Events at Physics
Events on Monday, February 13th, 2023
- Atomic Physics Seminar
- The geometry of quantum error correction under biased noise
- Time: 11:00 am
- Place: 5310 Chamberlin Hall
- Speaker: Arpit Dua
- Abstract: Quantum error correction is necessary because physical qubits have much higher error rates per gate operation than are needed for practical tasks. The popular choice is to encode a logical qubit in a large enough planar layout of many physical qubits, called the surface code, to have sufficiently low logical error rates. The optimal logical error rates depend on the statistical mechanics of logical operators. For example, under biased Pauli noise, having more higher-weight logical operator representations with a higher ratio of low-rate Pauli operators is better. Using this idea, I will discuss how, in active error correction, measuring Clifford-rotated Pauli stabilizers of the surface code can enhance code performance: higher error thresholds and lower subthreshold logical error rates, for biased Pauli noise. Using statistical mechanics and percolation theory, I will describe a phase diagram of 50% thresholds for random Clifford-rotated surface codes under pure dephasing noise. Using tensor network numerics, I will show that certain families of these random codes outperform the best-known translation invariant Clifford-rotated surface codes for finitely biased depolarizing noise.
- Host: Thad Walker
- Plasma Physics (Physics/ECE/NE 922) Seminar
- Multiscale Nature of Turbulence in Space Plasmas
- Time: 12:00 pm
- Place: 2241 Chamberlin Hall
- Speaker: Yan Yang, University of Delaware
- Abstract: Turbulence enters into space plasmas in many guises. The complexity and variability of the behavior of plasma turbulence are in large part due to the involvement of dynamics at many scales, ranging from macroscopic fluid to sub-electron scales. Based on what plasma properties we are interested in studying, be they dominant at small or large scales, plasma can be treated as tractable models in various limits, such as the kinetic theory and magnetohydrodynamic (MHD) theory. Turbulence flows are characterized by the nonlinear transfer of energy and other quantities across a huge range of scales. Observed turbulence in space is expected to involve cross-scale energy transfer and subsequent dissipation and heating. Space plasmas are frequently taken to be weakly collisional or collisionless. Therefore, an explicit form of viscous dissipation as in collisional (e.g., MHD) cases cannot be easily defined. A variety of approaches have attempted to characterize specific mechanisms (e.g., magnetic reconnection, wave-particle interaction, and turbulent-driven intermittency) and to quantify the dissipation. However, the community has not come to a consensus solution applicable to all systems. In this talk I will first give an overview of some basic properties for turbulence. Then I will briefly review turbulence theory application in space plasmas. I will discuss in detail how to disentangle multiscale properties, how plasma dynamics bridges multiple scales, what new ingredients are introduced in cross-scale transfer as models progress from fluid to kinetic, and how to identify key steps in energy transfer and estimate energy dissipation rate in weakly collisional plasmas. These also motivate several unresolved issues that may be addressed by future studies. Where feasible, examples are given from MHD, Particle in Cell, and hybrid Vlasov-Maxwell simulations, and from Magnetospheric Multiscale (MMS) observations.