117 research outputs found
Continuous-variable quantum enigma machines for long-distance key distribution
Quantum physics allows for unconditionally secure communication through
insecure communication channels. The achievable rates of quantum-secured
communication are fundamentally limited by the laws of quantum physics and in
particular by the properties of entanglement. For a lossy communication line,
this implies that the secret-key generation rate vanishes at least
exponentially with the communication distance. We show that this fundamental
limitation can be violated in a realistic scenario where the eavesdropper can
store quantum information for only a finite, yet arbitrarily long, time. We
consider communication through a lossy bononic channel (modeling linear loss in
optical fibers) and we show that it is in principle possible to achieve a
constant rate of key generation of one bit per optical mode over arbitrarily
long communication distances.Comment: 13 pages. V2: new title, new result on active attacks, increased
rigour in the security proo
Quantum-locked key distribution at nearly the classical capacity rate
Quantum data locking is a protocol that allows for a small secret key to
(un)lock an exponentially larger amount of information, hence yielding the
strongest violation of the classical one-time pad encryption in the quantum
setting. This violation mirrors a large gap existing between two security
criteria for quantum cryptography quantified by two entropic quantities: the
Holevo information and the accessible information. We show that the latter
becomes a sensible security criterion if an upper bound on the coherence time
of the eavesdropper's quantum memory is known. Under this condition we
introduce a protocol for secret key generation through a memoryless qudit
channel. For channels with enough symmetry, such as the d-dimensional erasure
and depolarizing channels, this protocol allows secret key generation at an
asymptotic rate as high as the classical capacity minus one bit.Comment: v2 is close to the published version and contains only the key
distribution protocols (4+5 pages), an extended version of the direct
communication protocol is posted in arXiv:1410.4748 Comments always welcom
Quantum Data Locking for Secure Communication against an Eavesdropper with Time-Limited Storage
Quantum cryptography allows for unconditionally secure communication against an eavesdropper endowed with unlimited computational power and perfect technologies, who is only constrained by the laws of physics. We review recent results showing that, under the assumption that the eavesdropper can store quantum information only for a limited time, it is possible to enhance the performance of quantum key distribution in both a quantitative and qualitative fashion. We consider quantum data locking as a cryptographic primitive and discuss secure communication and key distribution protocols. For the case of a lossy optical channel, this yields the theoretical possibility of generating secret key at a constant rate of 1 bit per mode at arbitrarily long communication distances.United States. Army Research Office (United States. Defense Advanced Research Projects Agency. Quiness Program (W31P4Q-12-1-0019
Methods for Estimating Capacities and Rates of Gaussian Quantum Channels
Optimization methods aimed at estimating the capacities of a general Gaussian
channel are developed. Specifically evaluation of classical capacity as maximum
of the Holevo information is pursued over all possible Gaussian encodings for
the lossy bosonic channel, but extension to other capacities and other Gaussian
channels seems feasible. Solutions for both memoryless and memory channels are
presented. It is first dealt with single use (single-mode) channel where the
capacity dependence from channel's parameters is analyzed providing a full
classification of the possible cases. Then it is dealt with multiple uses
(multi-mode) channel where the capacity dependence from the (multi-mode)
environment state is analyzed when both total environment energy and
environment purity are fixed. This allows a fair comparison among different
environments, thus understanding the role of memory (inter-mode correlations)
and phenomenon like superadditivity of the capacity. The developed methods are
also used for deriving transmission rates with heterodyne and homodyne
measurements at the channel output. Classical capacity and transmission rates
are presented within a unique framework where the rates can be treated as
logarithmic approximations of the capacity.Comment: 39 pages, 30 figures. New results and graphs were added. Errors and
misprints were corrected. To appear in IEEE Trans. Inf. T
Quantum data hiding in the presence of noise
When classical or quantum information is broadcast to separate receivers,
there exist codes that encrypt the encoded data such that the receivers cannot
recover it when performing local operations and classical communication, but
they can decode reliably if they bring their systems together and perform a
collective measurement. This phenomenon is known as quantum data hiding and
hitherto has been studied under the assumption that noise does not affect the
encoded systems. With the aim of applying the quantum data hiding effect in
practical scenarios, here we define the data-hiding capacity for hiding
classical information using a quantum channel. Using this notion, we establish
a regularized upper bound on the data hiding capacity of any quantum broadcast
channel, and we prove that coherent-state encodings have a strong limitation on
their data hiding rates. We then prove a lower bound on the data hiding
capacity of channels that map the maximally mixed state to the maximally mixed
state (we call these channels "mictodiactic"---they can be seen as a
generalization of unital channels when the input and output spaces are not
necessarily isomorphic) and argue how to extend this bound to generic channels
and to more than two receivers.Comment: 12 pages, accepted for publication in IEEE Transactions on
Information Theor
Robust quantum data locking from phase modulation
Quantum data locking is a unique quantum phenomenon that allows a relatively
short key to (un)lock an arbitrarily long message encoded in a quantum state,
in such a way that an eavesdropper who measures the state but does not know the
key has essentially no information about the encrypted message. The application
of quantum data locking in cryptography would allow one to overcome the
limitations of the one-time pad encryption, which requires the key to have the
same length as the message. However, it is known that the strength of quantum
data locking is also its Achilles heel, as the leakage of a few bits of the key
or the message may in principle allow the eavesdropper to unlock a
disproportionate amount of information. In this paper we show that there exist
quantum data locking schemes that can be made robust against information
leakage by increasing the length of the shared key by a proportionate amount.
This implies that a constant size key can still encrypt an arbitrarily long
message as long as a fraction of it remains secret to the eavesdropper.
Moreover, we greatly simplify the structure of the protocol by proving that
phase modulation suffices to generate strong locking schemes, paving the way to
optical experimental realizations. Also, we show that successful data locking
protocols can be constructed using random codewords, which very well could be
helpful in discovering random codes for data locking over noisy quantum
channels.Comment: A new result on the robustness of quantum data locking has been adde
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