496 research outputs found
What is "system": the information-theoretic arguments
The problem of "what is 'system'?" is in the very foundations of modern
quantum mechanics. Here, we point out the interest in this topic in the
information-theoretic context. E.g., we point out the possibility to manipulate
a pair of mutually non-interacting, non-entangled systems to employ
entanglement of the newly defined '(sub)systems' consisting the one and the
same composite system. Given the different divisions of a composite system into
"subsystems", the Hamiltonian of the system may perform in general
non-equivalent quantum computations. Redefinition of "subsystems" of a
composite system may be regarded as a method for avoiding decoherence in the
quantum hardware. In principle, all the notions refer to a composite system as
simple as the hydrogen atom.Comment: 13 pages, no figure
Entanglement can increase asymptotic rates of zero-error classical communication over classical channels
It is known that the number of different classical messages which can be
communicated with a single use of a classical channel with zero probability of
decoding error can sometimes be increased by using entanglement shared between
sender and receiver. It has been an open question to determine whether
entanglement can ever increase the zero-error communication rates achievable in
the limit of many channel uses. In this paper we show, by explicit examples,
that entanglement can indeed increase asymptotic zero-error capacity, even to
the extent that it is equal to the normal capacity of the channel.
Interestingly, our examples are based on the exceptional simple root systems E7
and E8.Comment: 14 pages, 2 figur
Irreversible Quantum Baker Map
We propose a generalization of the model of classical baker map on the torus,
in which the images of two parts of the phase space do overlap. This
transformation is irreversible and cannot be quantized by means of a unitary
Floquet operator. A corresponding quantum system is constructed as a completely
positive map acting in the space of density matrices. We investigate spectral
properties of this super-operator and their link with the increase of the
entropy of initially pure states.Comment: 4 pages, 3 figures include
Qubit Disentanglement and Decoherence via Dephasing
We consider whether quantum coherence in the form of mutual entanglement
between a pair of qubits is susceptible to decay that may be more rapid than
the decay of the coherence of either qubit individually. An instance of
potential importance for solid state quantum computing arises if embedded
qubits (spins, quantum dots, Cooper pair boxes, etc.) are exposed to global and
local noise at the same time. Here we allow separate phase-noisy channels to
affect local and non-local measures of system coherence. We find that the time
for decay of the qubit entanglement can be significantly shorter than the time
for local dephasing of the individual qubits.Comment: REVTeX, 9 pages, 1 figure, v2 with minor changes, reference adde
Detecting a stochastic background of gravitational radiation: Signal processing strategies and sensitivities
We analyze the signal processing required for the optimal detection of a
stochastic background of gravitational radiation using laser interferometric
detectors. Starting with basic assumptions about the statistical properties of
a stochastic gravity-wave background, we derive expressions for the optimal
filter function and signal-to-noise ratio for the cross-correlation of the
outputs of two gravity-wave detectors. Sensitivity levels required for
detection are then calculated. Issues related to: (i) calculating the
signal-to-noise ratio for arbitrarily large stochastic backgrounds, (ii)
performing the data analysis in the presence of nonstationary detector noise,
(iii) combining data from multiple detector pairs to increase the sensitivity
of a stochastic background search, (iv) correlating the outputs of 4 or more
detectors, and (v) allowing for the possibility of correlated noise in the
outputs of two detectors are discussed. We briefly describe a computer
simulation which mimics the generation and detection of a simulated stochastic
gravity-wave signal in the presence of simulated detector noise. Numerous
graphs and tables of numerical data for the five major interferometers
(LIGO-WA, LIGO-LA, VIRGO, GEO-600, and TAMA-300) are also given. The treatment
given in this paper should be accessible to both theorists involved in data
analysis and experimentalists involved in detector design and data acquisition.Comment: 81 pages, 30 postscript figures, REVTE
Weak force detection with superposed coherent states
We investigate the utility of non classical states of simple harmonic
oscillators, particularly a superposition of coherent states, for sensitive
force detection. We find that like squeezed states a superposition of coherent
states allows displacement measurements at the Heisenberg limit. Entangling
many superpositions of coherent states offers a significant advantage over a
single mode superposition states with the same mean photon number.Comment: 6 pages, no figures: New section added on entangled resources.
Changes to discussions and conclusio
Environment and classical channels in categorical quantum mechanics
We present a both simple and comprehensive graphical calculus for quantum
computing. In particular, we axiomatize the notion of an environment, which
together with the earlier introduced axiomatic notion of classical structure
enables us to define classical channels, quantum measurements and classical
control. If we moreover adjoin the earlier introduced axiomatic notion of
complementarity, we obtain sufficient structural power for constructive
representation and correctness derivation of typical quantum informatic
protocols.Comment: 26 pages, many pics; this third version has substantially more
explanations than previous ones; Journal reference is of short 14 page
version; Proceedings of the 19th EACSL Annual Conference on Computer Science
Logic (CSL), Lecture Notes in Computer Science 6247, Springer-Verlag (2010
Lagrangian evolution of global strings
We establish a method to trace the Lagrangian evolution of extended objects
consisting of a multicomponent scalar field in terms of a numerical calculation
of field equations in three dimensional Eulerian meshes. We apply our method to
the cosmological evolution of global strings and evaluate the energy density,
peculiar velocity, Lorentz factor, formation rate of loops, and emission rate
of Nambu-Goldstone (NG) bosons. We confirm the scaling behavior with a number
of long strings per horizon volume smaller than the case of local strings by a
factor of 10. The strategy and the method established here are
applicable to a variety of fields in physics.Comment: 5 pages, 2 figure
Entanglement Dynamics in Two-Qubit Open System Interacting with a Squeezed Thermal Bath via Quantum Nondemolition interaction
We analyze the dynamics of entanglement in a two-qubit system interacting
with an initially squeezed thermal environment via a quantum nondemolition
system-reservoir interaction, with the system and reservoir assumed to be
initially separable. We compare and contrast the decoherence of the two-qubit
system in the case where the qubits are mutually close-by (`collective regime')
or distant (`localized regime') with respect to the spatial variation of the
environment. Sudden death of entanglement (as quantified by concurrence) is
shown to occur in the localized case rather than in the collective case, where
entanglement tends to `ring down'. A consequence of the QND character of the
interaction is that the time-evolved fidelity of a Bell state never falls below
, a fact that is useful for quantum communication applications like
a quantum repeater. Using a novel quantification of mixed state entanglement,
we show that there are noise regimes where even though entanglement vanishes,
the state is still available for applications like NMR quantum computation,
because of the presence of a pseudo-pure component.Comment: 17 pages, 9 figures, REVTeX
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