Micro Cavity-based Spectroscopy of individual Nano Particles (M. Mader) / PEPS and local (gauge) symmetries (Dr. E. Zohar)

  • Double Feature!
  • Datum: 29.11.2016
  • Uhrzeit: 14:30 - 15:30
  • Vortragende(r): Mathias Mader, Doctoral Candidate, Laser Spectroscopy Division / Dr. Erez Zohar, Postdoc, Theory Division
  • Raum: Herbert Walther Lecture Hall
  • Gastgeber: MPQ

"Micro Cavity-based Spectroscopy of individual Nano Particles" (M. Mader)
Optical characterisation and imaging of individual nano systems beyond fluorescence bears the potential to provide a wealth of information. However it is very challenging to observe signals for extinction and dispersion of a single nano particle. At the same time, it is desirable to get information on the single particle level as nano systems can vary in size and shape. We present two complementary approaches for ultra-sensitive detection and characterisation of individual nano systems, both based on signal enhancement in a high-finesse optical cavity.Using a cavity made of a micro-machined and high reflectively coated mirror on the end-facet of an optical fiber and a plane mirror, that can be transversally scanned with respect to the fiber, the cavity mode can be used as a scanning probe to precisely measure the extinction of nano objects placed on the plane mirror. Furthermore, exploiting higher order resonances of the resonator, frequency shifts due to the object can be retrieved. Combining both measurements allows a full reconstruction of the complex polarizability tensor of the object.In an alternative approach, we integrate a fully fiber-based microcavity into a microfluidic cell to allow for ultra-sensitive spectroscopy of individual nano systems in an aqueous environment under well-controlled conditions. In addition to dispersive detection of the nano object by detecting a shift in the resonance frequency, absorption measurements are possible by observing the transmitted light. At the same time, the cavity mode can be used as an optical tweezer to trap particles. This opens the possibility to store a particle without binding to a surface and to investigate e.g. reaction dynamics on a single particle level.

"PEPS and local (gauge) symmetries" (Dr. E. Zohar)
Tensor networks, and in particular Projected Entangled Pair States (PEPS), are a powerful tool for the study of quantum many body physics, thanks to both their built-in ability of classifying and studying symmetries, and the efficient numerical calculations they allow. In the works I will present, we introduced a way to extend the set of symmetric PEPS in order to include local gauge invariance and investigate lattice gauge theories with fermionic matter.

I will review the basic ingredients of PEPS and the way of handling global symmetries with them, and show how to gauge them and make the symmetry local. As examples, I will present our construction of fermionic PEPS, based on Gaussian schemes, invariant under both global and local U(1) or SU(2) gauge transformations. The obtained states describe a truncated U(1) or SU(2) lattice gauge theory in 2+1 dimensions, involving both the gauge field and fermionic matter; I will present the methods which enabled us to study these states, and the results in which some well-known physical phases were identified (e.g. confinement of static charges and screening of dynamical ones).

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