92 research outputs found
Localization of relativistic particles and uncertainty relations
Localization of relativistic particles and their position-momentum
uncertainty relations are not yet fully understood. We discuss two schemes of
photon localization that are based on the energy density. One scheme produces a
positive operator-valued measure for localization. It coincides with the number
density operator and reproduces the effective 3x3 polarization density matrix.
Another scheme results in a probability distribution that is conditioned on the
detection. In both schemes the uncertainty relations for transversal position
and momentum approach the Heisenberg bound Comment: Published version; significantly modified and corrected with a focus
on photons and the experimental consequence
Inconsistency of quantum--classical dynamics, and what it implies
A new proof of the impossibility of a universal quantum-classical dynamics is
given. It has at least two consequences. The standard paradigm ``quantum system
is measured by a classical apparatus" is untenable, while a quantum matter can
be consistently coupled only with a quantum gravity.Comment: 5 pages, RevTeX
Quantum information and special relativity
Relativistic effects affect nearly all notions of quantum information theory.
The vacuum behaves as a noisy channel, even if the detectors are perfect. The
standard definition of a reduced density matrix fails for photon polarization
because the transversality condition behaves like a superselection rule. We can
however define an effective reduced density matrix which corresponds to a
restricted class of positive operator-valued measures. There are no pure photon
qubits, and no exactly orthogonal qubit states. Reduced density matrices for
the spin of massive particles are well-defined, but are not covariant under
Lorentz transformations. The spin entropy is not a relativistic scalar and has
no invariant meaning. The distinguishability of quantum signals and their
entanglement depend on the relative motion of observers.Comment: RevTex, 6 pages with one figure. Proceedings of TH-2002, Paris, 200
Reconstructing Quantum Geometry from Quantum Information: Area Renormalisation, Coarse-Graining and Entanglement on Spin Networks
After a brief review of spin networks and their interpretation as wave
functions for the (space) geometry, we discuss the renormalisation of the area
operator in loop quantum gravity. In such a background independent framework,
we propose to probe the structure of a surface through the analysis of the
coarse-graining and renormalisation flow(s) of its area. We further introduce a
procedure to coarse-grain spin network states and we quantitatively study the
decrease in the number of degrees of freedom during this process. Finally, we
use these coarse-graining tools to define the correlation and entanglement
between parts of a spin network and discuss their potential interpretation as a
natural measure of distance in such a state of quantum geometry.Comment: 27 pages, 12 figures, RevTex
Quantum causal histories in the light of quantum information
We use techniques of quantum information theory to analyze the quantum causal
histories approach to quantum gravity. We show that while it is consistent to
introduce closed timelike curves (CTCs), they cannot generically carry
independent degrees of freedom. Moreover, if the effective dynamics of the
chronology-respecting part of the system is linear, it should be completely
decoupled from the CTCs. In the absence of a CTC not all causal structures
admit the introduction of quantum mechanics. It is possible for those and only
for those causal structures that can be represented as quantum computational
networks. The dynamics of the subsystems should not be unitary or even
completely positive. However, we show that other commonly maid assumptions
ensure the complete positivity of the reduced dynamics.Comment: 9 pages, 8 eps figure
Proposal for a quantum delayed-choice experiment
Gedanken experiments are important conceptual tools in the quest to reconcile
our classical intuition with quantum mechanics and nowadays are routinely
performed in the laboratory. An important open question is the quantum
behaviour of the controlling devices in such experiments. We propose a
framework to analyse quantum-controlled experiments and illustrate the
implications by discussing a quantum version of Wheeler's delayed-choice
experiment. The introduction of a quantum-controlled device (i.e., quantum
beamsplitter) has several consequences. First, it implies that we can measure
complementary phenomena with a single experimental setup, thus pointing to a
redefinition of complementarity principle. Second, a quantum control allows us
to prove there are no consistent hidden-variable theories in which "particle"
and "wave" are realistic properties. Finally, it shows that a photon can have a
morphing behaviour between "particle" and "wave"; this further supports the
conclusion that "particle" and "wave" are not realistic properties but merely
reflect how we 'look' at the photon. The framework developed here can be
extended to other experiments, particularly to Bell-inequality tests
- …