345 research outputs found
On the Distributed Compression of Quantum Information
The problem of distributed compression for correlated quantum sources is considered. The classical version of this problem was solved by Slepian and Wolf, who showed that distributed compression could take full advantage of redundancy in the local sources created by the presence of correlations. Here it is shown that, in general, this is not the case for quantum sources, by proving a lower bound on the rate sum for irreducible sources of product states which is stronger than the one given by a naive application of Slepian–Wolf. Nonetheless, strategies taking advantage of correlation do exist for some special classes of quantum sources. For example, Devetak and Winter demonstrated the existence of such a strategy when one of the sources is classical. Optimal nontrivial strategies for a different extreme, sources of Bell states, are presented here. In addition, it is explained how distributed compression is connected to other problems in quantum information theory, including information-disturbance questions, entanglement distillation and quantum error correction
Superdense coding of quantum states
We describe a method to non-obliviously communicate a 2l-qubit quantum state
by physically transmitting l+o(l) qubits of communication, and by consuming l
ebits of entanglement and some shared random bits. In the non-oblivious
scenario, the sender has a classical description of the state to be
communicated. Our method can be used to communicate states that are pure or
entangled with the sender's system; l+o(l) and 3l+o(l) shared random bits are
sufficient respectively.Comment: 5 pages, revtex
Black holes as mirrors: quantum information in random subsystems
We study information retrieval from evaporating black holes, assuming that
the internal dynamics of a black hole is unitary and rapidly mixing, and
assuming that the retriever has unlimited control over the emitted Hawking
radiation. If the evaporation of the black hole has already proceeded past the
"half-way" point, where half of the initial entropy has been radiated away,
then additional quantum information deposited in the black hole is revealed in
the Hawking radiation very rapidly. Information deposited prior to the half-way
point remains concealed until the half-way point, and then emerges quickly.
These conclusions hold because typical local quantum circuits are efficient
encoders for quantum error-correcting codes that nearly achieve the capacity of
the quantum erasure channel. Our estimate of a black hole's information
retention time, based on speculative dynamical assumptions, is just barely
compatible with the black hole complementarity hypothesis.Comment: 18 pages, 2 figures. (v2): discussion of decoding complexity
clarifie
Geometric Quantum Computation
We describe in detail a general strategy for implementing a conditional
geometric phase between two spins. Combined with single-spin operations, this
simple operation is a universal gate for quantum computation, in that any
unitary transformation can be implemented with arbitrary precision using only
single-spin operations and conditional phase shifts. Thus quantum geometrical
phases can form the basis of any quantum computation. Moreover, as the induced
conditional phase depends only on the geometry of the paths executed by the
spins it is resilient to certain types of errors and offers the potential of a
naturally fault-tolerant way of performing quantum computation.Comment: 15 pages, LaTeX, uses cite, eepic, epsfig, graphicx and amsfonts.
Accepted by J. Mod. Op
Multi-Player Quantum Games
Recently the concept of quantum information has been introduced into game
theory. Here we present the first study of quantum games with more than two
players. We discover that such games can possess a new form of equilibrium
strategy, one which has no analogue either in traditional games or even in
two-player quantum games. In these `pure' coherent equilibria, entanglement
shared among multiple players enables new kinds of cooperative behavior: indeed
it can act as a contract, in the sense that it prevents players from
successfully betraying one-another.Comment: 5 pages, 2 figs. Substantial revisons inc. new result
Generalized remote state preparation: Trading cbits, qubits and ebits in quantum communication
We consider the problem of communicating quantum states by simultaneously
making use of a noiseless classical channel, a noiseless quantum channel and
shared entanglement. We specifically study the version of the problem in which
the sender is given knowledge of the state to be communicated. In this setting,
a trade-off arises between the three resources, some portions of which have
been investigated previously in the contexts of the quantum-classical trade-off
in data compression, remote state preparation and superdense coding of quantum
states, each of which amounts to allowing just two out of these three
resources. We present a formula for the triple resource trade-off that reduces
its calculation to evaluating the data compression trade-off formula. In the
process, we also construct protocols achieving all the optimal points. These
turn out to be achievable by trade-off coding and suitable time-sharing between
optimal protocols for cases involving two resources out of the three mentioned
