342 research outputs found
Observation of non-Markovian micro-mechanical Brownian motion
All physical systems are to some extent open and interacting with their
environment. This insight, basic as it may seem, gives rise to the necessity of
protecting quantum systems from decoherence in quantum technologies and is at
the heart of the emergence of classical properties in quantum physics. The
precise decoherence mechanisms, however, are often unknown for a given system.
In this work, we make use of an opto-mechanical resonator to obtain key
information about spectral densities of its condensed-matter heat bath. In
sharp contrast to what is commonly assumed in high-temperature quantum Brownian
motion describing the dynamics of the mechanical degree of freedom, based on a
statistical analysis of the emitted light, it is shown that this spectral
density is highly non-Ohmic, reflected by non-Markovian dynamics, which we
quantify. We conclude by elaborating on further applications of opto-mechanical
systems in open system identification.Comment: 5+6 pages, 3 figures. Replaced by final versio
Visualizing quantum entanglement and the EPR paradox during the photodissociation of a diatomic molecule using two ultrashort laser pulses
We investigate theoretically the dissociative ionization of a H2+ molecule
using two ultrashort laser (pump-probe) pulses. The pump pulse prepares a
dissociating nuclear wave packet on an ungerade surface of H2+. Next, an UV (or
XUV) probe pulse ionizes this dissociating state at large (R = 20 - 100 bohr)
internuclear distance. We calculate the momenta distributions of protons and
photoelectrons which show a (two-slit-like) interference structure. A general,
simple interference formula is obtained which depends on the electron and
protons momenta, as well as on the pump-probe delay on the pulses durations and
polarizations. This interference can be interpreted as visualization of an
electron state delocalized over the two-centres. This state is an entangled
state of a hydrogen atom with a momentum p and a proton with an opposite
momentum. -p dissociating on the ungerade surface of H2+. This pump-probe
scheme can be used to reveal the nonlocality of the electron which intuitively
should be localized on just one of the protons separated by the distance R much
larger than the atomic Bohr orbit
No Drama Quantum Theory?
This work builds on the following result of a previous article
(quant-ph/0509044): the matter field can be naturally eliminated from the
equations of the scalar electrodynamics (the Klein-Gordon-Maxwell
electrodynamics) in the unitary gauge. The resulting equations describe
independent dynamics of the electromagnetic field (they form a closed system of
partial differential equations). An improved derivation of this surprising
result is offered in the current work. It is also shown that for this system of
equations, a generalized Carleman linearization (Carleman embedding) procedure
generates a system of linear equations in the Hilbert space, which looks like a
second-quantized theory and is equivalent to the original nonlinear system on
the set of solutions of the latter. Thus, the relevant local realistic model
can be embedded into a quantum field theory. This model is equivalent to a
well-established model - the scalar electrodynamics, so it correctly describes
a large body of experimental data. Although it does not describe the electronic
spin and possibly some other experimental facts, it may be of great interest as
a "no drama quantum theory", as simple (in principle) as classical
electrodynamics. Possible issues with the Bell theorem are discussed.Comment: 4 page
Experimental violation of a cluster state Bell inequality
Cluster states are a new type of multiqubit entangled states with
entanglement properties exceptionally well suited for quantum computation. In
the present work, we experimentally demonstrate that correlations in a
four-qubit linear cluster state cannot be described by local realism. This
exploration is based on a recently derived Bell-type inequality [V. Scarani et
al., Phys. Rev A 71, 042325 (2005)] which is tailored, by using a combination
of three- and four-particle correlations, to be maximally violated by cluster
states but not violated at all by GHZ states. We observe a cluster state Bell
parameter of , which is more than 7 standard deviations larger
than the threshold of 2 imposed by local realism.Comment: 4 pages, 2 figure
Nonlocality of cluster states of qubits
We investigate cluster states of qubits with respect to their non-local
properties. We demonstrate that a Greenberger-Horne-Zeilinger (GHZ) argument
holds for any cluster state: more precisely, it holds for any partial, thence
mixed, state of a small number of connected qubits (five, in the case of
one-dimensional lattices). In addition, we derive a new Bell inequality that is
maximally violated by the 4-qubit cluster state and is not violated by the
4-qubit GHZ state.Comment: 5 pages; paragraph V.B contains a comparison with Guehne et al.,
quant-ph/041005
Large Quantum Superpositions and Interference of Massive Nanometer-Sized Objects
We propose a method to prepare and verify spatial quantum superpositions of a
nanometer-sized object separated by distances of the order of its size. This
method provides unprecedented bounds for objective collapse models of the wave
function by merging techniques and insights from cavity quantum optomechanics
and matter wave interferometry. An analysis and simulation of the experiment is
performed taking into account standard sources of decoherence. We provide an
operational parameter regime using present day and planned technology.Comment: 4 pages, 2 figures, to appear in PR
Radiation-pressure self-cooling of a micromirror in a cryogenic environment
We demonstrate radiation-pressure cavity-cooling of a mechanical mode of a
micromirror starting from cryogenic temperatures. To achieve that, a
high-finesse Fabry-Perot cavity (F\approx 2200) was actively stabilized inside
a continuous-flow 4He cryostat. We observed optical cooling of the fundamental
mode of a 50mu x 50 mu x 5.4 mu singly-clamped micromirror at \omega_m=3.5 MHz
from 35 K to approx. 290 mK. This corresponds to a thermal occupation factor of
\approx 1x10^4. The cooling performance is only limited by the mechanical
quality and by the optical finesse of the system. Heating effects, e.g. due to
absorption of photons in the micromirror, could not be observed. These results
represent a next step towards cavity-cooling a mechanical oscillator into its
quantum ground state
A high-reflectivity high-Q micromechanical Bragg-mirror
We report on the fabrication and characterization of a micromechanical
oscillator consisting only of a free-standing dielectric Bragg mirror with high
optical reflectivity and high mechanical quality. The fabrication technique is
a hybrid approach involving laser ablation and dry etching. The mirror has a
reflectivity of 99.6%, a mass of 400ng, and a mechanical quality factor Q of
approximately 10^4. Using this micromirror in a Fabry Perot cavity, a finesse
of 500 has been achieved. This is an important step towards designing tunable
high-Q high-finesse cavities on chip.Comment: 3 pages, 2 figure
- …