# Events at Physics

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### Sunday, February 19th, 2023

Wonders of Physics
The Wonders of Physics 40th annual show
Time: 1:00 pm
Place: 2103 Chamberlin
Abstract: The Wonders of Physics annual show is a fast-paced, engaging, and educational physics program, filled with demonstrations that help people better understand the physics in the world around them, while having fun at the same time.
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Wonders of Physics
The Wonders of Physics 40th annual show
Time: 4:00 pm
Place: 2103 Chamberlin
Abstract: The Wonders of Physics annual show is a fast-paced, engaging, and educational physics program, filled with demonstrations that help people better understand the physics in the world around them, while having fun at the same time.
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### Monday, February 20th, 2023

No events scheduled

### Tuesday, February 21st, 2023

Academic Calendar
Election Day for Wisconsin, including same-day registration
Time: 7:00 am
Abstract: Wisconsin Primary Election for Supreme Court, Mayor, Some Alders, and School Board.. Primary election for non-partisan offices that have two or more candidates: Supreme Court, School Board, Madison Mayor, and Alders. Go to MyVote.wi.gov to find your assigned polling place, see what is on your ballot, and check your registration. If you are not registered at your current address, you can register at your polling place on Election Day. See vote.wisc.edu for information on registration, and voter ID. (The final election for these contests will be April 4, 2023.) CONTACT: malischke@yahoo.com URL:
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R. G. Herb Condensed Matter Seminar
Interacting Opto-Moiré Quantum Matter
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Xi Wang, U Washington
Abstract: Moiré superlattices of 2D materials are an emerging platform for studying new physical
phenomena with high tunability. Strong excitonic responses in Transition Metal Dichalcogenides
(TMDs) allow optical access to the wealth of correlated physics. In this talk, I will present our
recent results about interactions between moiré excitons and charge carriers trapped in moiré
potentials. We have discovered a new interaction between exciton and charges enabled by
unusual quantum confinement in 2D moiré superlattices, which results in novel exciton many-
body ground states composed of moiré excitons and correlated electron lattices. The phase
diagram is further enriched when we investigate the magnetic interactions with optical
excitation. We have observed that the spin-spin interactions between moiré trapped holes can be
drastically tuned by optical excitation power, pointing to the excitons mediated long-range
exchange interaction between moiré trapped carriers. This discovery adds a new and dynamic
tuning knob to the rich many-body Hamiltonian of moiré quantum matter. Our work provides the
framework for understanding and engineering electronic and excitonic states in moiré quantum
matters.
Host: Victor Brar
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Thesis Defense
Photon Assisted Quasiparticle Poisoning and Single Flux Quantum-Based Digital Control of Superconducting Qubits
Time: 12:00 pm
Place: B343 Sterling
Speaker: Chuanhong Liu, Physics Graduate Student
Abstract: The ideal superconductor provides a pristine environment for the delicate states of a quantum computer: because there is an energy gap to excitations, there are no spurious modes with which the qubits can interact, causing irreversible decay of the quantum state. As a practical matter, however, there exists a high density of excitations out of the superconducting ground state even at ultralow temperature; these are known as quasiparticles. Observed quasiparticle densities are of order 1~$\mu$m$^{-3}$, tens of orders of magnitude greater than the equilibrium density expected from theory. Nonequilibrium quasiparticles extract energy from the qubit mode and can induce dephasing. Here we show that a dominant mechanism for quasiparticle poisoning is direct absorption of high-energy photons at the qubit junction. We use a Josephson junction-based photon source to controllably dose qubit circuits with millimeter-wave radiation, and we use an interferometric quantum gate sequence to reconstruct the charge parity of the qubit. We find that the structure of the qubit itself acts as a resonant antenna for millimeter-wave radiation, providing an efficient path for photons to generate quasiparticles. A deep understanding of this physics will pave the way to realization of next-generation superconducting qubits that are robust against quasiparticle poisoning. The single flux quantum (SFQ) digital superconducting logic family has been proposed for the scalable control of next-generation superconducting qubit arrays. In the initial implementation, SFQ-based gate fidelity was limited by quasiparticle (QP) poisoning induced by the dissipative on-chip SFQ driver circuit. In this work, we introduce a multi-chip module architecture to suppress phonon-mediated QP poisoning. Here, the SFQ elements and qubits are fabricated on separate chips that are joined with In bump bonds. We use interleaved randomized benchmarking to characterize the fidelity of SFQ-based gates, and we demonstrate an error per Clifford gate of 1.2(1)%, an order-of-magnitude reduction over the gate error achieved in the initial realization of SFQ-based qubit control. We use purity benchmarking to quantify the contribution of incoherent error at 0.96(2)%; we attribute this error to photon-mediated QP poisoning mediated by the resonant mm-wave antenna modes of the qubit and SFQ-qubit coupler. We anticipate that a straightforward redesign of the SFQ driver circuit to limit the bandwidth of the SFQ pulses will eliminate this source of infidelity, allowing SFQ-based gates with fidelity approaching theoretical limits, namely 99.9% for resonant sequences and 99.99% for more complex pulse sequences involving variable pulse-to-pulse separation.
Host: Robert McDermott
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### Wednesday, February 22nd, 2023

