59,614 research outputs found
Decoding Protocols for Classical Communication on Quantum Channels
We study the problem of decoding classical information encoded on quantum
states at the output of a quantum channel, with particular focus on increasing
the communication rates towards the maximum allowed by Quantum Mechanics. After
a brief introduction to the main theoretical formalism employed in the rest of
the thesis, i.e., continuous-variable Quantum Information Theory and Quantum
Communication Theory, we consider several decoding schemes. First, we treat the
problem from an abstract perspective, presenting a method to decompose any
quantum measurement into a sequence of easier nested measurements through a
binary-tree search. Furthermore we show that this decomposition can be used to
build a capacity-achieving decoding protocol for classical communication on
quantum channels and to solve the optimal discrimination of some sets of
quantum states. These results clarify the structure of optimal quantum
measurements, showing that it can be recast in a more operational and
experimentally-oriented fashion. Second, we consider a more practical approach
and describe three receiver structures for coherent states of the
electromagnetic field with applications to single-mode state discrimination and
multi-mode decoding at the output of a quantum channel. We treat the problem
bearing in mind the technological limitations faced nowadays in the field of
optical communications: we evaluate the performance of general decoding schemes
based on such technology and report increased performance of two schemes, the
first one employing a non-Gaussian transformation and the second one employing
a code tailored to be read out easily by the most common detectors. Eventually
we characterize a large class of multi-mode adaptive receivers based on common
technological resources, obtaining a no-go theorem for their capacity.Comment: PhD thesis. 171 pages, 16 figure
Contextual, Optimal and Universal Realization of the Quantum Cloning Machine and of the NOT gate
A simultaneous realization of the Universal Optimal Quantum Cloning Machine
(UOQCM) and of the Universal-NOT gate by a quantum injected optical parametric
amplification (QIOPA), is reported. The two processes, forbidden in their exact
form for fundamental quantum limitations, are found universal and optimal, and
the measured fidelity F<1 is found close to the limit values evaluated by
quantum theory. This work may enlighten the yet little explored
interconnections of fundamental axiomatic properties within the deep structure
of quantum mechanics.Comment: 10 pages, 2 figure
Quantum state decorrelation
We address the general problem of removing correlations from quantum states
while preserving local quantum information as much as possible. We provide a
complete solution in the case of two qubits, by evaluating the minimum amount
of noise that is necessary to decorrelate covariant sets of bipartite states.
We show that two harmonic oscillators in arbitrary Gaussian state can be
decorrelated by a Gaussian covariant map. Finally, for finite-dimensional
Hilbert spaces, we prove that states obtained from most cloning channels (e.g.,
universal and phase-covariant cloning) can be decorrelated only at the expense
of a complete erasure of information about the copied state. More generally, in
finite dimension, cloning without correlations is impossible for continuous
sets of states. On the contrary, for continuos variables cloning, a slight
modification of the customary set-up for cloning coherent states allows one to
obtain clones without correlations.Comment: 11 pages, 2 figures, RevTex
Confusability graphs for symmetric sets of quantum states
For a set of quantum states generated by the action of a group, we consider
the graph obtained by considering two group elements adjacent whenever the
corresponding states are non-orthogonal. We analyze the structure of the
connected components of the graph and show two applications to the optimal
estimation of an unknown group action and to the search for decoherence free
subspaces of quantum channels with symmetry.Comment: 7 pages, no figures, contribution to the Proceedings of the XXIX
International Colloquium on Group-Theoretical Methods in Physics, August
22-26, Chern Institute of Mathematics, Tianjin, Chin
Contextual Realization of the Universal Quantum Cloning Machine and of the Universal-NOT gate by Quantum Injected Optical Parametric Amplification
A simultaneous, contextual experimental demonstration of the two processes of
cloning an input qubit and of flipping it into the orthogonal qubit is
reported. The adopted experimental apparatus, a Quantum-Injected Optical
Parametric Amplifier (QIOPA) is transformed simultaneously into a Universal
Optimal Quantum Cloning Machine (UOQCM) and into a Universal NOT
quantum-information gate. The two processes, indeed forbidden in their exact
form for fundamental quantum limitations, will be found to be universal and
optimal, i.e. the measured fidelity of both processes F<1 will be found close
to the limit values evaluated by quantum theory. A contextual theoretical and
experimental investigation of these processes, which may represent the basic
difference between the classical and the quantum worlds, can reveal in a
unifying manner the detailed structure of quantum information. It may also
enlighten the yet little explored interconnections of fundamental axiomatic
properties within the deep structure of quantum mechanics. PACS numbers:
03.67.-a, 03.65.Ta, 03.65.UdComment: 27 pages, 7 figure
The structure of preserved information in quantum processes
We introduce a general operational characterization of information-preserving
structures (IPS) -- encompassing noiseless subsystems, decoherence-free
subspaces, pointer bases, and error-correcting codes -- by demonstrating that
they are isometric to fixed points of unital quantum processes. Using this, we
show that every IPS is a matrix algebra. We further establish a structure
theorem for the fixed states and observables of an arbitrary process, which
unifies the Schrodinger and Heisenberg pictures, places restrictions on
physically allowed kinds of information, and provides an efficient algorithm
for finding all noiseless and unitarily noiseless subsystems of the process
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