tag:www.physik.uni-hamburg.de,2005:/en/iqp/schmelcher/scientific-newsResearch Highlights2024-01-23T09:51:28ZNAGR-fakmin-36815765-production2024-01-21T23:00:00ZDiscovery of Peregrine solitons in Bose-Einstein condensates<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/36815788/peregrine-artistic-d29671f7cb09820120e8e4dd4a4d4690d501760a.jpg" />Off shore, deep-water wave phenomena can give rise to extreme wave events known as rogue waves -- monstrous walls of water that appear and disappear without a trace.
The sighting of such events was always considered mere exaggerations to enrich sailors' stories but, without going any further from reality, encounters with rogue waves are more frequent than one would expect and a thread to maritime structures, vessels, and the lives of those onboard.
The Peregrine soliton (PS), a wave localized in time and space, is one of the most celebrated candidates to rogue waves, and its mathematical description has allowed us to bring these monsters to the lab for study, not only in water-tank experiments but also in nonlinear optics and ultracold atoms.<p>Photo: AG Schmelcher</p>NAGR-fakmin-36251332-production2023-11-19T23:00:00ZQuantum Phases from Competing Van der Waals and Dipole-Dipole Interactions of Rydberg Atoms<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/36251776/quantum-phase-cd47e15799926d0796d0b3445e2df56cdf7c39e2.png" />The interplay of short- and long-range interactions are responsible for diverse phenomena having significance across various domains in science. For instance, short-range Van der Waals interactions help stabilizing the interior structure of proteins whereas dipole-dipole interactions influence the overall folding procedure as well as interactions with other molecules. Exotic quantum phases with competing orders arise from coexistence of short- and long-range interactions in many-body systems. However, the study of these phenomena in
their natural setting is challenging due to the limited control and the finite temperature environments. This has led to a rapid growth in the use of ultracold systems for quantum simulation of many-body problems. The unprecedented control achieved in these platforms over the system parameters has facilitated probing physical scenarios that are impossible if not difficult to achieve in conventional solid-state systems.<p>Photo: AG Schmelcher</p>NAGR-fakmin-35639909-production2023-09-18T22:00:00ZExploring disordered quantum spin models<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/35639957/peterfigs-rm-db984884786b7b57ff328f0b4da1668ff8c9db16.png" />Many phenomena that we find in nature are a result of the collective behaviour of constituent particles interacting with each other at the quantum level. Analytical descriptions of such strongly interacting many-body systems are very rare, and often a simplification to the actual picture. Thus, it is not uncommon to resort to sophisticated numerical methods for the theoretical investigation of many-body quantum systems. In this context, the physics of disordered many-body systems can be harder to investigate compared to ordered systems especially since the effects of disorder can have intriguing influence on the quantum correlations of the system. When compared to ordered systems, there is a shortage of numerical methods that can accurately model disordered many-body systems. In this work, we apply techniques borrowed from quantum chemistry to study such disordered systems and provide an alternative powerful numerical tool to study these systems.<p>Photo: AG Schmelcher</p>NAGR-fakmin-34454155-production2023-05-07T22:00:00ZHidden Symmetries in Wave SystemsOpposite to the well-known manifest geometrical symmetries of a device latent symmetries are hidden symmetries. They become accessible only by performing a reduction of a given system into an effective lower-dimensional one. We show how latent symmetries can be leveraged for continuous wave setups in the form of acoustic networks.
These are systematically designed to possess latent-symmetry induced point-wise amplitude parity between selected waveguide junctions for all low frequency eigenmodes. We develop a modular principle to interconnect latently symmetric networks to feature multiple latently symmetric junction pairs. By connecting such networks to a mirror symmetric subsystem, we design asymmetric setups featuring eigenmodes with domain-wise parity. Bridging the gap between discrete and continuous models, our work takes a pivotal step towards exploiting hidden geometrical symmetries in realistic wave setups.
