446 research outputs found
Atomic interferometer measurements of Berry's and Aharonov-Anandan's phases for isolated spins S > 1/2 non-linearly coupled to external fields
The aim of the present paper is to propose experiments for observing the
significant features of Berry's phases for S>1, generated by spin-Hamiltonians
endowed with two couplings, a magnetic dipole and an electric quadrupole one
with external B and E fields, as theoretically studied in our previous work.
The fields are assumed orthogonal, this mild restriction leading to geometric
and algebraic simplifications. Alkali atoms appear as good candidates for
interferometric measurements but there are challenges to be overcome. The only
practical way to generate a suitable E-field is to use the ac Stark effect
which induces an instability of the dressed atom. Besides atom loss, this might
invalidate Berry's phase derivation but this latter problem can be solved by an
appropriate detuning. The former puts an upper limit to the cycle duration,
which is bounded below by the adiabatic condition. By relying upon our previous
analysis of the non-adiabatic corrections, we have been able to reach a
compromise for the Rb hf level F=2, m=0 state, which is our candidate
for an interferometric measurement of the exotic Berry's phase generated by a
rotation of the E-field around the fixed B-field. By a numerical simulation we
have shown that the non-adiabatic corrections can be kept below the 0.1% level.
As an alternative candidate, we discuss the chromium ground state J=S=3, where
the instability problem is easily solved. We make a proposal to extend the
measurement of Aharonov-Anandan's phase beyond S=1/2 to the Rb hf level
F=m=1, by constructing, with the help of light-shifts, a Hamiltonian able to
perform a parallel transport along a closed circuit upon the density matrix
space, without any adiabatic constraint. In Appendix A, Berry's phase
difference for S=3/2 and 1/2, m=1/2 states is used to perform an entanglement
of 3 Qbits.Comment: 23 pages, 6 figures, modifications in the introduction, two
paragraphs adde
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Dynamical studies on the generation of periodic surface structures by femtosecond laser pulses
The dynamics of the formation of laser-induced periodic surface structures (LIPSS) on fused silica upon irradiation with linearly polarized fs-laser pulses (50 fs pulse duration, 800 nm center wavelength) is studied experimentally using a double pulse experiment with cross polarized pulse sequences and a trans illumination femtosecond time-resolved (0.1 ps - 1 ns) pump-probe diffraction approach. The results in both experiments confirm the importance of the ultrafast energy deposition and the laser-induced free-electron plasma in the conduction band of the solids for the formation of LIPSS
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Excitation and relaxation dynamics in ultrafast laser irradiated optical glasses
We discuss the dynamics of ultrashort pulsed laser excitation in bulk optical silica-based glasses (fused silica and borosilicate BK7) well-above the permanent modification threshold. We indicate subsequent structural and thermomechanical energy relaxation paths that translate into positive and negative refractive index changes, compression and rarefaction zones. If fast electronic decay occurs at low excitation levels in fused silica via self-trapping of excitons, for carrier densities in the vicinity of the critical value at the incident wavelength, persistent long-living absorptive states indicate the achievement of low viscosity matter states manifesting pressure relaxation, rarefaction, void opening and compaction in the neighboring domains. An intermediate ps-long excited carrier dynamics is observed for BK7 in the range corresponding to structural expansion and rarefaction. The amount of excitation and the strength of the subsequent hydrodynamic evolution is critically dependent on the pulse time envelope, indicative of potential optimization schemes
Exact Foldy-Wouthuysen transformation for spin 0 particle in curved space
Up to now, the only known exact Foldy- Wouthuysen transformation (FWT) in
curved space is that concerning Dirac particles coupled to static spacetime
metrics. Here we construct the exact FWT related to a real spin-0 particle for
the aforementioned spacetimes. This exact transformation exists independently
of the value of the coupling between the scalar field and gravity. Moreover,
the gravitational Darwin term written for the conformal coupling is one third
of the relevant term in the fermionic case.Comment: 10 pages, revtex, improved version to appear in Phys. Rev.
Pseudo-forces in quantum mechanics
Dynamical evolution is described as a parallel section on an infinite
dimensional Hilbert bundle over the base manifold of all frames of reference.
The parallel section is defined by an operator-valued connection whose
components are the generators of the relativity group acting on the base
manifold. In the case of Galilean transformations we show that the property
that the curvature for the fundamental connection must be zero is just the
Heisenberg equations of motion and the canonical commutation relation in
geometric language. We then consider linear and circular accelerating frames
and show that pseudo-forces must appear naturally in the Hamiltonian.Comment: 6 pages, 1 figure, revtex, new section added, to appear in PR
X-ray Near Field Speckle: Implementation and Critical Analysis
We have implemented the newly-introduced, coherence-based technique of x-ray
near-field speckle (XNFS) at 8-ID-I at the Advanced Photon Source. In the near
field regime of high-brilliance synchrotron x-rays scattered from a sample of
interest, it turns out, that, when the scattered radiation and the main beam
both impinge upon an x-ray area detector, the measured intensity shows
low-contrast speckles, resulting from interference between the incident and
scattered beams. We built a micrometer-resolution XNFS detector with a high
numerical aperture microscope objective and demonstrate its capability for
studying static structures and dynamics at longer length scales than
traditional far field x-ray scattering techniques. Specifically, we
characterized the structure and dynamics of dilute silica and polystyrene
colloidal samples. Our study reveals certain limitations of the XNFS technique,
which we discuss.Comment: 53 pages, 16 figure
Does Quantum Mechanics Clash with the Equivalence Principle - and Does it Matter?
With an eye on developing a quantum theory of gravity, many physicists have
recently searched for quantum challenges to the equivalence principle of
general relativity. However, as historians and philosophers of science are well
aware, the principle of equivalence is not so clear. When clarified, we think
quantum tests of the equivalence principle won't yield much. The problem is
that the clash/not-clash is either already evident or guaranteed not to exist.
Nonetheless, this work does help teach us what it means for a theory to be
geometric.Comment: 12 page
Decoherence, fluctuations and Wigner function in neutron optics
We analyze the coherence properties of neutron wave packets, after they have
interacted with a phase shifter undergoing different kinds of statistical
fluctuations. We give a quantitative (and operational) definition of
decoherence and compare it to the standard deviation of the distribution of the
phase shifts. We find that in some cases the neutron ensemble is more coherent,
even though it has interacted with a wider (i.e. more disordered) distribution
of shifts. This feature is independent of the particular definition of
decoherence: this is shown by proposing and discussing an alternative
definition, based on the Wigner function, that displays a similar behavior. We
briefly discuss the notion of entropy of the shifts and find that, in general,
it does not correspond to that of decoherence of the neutron.Comment: 18 pages, 7 figure
Lithium atom interferometer using laser diffraction : description and experiments
We have built and operated an atom interferometer of the Mach-Zehnder type.
The atomic wave is a supersonic beam of lithium seeded in argon and the mirrors
and beam-splitters for the atomic wave are based on elastic Bragg diffraction
on laser standing waves at 671 nm. We give here a detailed description of our
experimental setup and of the procedures used to align its components. We then
present experimental signals, exhibiting atomic interference effects with a
very high visibility, up to 84.5 %. We describe a series of experiments testing
the sensitivity of the fringe visibility to the main alignment defects and to
the magnetic field gradient.Comment: 8 avril 200
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