School of Physical Sciences

The term Gaussian universality refers to a class of results that are, loosely speaking, generalized central limit theorems (where, somewhat confusingly, the limit law is not necessarily Gaussian). They provide useful tools to study certain problems in machine learning. I will give a short overview of this idea and then present two types of results: One are upper and lower bounds that map out where Gaussian universality is applicable and what rates of convergence one can expect. The other is the use of these techniques to obtain quantitative results on the effects of data augmentation in machine learning problems.

Note unusual location

• Speaker: Peter Orbanz (UCL)
• Friday 14 February 2025, 14:00-15:00
• Venue: Centre for Mathematical Sciences MR15, CMS.
• Series: Statistics; organiser: Qingyuan Zhao.

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I’ll tell you about some of my favorite algebraic varieties, which are beautiful in their own right, and also have some dramatic applications to algebraic combinatorics. These include the top-heavy conjecture (one of the results for which June Huh was awarded the Fields Medal), as well as non-negativity of Kazhdan—Lusztig polynomials of matroids.

• Speaker: Nicholas Proudfoot, University of Oregon
• Wednesday 19 March 2025, 14:15-15:15
• Venue: CMS MR13.
• Series: Algebraic Geometry Seminar; organiser: Dhruv Ranganathan.

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Shankar and David will deliver the 2024 Novo Nordisk Lectures. Shankar’s will talk on Decoding DNA and David the applications of physical sciences to biomedicine, next generation DNA sequencing and beyond. This will be followed by a drinks reception hosted by the Novo Nordisk foundation.

• Speaker: Professors Sir Shankar Balasubramanian and Sir David Klenerman
• Thursday 03 April 2025, 16:00-17:30
• Venue: Dept of Chemistry, (BMS) Bristol Myers Squibb Lecture Theatre.
• Series: Chemistry Departmental-wide lectures; organiser: Balasubramanian-Admin.

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Funded by the Novo Nordisk Foundation and chaired by the 2024 Novo Nordisk Prize winners, Professors Sir Shankar Balasubramanian and Sir David Klenerman, this one day symposium brings together world leading scientists in the field of biophysics and chemical biology at the Yusuf Hamied Department of Chemistry.

The speakers are Professor Ed Boyden, MIT ; Professor Jason Chin, University of Cambridge; Professor Thomas Carrell from LMU , Munich; Professor Chuan He, University of Chicago; and Professor Xiaowei Zhang, Harvard University.

To register: https://www.eventbrite.co.uk/e/frontiers-in-biophysics-and-chemical-biology-tickets-1224872329109?aff=oddtdtcreator

• Speaker: Professor Ed Boyden, MIT; Professor Jason Chin, University of Cambridge; Professor Thomas Carrell from LMU, Munich; Professor Chuan He, University of Chicago; and Professor Xiaowei Zhang, Harvard University.
• Friday 04 April 2025, 10:30-17:00
• Venue: Dept of Chemistry, (BMS) Bristol Myers Squibb Lecture Theatre.
• Series: Chemistry Departmental-wide lectures; organiser: Balasubramanian-Admin.

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Sea ice plays a key role in Earth’s climate system and exhibits significant seasonal variability as it advances and retreats across the Arctic and Antarctic every year. The production of sea ice forecasts provides great scientific and practical value to stakeholders across the polar regions, informing shipping, conservation, logistics, and the daily lives of inhabitants of local communities. Machine learning offers a promising means by which to develop such forecasts, capturing the nonlinear dynamics and subtle spatiotemporal patterns at play as effectively—if not more effectively—than conventional physics-based models. In particular, the ability of deep generative models to produce probabilistic forecasts which acknowledge the inherent stochasticity of sea ice processes and represent uncertainty by design make them a sensible choice for the task of sea ice forecasting. Diffusion models, a class of deep generative models, present a strong option given their state-of-the-art performance on computer vision tasks and their strong track record when adapted to spatiotemporal modelling tasks in weather and climate domains. In this talk, I will present preliminary results from a IceNet-like [1] diffusion model trained to autoregressively forecast daily, 6.25 km resolution sea ice concentration in the Bellingshausen Sea along the Antarctic Peninsula. I will also touch on the downstream applications for these forecasts, from conservation to marine route planning, which are under development at the British Antarctic Survey (BAS). I welcome ideas and suggestions for improvement and look forward to discussing opportunities for collaboration within and beyond BAS .

