1,472 research outputs found
Is Amino-Acid Homochirality Due To Asymmetric Photolysis In Space?
Amino acids occurring in proteins are, with rare exceptions, exclusively of
the L-configuration. Among the many scenarios put forward to explain the origin
of this chiral homogeneity (i.e., homochirality), one involves the asymmetric
photolysis of amino acids present in space, triggered by circularly polarized
UV radiation. The recent observation of circularly polarized light (CPL) in the
Orion OMC-1 star-forming region (Bailey et al. 1998, Science 281, 672) has been
presented as providing a strong validation of this scenario. The present paper
reviews the situation. It is stressed for example that one important condition
for the asymmetric photolysis by CPL to be at the origin of the terrestrial
homochirality of natural amino acids is generally overlooked, namely, the
asymmetric photolysis should favour the L-enantiomer for ALL the primordial
amino acids involved in the genesis of life (i.e., biogenic amino acids).
Although this condition is probably satisfied for aliphatic amino acids, some
non-aliphatic amino acids like tryptophan and proline may violate the condition
and thus invalidate the asymmetric photolysis scenario, assuming they were
among the primordial amino acids. Alternatively, if CPL photolysis in space is
indeed the source of homochirality of amino acids, then tryptophan and proline
may be crossed out from the list of biogenic amino acids.Comment: To appear in Space Science Reviews, 11 pages, 1 figure (LaTeX
Quantum conditional operator and a criterion for separability
We analyze the properties of the conditional amplitude operator, the quantum
analog of the conditional probability which has been introduced in
[quant-ph/9512022]. The spectrum of the conditional operator characterizing a
quantum bipartite system is invariant under local unitary transformations and
reflects its inseparability. More specifically, it is shown that the
conditional amplitude operator of a separable state cannot have an eigenvalue
exceeding 1, which results in a necessary condition for separability. This
leads us to consider a related separability criterion based on the positive map
, where is an Hermitian operator. Any
separable state is mapped by the tensor product of this map and the identity
into a non-negative operator, which provides a simple necessary condition for
separability. In the special case where one subsystem is a quantum bit,
reduces to time-reversal, so that this separability condition is
equivalent to partial transposition. It is therefore also sufficient for
and systems. Finally, a simple connection between this
map and complex conjugation in the "magic" basis is displayed.Comment: 19 pages, RevTe
Multipartite Asymmetric Quantum Cloning
We investigate the optimal distribution of quantum information over
multipartite systems in asymmetric settings. We introduce cloning
transformations that take identical replicas of a pure state in any
dimension as input, and yield a collection of clones with non-identical
fidelities. As an example, if the clones are partitioned into a set of
clones with fidelity and another set of clones with fidelity ,
the trade-off between these fidelities is analyzed, and particular cases of
optimal cloning machines are exhibited. We also present an
optimal cloning machine, which is the first known example of a
tripartite fully asymmetric cloner. Finally, it is shown how these cloning
machines can be optically realized.Comment: 5 pages, 2 figure
Cloning the entanglement of a pair of quantum bits
It is shown that any quantum operation that perfectly clones the entanglement
of all maximally-entangled qubit pairs cannot preserve separability. This
``entanglement no-cloning'' principle naturally suggests that some approximate
cloning of entanglement is nevertheless allowed by quantum mechanics. We
investigate a separability-preserving optimal cloning machine that duplicates
all maximally-entangled states of two qubits, resulting in 0.285 bits of
entanglement per clone, while a local cloning machine only yields 0.060 bits of
entanglement per clone.Comment: 4 pages Revtex, 2 encapsulated Postscript figures, one added autho
Extremal quantum cloning machines
We investigate the problem of cloning a set of states that is invariant under
the action of an irreducible group representation. We then characterize the
cloners that are "extremal" in the convex set of group covariant cloning
machines, among which one can restrict the search for optimal cloners. For a
set of states that is invariant under the discrete Weyl-Heisenberg group, we
show that all extremal cloners can be unitarily realized using the so-called
"double-Bell states", whence providing a general proof of the popular ansatz
used in the literature for finding optimal cloners in a variety of settings.