NAGR-fakmin-33467403-production2023-01-28T23:00:00ZDynamical excitation processes and correlations of three-body two-dimensional mixtures<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/33473612/dynamics-sketch-revised-f5a2016c1e3ca439a477f24cb26a3aa24025d5ca.png" />Few-body bound states are instrumental in understanding distinct phases of matter and universal phenomena like the Efimov effect. Quantum gases cooled down close to absolute zero offer an exquisite research platform to study bound state formation, given the tunability of interparticle interactions by means of magnetic fields. Several recent experimental breakthroughs allowed the dynamical association of trimers in strongly interacting Bose gases, which sparked the theoretical investigation of efficient protocols for forming these states. However, trimers are metastable and they decay very fast, rendering their detection an experimental and theoretical challenge. One possible way out is to confine the gases along one dimension using strong magnetic fields, rendering them effectively two-dimernsional. It is predicted that longer lived trimer states can thus form.<p>Photo: AG Schmelcher</p>NAGR-fakmin-31451731-production2022-07-19T22:00:00ZCounterflow dynamics of two correlated impurities immersed in a bosonic gas<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/31466917/cover-fig-0f5a4c21cbe12dc0ca0cb2c85b11ad719edcdcd9.png" />The immersion of impurity atoms into an ultracold environment is a widely studied research field and sheds light on many fundamental aspects in quantum mechanics. For instance, the development of correlations between the bath and the impurities can lead to the dressing of the latter by the excitations of the bath such that they can be interpreted as quasi-particles (polarons). Moreover, this impurity-bath correlation can induce correlations among the impurities themselves, even when they are initially non-interacting. In the case of bosonic impurities, this effect is expressed in an induced attractive force between them and, eventually, lead to their coalescence.<p>Photo: AG Schmelcher</p>NAGR-fakmin-28698808-production2021-12-09T23:00:00ZFew-body correlations for Bose and Fermi mixtures in two dimensions<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/28744126/few-body-correlations-c48b9134e104e788e1b1268a2b41fa57590c59a4.png" />Few-body ultracold systems serve as building blocks in understanding the fundamental processes occurring in ultracold gases. In particular, it was recently shown that a few parameters called Tan contacts, defined through the asymptotic expansion of the momentum distribution of an interacting gas, define its thermodynamic properties as well as its spectroscopic features via a set of universal relations. These relations are universal in the sense that they hold regardless of the quantum statistics of the gas or the interaction strength or even the particle number. The Tan contacts have a microscopic character originating from two- and three-body collisions, and thus few-body models are essential in unveiling the behaviour of these parameters. Another aspect, shown recently, connects the contact parameters with the onset of few-body correlations and the dynamical formation of few-body bound states.<p>Photo: AG Schmelcher</p>NAGR-fakmin-28724127-production2021-09-09T22:00:00ZSpontaneous Formation of Surface Patterns in a Driven Bose-Einstein Condensate<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/28742884/spontaneous-formation-4cab7a7e6039f90817d07d4f8a08e251b05d6e49.jpg" />In a collaboration of theory and experiment this work explored processes of
pattern formation in radiofrequency driven Bose-Einstein condensates.
Two-dimensional star-shaped patterns with l-fold symmetry, ranging from
quadrupole (l=2) to heptagon modes (l=7), have been parametrically excited
by modulating the atomic interactions via so-called Feshbach resonances.
Mathematical techniques based on an effective Mathieu equation and Floquet
analysis are utilized here to unravel the instability conditions and study
the surface modes in a trapped superfluid. The resonant frequencies of the
patterns helped us to understand the dispersion relation of the collective excitations.