[1] Andersson, Tom R., et al. “Seasonal Arctic sea ice forecasting with probabilistic deep learning.” Nature communications 12.1 (2021): 5124. https://www.nature.com/articles/s41467-021-25257-4

• Speaker: Andrew McDonald, University of Cambridge and British Antarctic Survey
• Wednesday 12 February 2025, 14:00-15:00
• Venue: BAS Seminar Room 2; https://ukri.zoom.us/j/96472472041.
• Series: British Antarctic Survey - Polar Oceans seminar series; organiser: Dr Birgit Rogalla.

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Porous semiconducting nitrides are effectively a new class of semiconducting material, with properties distinct from the monolithic nitride layers from which devices from light emitting diodes (LEDs) to high electron mobility transistors are increasingly made. The introduction of porosity provides new opportunities to engineer a range of properties including refractive index, thermal and electrical conductivity, stiffness and piezoelectricity. Quantum structures may be created within porous architectures and novel composites may be created via the infiltration of other materials into porous nitride frameworks. A key example of the application of porous nitrides in photonics is the fabrication of high reflectivity distributed Bragg reflectors (DBRs) from alternating layers of porous and non-porous GaN. These reflectors are fabricated from epitaxial structures consisting of alternating doped and undoped layers, in which only the conductive, doped layers are electrochemically etched. Conventionally, trenches are formed using a dry-etching process, penetrating through the multilayer, and the electrochemical etch then proceeds laterally from the trench sidewalls. The need for these trenches then limits the device designs and manufacturing processes within which the resulting reflectors can be used. We have developed a novel alternative etching process, which removes the requirement for the dry-etched trenches, with etching proceeding vertically from the top surface through channels formed at naturally-occurring defects in the crystal structure of GaN (see Figure). This etch process leaves an undoped top surface layer almost unaltered and suitable for further epitaxy. This new defect-based etching process provides great flexibility for the creation of a variety of sub-surface porous architectures on top of which a range of devices may be grown. Whilst DBR structures enable improved light extraction from LEDs and the formation of resonant cavities for lasers and single photon sources, recent development also suggests that thick, sub-surface porous layers may enable strain relaxation to help improve the efficiency of red microLEDs for augmented reality displays. Meanwhile, the option of filling pores in nitride layers with other materials provides new opportunities for the integration of nitrides with emerging photonic materials, such as the hybrid-perovskite semiconductors, with perovskites encapsulated in porous nitride layers demonstrating greatly improved robustness against environmental degradation.

• Speaker: Rachel Oliver (DMSM)
• Wednesday 26 February 2025, 16:15-17:15
• Venue: Small Lecture Theatre, Cavendish Laboratory, J.J. Thomson Avenue.
• Series: Cavendish Quantum Colloquium; organiser: Lucas Sá.

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Abstract not available

• Speaker: Professors Sir Shankar Balasubramanian and Sir David Klenerman
• Thursday 03 April 2025, 16:00-17:30
• Venue: Dept of Chemistry, (BMS) Bristol Myers Squibb Lecture Theatre.
• Series: Chemistry Departmental-wide lectures; organiser: Balasubramanian-Admin.

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Abstract not available

• Speaker: Wendelin Werner (Cambridge)
• Tuesday 11 February 2025, 14:00-15:00
• Venue: MR12.
• Series: Probability; organiser: ww295.

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Abstract not available

• Speaker: Dr Bjarne Bergh, DAMTP, University of Cambridge
• Wednesday 30 April 2025, 14:00-15:00
• Venue: MR5, CMS Pavilion A.
• Series: Information Theory Seminar; organiser: Prof. Ramji Venkataramanan.

