6,163 research outputs found
Beating the PNS attack in practical quantum cryptography
In practical quantum key distribution, weak coherent state is often used and
the channel transmittance can be very small therefore the protocol could be
totally insecure under the photon-number-splitting attack. We propose an
efficient method to verify the upper bound of the fraction of counts caused by
multi-photon pluses transmitted from Alice to Bob, given whatever type of Eve's
action. The protocol simply uses two coherent states for the signal pulses and
vacuum for decoy pulse. Our verified upper bound is sufficiently tight for QKD
with very lossy channel, in both asymptotic case and non-asymptotic case. The
coherent states with mean photon number from 0.2 to 0.5 can be used in
practical quantum cryptography. We show that so far our protocol is the
decoy-state protocol that really works for currently existing set-ups.Comment: So far this is the unique decoy-state protocol which really works
efficiently in practice. Prior art results are commented in both main context
and the Appendi
Insecurity of position-based quantum cryptography protocols against entanglement attacks
Recently, position-based quantum cryptography has been claimed to be
unconditionally secure. In contrary, here we show that the existing proposals
for position-based quantum cryptography are, in fact, insecure if entanglement
is shared among two adversaries. Specifically, we demonstrate how the
adversaries can incorporate ideas of quantum teleportation and quantum secret
sharing to compromise the security with certainty. The common flaw to all
current protocols is that the Pauli operators always map a codeword to a
codeword (up to an irrelevant overall phase). We propose a modified scheme
lacking this property in which the same cheating strategy used to undermine the
previous protocols can succeed with a rate at most 85%. We conjecture that the
modified protocol is unconditionally secure and prove this to be true when the
shared quantum resource between the adversaries is a two- or three- level
system
Secure two-party quantum evaluation of unitaries against specious adversaries
We describe how any two-party quantum computation, specified by a unitary
which simultaneously acts on the registers of both parties, can be privately
implemented against a quantum version of classical semi-honest adversaries that
we call specious. Our construction requires two ideal functionalities to
garantee privacy: a private SWAP between registers held by the two parties and
a classical private AND-box equivalent to oblivious transfer. If the unitary to
be evaluated is in the Clifford group then only one call to SWAP is required
for privacy. On the other hand, any unitary not in the Clifford requires one
call to an AND-box per R-gate in the circuit. Since SWAP is itself in the
Clifford group, this functionality is universal for the private evaluation of
any unitary in that group. SWAP can be built from a classical bit commitment
scheme or an AND-box but an AND-box cannot be constructed from SWAP. It follows
that unitaries in the Clifford group are to some extent the easy ones. We also
show that SWAP cannot be implemented privately in the bare model
Entanglement of 2xK quantum systems
We derive an analytical expression for the lower bound of the concurrence of
mixed quantum states of composite 2xK systems. In contrast to other, implicitly
defined entanglement measures, the numerical evaluation of our bound is
straightforward. We explicitly evaluate its tightness for general mixed states
of 2x3 systems, and identify a large class of states where our expression gives
the exact value of the concurrence.Comment: 7 pages, 1 figure, to be published in Europhysics Lette
A decoy-state protocol for quantum cryptography with 4 intensities of coherent states
In order to beat any type of photon-number-splitting attack, we propose a
protocol for quantum key distributoin (QKD) using 4 different intensities of
pulses. They are vacuum and coherent states with mean photon number
and . is around 0.55 and this class of pulses are used as the
main signal states. The other two classes of coherent states () are
also used signal states but their counting rates should be studied jointly with
the vacuum. We have shown that, given the typical set-up in practice, the key
rate from the main signal pulses is quite close to the theoretically allowed
maximal rate in the case given the small overall transmittance of
Quantum circuit for security proof of quantum key distribution without encryption of error syndrome and noisy processing
One of the simplest security proofs of quantum key distribution is based on
the so-called complementarity scenario, which involves the complementarity
control of an actual protocol and a virtual protocol [M. Koashi, e-print
arXiv:0704.3661 (2007)]. The existing virtual protocol has a limitation in
classical postprocessing, i.e., the syndrome for the error-correction step has
to be encrypted. In this paper, we remove this limitation by constructing a
quantum circuit for the virtual protocol. Moreover, our circuit with a shield
system gives an intuitive proof of why adding noise to the sifted key increases
the bit error rate threshold in the general case in which one of the parties
does not possess a qubit. Thus, our circuit bridges the simple proof and the
use of wider classes of classical postprocessing.Comment: 8 pages, 2 figures. Typo correcte
Revival of quantum correlations without system-environment back-action
Revivals of quantum correlations have often been explained in terms of
back-action on quantum systems by their quantum environment(s). Here we
consider a system of two independently evolving qubits, each locally
interacting with a classical random external field. The environments of the
qubits are also independent, and there is no back-action on the qubits.
