Superconductivity, the ability to carry electrical current without any resistance whatsoever, is perhaps the longest known example of a quantal order—defined by an order parameter, the phase of the electronic wave function, that does not have a classical analogue. While the problem of classifying superconducting states is largely solved, the question of mechanism remains in many respects open....
The physical world is hierarchical. Kenneth Wilson mathematically formulated this idea in the language of effective theory. For each energy scale, there is a set of laws called an effective theory that describes physical phenomena at that scale, and we can be ignorant of more fundamental laws at higher energies. For example, the pion theory is an effective theory of QCD; we do not need to know...
Holography posits a radical way to quantify gravitational physics. It claims that all information of a gravitational theory in a region of space can be encoded by a quantum field theory at the boundary of this region. Here I will discuss quantum gravity from this perspective. We will see how one can engineer - i.e., design and build - gravity through this relationship, using possible quantum...
Nanoscale engines are becoming increasingly common, manifested for example in on-chip processors that transduce energy and particles between reservoirs to perform tasks like cooling. However, when compared to macroscopic energy-transducing engines, quantum thermoelectric devices might operate on time and length scales where their environmental energy reservoirs cannot be accurately described...
The origin of complexity remains one of the most important and, at the same time, the most controversial scientific problems. Earlier attempts were based on theory of dynamical systems but did not lead to a satisfactory solution of the problem. I believe that a deeper understanding is possible based on a recent development of statistical physics, combining it with relevant ideas from...
Four-dimensional black holes are know not to deform under weak static perturbations, i.e., they have a vanishing polarizability. This constitutes a sort of classical naturalness problem. I will present a solution to this problem, which shows that this surprising fact remains true for arbitrarily large perturbations. I will end commenting on our ability to test this results in observations and...
Effective Field Theory (EFT) gives a universal language to parameterize the effects of heavy particles at low energies in an expansion in powers of E/M, where E is the experimental energy scale and M is the mass of the heavy particle. The most general corrections to the Standard Model can be fully classified, and searches for the effects of these corrections can be performed. However,...
The Hubbard model is a paradigm of the `strong correlation problem’, with relevance to high-Tc superconductors and ultra-cold atoms in optical lattices. Key aspects of its physics in two dimensions can now be established beyond doubt, thanks to the development of controlled and accurate computational methods working in synergy (such as quantum embedding, tensor networks, various flavors of...
We will discuss recent experiments in the pseudogap phase of the high Tc cuprate YBCO that have been interpreted as the light induced Meissner effect. A special feature of these materials is a bilayer structure with a large difference of Josephson couplings within the bilayers and between them. Motivated by this hierarchy of scales, we introduce a model that consists of bilayers of...
Ordered phases of matter, such as solids, ferromagnets, superfluids, or quantum topological order, typically only exist at low temperatures. Upending the conventional wisdom, we present explicit local models in which all such phases persist to arbitrarily high temperature. The physical mechanism is that order in one degree of freedom can enable many more to freely fluctuate, leading to...
The conformal bootstrap program has emerged as one of the most powerful non-perturbative tools for studying critical phenomena in quantum and statistical field theories. By exploiting the symmetries and consistency conditions of conformal field theory (CFT), such as crossing symmetry and unitarity, this approach enables the determination of scaling dimensions and operator product expansion...
We have strived for a general understanding of complex general chemical problems in the last 25 years. This endeavor has led my group to touch upon various situations that point to unsolved problems of quantum chemistry. In this talk, I will present a compilation of these problems that will lead us from relativistic quantum chemistry to electronic structure theory and reaction chemistry, up to...
Employing quantum light in nonlinear multidimensional spectroscopy is opening up many exciting opportunities to enhance the signal-to-noise ratio, improve the combined temporal, spatial, and spectral resolutions, and simplify nonlinear optical signals by selecting desired transition pathways in second and third order signals. We show how photoelectron signals generated by time-energy entangled...
In ultracold atomic physics and in nuclear physics, the interactions are often very close to the regime of universality, with near-infinite scattering lengths. Using a variety of tools, such as correlated Gaussian basis set expansions, effective field theory, and Monte Carlo methods, significant progress has been achieved during the past decade to compute bound state properties and in some...
Polaritons are hybrid light-matter states that form when material excitations strongly interact with “trapped or confined light”, such as in optical cavities or plasmonic nanostructures. While the physics community has studied polaritonic phenomena with atoms and inorganic semiconductors for several decades, significant interest in molecular polaritons has only emerged over the past decade....
There has been recent experimental evidence demonstrating quantum optical signatures in nonperturbative, extremely nonlinear optical processes such as high-order harmonic generation (HHG). These results came as a surprise, as typically, quantum optics deals with single or few-photon states; light states with increasing photon numbers behave more and more classically. Strong-field physics...
Please visit the Festive Colloquium webpage (https://indico.desy.de/event/47388/) for all information, including registration.
Quantum impurity problems first appeared in the treatment of magnetic atoms embedded in metals, and now act as auxiliary objects within the dynamical mean field theory of correlated materials. Among Prof. Lichtenstein's many notable contributions to our current understanding of the physics of materials are his seminal work in developing Monte Carlo techniques for simulating impurity models....
In physics and chemistry it is often observed that the curves of a physical quantity f(x,p) cross at one or more points, when plotted as a function of x for different values of the parameter p. Sometimes these crossing points are confined to a remarkably narrow region, or are even located at a single point, called "isosbestic point". For example, crossing points are found in the curves of the...
