6 research outputs found
Device-independent two-party cryptography secure against sequential attacks
The goal of two-party cryptography is to enable two parties, Alice and Bob,
to solve common tasks without the need for mutual trust. Examples of such tasks
are private access to a database, and secure identification. Quantum
communication enables security for all of these problems in the noisy-storage
model by sending more signals than the adversary can store in a certain time
frame. Here, we initiate the study of device-independent protocols for
two-party cryptography in the noisy-storage model. Specifically, we present a
relatively easy to implement protocol for a cryptographic building block known
as weak string erasure and prove its security even if the devices used in the
protocol are prepared by the dishonest party. Device-independent two-party
cryptography is made challenging by the fact that Alice and Bob do not trust
each other, which requires new techniques to establish security. We fully
analyse the case of memoryless devices (for which sequential attacks are
optimal) and the case of sequential attacks for arbitrary devices. The key
ingredient of the proof, which might be of independent interest, is an explicit
(and tight) relation between the violation of the Clauser-Horne-Shimony-Holt
inequality observed by Alice and Bob and uncertainty generated by Alice against
Bob who is forced to measure his system before finding out Alice's setting
(guessing with postmeasurement information). In particular, we show that
security is possible for arbitrarily small violation.Comment: 18 pages, 7 figures, published versio
Analytic and nearly optimal self-testing bounds for the Clauser-Horne-Shimony-Holt and Mermin inequalities
Self-testing refers to the phenomenon that certain extremal quantum
correlations (almost) uniquely identify the quantum system under consideration.
For instance observing the maximal violation of the CHSH inequality certifies
that the two parties share a singlet. While self-testing results are known for
several classes of states, in many cases they are only applicable if the
observed statistics are almost perfect, which makes them unsuitable for
practical applications. Practically relevant self-testing bounds are much less
common and moreover they all result from a single numerical method (with one
exception which we discuss in detail). In this work we present a new technique
for proving analytic self-testing bounds of practically relevant robustness. We
obtain improved bounds for the case of self-testing the singlet using the CHSH
inequality (in particular we show that non-trivial fidelity with the singlet
can be achieved as long as the violation exceeds ). In case of self-testing the tripartite GHZ state
using the Mermin inequality we derive a bound which not only improves on
previously known results but turns out to be tight. We discuss other scenarios
to which our technique can be immediately applied.Comment: 4 pages, 2 figures. v2: fixed an inconsistency in specifying the
extraction channel, improved presentation; v3: published versio
Near-maximal two-photon entanglement for quantum communications at 2.1 μm
Owing to a reduced solar background and low propagation losses in the atmosphere, the 2- to 2.5-μm waveband is a promising candidate for daylight quantum communication. This spectral region also offers low losses and low dispersion in hollow-core fibers and in silicon waveguides. We demonstratenear-maximally entangled photon pairs at 2.1 μm that could support device-independent quantum key distribution (DIQKD), assuming sufficiently high channel efficiencies. The state corresponds to a positive secure-key rate (0.254 bits/pair, with a quantum bit error rate of 3.8%) based on measurements in alaboratory setting with minimal channel loss and transmission distance. This is promising for the future implementation of DIQKD at 2.1 μm