861 research outputs found
Local Quantum Measurement and No-Signaling Imply Quantum Correlations
We show that, assuming that quantum mechanics holds locally, the finite speed
of information is the principle that limits all possible correlations between
distant parties to be quantum mechanical as well. Local quantum mechanics means
that a Hilbert space is assigned to each party, and then all local
positive-operator-valued measurements are (in principle) available; however,
the joint system is not necessarily described by a Hilbert space. In
particular, we do not assume the tensor product formalism between the joint
systems. Our result shows that if any experiment would give nonlocal
correlations beyond quantum mechanics, quantum theory would be invalidated even
locally.Comment: Published version. 5 pages, 1 figure
A violation of the uncertainty principle implies a violation of the second law of thermodynamics
Uncertainty relations state that there exist certain incompatible
measurements, to which the outcomes cannot be simultaneously predicted. While
the exact incompatibility of quantum measurements dictated by such uncertainty
relations can be inferred from the mathematical formalism of quantum theory,
the question remains whether there is any more fundamental reason for the
uncertainty relations to have this exact form. What, if any, would be the
operational consequences if we were able to go beyond any of these uncertainty
relations? We give a strong argument that justifies uncertainty relations in
quantum theory by showing that violating them implies that it is also possible
to violate the second law of thermodynamics. More precisely, we show that
violating the uncertainty relations in quantum mechanics leads to a
thermodynamic cycle with positive net work gain, which is very unlikely to
exist in nature.Comment: 8 pages, revte
Compressibility of Mixed-State Signals
We present a formula that determines the optimal number of qubits per message
that allows asymptotically faithful compression of the quantum information
carried by an ensemble of mixed states. The set of mixed states determines a
decomposition of the Hilbert space into the redundant part and the irreducible
part. After removing the redundancy, the optimal compression rate is shown to
be given by the von Neumann entropy of the reduced ensemble.Comment: 7 pages, no figur
Linking a distance measure of entanglement to its convex roof
An important problem in quantum information theory is the quantification of
entanglement in multipartite mixed quantum states. In this work, a connection
between the geometric measure of entanglement and a distance measure of
entanglement is established. We present a new expression for the geometric
measure of entanglement in terms of the maximal fidelity with a separable
state. A direct application of this result provides a closed expression for the
Bures measure of entanglement of two qubits. We also prove that the number of
elements in an optimal decomposition w.r.t. the geometric measure of
entanglement is bounded from above by the Caratheodory bound, and we find
necessary conditions for the structure of an optimal decomposition.Comment: 11 pages, 4 figure
The quantum capacity is properly defined without encodings
We show that no source encoding is needed in the definition of the capacity
of a quantum channel for carrying quantum information. This allows us to use
the coherent information maximized over all sources and and block sizes, but
not encodings, to bound the quantum capacity. We perform an explicit
calculation of this maximum coherent information for the quantum erasure
channel and apply the bound in order find the erasure channel's capacity
without relying on an unproven assumption as in an earlier paper.Comment: 19 pages revtex with two eps figures. Submitted to Phys. Rev. A.
Replaced with revised and simplified version, and improved references, etc.
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Quantum Copying: Beyond the No-Cloning Theorem
We analyze to what extent it is possible to copy arbitrary states of a
two-level quantum system. We show that there exists a "universal quantum
copying machine", which approximately copies quantum mechanical states in such
a way that the quality of its output does not depend on the input. We also
examine a machine which combines a unitary transformation with a selective
measurement to produce good copies of states in a neighborhood of a particular
state. We discuss the problem of measurement of the output states.Comment: RevTex, 26 pages, to appear in Physical Review
Lossless quantum data compression and variable-length coding
In order to compress quantum messages without loss of information it is
necessary to allow the length of the encoded messages to vary. We develop a
general framework for variable-length quantum messages in close analogy to the
classical case and show that lossless compression is only possible if the
message to be compressed is known to the sender. The lossless compression of an
ensemble of messages is bounded from below by its von-Neumann entropy. We show
that it is possible to reduce the number of qbits passing through a quantum
channel even below the von-Neumann entropy by adding a classical side-channel.
We give an explicit communication protocol that realizes lossless and
instantaneous quantum data compression and apply it to a simple example. This
protocol can be used for both online quantum communication and storage of
quantum data.Comment: 16 pages, 5 figure
Quantum channels with a finite memory
In this paper we study quantum communication channels with correlated noise
effects, i.e., quantum channels with memory. We derive a model for correlated
noise channels that includes a channel memory state. We examine the case where
the memory is finite, and derive bounds on the classical and quantum
capacities. For the entanglement-assisted and unassisted classical capacities
it is shown that these bounds are attainable for certain classes of channel.
Also, we show that the structure of any finite memory state is unimportant in
the asymptotic limit, and specifically, for a perfect finite-memory channel
where no nformation is lost to the environment, achieving the upper bound
implies that the channel is asymptotically noiseless.Comment: 7 Pages, RevTex, Jrnl versio
The Holevo bound and Landauer's principle
Landauer's principle states that the erasure of information generates a
corresponding amount of entropy in the environment. We show that Landauer's
principle provides an intuitive basis for Holevo bound on the classical
capacity of a quantum channel.Comment: to appear in Physics Letters
Bostonia. Volume 14
Founded in 1900, Bostonia magazine is Boston University's main alumni publication, which covers alumni and student life, as well as university activities, events, and programs
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