HS 2025

16.09.2025 Daniel Arrufat Vicente (ETH Zurich)

Location: HPF G6, Time: 14:30

Ensemble Inequivalence in Long-Range Quantum Spin Systems

Ensemble inequivalence, where a system’s thermodynamic properties depend on the statistical ensemble used to describe it, is a well-established phenomenon in classical systems with long-range interactions. Here, we present a detailed analysis of a quantum ferromagnet spin model that exhibits this same behaviour. We find that the microcanonical and canonical phase diagrams are identical at zero temperature but differ significantly at finite temperatures. Our results highlight a breakdown of ensemble equivalence in a quantum long-range spin model, contrasting with what is observed for short-range.

14.10.2025 Radu Andrei (ETH Zurich)

Location: HPF G6, Time: 14:00

Evolution of antiferromagnetism with doping in a strongly
correlated insulator

What is the fate of magnetic order in a strongly correlated insulator once it is doped with charge carriers?
Answering this deceptively simple question is one of the most challenging problems in condensed matter physics,
but understanding its solution could potentially hold the key to understanding high-temperature superconductivity.
Recent progress in systems of ultracold atoms in optical lattices has shed new light on this long-standing problem by enabling the direct quantum simulation of the Fermi Hubbard model and its dynamics as a function of hole doping concentration. Inspired by this, we reexamine this problem by developing a self-consistent theory of magnons in a Heisenberg antiferromagnet and their interaction with doped holes. Using a combination of self-consistent
diagrammatics and tensor-network simulations, we show that this surprisingly simple theory captures much of the correct physics. In particular, we study how the spin-stiffness of the magnons is renormalized by itinerant holes, which disfavor the Neel order leading to a softening of the magnon bandwidth and spin-wave velocity. We also study how this
softening manifests in the two-magnon response, which can be measured in both Raman scattering in solid-state systems, as well as modulation spectroscopy in quantum simulators. We find that doping also leads to a pronounced softening and broadening of this peak, painting a consistent picture that as holes are doped into the insulator, the magnetic order
slowly melts and ultimately gives way to an instability.

21.10.2025 András Szabó (Max Planck Institute for Solid State Research)

Location: HPF G6, Time: 14:00

Intertwined and Induced Orders in Correlated Quantum Matter

The notion of competing symmetry-breaking instabilities is one often  associated with strongly correlated quantum systems. However, ordered  phases need not always compete, they can also cooperate or intertwine,  giving rise to composite or emergent forms of order. In this talk, we review the basic concepts underlying intertwined and induced orders, and illustrate them through a few exotic examples relevant to strongly  correlated systems of current interest.
We consider the case of heavy-fermion superconductors UTe2 and  CeRh2As2, both of which exhibit a notoriously complex confluence of  ordered phases. In UTe2, a bulk homogeneous superconducting phase  intertwines with a spatially modulated charge-density wave order that  is only present on the surface, yielding a surface-localized modulated  pair-density wave. We show that the induced pair-density wave can have emergent properties enabled by intertwining, with the potential to  resolve the long standing puzzle of time-reversal symmetry breaking at  the superconducting critical temperature.
Using CeRh2As2 as a second example, we demonstrate how nonsymmorphic  symmetries give rise to novel types of composite orders, specifically  two-component order parameters with components of opposite parity,  despite the centrosymmetric crystal structure. This enables superconductivity to induce odd-parity orders such as electric  polarization or antiferromagnetism, which is symmetry forbidden in  systems with symmorphic space groups.
If time permits we venture into the time domain, and show that  intertwining can also occur between conventional Landau order  parameters and time crystalline order, the latter breaking discrete  time translation symmetry in a driven-dissipative matter-cavity system.

[1] A. Szabo, A. Ramires, arXiv:2503.24390
[2] A. Szabo, A. Ramires, PRB 110, L180503 (2024)
[3] A. Szabo, R. Chitra, arXiv:2505.21504

28.10.2025 Andrew Jreissaty (ETH Zurich)

Location: HPF G6, Time: 14:00

Entanglement and optimization within autoregressive neural quantum states

Neural quantum states (NQS) are powerful variational ans ̈atze capable of representing highly entangled quantum many-body wavefunctions. While the average entanglement properties of ensembles of restricted Boltzmann machines are well understood, the entanglement structure of autoregressive NQS such as recurrent neural networks and transformers remains largely unexplored. We perform large-scale simulations of ensembles of random autoregressive wavefunctions for chains
of up to 256 spins and uncover signatures of transitions in their average entanglement scaling, entanglement spectra, and correlation functions. We show that the standard softmax normalization of the wavefunction suppresses entanglement and fluctuations, and introduce a square modulus normalization function that restores them. Finally, we connect the insights gained from our entanglement and activation function analysis to initialization strategies for finding the ground states of strongly correlated Hamiltonians via variational Monte Carlo.

06.11.2025 Bernardo Barrera (Boston University)

Location: HIT H 51, Time: 14:00

Non-adiabatic Dynamics Beyond the Born-Oppenheimer Approximation

We present a framework for describing the dynamics of systems composed of slow and fast degrees of freedom (DOFs) beyond the standard Born–Oppenheimer approximation. By systematically incorporating non-adiabatic corrections to the instantaneous state of the fast subsystem, we uncover qualitatively new dynamical behavior absent in the adiabatic limit. As a concrete example, we analyze a model consisting of a slow molecular coordinate coupled to an ensemble of fast spins. We show that velocity fluctuations of the molecule induce effective antiferromagnetic interactions among the spins, leading to the generation of correlations and entanglement. Conversely, the backaction of the spins renormalizes the molecule’s effective inertial mass.

