544 research outputs found
Kak's three-stage protocol of secure quantum communication revisited: Hitherto unknown strengths and weaknesses of the protocol
Kak's three-stage protocol for quantum key distribution is revisited with
special focus on its hitherto unknown strengths and weaknesses. It is shown
that this protocol can be used for secure direct quantum communication.
Further, the implementability of this protocol in the realistic situation is
analyzed by considering various Markovian noise models. It is found that the
Kak's protocol and its variants in their original form can be implemented only
in a restricted class of noisy channels, where the protocols can be transformed
to corresponding protocols based on logical qubits in decoherence free
subspace. Specifically, it is observed that Kak's protocol can be implemented
in the presence of collective rotation and collective dephasing noise, but
cannot be implemented in its original form in the presence of other types of
noise, like amplitude damping and phase damping noise. Further, the performance
of the protocol in the noisy environment is quantified by computing average
fidelity under various noise models, and subsequently a set of preferred states
for secure communication in noisy environment have also been identified.Comment: Kak's protocol is not suitable for quantum cryptography in presence
of nois
Quantum Communication and Decoherence
In this contribution we will give a brief overview on the methods used to
overcome decoherence in quantum communication protocols. We give an
introduction to quantum error correction, entanglement purification and quantum
cryptography. It is shown that entanglement purification can be used to create
``private entanglement'', which makes it a useful tool for cryptographic
protocols.Comment: 31 pages, 10 figures, LaTeX, book chapter to appear in ``Coherent
Evolution in Noisy Environments'', Lecture Notes in Physics, (Springer
Verlag, Berlin-Heidelberg-New York). Minor typos correcte
Reexamination of Quantum Bit Commitment: the Possible and the Impossible
Bit commitment protocols whose security is based on the laws of quantum
mechanics alone are generally held to be impossible. In this paper we give a
strengthened and explicit proof of this result. We extend its scope to a much
larger variety of protocols, which may have an arbitrary number of rounds, in
which both classical and quantum information is exchanged, and which may
include aborts and resets. Moreover, we do not consider the receiver to be
bound to a fixed "honest" strategy, so that "anonymous state protocols", which
were recently suggested as a possible way to beat the known no-go results are
also covered. We show that any concealing protocol allows the sender to find a
cheating strategy, which is universal in the sense that it works against any
strategy of the receiver. Moreover, if the concealing property holds only
approximately, the cheat goes undetected with a high probability, which we
explicitly estimate. The proof uses an explicit formalization of general two
party protocols, which is applicable to more general situations, and a new
estimate about the continuity of the Stinespring dilation of a general quantum
channel. The result also provides a natural characterization of protocols that
fall outside the standard setting of unlimited available technology, and thus
may allow secure bit commitment. We present a new such protocol whose security,
perhaps surprisingly, relies on decoherence in the receiver's lab.Comment: v1: 26 pages, 4 eps figures. v2: 31 pages, 5 eps figures; replaced
with published version; title changed to comply with puzzling Phys. Rev.
regulations; impossibility proof extended to protocols with infinitely many
rounds or a continuous communication tree; security proof of decoherence
monster protocol expanded; presentation clarifie
Quantum cryptography over non-Markovian channels
A set of schemes for secure quantum communication are analyzed under the
influence of non-Markovian channels. By comparing with the corresponding
Markovian cases, it is seen that the average fidelity in all these schemes can
be maintained for relatively longer periods of time. The effects of
non-Markovian noise on a number of facets of quantum cryptography, such as
quantum secure direct communication, deterministic secure quantum communication
and their controlled counterparts, quantum dialogue, quantum key distribution,
quantum key agreement, etc., have been extensively investigated. Specifically,
a scheme for controlled quantum dialogue (CQD) is analyzed over damping,
dephasing and depolarizing non-Markovian channels, and subsequently, the effect
of these non-Markovian channels on the other schemes of secure quantum
communication is deduced from the results obtained for CQD. The damped
non-Markovian channel causes, a periodic revival in the fidelity; while
fidelity is observed to be sustained under the influence of the dephasing
non-Markovian channel. The depolarizing channel, as well as the other
non-Markovian channels discussed here, show that the obtained average fidelity
subjected to noisy environment depends on the strength of coupling between the
quantum system with its surroundings and the number of rounds of quantum
communication involved in a particular scheme.Comment: 11 pages, 6 figure
Which verification qubits perform best for secure communication in noisy channel?
