793 research outputs found
Limitations of entropic inequalities for detecting nonclassicality in the postselected Bell causal structure
Classical and quantum physics impose different constraints on the joint
probability distributions of observed variables in a causal structure. These
differences mean that certain correlations can be certified as non-classical,
which has both foundational and practical importance. Rather than working with
the probability distribution itself, it can instead be convenient to work with
the entropies of the observed variables. In the Bell causal structure with two
inputs and outputs per party, a technique that uses entropic inequalities is
known that can always identify non-classical correlations. Here we consider the
analogue of this technique in the generalization of this scenario to more
outcomes. We identify a family of non-classical correlations in the Bell
scenario with two inputs and three outputs per party whose non-classicality
cannot be detected through the direct analogue of the previous technique. We
also show that use of Tsallis entropy instead of Shannon entropy does not help
in this case. Furthermore, we give evidence that natural extensions of the
technique also do not help. More precisely, our evidence suggests that even if
we allow the observed correlations to be post-processed according to a natural
class of non-classicality non-generating operations, entropic inequalities for
either the Shannon or Tsallis entropies cannot detect the non-classicality, and
hence that entropic inequalities are generally not sufficient to detect
non-classicality in the Bell causal structure. In addition, for the bipartite
Bell scenario with two inputs and three outputs we find the vertex description
of the polytope of non-signalling distributions that satisfy all of the
CHSH-type inequalities, which is one of the main regions of investigation in
this work.Comment: 14+7 pages, 3 figures, v2: new results added and parts of the text
restructured, v3: version accepted for publication (title differs from
published version due to editorial convention
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
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
Quantum Gambling Using Three Nonorthogonal States
We provide a quantum gambling protocol using three (symmetric) nonorthogonal
states. The bias of the proposed protocol is less than that of previous ones,
making it more practical. We show that the proposed scheme is secure against
non-entanglement attacks. The security of the proposed scheme against
entanglement attacks is shown heuristically.Comment: no essential correction, 4 pages, RevTe
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
Experimental quantum tossing of a single coin
The cryptographic protocol of coin tossing consists of two parties, Alice and
Bob, that do not trust each other, but want to generate a random bit. If the
parties use a classical communication channel and have unlimited computational
resources, one of them can always cheat perfectly. Here we analyze in detail
how the performance of a quantum coin tossing experiment should be compared to
classical protocols, taking into account the inevitable experimental
imperfections. We then report an all-optical fiber experiment in which a single
coin is tossed whose randomness is higher than achievable by any classical
protocol and present some easily realisable cheating strategies by Alice and
Bob.Comment: 13 page
Secure gated detection scheme for quantum cryptography
Several attacks have been proposed on quantum key distribution systems with
gated single-photon detectors. The attacks involve triggering the detectors
outside the center of the detector gate, and/or using bright illumination to
exploit classical photodiode mode of the detectors. Hence a secure detection
scheme requires two features: The detection events must take place in the
middle of the gate, and the detector must be single-photon sensitive. Here we
present a technique called bit-mapped gating, which is an elegant way to force
the detections in the middle of the detector gate by coupling detection time
and quantum bit error rate. We also discuss how to guarantee single-photon
sensitivity by directly measuring detector parameters. Bit-mapped gating also
provides a simple way to measure the detector blinding parameter in security
proofs for quantum key distribution systems with detector efficiency mismatch,
which up until now has remained a theoretical, unmeasurable quantity. Thus if
single-photon sensitivity can be guaranteed within the gates, a detection
scheme with bit-mapped gating satisfies the assumptions of the current security
proofs.Comment: 7 pages, 3 figure
Bell inequalities for three systems and arbitrarily many measurement outcomes
We present a family of Bell inequalities for three parties and arbitrarily
many outcomes, which can be seen as a natural generalization of the Mermin Bell
inequality. For a small number of outcomes, we verify that our inequalities
define facets of the polytope of local correlations. We investigate the quantum
violations of these inequalities, in particular with respect to the Hilbert
space dimension. We provide strong evidence that the maximal quantum violation
can only be reached using systems with local Hilbert space dimension exceeding
the number of measurement outcomes. This suggests that our inequalities can be
used as multipartite dimension witnesses.Comment: v1 6 pages, 4 tables; v2 Published version with minor typos correcte
Universal teleportation with a twist
We give a transfer theorem for teleportation based on twisting the
entanglement measurement. This allows one to say what local unitary operation
must be performed to complete the teleportation in any situation, generalizing
the scheme to include overcomplete measurements, non-abelian groups of local
unitary operations (e.g., angular momentum teleportation), and the effect of
non-maximally entangled resources.Comment: 4 pages, 1 figur
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