Niklas Mueller, Research Associate

Research Interests

Quantum computers and analog quantum simulators offer us the possibility to address outstanding problems in high energy and nuclear physics. Noisy Intermediate-Scale (NISQ) quantum technology, based on trapped ion, superconducting qubit or cold atom technology, will be available in the near future and will allow computation/simulation of small quantum systems with reasonable error tolerance and to address phenomena that are relevant in nature.

An example where quantum devices can overcome the limitations of classical computers are hadron structure and scattering amplitudes in high-energy physics. I explore novel discretization/regularizations of quantum field theory using worldline/heat-kernel methods to compute for example the partonic structure of the proton.

I also explore the real-time dynamics of gauge theories, using Kogut-Susskind Hamiltonian formulations of QED and QCD, with classical computational techniques and I study the possibilities of using quantum devices to do so. For example, I study lower dimensional systems, such as 1+1 dimensional QED with a theta term, as a prototype model for CP violation in Quantum Chromodynamics (QCD). This model shows intriguing nonequilibrium behavior, including a dynamical topological quantum phase transition, which makes it an attractive potential benchmark study for table-top quantum simulator experiments.

I am interested in all aspects of nonequilibrium dynamics of gauge theories. These include theoretical descriptions of transport phenomena involving chiral fermions QED and QCD in 3+1 dimensions. Using semi-classical expansions of the worldline formulation for fermions, my collaborators and I have developed quantum (chiral) kinetic theory for fermions including internal (anomalous) symmetries.
I also utilize large-scale supercomputer resources provided by the Department of Energy’s National Energy Research Scientific Computing Center to perform real-time lattice simulations using the classical statistical approximation. These studies are important to understand the outcome of experimental searches for the Chiral Magnetic and related Effects in ultra-relativistic heavy ion collisions at Brookhaven’s Relativistic Heavy Ion Collider (RHIC) and at CERN’s LHC, for strong field phenomena in future laser experiments, and for the dynamics of astrophysical objects such as neutron stars and supernovae.

I am intrigued by the experimental potential of a future Electron Collider to measure the origin of the proton’s spin in the small x limit and to access the proton’s parton Wigner distribution, for example in diffractive dijet production, which my collaborators and I compute in the high energy limit of QCD using the Color Glass Condensate effective theory.