18,214 research outputs found
Simulation of interaction Hamiltonians by quantum feedback: a comment on the dynamics of information exchange between coupled systems
Since quantum feedback is based on classically accessible measurement
results, it can provide fundamental insights into the dynamics of quantum
systems by making available classical information on the evolution of system
properties and on the conditional forces acting on the system. In this paper,
the feedback-induced interaction dynamics between a pair of quantum systems is
analyzed. It is pointed out that any interaction Hamiltonian can be simulated
by local feedback if the levels of decoherence are sufficiently high. The
boundary between genuine entanglement generating quantum interactions and
non-entangling classical interactions is identified and the nature of the
information exchange between two quantum systems during an interaction is
discussed.Comment: 14 pages, 4 figures; invited paper for the special issue of J. Opt. B
on quantum contro
Generation of a highly phase sensitive polarization squeezed N-photon state by collinear parametric downconversion and coherent photon subtraction
It is shown that a highly phase sensitive polarization squeezed (2n-1)-photon
state can be generated by subtracting a diagonally polarized photon from the 2n
photon component generated in collinear type II downconversion. This
polarization wedge state has the interesting property that its photon number
distribution in the horizontal and vertical polarizations remains sharply
defined for phase shifts of up to 1/n between the circularly polarized
components. Phase shifts at the Heisenberg limit are therefore observed as
nearly deterministic transfers of a single photon between the horizontal and
vertical polarization components.Comment: 7 pages, including 5 figures, improved explanation of interferometry
(one new figure
How weak values emerge in joint measurements on cloned quantum systems
A statistical analysis of optimal universal cloning shows that it is possible
to identify an ideal (but non-positive) copying process that faithfully maps
all properties of the original Hilbert space onto two separate quantum systems.
The joint probabilities for non-commuting measurements on separate clones then
correspond to the real parts of the complex joint probabilities observed in
weak measurements on a single system, where the measurements on the two clones
replace the corresponding sequence of weak measurement and post-selection. The
imaginary parts of weak measurement statics can be obtained by replacing the
cloning process with a partial swap operation. A controlled-swap operation
combines both processes, making the complete weak measurement statistics
accessible as a well-defined contribution to the joint probabilities of fully
resolved projective measurements on the two output systems.Comment: 4 pages, major changes to title and introduction, improved
explanation of weak measurement statistic
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