368 research outputs found
On quantum time crystals and interacting gauge theories in atomic Bose-Einstein condensates
We study the dynamics of a Bose-Einstein condensate trapped circumferentially
on a ring, and which is governed by an interacting gauge theory. We show that
the associated density-dependent gauge potential and concomitant current
nonlinearity permits a ground state in the form of a rotating chiral bright
soliton. This chiral soliton is constrained to move in one direction by virtue
of the current nonlinearity, and represents a time crystal in the same vein as
Wilczek's original proposal.Comment: New version with general improvements and correction
Experimental Tests of the New Paradigm for Laser Filamentation in Gases
Since their discovery in the mid-1990s, ultrafast laser filaments in gases
have been described as products of a dynamic balance between Kerr self-focusing
and defocusing by free electric charges that are generated via multi-photon
ionization on the beam axis. This established paradigm has been recently
challenged by a suggestion that the Kerr effect saturates and even changes sign
at high intensity of light, and that this sign reversal, not free-charge
defocusing, is the dominant mechanism responsible for the extended propagation
of laser filaments. We report qualitative tests of the new theory based on
electrical and optical measurements of plasma density in femtosecond laser
filaments in air and argon. Our results consistently support the established
paradigm.Comment: 4 pages, 4 figure
Non-collinear interaction of photons with orbital angular momentum
We elucidate the consequences of a phase-matching theory that describes
second-harmonic generation of two non-collinear incident light beams that carry
orbital angular momentum (OAM). More specifically, the two incident beams
generate a third that, depending on the incident OAM, may experience a
significantly smaller conversion efficiency in comparison to that based on the
conventional phase-matching theory. This is the case even for incident angles
substantially less than those required for non-conservation of OAM in the
nonlinear interaction. Experiments are performed under different conditions and
are in excellent agreement with the theory. Our results have implications
beyond the specific case studied here of second-harmonic generation, in
particular for parametric down-conversion of photons.Comment: 6 pages, 4 figure
- …