Home on David A. Reiss

dg-solvax

Sun, 06 Sep 2026 00:00:00 +0000

Discontinuous Galerkin and Riemann solver in JAX.

First thoughts on "Gödel, Escher, Bach" and Lean

Sun, 06 Sep 2026 00:00:00 +0000

Currently I’m reading the classic "Gödel, Escher, Bach" (GEB) by Douglas R. Hofstadter, years ago recommended to me by my stepfather after he read it when he was studying computer science besides working as a math and physics teacher (in retrospect, him doing so has impressed and inspired me, until today). Since then, I had GEB on my bookshelf, but never read it because I always thought: “This is not the right time, because…” “I’ve got too many other things on my mind”, “there’s so much other stuff to do”, and so on… (To be honest, I may also have gotten a little bit intimidated by the intellectual level of the book as he described it, but as it turned out, I actually find it quite approachable.)

Further project descriptions'

Sun, 06 Sep 2026 00:00:00 +0000

…summaries will follow.

NVIDIA GTC Berlin 2026

Sun, 06 Sep 2026 00:00:00 +0000

I’m excited to announce that I’ll be able to attend the AI conference NVIDIA GTC Berlin 2026 on October 21–22, 2026 (which is part of NVIDIA’s global GPU Technology Conference series) thanks to my employer TNG Technology Consulting GmbH! It’s hard to tell which aspect of it excites me most, but probably: to get to know the people there.

So if you’ll be at the NVIDIA GTC Berlin as well, feel free to drop me a note, I’d be happy to meet you!

About me

Wed, 02 Sep 2026 00:00:00 +0000

I’m David, a theoretical physicist turned AI researcher, working as a software consultant for TNG Technology Consulting GmbH (formerly as a researcher at Freie Universität Berlin).
Loving deep dives, both technical & personal, and spending time with my son. Passionate about science & technology, books & writing, art & theatre (great to live near Berlin!).
There would be so much more to say about me… But these pages are mainly about my work and thoughts—I hope you find them inspiring!

Datenschutzerklärung

Wed, 02 Sep 2026 00:00:00 +0000

Diese Datenschutzerklärung informiert Sie gemäß Art. 13 DSGVO über die Verarbeitung personenbezogener Daten auf dieser Website.

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David A. Reiss

Zum Kahleberg 28
14478 Potsdam
Deutschland

E-Mail: dav.reiss[at]t-online.de

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Impressum

Wed, 02 Sep 2026 00:00:00 +0000

Angaben gemäß § 5 DDG

Website-Betreiber / Diensteanbieter:

David A. Reiss

Zum Kahleberg 28
14478 Potsdam
Deutschland

E-Mail: dav.reiss[at]t-online.de

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David A. Reiss (Anschrift wie oben)

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Workshop series "Reinforcement learning fundamentals"

Fri, 24 Apr 2026 00:00:00 +0000

In these workshops, TNG colleagues gained a foundational understanding of reinforcement learning.

Talk "Tales from the machinery room - Customizing LLMs @ TNG AI research"

Fri, 24 Oct 2025 00:00:00 +0000

"The Germans just Frankensteined DeepSeek's R1 and V3 into something called R1T Chimera".

Assembly of Experts: Linear-time construction of the Chimera LLM variants with emergent and adaptable behaviors

Sat, 31 May 2025 00:00:00 +0000

Requiring 10^13-10^15 FLOPs to calculate one 8 bit weight in an LLM during pretraining is extremely expensive and seems inefficient. To better leverage the huge investments made into pretrained models, we develop the new “Assembly-of-Experts” (AoE) construction method to create capable child variants of existing Mixture-of-Experts parent models in linear time. Model weight tensors get interpolated individually, allowing to enhance or suppress semantic features of the parents. Varying the proportion of weights taken from the parent models, we observe some properties of the AoE child model changing gradually, while other behavioral traits emerge with a sharp transition. Surprisingly, nearly every generated model is functional and capable, which makes searching the model space straightforward. We construct the DeepSeek R1T “Chimera”, a 671B open-weights hybrid model combining DeepSeek’s V3-0324 and R1 model variants. The child inherits only the routed expert tensors of R1, but still achieves about R1-level intelligence. At the same time, it uses about 40% fewer output tokens, close to V3 speed. Constructed without any fine-tuning or distillation, the Chimera exhibits surprisingly compact, orderly reasoning compared to its parent models. The DeepSeek R1T Chimera model is available as open weights on Hugging Face.

