciate
Practical decoy state method in quantum key distribution with heralded single photon source
We propose a practical decoy state method with heralded single photon source
for quantum key distribution (QKD). In the protocol, 3 intensities are used and
one can estimate the fraction of single-photon counts. The final key rate over
transmission distance is simulated under various parameter sets. Due to the
lower dark count than that of a coherent state, it is shown that a 3-intensity
decoy-state QKD with a heralded source can work for a longer distance than that
of a coherent state.Comment: 10 pages, 4 Postscript figure
Entanglement capabilities of non-local Hamiltonians
We quantify the capability of creating entanglement for a general physical
interaction acting on two qubits. We give a procedure for optimizing the
generation of entanglement. We also show that a Hamiltonian can create more
entanglement if one uses auxiliary systems.Comment: replaced with published version, 4 pages, no figure
Experimental Test of Relativistic Quantum State Collapse with Moving Reference Frames
An experimental test of relativistic wave-packet collapse is presented. The
tested model assumes that the collapse takes place in the reference frame
determined by the massive measuring detectors. Entangled photons are measured
at 10 km distance within a time interval of less than 5 ps. The two apparatuses
are in relative motion so that both detectors, each in its own inertial
reference frame, are first to perform the measurement. The data always
reproduces the quantum correlations and thus rule out a class of collapse
models. The results also set a lower bound on the "speed of quantum
information" to 0.66 x 10^7 and 1.5 x 10^4 times the speed of light in the
Geneva and the background radiation reference frames, respectively. The very
difficult and deep question of where the collapse takes place - if it takes
place at all - is considered in a concrete experimental context.Comment: 4 pages + 2 ps figure
Countering Quantum Noise with Supplementary Classical Information
We consider situations in which i) Alice wishes to send quantum information
to Bob via a noisy quantum channel, ii) Alice has a classical description of
the states she wishes to send and iii) Alice can make use of a finite amount of
noiseless classical information. After setting up the problem in general, we
focus attention on one specific scenario in which Alice sends a known qubit
down a depolarizing channel along with a noiseless cbit. We describe a protocol
which we conjecture is optimal and calculate the average fidelity obtained. A
surprising amount of structure is revealed even for this simple case which
suggests that relationships between quantum and classical information could in
general be very intricate.Comment: RevTeX, 5 pages, 2 figures Typo in reference 9 correcte
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