Theory Seminar (High Energy/Cosmology)
Title to be announced
Time: 1:00 pm
Place: Chamberlin 5280
Host: George Wojcik
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### Thursday, February 23rd, 2023

R. G. Herb Condensed Matter Seminar
VdW heterostructures: a new route to designing quantum matters
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Xiameng Liu, Princeton
Abstract: From superconductivity to fractionalized particles, fascinating phenomena arise in quantum materials due to the collective behaviors of electrons. These quantum effects challenge our intuition about nature and provide new opportunities for future quantum information technologies. Recently, two-dimensional materials and their heterostructures have become a leading platform for realizing new quantum states of matter. Mechanically assembled layer-by-layer and held together by the van der Waals (vdW) force, vdW heterostructures break through the limitations of traditional material synthesis and offer entirely new ways to create quantum matters. My talk will feature two examples of designing quantum matters with vdW heterostructures. In the first example, I will illustrate how Coulomb interactions across separate atomic layers pair fermions (electrons and holes) into bosons to achieve a superfluid condensate state. Thanks to the tunability of the vdW platform, we can vary the pairing strength and change the nature of this fermion condensate from strong coupling to weak coupling, demonstrating a long-sought paradigm known as the BEC-BCS crossover. In the second example, I will introduce the idea of moiré band engineering, where the interference between two atomic lattices—named the moiré pattern—defines a new periodicity and reforms electronic band structures. In twisted double bilayer graphene, such moiré periodicity creates highly-degenerate bands tunable by a perpendicular electric field. We observed electron correlation effects, including interaction-driven insulation and spontaneous symmetry breaking of spins. Their evolution with the electric fields reveals their close connection with the moiré band features. Finally, I will briefly discuss applications of local probe techniques to uncover hidden quantum properties in vdW platforms and share visions of leveraging rich interplays across atomic interfaces to access major themes in condensed matter physics.
Host: Victor Brar
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Entropy, molecular motors, and non-thermal equilibrium statistical physics
Time: 4:00 pm
Place: 2241 Chamberlin Hall -
Speaker: Steven Chu, Stanford University
Abstract: The transport of molecular cargos in neuronal cells is analyzed in the context of new developments in statistical physics. We developed bright optical probes which enabled the long-term single tracking of molecular cargos in live neurons for tens of minutes. The number of dynein motors transporting a cargo in a neuron was found to switch stochastically from one to five motors. We were able to resolve individual molecular steps, and new a quantitative chemo-mechanical model where a single step requires the hydrolysis of two ATP molecules.

We also find that the movement approaches a steady-state non-thermal equilibrium with effective temperature, T_eff=〖6×T〗_cell=6×310 K. Also, the minimum “uncertainty principle” limit, ΔQ⋅ϵ^2≥2k_B T_eff, where ΔQ=T_eff ΔS is the heat entropy needed to achieve movement with a normalized precision 〖ϵ(x)〗^2≡((〈x^2 〉-〈x〉^2))⁄〈x〉^2 . This uncertainty limit sets the minimum heat energy needed to achieve a given precision in any physical operation. In the context of intercellular molecular transport, a more uniform motion of the cargo requires a greater expenditure of energy.

Host: Uwe Bergmann, Alessandro Senes
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### Friday, February 24th, 2023

Physics Department Colloquium
Climate Change and innovative paths to a sustainable future
Time: 12:00 pm
Place: 2103 Chamberlin Hall -
Speaker: Steven Chu, Stanford University
Abstract: “Climate Change and innovative paths to a sustainable future” The multiple industrial and agricultural revolutions have transformed the world. However, an unintended consequence of this progress is that we are changing the climate of our planet. In addition to the climate risks, we will need to provide enough clean energy, water and food of a more prosperous world that may grow to 11 billion by 2100. The talk will discuss the significant technical challenges and potential solutions that could provide better paths to a more sustainable future. How we transition to from where we are now to where we need to be within 50 years is arguably the most pressing set of issues that science and innovation has to address.
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