1,554 research outputs found
Unconditional security of coherent-state quantum key distribution with strong phase-reference pulse
We prove the unconditional security of a quantum key distribution protocol in
which bit values are encoded in the phase of a weak coherent-state pulse
relative to a strong reference pulse. In contrast to implementations in which a
weak pulse is used as a substitute for a single-photon source, the achievable
key rate is found to decrease only linearly with the transmission of the
channel.Comment: 4 pages, 3 figure
Unconditional security at a low cost
By simulating four quantum key distribution (QKD) experiments and analyzing
one decoy-state QKD experiment, we compare two data post-processing schemes
based on security against individual attack by L\"{u}tkenhaus, and
unconditional security analysis by Gottesman-Lo-L\"{u}tkenhaus-Preskill. Our
results show that these two schemes yield close performances. Since the Holy
Grail of QKD is its unconditional security, we conclude that one is better off
considering unconditional security, rather than restricting to individual
attacks.Comment: Accepted by International Conference on Quantum Foundation and
Technology: Frontier and Future 2006 (ICQFT'06
Heralded qubit amplifiers for practical device-independent quantum key distribution
Device-independent quantum key distribution does not need a precise quantum
mechanical model of employed devices to guarantee security. Despite of its
beauty, it is still a very challenging experimental task. We compare a recent
proposal by Gisin et al. [Phys. Rev. Lett. 105, 070501 (2010)] to close the
detection loophole problem with that of a simpler quantum relay based on
entanglement swapping with linear optics. Our full-mode analysis for both
schemes confirms that, in contrast to recent beliefs, the second scheme can
indeed provide a positive key rate which is even considerably higher than that
of the first alternative. The resulting key rates and required detection
efficiencies of approx. 95% for both schemes, however, strongly depend on the
underlying security proof.Comment: 5 pages, 3 figure
Security of EPR-based Quantum Cryptography against Incoherent Symmetric Attacks
We investigate a new strategy for incoherent eavesdropping in Ekert's
entanglement based quantum key distribution protocol. We show that under
certain assumptions of symmetry the effectiveness of this strategy reduces to
that of the original single qubit protocol of Bennett and Brassard
Security of differential phase shift quantum key distribution against individual attacks
We derive a proof of security for the Differential Phase Shift Quantum Key
Distribution (DPSQKD) protocol under the assumption that Eve is restricted to
individual attacks. The security proof is derived by bounding the average
collision probability, which leads directly to a bound on Eve's mutual
information on the final key. The security proof applies to realistic sources
based on pulsed coherent light. We then compare individual attacks to
sequential attacks and show that individual attacks are more powerful
Effects of detector efficiency mismatch on security of quantum cryptosystems
We suggest a type of attack on quantum cryptosystems that exploits variations
in detector efficiency as a function of a control parameter accessible to an
eavesdropper. With gated single-photon detectors, this control parameter can be
the timing of the incoming pulse. When the eavesdropper sends short pulses
using the appropriate timing so that the two gated detectors in Bob's setup
have different efficiencies, the security of quantum key distribution can be
compromised. Specifically, we show for the Bennett-Brassard 1984 (BB84)
protocol that if the efficiency mismatch between 0 and 1 detectors for some
value of the control parameter gets large enough (roughly 15:1 or larger), Eve
can construct a successful faked-states attack causing a quantum bit error rate
lower than 11%. We also derive a general security bound as a function of the
detector sensitivity mismatch for the BB84 protocol. Experimental data for two
different detectors are presented, and protection measures against this attack
are discussed.Comment: v3: identical to the journal version. However, after publication we
have discovered that Eq. 11 is incorrect: the available bit rate after
privacy amplification is reduced even in the case (QBER)=0 [see Quant. Inf.
Comp. 7, 73 (2007)
Coin Tossing is Strictly Weaker Than Bit Commitment
We define cryptographic assumptions applicable to two mistrustful parties who
each control two or more separate secure sites between which special relativity
guarantees a time lapse in communication. We show that, under these
assumptions, unconditionally secure coin tossing can be carried out by
exchanges of classical information. We show also, following Mayers, Lo and
Chau, that unconditionally secure bit commitment cannot be carried out by
finitely many exchanges of classical or quantum information. Finally we show
that, under standard cryptographic assumptions, coin tossing is strictly weaker
than bit commitment. That is, no secure classical or quantum bit commitment
protocol can be built from a finite number of invocations of a secure coin
tossing black box together with finitely many additional information exchanges.Comment: Final version; to appear in Phys. Rev. Let
Efficient Heralding of Photonic Qubits with Apllications to Device Independent Quantum Key Distribution
We present an efficient way of heralding photonic qubit signals using linear
optics devices. First we show that one can obtain asymptotically perfect
heralding and unit success probability with growing resources. Second, we show
that even using finite resources, we can improve qualitatively and
quantitatively over earlier heralding results. In the latte r scenario, we can
obtain perfect heralded photonic qubits while maintaining a finite success
probability. We demonstrate the advantage of our heralding scheme by predicting
key rates for device independent quantum key distribution, taking imperfections
of sources and detectors into account
Unconditionally secure quantum bit commitment is impossible
The claim of quantum cryptography has always been that it can provide
protocols that are unconditionally secure, that is, for which the security does
not depend on any restriction on the time, space or technology available to the
cheaters. We show that this claim does not hold for any quantum bit commitment
protocol. Since many cryptographic tasks use bit commitment as a basic
primitive, this result implies a severe setback for quantum cryptography. The
model used encompasses all reasonable implementations of quantum bit commitment
protocols in which the participants have not met before, including those that
make use of the theory of special relativity.Comment: 4 pages, revtex. Journal version replacing the version published in
the proceedings of PhysComp96. This is a significantly improved version which
emphasis the generality of the resul
Is Quantum Bit Commitment Really Possible?
We show that all proposed quantum bit commitment schemes are insecure because
the sender, Alice, can almost always cheat successfully by using an
Einstein-Podolsky-Rosen type of attack and delaying her measurement until she
opens her commitment.Comment: Major revisions to include a more extensive introduction and an
example of bit commitment. Overlap with independent work by Mayers
acknowledged. More recent works by Mayers, by Lo and Chau and by Lo are also
noted. Accepted for publication in Phys. Rev. Let
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