Quantum reinforcement learning of classical rare dynamics: Enhancement by intrinsic Fourier features

Tue, 22 Apr 2025 00:00:00 +0000

Rare events are essential for understanding the behavior of non-equilibrium and industrial systems. It is of ongoing interest to develop methods for effectively searching for rare events. With the advent of quantum computing and its potential advantages over classical computing for applications like sampling certain probability distributions, the question arises whether quantum computers could also provide an advantage or inspire new methods for sampling the statistics of rare events. In this work, we propose a quantum reinforcement learning (QRL) method for studying rare dynamics, and we investigate their benefits over classical approaches based on neural networks. As a proof-of-concept example, we demonstrate that our QRL agents can learn and generate the rare dynamics of random walks, and we are able to explain this success as well as the different contributing factors to it via the intrinsic Fourier features of the parameterized quantum circuit. Furthermore, we show better learning behavior with fewer parameters compared to classical approaches. This is the first investigation of QRL applied to generating rare events and suggests that QRL is a promising method to study their dynamics and statistics.

Past poster presentations in academia

Wed, 01 Jan 2025 00:00:00 +0000

...will follow soon.

Past talks in academia

Wed, 01 Jan 2025 00:00:00 +0000

...will follow soon.

Past teaching in academia

Wed, 01 Jan 2025 00:00:00 +0000

...will follow soon.

Theory of ac magnetoelectric transport in normal-metal - magnetic-insulator heterostructures

Fri, 23 Aug 2024 00:00:00 +0000

(Note that cited here as the arXiv v1 preprint. The content was later reorganized (after disagreements about this I decided to be not included in the list of authors anymore) and published as Phys. Rev. B 113, 014419 (2026).) Electron-magnon coupling at the interface between a normal metal and a magnetically ordered insulator modifies the electrical conductivity of the normal metal, an effect known as spin-Hall magnetoresistance. It can also facilitate magnon-mediated current drag, the nonlocal electric current response of two normal metal layers separated by a magnetic insulator. In this article, we present a theory of these two spintronic effects and their nonlinear counterparts for time-dependent applied electric fields $E(\omega)$, with driving frequencies $\omega$ up to the THz regime. We compare various mechanisms leading to a quadratic-in-$E$ response - Joule heating, spin-torque diode effect, and magnonic unidirectional spin-Hall magnetoresistance - and show how these can be disentangled by their characteristic dependences on $\omega$ and the magnetization direction.

Finite-frequency spin conductance of the interface between a ferro- or ferrimagnetic insulator and a normal metal

Tue, 02 Aug 2022 00:00:00 +0000

The interface between a ferro-/ferrimagnetic insulator and a normal metal can support spin currents polarized collinear with and perpendicular to the magnetization direction. The flow of angular momentum perpendicular to the magnetization direction (“transverse” spin current) takes place via spin torque and spin pumping. The flow of angular momentum collinear with the magnetization (“longitudinal” spin current) requires the excitation of magnons. In this article we extend the existing theory of longitudinal spin transport in the zero-frequency weak-coupling limit in two directions. We calculate the longitudinal spin conductance non-perturbatively (but in the low-frequency limit) and at finite frequency (but in the limit of low interface transparency). For the paradigmatic spintronic material system YIG|Pt, we find that non-perturbative effects lead to a longitudinal spin conductance that is ca. 40% smaller than the perturbative limit, whereas finite-frequency corrections are relevant at low temperatures < 100 K only, when only few magnon modes are thermally occupied.

Ultrafast demagnetization of iron induced by optical versus terahertz pulses

Thu, 03 Jun 2021 00:00:00 +0000

We study ultrafast magnetization quenching of ferromagnetic iron following excitation by an optical vs a terahertz pump pulse. While the optical pump (photon energy of 3.1 eV) induces a strongly nonthermal electron distribution, terahertz excitation (~4 meV) results in a quasi-thermal perturbation of the electron population. The pump-induced spin and electron dynamics are interrogated by the magneto-optic Kerr effect (MOKE). A deconvolution procedure allows us to push the time resolution down to 130 fs, even though the driving terahertz pulse is more than 0.5 ps long. Remarkably, the MOKE signals exhibit an almost identical time evolution for both optical and terahertz pump pulses, despite the three orders of magnitude different number of excited electrons. We are able to quantitatively explain our results using a model based on quasi-elastic spin-flip scattering. It shows that in the small-perturbation limit, the rate of demagnetization of a metallic ferromagnet is proportional to the excess energy of the electrons, independent of the precise shape of their distribution. Our results reveal that the dynamics of ultrafast demagnetization and of the closely related terahertz spin transport do not depend on the pump photon energy.

