Division of Theoretical Physics

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Previous Theoretical Physics Seminars
March 26, 2024
Singular quantum Fisher information matrix in critical metrology
dr Karol Gietka
(Institut für Theoretische Physik, Universität Innsbruck)

Critical metrology relies on the extreme sensitivity of the system's eigenstates close to the critical point to Hamiltonian parameter perturbations. Typically, however, the critical point, at which the phase transition occurs, is a function of many if not all the parameters of the system. This suggests that the quantum Fisher information matrix might be singular at the critical point, which in the context of parameter estimation is typically representing a significant complication. On the example of a toy-model Landau-Zener Hamiltonian, the Ising Hamiltonian, and the thermodynamic limit of the Lipkin-Meshkov-Glick Hamiltonian, we show that the quantum Fisher information matrix in critical metrology is always singular regardless of the system size and the distance to the critical point. However, contrary to the regular approach to metrology, where the singularity is detrimental, we argue that critical metrology works precisely because the quantum Fisher information matrix is singular.

March 19, 2024
On the Ideal Shapes of Stalagmites
prof. dr hab. Piotr Szymczak
(Faculty of Physics, University of Warsaw)

Stalagmites are column-like formations that rise from the floor of caves. They are formed by the buildup of minerals deposited from water dripping from the ceiling. The water dissolves minerals, such as calcium carbonate, from the rock above. As the water drips down, it loses carbon dioxide to the cave air. This causes the minerals to come out of solution and precipitate onto the cave floor, slowly building up the stalagmite.

Nearly sixty years ago, Franke formulated a mathematical model for the growth of stalagmites. In this model, the local growth rate of a stalagmite is proportional to the oversaturation of calcium ions in the solution dripping down the stalagmite's surface. Franke postulated that - provided the physical conditions in the cave remain constant - after a sufficiently long period, the stalagmite will assume an ideal shape, which in later stages of growth will only move upwards without further change in its form. These conclusions were later confirmed in computer simulations yet the mathematical form of this ideal shape was not discovered.

As we will show, Franke's model for stalagmite growth can be solved analytically, finding invariant, Platonic forms of stalagmites that could be observed in an "ideal cave", under constant physical conditions and with a constant flow of water dripping from an associated stalactite. Interestingly, it turns out that the shape numerically found in previous numerical studies is just one of a whole family of solutions. These new solutions describe stalagmites with a flat area at their peak of a certain fixed diameter, and conical stalagmites, with sharply pointed tops. All of these forms are observed in caves.

March 12, 2024
Interacting Networks of Liquid Light
dr Helgi Sigurðsson
(Faculty of Physics, University of Warsaw & Science Institute, University of Iceland)

Recent years have seen a surge of advancements in optical manipulation over bosonic light-matter quasiparticles known as exciton-polaritons in semiconductor microcavities. These particles appear under strong-coupling conditions between confined cavity photons and embedded quantum-well excitons. Characterised by very high interaction strengths, nonlinearities, and picosecond timescales, they provide an exciting testbed to explore room-temperature nonequilibrium Bose-Einstein condensation in the optical regime.

In this talk, I will present results on all-optically engineered macroscopic networks of connected exciton-polariton condensates, which permit studies on fundamental emergent behaviours in nonequilibrium quantum fluidic systems that are subject to an external drive and dissipation. I will explain how pumped polariton fluids give rise to so-called “ballistic condensates” which can interfere to form a bosonic analog of time-delay coupled oscillations, a behavior found all across nature. I will present experimental and theoretical results on large-scale condensate networks displaying aforementioned emergent behaviors, including: spontaneous synchronization with unprecedented long-range spatial and temporal correlations [1,2], formation of persistent circulating mass currents [3], non-invasive optical control of the network coupling weights [4], synthesis of artificial lattices for studies of non-Hermitian topological physics and Bloch band formation [5,6], and vortex frustration [7].

Lastly, I will discuss recent developments on the role of polariton condensate networks as nonlinear information processing elements in the optical computing paradigm. I will address three examples: room-temperature optical logic, analog spin simulators, and as neuromorphic computing hardware.

[1] Töpfer et al., Communication Physics 3, 2 (2020).
[2] Töpfer et al., Optica 8, 106 (2021).
[3] Cookson et al., Nature Communications 12, 2120 (2021).
[4] Alyatkin et al., Physical Review Letters 124, 207402 (2020).
[5] Pickup et al., Nature Communications 11, 4431 (2020).
[6] Alyatkin et al., Nature Communications 12, 5571 (2021).
[7] Alyatkin et al., arXiv:2207.01850 (2022).

March 5, 2024
On the dynamics of progress
prof. Zbigniew R. Struzik
(University of Tokyo)

Autonomy is a unique defining feature of living systems, from bacteria -- or even from viruses, the organisms at the 'edge of life' -- striving to survive in evolving environment. Yet, evolution is a collective phenomenon requiring interaction, 'counterintuitively' to autonomic thinking of individual organisms allowing their species to survive.
Such an apparent dichotomy and the struggle of the 'opposites' is ubiquitous in most complex dynamical systems, entailing complex adaptive systems which encompass living systems. The struggle of the opposites underlies most if not all roads to progress, understood as the evolution of 'fitness' -- be it adaptation to environment for species survival or evolution of scientific paradigms, or indeed, the evolution of the market share for those financially challenged.

Curiously, the concept of the 'roads to progress' has to date not sufficiently been studied using the language of physics of complexity. Indeed, physics traditionally understood, stayed away from investigating phenomena involving engaging in any laws of -- and due to -- individual and individualised behaviour. Yet, as becomes apparent, such seemingly impenetrable problems as evolution of social standards such as morality or religion are not principally different from evolution of scientific paradigms and innovation, these in turn not being that different from the evolution of generic actors in a competitive company market.

Such universality is where physics can flourish and indeed, a rapidly growing range of reports shows intriguing beauty of the phenomena characterising the dynamics of the road to progress. In my talk, I will present our own recent work on the game-of-life like system which - to our surprise - showed exciting richness of critical phenomena. I will also refer to selected works in order to encourage listeners to engage in research of the 'physics of struggle'.

February 20, 2024
Non-Hermitian physics and phase transitions
dr Amir Rahmani
(Institute of Physics PAS)

A physical system may become non-conservative when it is interacting with its environment. One way to introduce openness is by breaking the Hermiticity, that is, by involving non-Hermitian matrices/operators. This seminar will provide a quick overview of the theoretical background of non-Hermitian physics, followed by a focus on the topic of non-Hermitian phase transitions intertwined with the physics of light-matter interactions.

February 13, 2024
Spin Squeezing for Ultracold Fermions in Optical Lattices
prof. Gediminas Juzeliūnas
(Institute of Theoretical Physics and Astronomy, Vilnius University, Vilnius, Lithuania)

In the initial part of the talk an extended overview will be presented on individual and collective spins, the states of the collective spin, squeezing the collective spin states. We will also talk about different spin squeezing mechanisms including one axis twisting (OAT), and two axis countertwisting (TACT) spin squeezing models. Subsequently we will discuss possibilities to produce spin squeezing for spinful atomic fermions in optical lattices. It is shown that by applying laser radiation one can simulate not only OAT but also TACT spin squeezing models, the latter TACT model providing better squeezing. The spin squeezing generated in this way is mediated by spin waves playing a role of the intermediate states facilitating the squeezing process. The spin squeezing can be used for increasing sensitivity of atomic clocks.

More about this work:
Phys. Rev. Lett. 129, 090403 (2022)
Phys. Rev. B 108, 104301 (2023)