Our result can also be generalized to continuous-variable optimal cloning in
infinite dimensions, where the covariance group is the customary
Weyl-Heisenberg group of displacements.Comment: revised version accepted for publicatio
Quantum key distribution for d-level systems with generalized Bell states
Using the generalized Bell states and controlled not gates, we introduce an
enatanglement-based quantum key distribution (QKD) of d-level states (qudits).
In case of eavesdropping, Eve's information gain is zero and a quantum error
rate of (d-1)/d is introduced in Bob's received qudits, so that for large d,
comparison of only a tiny fraction of received qudits with the sent ones can
detect the presence of Eve.Comment: 8 pages, 3 figures, REVTEX, references added, extensive revision, to
appear in Phys. Rev.
Information-theoretic interpretation of quantum error-correcting codes
Quantum error-correcting codes are analyzed from an information-theoretic
perspective centered on quantum conditional and mutual entropies. This approach
parallels the description of classical error correction in Shannon theory,
while clarifying the differences between classical and quantum codes. More
specifically, it is shown how quantum information theory accounts for the fact
that "redundant" information can be distributed over quantum bits even though
this does not violate the quantum "no-cloning" theorem. Such a remarkable
feature, which has no counterpart for classical codes, is related to the
property that the ternary mutual entropy vanishes for a tripartite system in a
pure state. This information-theoretic description of quantum coding is used to
derive the quantum analogue of the Singleton bound on the number of logical
bits that can be preserved by a code of fixed length which can recover a given
number of errors.Comment: 14 pages RevTeX, 8 Postscript figures. Added appendix. To appear in
Phys. Rev.
Reversibility of continuous-variable quantum cloning
We analyze a reversibility of optimal Gaussian quantum cloning of a
coherent state using only local operations on the clones and classical
communication between them and propose a feasible experimental test of this
feature. Performing Bell-type homodyne measurement on one clone and anti-clone,
an arbitrary unknown input state (not only a coherent state) can be restored in
the other clone by applying appropriate local unitary displacement operation.
We generalize this concept to a partial LOCC reversal of the cloning and we
show that this procedure converts the symmetric cloner to an asymmetric cloner.
Further, we discuss a distributed LOCC reversal in optimal Gaussian
cloning of coherent states which transforms it to optimal cloning for
. Assuming the quantum cloning as a possible eavesdropping attack on
quantum communication link, the reversibility can be utilized to improve the
security of the link even after the attack.Comment: 7 pages, 5 figure
Phase-Conjugated Inputs Quantum Cloning Machines
A quantum cloning machine is introduced that yields identical optimal
clones from replicas of a coherent state and replicas of its phase
conjugate. It also optimally produces phase-conjugated clones at no
cost. For well chosen input asymmetries , this machine is shown to
provide better cloning fidelities than the standard cloner. The
special cases of the optimal balanced cloner () and the optimal
measurement () are investigated.Comment: 4 pages (RevTex), 2 figure
Entropy production in Gaussian bosonic transformations using the replica method: application to quantum optics
In spite of their simple description in terms of rotations or symplectic
transformations in phase space, quadratic Hamiltonians such as those modeling
the most common Gaussian operations on bosonic modes remain poorly understood
in terms of entropy production. For instance, determining the von Neumann
entropy produced by a Bogoliubov transformation is notably a hard problem, with
generally no known analytical solution. Here, we overcome this difficulty by
using the replica method, a tool borrowed from statistical physics and quantum
field theory. We exhibit a first application of this method to the field of
quantum optics, where it enables accessing entropies in a two-mode squeezer or
optical parametric amplifier. As an illustration, we determine the entropy
generated by amplifying a binary superposition of the vacuum and an arbitrary
Fock state, which yields a surprisingly simple, yet unknown analytical
expression
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