4,031 research outputs found
On the role of coherent attacks in a type of strategic problem related to quantum key distribution
We consider a strategic problem of the Evesdropping to quantum key
distribution. Evesdropper hopes to obtain the maxium information given the
disturbance to the qubits is often For this strategy, the optimized individual
attack have been extensively constructed under various conditions. However, it
seems a difficult task in the case of coherent attack, i.e., Eve may treat a
number of intercepted qubits collectively, including the collective unitary
transformations and the measurements. It was conjectured by Cirac and Gisin
that no coherent attack can be more powerful for this strategy for BB84
protocol. In this paper we give a general conclusion on the role of coherent
attacks for the strategy of maxmizing the information given the disturbance.
Suppose in a quantum key distribution(QKD) protocol, all the transmitted bits
from Alice are independent and only the individual disturbances to each qubits
are examined by Alice and Bob. For this type of protocols(so far almost all QKD
protocols belong to this type), in principle no coherent attack is more
powerful than the product of optimized individual attack to each individual
qubits. All coherent attacks to the above QKD protocols can be disregarded for
the strategy above.Comment: This is identical to quant-ph/0110073. The the title of the old
version is inappropriate. What I have proven is the Cirac-Gisin conjecture in
Phys. Lett. A229, 1(1997). The actual conclusion is that coherent attack can
be ignored for the strategy of maximizing Eve's total information given
disturbance. But this does not mean coherent attack can be ignored in QK
A review on the decoy-state method for practical quantum key distribution
We present a review on the historic development of the decoy state method,
including the background, principles, methods, results and development. We also
clarify some delicate concepts. Given an imperfect source and a very lossy
channel, the photon-number-splitting (PNS) attack can make the quantum key
distribution (QKD) in practice totally insecure. Given the result of ILM-GLLP,
one knows how to distill the secure final key if he knows the fraction of
tagged bits. The purpose of decoy state method is to do a tight verification of
the the fraction of tagged bits. The main idea of decoy-state method is
changing the intensities of source light and one can verify the fraction of
tagged bits of certain intensity by watching the the counting rates of pulses
of different intensities. Since the counting rates are small quantities, the
effect of statistical fluctuation is very important. It has been shown that
3-state decoy-state method in practice can work even with the fluctuations and
other errors.Comment: 17 page
Criteria for unconditional entanglement purification
We show that the fidelity result of advantage distillation (Bennett et al,
Phys. Rev. Lett., 76, 722(1996)) is not only for the product state of raw
pairs, it is actually correct with whatever form of state of raw pairs. We then
give a general theorem for unconditional entanglement purification. This
theorem lists the conditions on which the fidelity result of a purification
protocol keeps unchanged from product form of raw-pair state and arbitrary form
of raw-pair state. Using this theorem, we find that all existing purification
puricfication can work for arbitrary initial state of raw pairs.Comment: The motivation and the title are change
On quantum key distribution in decoherence-free subspace
We propose easy implementable protocols for robust quantum key distribution
with the collective dephasing channel or collective rotating channel.
In these protocols, Bob only takes passive photon detection to measure the
polarization qubits in the random bases. The source for the protocol with
collective rotating channel is made by type 2 spontaneous parametric down
conversion with random unitary rotation and phase shifter, no quantum
disentangler is required.
A simple proof for unconditionally security is shown.Comment: 5 pages, 3 figur
On the average fidelity criterion for the non-classicality of the continuous variable quantum teleportation
We show that the existing argument on the non-classicality of the continuous
variable quantum teleportation (CVQT) experiment by the average fidelity
criterion is incomplete therefore so far it is still unclear whether the CVQT
experiment(Furusawa et al, Science,282, 706(1998)) is really non-classical.Comment: 5 page
Perfect random number generator is unnecessary for secure quantum key distribution
Game G: Clare passes a string s which is either from perfect random number
generator R0 or from good imperfect number generator R1, with equal
probability. Alice's information about whether it is from R0 or R1 is bounded
by small value h. Alice use s as the input random numbers for QKD protocol with
Bob. Suppose Eve may have very small information about the final key if s is
from R0 and Eve has large information if s is from R1, then after the protocol,
Alice announce the final key, Eve's information about whether s is from R0 or
R1 is unreasonablly large, i.e., breaks the known bound, h. Explicit formulas
are given in the article.Comment: The proof is VERY simpl
A practically feasible entanglement assisted quantum key distribution protocol
We give an entanglement assisted scheme for quantum key distribution.
The scheme requires the maximally entangled 2-qubit state but does not
require any quantum storage. The scheme is unconditionally secure under
whatever Eve's attack.
Given the symmetric noisy channel with uncorrelated noise, our scheme can
tolerate the bit error rate up to
26% in the 4-state case and 30% in the
6-state respectively, respectively. These values are higher than those of all
currently known two-level-state schemes without using a quantum storage.Comment: 13 pages, 3 figure
BDSW protocol revisited: an efficient method for the key distillation without classical computational complexity
In quantum key distribution(QKD), one can use a classical CSS code to distill
the final key. However, there is a constraint for the two codes in CSS code and
so far it is unknown how to construct a large CSS code efficiently. Here we
show that the BDSW method given by Bennett et al can be modified and the error
correction and privacy amplification can be done separately with two
INDEPENDENT parity matrices. With such a modification, BDSW method can be used
to distill the final key without any classical computational complexity. We
also apply the method to the case of imperfect source where a small fraction of
signals are tagged by Eve
Quantum entanglement swapping can be tested with type I parametric down conversion
We propose a very simple scheme to test the quantum enatanglement swapping in
a priori. In the scheme, we only assume the photon detector can distinguish the
vacuum and non-vacuum Fock state rather than the exact Fock number states.Comment: To appear in Phys. Rev. A. We shall NOT do the proposed experiment.
We hope any experimental group to do i
Producing the entangled photon pairs without type II down conversion
We propose a simple scheme to produce the polarization entangled photon pairs
without the type II phase match. The same scheme can also be used to produce
the macroscopic entangled photon states in both photon number space and the
polarization space(A. Lamas-Linares, J.C. Howell and D. Bouwmeester, Nature,
412887(2001)). Advantages and applications of our scheme in quantum key
distribution are discussed.Comment: 3 figure
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