433 research outputs found
A triangle of dualities: reversibly decomposable quantum channels, source-channel duality, and time reversal
Two quantum information processing protocols are said to be dual under
resource reversal if the resources consumed (generated) in one protocol are
generated (consumed) in the other. Previously known examples include the
duality between entanglement concentration and dilution, and the duality
between coherent versions of teleportation and super-dense coding. A quantum
feedback channel is an isometry from a system belonging to Alice to a system
shared between Alice and Bob. We show that such a resource may be reversibly
decomposed into a perfect quantum channel and pure entanglement, generalizing
both of the above examples. The dual protocols responsible for this
decomposition are the ``feedback father'' (FF) protocol and the ``fully quantum
reverse Shannon'' (FQRS) protocol. Moreover, the ``fully quantum Slepian-Wolf''
protocol (FQSW), a generalization of the recently discovered ``quantum state
merging'', is related to FF by source-channel duality, and to FQRS by time
reversal duality, thus forming a triangle of dualities. The source-channel
duality is identified as the origin of the previously poorly understood
``mother-father'' duality. Due to a symmetry breaking, the dualities extend
only partially to classical information theory.Comment: 5 pages, 5 figure
Vacuum state truncation via the quantum Zeno effect
In the context of quantum state engineering we analyze the effect of
observation on nonlinear optical -photon Fock state generation. We show that
it is possible to truncate the vacuum component from an arbitrary photon number
superposition without modifying its remaining parts. In the course of the full
dynamical analysis of the effect of observation, it is also found that the Zeno
and the anti-Zeno effects repeat periodically. We discuss the close
relationship between vacuum state truncation and so-called "interaction-free"
measurement.Comment: 4 pages, 2 figures, LaTeX; TeX errors fixe
Stabilisation of Quantum Computations by Symmetrisation
We propose a method for the stabilisation of quantum computations (including
quantum state storage). The method is based on the operation of projection into
, the symmetric subspace of the full state space of redundant
copies of the computer. We describe an efficient algorithm and quantum network
effecting --projection and discuss the stabilising effect of the
proposed method in the context of unitary errors generated by hardware
imprecision, and nonunitary errors arising from external environmental
interaction. Finally, limitations of the method are discussed.Comment: 20 pages LaTeX, 2 postscript figure
Distinguishability of States and von Neumann Entropy
Consider an ensemble of pure quantum states |\psi_j>, j=1,...,n taken with
prior probabilities p_j respectively. We show that it is possible to increase
all of the pairwise overlaps || i.e. make each constituent pair
of the states more parallel (while keeping the prior probabilities the same),
in such a way that the von Neumann entropy S is increased, and dually, make all
pairs more orthogonal while decreasing S. We show that this phenomenon cannot
occur for ensembles in two dimensions but that it is a feature of almost all
ensembles of three states in three dimensions. It is known that the von Neumann
entropy characterises the classical and quantum information capacities of the
ensemble and we argue that information capacity in turn, is a manifestation of
the distinguishability of the signal states. Hence our result shows that the
notion of distinguishability within an ensemble is a global property that
cannot be reduced to considering distinguishability of each constituent pair of
states.Comment: 18 pages, Latex, 2 figure
On quantum coding for ensembles of mixed states
We consider the problem of optimal asymptotically faithful compression for
ensembles of mixed quantum states. Although the optimal rate is unknown, we
prove upper and lower bounds and describe a series of illustrative examples of
compression of mixed states. We also discuss a classical analogue of the
problem.Comment: 23 pages, LaTe
Optical Communication Noise Rejection Using Correlated Photons
This paper describes a completely new way to perform noise rejection using a
two-photon sensitive detector and taking advantage of the properties of
correlated photons to improve an optical communications link in the presence of
uncorrelated noise. In particular, a detailed analysis is made of the case
where a classical link would be saturated by an intense background, such as
when a satellite is in front of the sun,and identifies a regime where the
quantum correlating system has superior performance.Comment: 12 pages, 1 figure, 1 tabl
Electronic spin drift in graphene field effect transistors
We studied the drift of electron spins under an applied DC electric field in
single layer graphene spin valves in a field effect transport geometry at room
temperature. In the metallic conduction regime (
m), for DC fields of about 70 kV/m applied between the spin
injector and spin detector, the spin valve signals are increased/decreased,
depending on the direction of the DC field and the carrier type, by as much as
50%. Sign reversal of the drift effect is observed when switching from
hole to electron conduction. In the vicinity of the Dirac neutrality point the
drift effect is strongly suppressed. The experiments are in quantitative
agreement with a drift-diffusion model of spin transport.Comment: 4 figure
Counterfactual Quantum Cryptography
Quantum cryptography allows one to distribute a secret key between two remote
parties using the fundamental principles of quantum mechanics. The well-known
established paradigm for the quantum key distribution relies on the actual
transmission of signal particle through a quantum channel. This paper shows
that the task of a secret key distribution can be accomplished even though a
particle carrying secret information is not in fact transmitted through the
quantum channel. The proposed protocols can be implemented with current
technologies and provide practical security advantages by eliminating the
possibility that an eavesdropper can directly access the entire quantum system
of each signal particle.Comment: 19 pages, 1 figure; a little ambiguity in the version 1 removed;
abstract, text, references, and appendix revised; suggestions and comments
are highly appreciate
Quantum Key Distribution with Classical Bob
Secure key distribution among two remote parties is impossible when both are
classical, unless some unproven (and arguably unrealistic)
computation-complexity assumptions are made, such as the difficulty of
factorizing large numbers. On the other hand, a secure key distribution is
possible when both parties are quantum.
What is possible when only one party (Alice) is quantum, yet the other (Bob)
has only classical capabilities? We present a protocol with this constraint,
and prove its robustness against attacks: we prove that any attempt of an
adversary to obtain information (and even a tiny amount of information)
necessarily induces some errors that the legitimate users could notice.Comment: 4 and a bit pages, 1 figure, RevTe
Secure quantum key distribution with an uncharacterized source
We prove the security of the Bennett-Brassard (BB84) quantum key distribution
protocol for an arbitrary source whose averaged states are basis-independent, a
condition that is automatically satisfied if the source is suitably designed.
The proof is based on the observation that, to an adversary, the key extraction
process is equivalent to a measurement in the sigma_x-basis performed on a pure
sigma_z-basis eigenstate. The dependence of the achievable key length on the
bit error rate is the same as that established by Shor and Preskill for a
perfect source, indicating that the defects in the source are efficiently
detected by the protocol.Comment: 4 pages, 1 figure, REVTeX, minor revision
- …