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Condensed Matter Physics - Biophysics Seminars |

For more information contact Stephan Haas (213) 740-4528.

Environmental regulation, benefits and costs of bacterial motility

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Victor Sourjik (Max Planck Institute Marburg)
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Microorganisms possess diverse mechanisms to regulate investment into individual cellular processes according to their extra- and intracellular environment. How these regulatory strategies reflect the inherent tradeoff between the benefit and cost of resource investment remains largely unknown, particularly for many cellular functions that are not immediately related to growth. I will present our recent work on understanding the physiological importance and environmental regulation of motility and chemotaxis of Escherichia coli, one of the most complex and costly bacterial behaviors. I will particularly focus on the regulation of motility as a function of bacterial growth rate. Our work showed that in poor nutritional conditions bacteria increase their investment in motility in proportion to the selective advantage provided by chemotaxis. Thus, bacteria appear to pre-invest resources into the motile behavior in proportion to the anticipated benefit that can be provided by chemotaxis when gradients of secondary nutrients are either introduced in their environment or created by bacterial communities through excretion and consumption of metabolites.

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Jan 24, 2pm **

Condensation and interactions of molecules on nanoparticles

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Michal Farnik (J. Heyrovsky Institute of Physical Chemistry, Prague)
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Clusters represent a bridge between individual molecules and bulk. Their investigations provide a detailed molecular-level understanding of bulk properties and complex processes in condensed matter. Our versatile cluster beam apparatus (CLUB) in Prague allows for a large variety of experiments with clusters and nanoparticles: adsorption of molecules; photoionization mass spectrometry; electron ionization and attachment; photodissociation and velocity map imaging including IR-UV pump-probe experiments, etc. Some of these experiments will be introduced in the talk. The first part will concentrate on molecular adsorption in relevance to aerosol particle formation in the atmosphere. The second part will show how intermolecular reactions are affected by the cluster medium. Using examples, we will demonstrate how the investigated phenomena are relevant to atmospheric processes, to astronomy and astrochemistry, and even to surface-assisted technologically relevant processes such as FEBID (focused electron beam induced deposition).

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Jan 31, 2pm **

From nanotech to living sensors: unraveling the spin physics of biosensing at the nanoscale

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Clarice Aiello (UCLA)
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Substantial in vitro and physiological experimental results suggest that similar coherent spin physics might underlie phenomena as varied as the biosensing of magnetic fields in animal navigation and the magnetosensitivity of metabolic reactions related to oxidative stress in cells. If this is correct, organisms might behave, for a short time, as “living quantum sensors” and might be studied and controlled using quantum sensing techniques developed for technological sensors. I will outline our approach towards performing coherent quantum measurements and control on proteins, cells and organisms in order to understand how they interact with their environment, and how physiology is regulated by such interactions. Can coherent spin physics be established – or refuted! – to account for physiologically relevant biosensing phenomena, and be manipulated to technological and therapeutic advantage?

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Feb 25, 1pm **

Investigation of connectivity in quantum annealers

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Adrian Lupascu
(University of Waterloo)
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Tntum annealing is a computational paradigm for solving combinatorial problems based on finding the ground state of a Hamiltonian. The common approach is to encode problems using Ising spin Hamiltonians. The connectivity of the spins in the Ising system, which is the number of controllable spin-spin interactions, is important for the computational power of an annealer.
We present our work on the implementation of long-range interactions in a quantum annealer. This work is done in the context of an implementation of quantum annealers based on superconducting flux quantum bits. The usual approach for implementation of spin-spin (qubit-qubit) interactions is based on the use of superconducting loops interrupted by a Josephson junction (RF-SQUIDs). This approach breaks down for coupling qubits at large distances. The coupler tree architecture is a proposal for implementation of long-range interactions between superconducting flux qubits using a network of coupled RF-SQUIDs.
We present the results of experiments with two capacitively shunted flux qubits connected by a chain of RF-SQUIDs. We demonstrated propagation of a magnetic flux signal through the chain, a first step towards demonstration of long range qubit-qubit interactions. We will discuss future prospects for this work and other directions for implementation of high connectivity.

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March 6, 2pm **

Non-Hermitian Photonics: Optics at an Exceptional Point

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Mercehdeh Khajavikhan (USC EE)
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In recent years, non-Hermitian degeneracies, also known as exceptional points (EPs), have emerged as a new paradigm for engineering the response of optical systems. At such points, an N-dimensional space can be represented by a single eigenvalue and one eigenvector. As a result, these points are associated with abrupt phase transitions in parameter space. Among many different non-conservative photonic configurations, parity-time (PT) symmetric systems are of particular interest since they provide a powerful platform to explore and consequently utilize the physics of exceptional points in a systematic manner. In this talk, I will review some of our recent works in the area of non-Hermitian (mainly PT-symmetric) active photonics. For example, in a series of works, we have demonstrated how the generation and judicial utilization of these points in laser systems can result in unexpected dynamics, unusual linewidth behavior, and improved modal response. On the other hand, biasing a photonic system at an exceptional point can lead to orders of magnitude enhancement in sensitivity- an effect that may enable a new generation of ultrasensitive optical sensors on-chip. Non-Hermiticity can also be used as a means to promote or single out an edge mode in photonic topological insulator lattices.
Rotation sensors play a crucial role in a diverse set of applications associated with navigation, positioning, and inertial sensing. Most optical gyroscopes rely on the Sagnac effect induced phase shift that scales linearly with the rotational velocity. In ring laser gyroscopes (RLGs), this shift manifests itself as a resonance splitting in the emission spectrum that can be detected as a beat frequency. The need for ever-more precise RLGs has fueled research activities towards devising new approaches aimed to boost the sensitivity beyond what is dictated by geometrical constraints. In this respect, attempts have been made in the past to use either dispersive or nonlinear effects. Here, we propose a new scheme for ultrasensitive laser gyroscopes that utilizes the physics of exceptional points. By exploiting the properties of such non-Hermitian degeneracies, we show that the rotation-induced frequency splitting becomes proportional to the square root of the gyration speed- thus enhancing the sensitivity to low angular rotations by orders of magnitudes. We will then describe a possible modification of a standard RLG to support an exceptional point, and measure the resulting enhanced sensitivity in the proposed system.

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March 27, 2pm **

Photonics in complex systems

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Chia Wei Hsu (USC EE)
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The interactions between light and complex systems provide vast opportunities for exploring fundamental physics and for a wide range of applications. Materials, structures, and the light fields themselves can all be designed to tailor the optical properties of a system. Inversely, when a system's optical properties are fully characterized, one may infer its underlying material composition and structure. In this talk, I will describe some of our work on photonics in complex systems. The first part concerns "bound states in the continuum," which are special eigenstates that remain perfectly confined even when its eigenfrequency lies in the continuous spectrum of the extended states of an open system. I will describe the first experimental realization of bound states in the continuum not protected by symmetry, their topological property, and potential realization in fiber Bragg gratings. The second part concerns non-Hermitian degeneracies (called exceptional points) in periodic structures, where we predict and experimentally realize rings of exceptional points in the momentum space as well as paired exceptional points connected by bulk Fermi arcs. In the third part, I will describe the control of light transport via wavefront shaping, the long-range correlations between multiply scattered photons, and a transverse localization phenomenon for the transmission eigenchannels that is distinct from Anderson localization.

Zoom link: https://usc.zoom.us/j/592138702 Recorded Seminar: https://usc.zoom.us/rec/share/5vYkEq7TrWZISY2V0XjDAbICN47HT6a8gCgXrvBfnh7Woau_yV2eUspZ6J-4xlq9

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