47 research outputs found
Topics in Modern Quantum Optics
This is the written version of lectures presented at "The 17th Symposium on
Theoretical Physics - Applied Field Theory", 29 June - 1 July, 1998, the
Sangsan Mathematical Science Building, Seoul National University, Seoul, Korea.Comment: 97 pages, 23 figures, 187 references. Misprints corrected, most
figures redrawn and references update
Thermally induced photon splitting
We calculate thermal corrections to the non-linear QED effective action for
low-energy photon interactions in a background electromagnetic field. The
high-temperature expansion shows that at the vacuum contribution is
exactly cancelled to all orders in the external field except for a non-trivial
two-point function contribution. The high-temperature expansion derived reveals
a remarkable cancellation of infrared sensitive contributions. As a result
photon-splitting in the presence of a magnetic field is suppressed in the
presence of an electron-positron QED-plasma at very high temperatures. In a
cold and dense plasma a similar suppression takes place. At the same time
Compton scattering dominates for weak fields and the suppression is rarely
important in physical situations.Comment: 15 pages, 2 ps figures, Late
Atomic beam correlations and the quantum state of the micromaser
Correlation measurements on the states of two-level atoms having passed
through a micromaser at different times can be used to infer properties of the
quantum state of the radiation field in the cavity. Long(short) correlation
length in time is to some extent associated with super(sub)-Poissonian photon
statistics. The correlation length is also an indicator of a phase structure
much richer than what is revealed by the usual single-time observables, like
the atomic inversion or the Mandel quality factor. In realistic experimental
situations the correlations may extend over many times the decay time of the
cavity. Our assertions are verified by comparing theoretical calculations with
a high-precision Monte-Carlo simulation of the micromaser system.Comment: 4 pages, styles: aps, latex, times, epsf, More physical insight
added, title and figures changed, more references. The paper can be retrieved
as compressed file called elmfors.maser.ps.Z from
http://connect.nbi.dk/pub/lautrup/ or via anonymous ftp at
ftp://connect.nbi.dk/pub/lautrup
Anomalous Paths in Quantum Mechanical Path-Integrals
We investigate modifications of the discrete-time lattice action, for a
quantum mechanical particle in one spatial dimension, that vanish in the
na\"ive continuum limit but which, nevertheless, induce non-trivial effects due
to quantum fluctuations. These effects are seen to modify the geometry of the
paths contributing to the path-integral describing the time evolution of the
particle, which we investigate through numerical simulations. In particular, we
demonstrate the existence of a modified lattice action resulting in paths with
any fractal dimension, d_f, between one and two. We argue that d_f=2 is a
critical value, and we exhibit a type of lattice modification where the
fluctuations in the position of the particle becomes independent of the time
step, in which case the paths are interpreted as superdiffusive L\'{e}vy
flights. We also consider the jaggedness of the paths, and show that this gives
an independent classification of lattice theories.Comment: 7 pages,6 figure