In secure quantum communication protocols, a set of single qubits prepared
using 2 or more mutually unbiased bases or a set of -qubit ()
entangled states of a particular form are usually used to form a verification
string which is subsequently used to detect traces of eavesdropping. The qubits
that form a verification string are referred to as decoy qubits, and there
exists a large set of different quantum states that can be used as decoy
qubits. In the absence of noise, any choice of decoy qubits provides equivalent
security. In this paper, we examine such equivalence for noisy environment
(e.g., in amplitude damping, phase damping, collective dephasing and collective
rotation noise channels) by comparing the decoy-qubit assisted schemes of
secure quantum communication that use single qubit states as decoy qubits with
the schemes that use entangled states as decoy qubits. Our study reveals that
the single qubit assisted scheme perform better in some noisy environments,
while some entangled qubits assisted schemes perform better in other noisy
environments. Specifically, single qubits assisted schemes perform better in
amplitude damping and phase damping noisy channels, whereas a few
Bell-state-based decoy schemes are found to perform better in the presence of
the collective noise. Thus, if the kind of noise present in a communication
channel (i.e., the characteristics of the channel) is known or measured, then
the present study can provide the best choice of decoy qubits required for
implementation of schemes of secure quantum communication through that channel.Comment: 11 pages, 4 figure
Why Quantum Bit Commitment And Ideal Quantum Coin Tossing Are Impossible
There had been well known claims of unconditionally secure quantum protocols
for bit commitment. However, we, and independently Mayers, showed that all
proposed quantum bit commitment schemes are, in principle, insecure because the
sender, Alice, can almost always cheat successfully by using an
Einstein-Podolsky-Rosen (EPR) type of attack and delaying her measurements. One
might wonder if secure quantum bit commitment protocols exist at all. We answer
this question by showing that the same type of attack by Alice will, in
principle, break any bit commitment scheme. The cheating strategy generally
requires a quantum computer. We emphasize the generality of this ``no-go
theorem'': Unconditionally secure bit commitment schemes based on quantum
mechanics---fully quantum, classical or quantum but with measurements---are all
ruled out by this result. Since bit commitment is a useful primitive for
building up more sophisticated protocols such as zero-knowledge proofs, our
results cast very serious doubt on the security of quantum cryptography in the
so-called ``post-cold-war'' applications. We also show that ideal quantum coin
tossing is impossible because of the EPR attack. This no-go theorem for ideal
quantum coin tossing may help to shed some lights on the possibility of
non-ideal protocols.Comment: We emphasize the generality of this "no-go theorem". All bit
commitment schemes---fully quantum, classical and quantum but with
measurements---are shown to be necessarily insecure. Accepted for publication
in a special issue of Physica D. About 18 pages in elsart.sty. This is an
extended version of an earlier manuscript (quant-ph/9605026) which has
appeared in the proceedings of PHYSCOMP'9
A comparative study of protocols for secure quantum communication under noisy environment: single-qubit-based protocols versus entangled-state-based protocols
The effect of noise on various protocols of secure quantum communication has
been studied. Specifically, we have investigated the effect of amplitude
damping, phase damping, squeezed generalized amplitude damping, Pauli type as
well as various collective noise models on the protocols of quantum key
distribution, quantum key agreement,quantum secure direct quantum communication
and quantum dialogue. From each type of protocol of secure quantum
communication, we have chosen two protocols for our comparative study; one
based on single qubit states and the other one on entangled states. The
comparative study reported here has revealed that single-qubit-based schemes
are generally found to perform better in the presence of amplitude damping,
phase damping, squeezed generalized amplitude damping noises, while
entanglement-based protocols turn out to be preferable in the presence of
collective noises. It is also observed that the effect of noise entirely
depends upon the number of rounds of quantum communication involved in a scheme
of quantum communication. Further, it is observed that squeezing, a completely
quantum mechanical resource present in the squeezed generalized amplitude
channel, can be used in a beneficial way as it may yield higher fidelity
compared to the corresponding zero squeezing case.Comment: 23 pages 7 figure
Applications of quantum cryptographic switch: Various tasks related to controlled quantum communication can be performed using Bell states and permutation of particles
Recently, several aspects of controlled quantum communication (e.g.,
bidirectional controlled state teleportation, controlled quantum secure direct
communication, controlled quantum dialogue, etc.) have been studied using
-qubit () entanglement. Specially, a large number of schemes for
bidirectional controlled state teleportation are proposed using -qubit
entanglement (). Here, we propose a set of protocols to
illustrate that it is possible to realize all these tasks related to controlled
quantum communication using only Bell states and permutation of particles
(PoP). As the generation and maintenance of a Bell state is much easier than a
multi-partite entanglement, the proposed strategy has a clear advantage over
the existing proposals. Further, it is shown that all the schemes proposed here
may be viewed as applications of the concept of quantum cryptographic switch
which was recently introduced by some of us. The performances of the proposed
protocols as subjected to the amplitude damping and phase damping noise on the
channels are also discussed.Comment: 12 pages, 3 figure
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