Session C02: Physics Education, Laser and Condensed Matter Physics

Guided Group Work in Graduate-Level Quantum Mechanics

Guided group work (GGW) has been effectively used in undergraduate physical classrooms for years. Given the substantial selection effects between graduate and undergraduate populations, it is an open question whether group work might be useful at the graduate level. At The Ohio State University, GGW sessions have been developed and run over the past five years for each core course, but this work will focus on quantum mechanics. Students were given pretests and posttests that consist of some calculations, but mostly of conceptual questions. We will discuss trends in student performance across four years (\textasciitilde 160 students), using many assessment questions covering various standard quantum mechanics content areas. We will note some prevalent misconceptions. We find a statistically significant effect of GGW attendance on student performance on related conceptual questions, even many weeks after instruction. Potential confounding effects are discussed, including student self-selection into treatment groups.   

Many-Body Quantum Physics with Tensor Networks

The accurate prediction of the ground state and interesting exciting states in many-body quantum systems is a daunting task due to an exponential problem size growth with the number of interacting atoms. To deal with this problem, in condensed matter physics either approximate mean-field theories are proposed, or fast numerical algorithms are developed which consume a lot of computational resources. In the 1990s with the advent of quantum computing, entanglement has been recognized as one of the main resources in quantum information sciences. In the early 2000s, the physicists introduced the Tensor Network (TN) theory (and the notion of matrix product states) $^{\mathrm{[\ast ]}}$ as a novel way to study interesting states of many-body quantum systems. TN theory makes use of the structure of entanglement of the many-body systems to make predictions about the energy configuration of the system. In this work, we present a brief overview of TN theory accessible to a graduate-level or an advanced undergraduate-level audience. [*] Rom\'{a}n Or\'{u}s. "A practical introduction to tensor networks: Matrix product states and projected entangled pair states." Annals of Physics 349 (2014): 117-158.   

Nested Sampling on Lennard-Jones Particles

One of the most sought after qualities of a system in statistical mechanics is the partition function, which can be used to find many useful qualities of a system, including the heat capacity. This function is defined as defined by the sum over all energies of the weighted Boltzmann Factor. The weighting factor is almost always the most difficult part of the partition function to find. One way to approximate this is Nested Sampling. This method involves taking configurations of the atoms, cooling them down and recording their energy. The data we collect from this process gives us a set of energies and the corresponding weighting factors, allowing us to approximate the partition function.   

Are two laser pulses (with half energy) better than one for ion acceleration?

Ultra-intense lasers can be used as a source of energetic ions for a wide range of applications. Many efforts are being made in order to optimize the usage of lasers to accelerate ions. An interesting approach is described in Ferri et al. 2019, in which two laser pulses of half energy arriving at an angle of incidence close to 45 degrees can achieve greater maximum proton energy than one pulse with the same total energy. For a variety of practical reasons, this enhancement has not yet been demonstrated experimentally. Through 2D3V Particle in Cell simulations, we examine whether this approach would enhance ion acceleration for the “Extreme Light” laser system at Wright-Patterson Air Force Base. This laser system has an intensity that is lower than the range explored by Ferri et al. 2019. We find that for this system that ion acceleration would be substantially enhanced by this approach.   

Rubidium Isotope Shift Measurement using Noisy Lasers

We describe theoretically why the typical advanced undergraduate rubidium SAS laboratory works well with free-running laser diodes, demonstrate it experimentally using these lasers tuned to either principal near-infrared transitions, and show an extension of the laboratory using the modulation transfer spectroscopy method.   

Finding Size and Structure of Polymeric Microgels Using Light Scattering

The effect of the amount of crosslinker on the structure and dynamics of polysaccharide microgels was studied below and above volume phase transition using Dynamic and Static Light Scattering (DLS and SLS) techniques. When the relative amount of crosslinker was varied by a factor of a hundred, three (possibly four) apparent behavioral regimes emerged. At low crosslinker concentrations, behavior was found to be consistent with soft or fuzzy sphere microgel models that displayed significant particle deswelling. At high crosslinker concentrations, instead of typical deswelling, microgels grew in size with the temperature increase, suggesting inhomogeneous structure and crosslinking densities. At intermediate crosslinker concentrations, microgels behaved erratically and did not significantly grow or deswell at the volume phase transition. The apparent regimes are likely due to nonuniform crosslinker distribution in the polymer microgel, which leads to nonuniform density of microgel particles, especially at large concentrations of the crosslinker. Microgels' molecular weight in all crosslinker regimes was measured below and above volume phase transition leading to the estimates of density and preliminary conclusions on microgel architecture at various crosslinking densities.   

Quantum Coherence in Photosynthetic Reaction Centers: A Quantum Heat Engine Perspective

Recent studies have suggested that the quantum effects can enhance the photosynthetic yield. In this work, we present a heat engine-based model of photosynthetic reaction centers. In our model, we consider three chlorophyll molecules, with two closely separated electron donor molecules and a single electron acceptor. In our heat engine, thermal light from the sun acts as a reservoir that promotes the excitation of the donor molecules. The electron transfer to the acceptor involves photon emission into a heat sink, while the useful work occurs during the deexcitation of the acceptor. Based on such a process, the reaction center can be configured as a five-level quantum system\footnote{K. E. Dorfman, D. V. Voronine, S. Mukamel, and M. O. Scully, ``Photosynthetic reaction center as a quantum heat engine'', PNAS, \textbf{110}, 2746-2751 (2013).}. Recently, we have derived a master equation for our five-level system under the standard Born-Markov approximation. Next, we are planning to pay close attention to the quantum coherence terms in our master equation which can be held responsible for the enhancement of the photosynthetic yield.