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Abstract not available

• Speaker: Paul Goddard - University of Warwick
• Wednesday 28 May 2025, 11:15-12:00
• Venue: Mott Seminar Room (531), Cavendish Laboratory, Department of Physics.
• Series: Quantum Matter Seminar; organiser: Mads Fonager Hansen.

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Combinatorial optimisation (CO) is a sub-area of discrete mathematics. Basic examples for CO problems are finding a shortest path or a minimum spanning tree in a graph. So-called network flows or variations of matching would be more advanced problems. There are also abstract concepts like matroids that offer an algebraic point of view and a uniform foundation for some of the more concrete problems.

Since the considered structures are finite, it is a natural aim to compute a solution efficiently. That implies an overlap with the theory of algorithms, especially running time analysis.

This talk is mainly about the Isabelle/HOL formalisation of a specific CO problem, namely, minimum cost flows, which are a subtype of network flows. Among others, this includes Orlin’s Algorithm, which is a most efficient method to compute a minimum cost flow in general networks. Also, the running time argument for this advanced algorithm and some reductions among flow problems were formalised.

- The Isabelle proof scripts can be found in this GitHub repo: https://github.com/mabdula/Isabelle-Graph-Library

- The formalisation is described in this paper: A Formal Analysis of Capacity Scaling Algorithms for Minimum Cost Flows by Mohammad Abdulaziz and Thomas Ammer, ITP 2024

=== Hybrid talk ===

Join Zoom Meeting https://cam-ac-uk.zoom.us/j/87143365195?pwd=SELTNkOcfVrIE1IppYCsbooOVqenzI.1

Meeting ID: 871 4336 5195

Passcode: 541180

• Speaker: Thomas Ammer (King's College London)
• Thursday 06 February 2025, 17:00-18:00
• Venue: MR14 Centre for Mathematical Sciences.
• Series: Formalisation of mathematics with interactive theorem provers ; organiser: Jonas Bayer.

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Abstract not available

Please note the unusual time and place.

• Speaker: Jonny Evans (Lancaster)
• Friday 14 March 2025, 10:30-11:30
• Venue: CMS, MR15.
• Series: Differential Geometry and Topology Seminar; organiser: Ailsa Keating.

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Abstract not available

Please note the unusual time and place.

• Speaker: Jonny Evans (Lancaster)
• Thursday 13 March 2025, 11:15-12:15
• Venue: CMS, MR14.
• Series: Differential Geometry and Topology Seminar; organiser: Ailsa Keating.

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Abstract not available

• Speaker: Jonny Evans (Lancaster)
• Wednesday 12 March 2025, 16:00-17:00
• Venue: MR13.
• Series: Differential Geometry and Topology Seminar; organiser: Ailsa Keating.

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Abstract not available

• Speaker: Paul Goddard
• Wednesday 28 May 2025, 11:15-12:00
• Venue: Mott Seminar Room (531), Cavendish Laboratory, Department of Physics.
• Series: Quantum Matter Seminar; organiser: Mads Fonager Hansen.

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Abstract not available

• Speaker: Sven Friedemann - University of Bristol
• Wednesday 14 May 2025, 11:15-12:00
• Venue: Mott Seminar Room (531), Cavendish Laboratory, Department of Physics.
• Series: Quantum Matter Seminar; organiser: Mads Fonager Hansen.

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Abstract not available

• Speaker: Marie-Aude Measson - CNRS Institut Neel
• Wednesday 19 March 2025, 11:15-12:00
• Venue: Mott Seminar Room (531), Cavendish Laboratory, Department of Physics.
• Series: Quantum Matter Seminar; organiser: Mads Fonager Hansen.