above.Comment: 15 pages, 2 figures, 1 tabl
Possibility, Impossibility and Cheat-Sensitivity of Quantum Bit String Commitment
Unconditionally secure non-relativistic bit commitment is known to be
impossible in both the classical and the quantum worlds. But when committing to
a string of n bits at once, how far can we stretch the quantum limits? In this
paper, we introduce a framework for quantum schemes where Alice commits a
string of n bits to Bob in such a way that she can only cheat on a bits and Bob
can learn at most b bits of information before the reveal phase. Our results
are two-fold: we show by an explicit construction that in the traditional
approach, where the reveal and guess probabilities form the security criteria,
no good schemes can exist: a+b is at least n. If, however, we use a more
liberal criterion of security, the accessible information, we construct schemes
where a=4log n+O(1) and b=4, which is impossible classically. We furthermore
present a cheat-sensitive quantum bit string commitment protocol for which we
give an explicit tradeoff between Bob's ability to gain information about the
committed string, and the probability of him being detected cheating.Comment: 10 pages, RevTex, 2 figure. v2: title change, cheat-sensitivity adde
Enhanced detection of groundwater contamination from a leaking waste disposal site by microbial community profiles
Groundwater biogeochemistry is adversely impacted when municipal solid waste leachate, rich in nutrients and anthropogenic compounds, percolates into the subsurface from leaking landfills. Detecting leachate contamination using statistical techniques is challenging because well strategies or analytical techniques may be insufficient for detecting low levels of groundwater contamination. We sampled profiles of the microbial community from monitoring wells surrounding a leaking landfill using terminal restriction fragment length polymorphism (T-RFLP) targeting the 16S rRNA gene. Results show in situ monitoring of bacteria, archaea, and the family Geobacteraceae improves characterization of groundwater quality. Bacterial T-RFLP profiles showed shifts correlated to known gradients of leachate and effectively detected changes along plume fringes that were not detected using hydrochemical data. Experimental sediment microcosms exposed to leachate-contaminated groundwater revealed a shift from a -Proteobacteria and Actinobacteria dominated community to one dominated by Firmicutes and δ-Proteobacteria. This shift is consistent with the transition from oxic conditions to an anoxic, iron-reducing environment as a result of landfill leachate-derived contaminants and associated redox conditions. We suggest microbial communities are more sensitive than hydrochemistry data for characterizing low levels of groundwater contamination and thus provide a novel source of information for optimizing detection and long-term monitoring strategies at landfill sites. Copyright 2010 by the American Geophysical Union
Security of quantum bit string commitment depends on the information measure
Unconditionally secure non-relativistic bit commitment is known to be
impossible in both the classical and the quantum world. However, when
committing to a string of n bits at once, how far can we stretch the quantum
limits? In this letter, we introduce a framework of quantum schemes where Alice
commits a string of n bits to Bob, in such a way that she can only cheat on a
bits and Bob can learn at most b bits of information before the reveal phase.
Our results are two-fold: we show by an explicit construction that in the
traditional approach, where the reveal and guess probabilities form the
security criteria, no good schemes can exist: a+b is at least n. If, however,
we use a more liberal criterion of security, the accessible information, we
construct schemes where a=4 log n+O(1) and b=4, which is impossible
classically. Our findings significantly extend known no-go results for quantum
bit commitment.Comment: To appear in PRL. Short version of quant-ph/0504078, long version to
appear separately. Improved security definition and result, one new lemma
that may be of independent interest. v2: added funding reference, no other
change
Quantum Communication in Rindler Spacetime
A state that an inertial observer in Minkowski space perceives to be the
vacuum will appear to an accelerating observer to be a thermal bath of
radiation. We study the impact of this Davies-Fulling-Unruh noise on
communication, particularly quantum communication from an inertial sender to an
accelerating observer and private communication between two inertial observers
in the presence of an accelerating eavesdropper. In both cases, we establish
compact, tractable formulas for the associated communication capacities
assuming encodings that allow a single excitation in one of a fixed number of
modes per use of the communications channel. Our contributions include a
rigorous presentation of the general theory of the private quantum capacity as
well as a detailed analysis of the structure of these channels, including their
group-theoretic properties and a proof that they are conjugate degradable.
Connections between the Unruh channel and optical amplifiers are also
discussed.Comment: v3: 44 pages, accepted in Communications in Mathematical Physic
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