Theory of spin-Hall magnetoresistance in the ac terahertz regime

Tue, 13 Apr 2021 00:00:00 +0000

In bilayers consisting of a normal metal (N) with spin-orbit coupling and a ferromagnet (F), the combination of the spin-Hall effect, the spin-transfer torque, and the inverse spin-Hall effect gives a small correction to the in-plane conductivity of N, which is referred to as spin-Hall magnetoresistance (SMR). We here present a theory of the SMR and the associated off-diagonal conductivity corrections for frequencies up to the terahertz regime. We show that the SMR signal has pronounced singularities at the spin-wave frequencies of F, which identifies it as a potential tool for all-electric spectroscopy of magnon modes. A systematic change of the magnitude of the SMR at lower frequencies is associated with the onset of a longitudinal magnonic contribution to spin transport across the F-N interface.

Laser-induced terahertz spin transport in magnetic nanostructures arises from the same force as ultrafast demagnetization

Mon, 22 Mar 2021 00:00:00 +0000

Laser-induced terahertz spin transport (TST) and ultrafast demagnetization (UDM) are central but so far disconnected phenomena in femtomagnetism and terahertz spintronics. Here, we use broadband terahertz emission spectroscopy to reliably measure both processes in one setup. We find that the rate of UDM of a single ferromagnetic metal film F has the same time evolution as the flux of TST from F into an adjacent normal-metal layer N. This remarkable agreement shows that UDM and TST are driven by the same force, which is fully determined by the state of the ferromagnet. An analytical model consistently and quantitatively explains our observations. It reveals that both UDM in F and TST in the F|N stack arise from a generalized spin voltage, which is defined for arbitrary, nonthermal electron distributions. We also conclude that contributions due to a possible temperature difference between F and N are minor and that the spin-current amplitude can, in principle, be increased by one order of magnitude. In general, our findings allow one to apply the vast knowledge of UDM to TST, thereby opening up new pathways toward large-amplitude terahertz spin currents and, thus, energy-efficient ultrafast spintronic devices.

Lower bounds for the time-bandwidth product of a single-photon pulse

Mon, 10 Feb 2020 00:00:00 +0000

Two complementary properties, such as frequency and duration of a pulse, cannot be precisely measured simultaneously. The lower bound on the time-bandwidth product, known also as uncertainty relation, plays a very important role in quantum theory. In this work, we consider single-photon pulses with arbitrary temporal and spectral profiles, and derive a lower bound for the single-photon time-bandwidth product.

Quantum robustness of fracton phases

Fri, 29 Nov 2019 00:00:00 +0000

The quantum robustness of fracton phases is investigated by studying the influence of quantum fluctuations on the X-Cube model and Haah’s code, which realize a type-I and type-II fracton phase, respectively. To this end a finite uniform magnetic field is applied to induce quantum fluctuations in the fracton phase resulting in zero-temperature phase transitions between fracton phases and polarized phases. Using high-order series expansions and a variational approach, all phase transitions are classified as strongly first order, which turns out to be a consequence of the (partial) immobility of fracton excitations. Indeed, single fractons as well as few-fracton composites can hardly lower their excitation energy by delocalization due to the intriguing properties of fracton phases, as demonstrated in this work explicitly in terms of fracton quasi-particles.

Quantum robustness and phase transitions of the 3D toric code in a field

Mon, 11 Feb 2019 00:00:00 +0000

We study the robustness of 3D intrinsic topological order under external perturbations by investigating the paradigmatic microscopic model, the 3D toric code in an external magnetic field. Exact dualities as well as variational calculations reveal a ground-state phase diagram with first and second-order quantum phase transitions. The variational approach can be applied without further approximations only for certain field directions. In the general field case, an approximative scheme based on an expansion of the variational energy in orders of the variational parameters is developed. For the breakdown of the 3D intrinsic topological order, it is found that the (im-)mobility of the quasiparticle excitations is crucial in contrast to their fractional statistics.

Quantum phase transitions to topological Haldane phases in spin-one chains studied by linked-cluster expansions

Mon, 09 Oct 2017 00:00:00 +0000

We use linked-cluster expansions to analyze the quantum phase transitions between symmetry unbroken trivial and topological Haldane phases in two different spin-one chains. The first model is the spin-one Heisenberg chain in the presence of a single-ion anisotropy while the second one is the dimerized spin-one Heisenberg chain. For both models we determine the ground-state energy and the one-particle gap inside the non-topological phase as a high-order series using perturbative continuous unitary transformations. Extrapolations of the gap series are applied to locate the quantum critical point and to extract the associated critical exponent. We find that this approach works unsatisfactory for the anisotropic chain, since the quality of the extrapolation appears insufficient due to the large correlation length exponent. In contrast, extrapolation schemes display very good convergence for the gap closing in the case of the dimerized spin-one Heisenberg chain.