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Electrons typically traverse a conductive medium in a diffusive manner, resulting in a linear relationship between the measured voltage and applied current – known as Ohm’s law. However, violations of Ohm’s law may be found when the inherent symmetries of the underlying system are broken. Examples include the sliding motion of density waves; ballistic or hydrodynamic electron transport; or the symmetry-breaking realised by lattice or magnetic order. Focused ion beam (FIB) fabrication methods enable precise nanoscale devices to be fashioned from high-quality single crystalline materials, ideal for exploring these nonlinear phenomena. Such nanoengineering offers vast potential for the investigation of both fundamental physics and the development novel quantum devices. In this talk, I will introduce three specific examples. Firstly, we will explore the current-induced sliding motion of a skyrmion lattice in Gd2PdSi3 and the resulting emergent electrodynamics, which originate from a time-dependent Berry phase. Secondly, I will highlight our latest breakthrough to develop FIB fabrication of three dimensional nanostructures, in the form of helical-shaped devices of the high-mobility Weyl magnet CoSn2S2. By breaking inversion symmetry on the length scale of the electron mean free path, we observe large nonreciprocal transport, resulting in a switchable diode effect. Finally, if time permits, I will discuss the possibility to fabricate highly symmetrical devices, which allows the probing of symmetry breaking along multiple directions of a material simultaneously – in this case exploited to study signatures of p-wave magnetism in Gd3Ru4Al12.

• Speaker: Max Birch - RIKEN Center for Emergent Matter Science
• Wednesday 05 March 2025, 11:15-12:00
• Venue: Mott Seminar Room (531), Cavendish Laboratory, Department of Physics.
• Series: Quantum Matter Seminar; organiser: Mads Fonager Hansen.

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Anomalous fluctuations are phenomena where hydrodynamic fluctuations in the system behave in a way that violates usual expectations, e.g. typical fluctuations that are Gaussian. It was discovered recently that certain many-body systems exhibit such fluctuations, and one of the most notable examples is the isotropic spin-1/2 Heisenberg chain whose spin transport shows a surprising “partial” Kardar-Parisi-Zhang (KPZ) physics. Such partial KPZ behaviour has been also experimentally confirmed using superconducting qubits, where it was observed that the higher spin cumulants behave in a way that is not controlled by any known KPZ sub universality class (e.g. GUE or Baik-Rains). In this talk, I will introduce a hydrodynamic framework based the ballistic macroscopic fluctuation theory to describe anomalous fluctuations and apply it to a class of stochastic cellular automata. The cellular automata, which have been solved microscopically, conserve a charge and it has been demonstrated that the charge fluctuations in these systems and the spin fluctuations in the easy-axis Heisenberg chain are both anomalous with the same non-Gaussian probability distribution function. I will show how our approach successfully reproduces the known typical and large charge fluctuations in the systems and explain how one can understand the phenomena hydrodynamically in systems with a Z_2 charge.

• Speaker: Takato Yoshimura, Oxford
• Friday 14 February 2025, 14:00-15:30
• Venue: TCM Seminar Room.
• Series: Theory of Condensed Matter; organiser: Gaurav.

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Novel reactivities of organoboron compounds have been identified as a source for creating new chemical spaces with functional value. Previously uncharted approaches of activation including, non-covalent interactions, electrochemical redox processes, and photoexcitation have been exploited for the functionalization of organoboron compounds. Combined experimental and computational studies provide insight into the fundamental understanding of the process. Special emphasis of the research program has been devoted to the formation of products with stereochemically-enriched C(sp3) center.

References [1] Go, S. Y.; Chung, H.; Shin, S. J.; An, S.; Youn, J. H.; Im, T. Y.; Kim, J. Y.; Chung, T. D.; Lee, H. G. J. Am. Chem. Soc. 2022, 144, 9149. [2] Roh, B.; Farah, A. O.; Kim, B.; Feoktistova, T.; Moller, F.; Cheong, P. H.; Lee, H. G. J. Am. Chem. Soc. 2023, 145, 7075. [3] Koo, J.; Kim, W.; Jhun, B. H.; Park, S.; Song, D.; You, Y.; Lee, H. G. J. Am. Chem. Soc. 2024, 146, 22874.

• Speaker: Prof Hong Geun Lee (Seoul National University)
• Thursday 15 May 2025, 14:00-15:00
• Venue: Dept. of Chemistry, Pfizer Lecture Theatre.
• Series: Synthetic Chemistry Research Interest Group; organiser: Jasmine Mitchell.

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