Nevertheless, entanglement, quantum discord and classical correlations between
the two qubits may revive in this model. We explain the revivals in terms of
correlations in a classical-quantum state of the environments and the qubits.
Although classical states cannot store entanglement on their own, they can play
a role in storing and reviving entanglement. It is important to know how the
absence of back-action, or modelling an environment as classical, affects the
kind of system time evolutions one is able to describe. We find a class of
global time evolutions where back-action is absent and for which there is no
loss of generality in modelling the environment as classical. Finally, we show
that the revivals can be connected with the increase of a parameter used to
quantify non-Markovianity of the single-qubit dynamics.Comment: 8 pages, 4 figures; this version to appear in Phys. Rev.
Security proof of a three-state quantum key distribution protocol without rotational symmetry
Standard security proofs of quantum key distribution (QKD) protocols often
rely on symmetry arguments. In this paper, we prove the security of a
three-state protocol that does not possess rotational symmetry. The three-state
QKD protocol we consider involves three qubit states, where the first two
states, |0_z> and |1_z>, can contribute to key generation and the third state,
|+>=(|0_z>+|1_z>)/\sqrt{2}, is for channel estimation. This protocol has been
proposed and implemented experimentally in some frequency-based QKD systems
where the three states can be prepared easily. Thus, by founding on the
security of this three-state protocol, we prove that these QKD schemes are, in
fact, unconditionally secure against any attacks allowed by quantum mechanics.
The main task in our proof is to upper bound the phase error rate of the qubits
given the bit error rates observed. Unconditional security can then be proved
not only for the ideal case of a single-photon source and perfect detectors,
but also for the realistic case of a phase-randomized weak coherent light
source and imperfect threshold detectors. Our result on the phase error rate
upper bound is independent of the loss in the channel. Also, we compare the
three-state protocol with the BB84 protocol. For the single-photon source case,
our result proves that the BB84 protocol strictly tolerates a higher quantum
bit error rate than the three-state protocol; while for the coherent-source
case, the BB84 protocol achieves a higher key generation rate and secure
distance than the three-state protocol when a decoy-state method is used.Comment: 10 pages, 3 figures, 2 column
Bell Measurements and Observables
A general matrix approach to study entangled states is presented, based on
operator completeness relations. Bases of unitary operators are considered,
with focus on irreducible representations of groups. Bell measurements for
teleportation are considered, and robustness of teleportation to various kinds
of non idealities is shown.Comment: 11 pages. Elsart styl
Two-way quantum communication channels
We consider communication between two parties using a bipartite quantum
operation, which constitutes the most general quantum mechanical model of
two-party communication. We primarily focus on the simultaneous forward and
backward communication of classical messages. For the case in which the two
parties share unlimited prior entanglement, we give inner and outer bounds on
the achievable rate region that generalize classical results due to Shannon. In
particular, using a protocol of Bennett, Harrow, Leung, and Smolin, we give a
one-shot expression in terms of the Holevo information for the
entanglement-assisted one-way capacity of a two-way quantum channel. As
applications, we rederive two known additivity results for one-way channel
capacities: the entanglement-assisted capacity of a general one-way channel,
and the unassisted capacity of an entanglement-breaking one-way channel.Comment: 21 pages, 3 figure
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