11.11.2025 Jon Curtis (ETH Zurich)

Location: HPF G6, Time: 14:00

Listening to Quantum Materials Using Nonlinear Noise Spectroscopy

Quantum materials such as superconductors, magnets, and topological insulators have great potential for realizing advanced electronic and computing devices, but characterizing and understanding these materials has remained a challenge. Conventional tools like nonlinear
optics or transport are often difficult to employ or interpret since they are severely limited in spatial resolution. Especially given the rise of two-dimensional moiré materials, it is more important than ever to find probes which can resolve the rich spatial and dynamical correlations present in these materials. One such technique is noise magnetometry, which utilizes nanoscale
spin qubits to map out the landscape of magnetic fluctuations in a material with spatial resolution on the order of 10’s of nm. In this talk I will provide a theoretical example of how nanoscale noise spectroscopy can be used to uncover superconducting fluctuations in a
two-dimensional material, before highlighting the need for nonlinear probes. I will then unveil a new set of protocols which can be used to study nonlinear correlations in the magnetic noise, blending the crisp spatial resolution of noise spectroscopy with the insights of nonlinear
spectroscopy. As an example, I will show how this technique can be used to study critical magnetic fluctuations in a two-dimensional material, such as a Van der Waals magnet, and how cooperative noise onsets near a phase transition. I will then conclude by providing an outlook on the next generation of probes which can use multiple qubits to detect spatially non-local correlations, and what we can look for using these tools.

18.11.2025 Clemens Lindner (University of Jyväskylä)

Location: HPF G6, Time: 14:00

Kernels from Quantum Hilbert Space

Classical machine learning encompasses a variety of methods. One family is kernel methods, which rely on a similarity measure between data points (known as a kernel function) to solve tasks such as classification or regression. This similarity measure can be viewed as an inner product in an implicit high-dimensional feature space. The advent of quantum computers enables the implementation of quantum kernels, where a quantum device is used to evaluate the kernel function, typically by computing the overlap between quantum feature states, followed by classical post-processing. In this talk, I will introduce Quantum Kernel Methods by first providing an intuitive overview of the classical case, then linking it to quantum computation, and finally concluding with a discussion of the current state of the field.

25.11.2025 Jannes Nys (ETH Zurich)

Location: HPF G6, Time: 14:00

Neural representations of fermionic matter in and out of equilibrium

Understanding how quantum many-body systems interact and give rise to emergent collective phenomena is essential in various fields of science and technology development, including designing new quantum materials with targeted properties, advancing analog quantum simulations, and designing quantum computational devices. However, when strong correlations play a significant role, the existing computational and theoretical tools inevitably face challenges. In this talk, I will introduce a class of machine-learning-inspired computational methods to advance our understanding of both equilibrium and non-equilibrium properties of strongly correlated matter, with a focus on fermionic systems.

Location: HPF G6, Time: 14:00

Universality and weak-ergodicity breaking in quantum quenches

Sudden quenches in quantum many-body systems can generate rich non-equilibrium phenomena, often revealing surprising physical behaviors. In this seminar, I will argue that the emergence of weak ergodicity breaking following quantum quenches in certain local many-body systems is a direct consequence of lattice discretization. To support this claim, I will examine the out-of-equilibrium dynamics of quantum $O(n)$ models, in the large n limit, defined on a lattice. Along the way, I will also revisit two puzzling results on quantum $O(n)$ models concerning the universal critical scaling at the dynamical phase transition, and the equilibration to a Generalised Gibbs ensemble. I will show how apparent contradictions in the previous literature can be resolved by properly accounting for lattice effects.

Ref: Universality and weak-ergodicity breaking in quantum quenches, Guido Giachetti, Andrea Solfanelli, Nicolò Defenu, arXiv:2511.08687 (2025)

16.12.2025 Arta Safari (ETH Zurich)

Location: HIT F.12, Time: 16:30

Exploring Low-Energy Manifolds with Effective Hamiltonians

Within the last twenty years, numerical electronic structure techniques have reached the point where exchange processes in strongly-correlated transition metal clusters can be studied with meV accuracy. In this talk, it will be shown
how a judicious choice of angular momentum coupling scheme allows to derive and validate effective models, like the Heisenberg or orbitally-dependent exchange Hamiltonians, from first principles.

As application examples, I will discuss recent results on systems from bioinorganic chemistry and solid state physics: a [CaMn $$_4$$ O $$_5$$] complex resembling the active centre of photosystem II and a square-plaquette cluster
model of the spin-orbit Mott insulator Sr $$_2$$ IrO $$_4$$.

27.01.2026 Irene García Martínez (Universitat de València)

Location: HIT K.51, Time: 14:00

Spin squeezing parameter in quantum metrology

Quantum metrology explores methods to enhance measurement precision beyond classical limits, with applications ranging from atomic clocks to gravitational wave detection. An important and widely used tool in quantum metrology is the spin squeezing parameter. Its development was mainly motivated by two applications: the improvement of precision measurements beyond the classical limit and the study of particle correlations and entanglement.

In quantum metrology, the spin squeezing parameter determines the sensitivity that can be achieved through the measurement of a fixed, possibly suboptimal observable. It therefore determines a lower bound on the quantum Fisher information, which expresses the maximal sensitivity achievable with an optimal observable. In this seminar, I will introduce the concept of spin squeezing in quantum metrology, outline its standard derivation (method of moments), and discuss a proposed generalization of the parameter.