The experimental results are in excellent agreement with the theoretical
mean-field framework. This work opens a new pathway
for generating higher-lying collective excitations with applications, such as
the probing of exotic properties of quantum fluids and providing a generation
mechanism of quantum turbulence.<p>Photo: AG Schmelcher</p>NAGR-fakmin-27334822-production2021-08-31T09:00:00ZFlat bands by latent symmetry<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/27345487/lasyflatbands-science-highlight--33x414-0a09638dcec6ac509906eecd9a20fd93b89faa72.png" />Flat bands of discrete lattice models are energy eigenvalue bands which are dispersionless, that is, independent of the crystal momentum. They play a pivoting role in designing systems with suppressed wave transport or dispersion and, more recently, in understanding strongly correlated states of matter and superconductivity. Flat bands originate from corresponding eigenstates which are compactly localized on only a small number of lattice sites and vanish on the rest of the lattice. Such compact localized states (CLSs) occur due to destructive interference of eigenstate amplitudes on surrounding sites, in turn caused by the special lattice geometry of the flat band system. A large effort has been made recently to identify schemes to generate CLSs and concomitant flat bands, a class of which relies on basic geometrical symmetries of the lattice unit cell. Using concepts and tools from spectral graph theory, researchers at the Center for Optical Quantum Technologies of the University of Hamburg have now demonstrated that a certain kind of hidden geometrical symmetries, called ``latent symmetries'', can be utilized to construct CLSs and flat bands. This opens up a new perspective on exploiting fundamental symmetry properties of model Hamiltonians to explain and design flat band systems.<p>Photo: AG Schmelcher</p>NAGR-fakmin-25989146-production2021-04-26T22:00:00ZQuantum Hidden Symmetries<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/25989191/quantum-hidden-symmetries-69644f7e3b59125c24dacc776d42af13c562f61b.png" />Symmetries are of fundamental importance for the understanding and modeling
of physical systems, virtually across all length and energy scales. In particular,
symmetries may induce degeneracies in the frequency spectrum of wavemechanical systems. Researchers at the Center for Optical Quantum Technologies of the University of Hamburg have demonstrated a novel route to address symmetry-induced degeneracies, by exploring certain hidden symmetries in discrete and lattice-like models. Such so-called "latent symmetries" are generally unveiled when reducing a system’s description to a suitable effective model without affecting its eigenspectrum. They may therefore be used to explain, e.g., energy degeneracy points which occur in band structures and whose symmetry origin is obscured at first sight.<p>Photo: AG Schmelcher</p>NAGR-fakmin-25768714-production2021-02-18T23:00:00ZUltrafast radiationless transitions of Rydberg atoms and vibrational dynamics of Rydberg molecules<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/25778809/scheme-news-ff8d8c1eb05becb09e2bd023bcfee0bdc759765c.png" />Rydberg atoms embody versatile tools for ultracold gas experiments, quantum optics, and quantum simulation with neutral atoms. Researchers from the University of Hamburg and the Max Planck Institute for the Physics of Complex Systems in Dresden have studied the interplay of Rydberg atoms and ground-state atoms. At huge internuclear distances, the characteristic Born-Oppenheimer potentials feature conical intersections, such that ultrafast changes of electronic angular momentum occur, a process that can be controlled via the principle quantum number n. Furthermore, quantum dynamics calculations of vibrational bound states in these potentials that lead to ultra-long-range Rydberg molecules are established.<p>Photo: AG Schmelcher</p>NAGR-fakmin-25174318-production2020-09-13T22:00:00ZParametrically excited star-shaped patterns at the interface of binary Bose-Einstein condensates<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/25175617/star-patterns-f0e3568d3f8de0038fe30db9aa944e712bbf4c7d.png" />Liquid drops being weakly affixed to a vertically oscillating surface can display star-shaped patterns. This symmetry breaking mechanism of both the spatial and the temporal symmetries dates back to 1831 of the original Faraday experiment for a fluid in a vertically shaken vessel. In direct correspondence to this experiment, such symmetry-breaking related instabilities have been extensively investigated in diverse contexts including classical fluids, periodically modulated chemical systems, granular media, nonlinear fiber optics and within the realm of Bose-Einstein condensates (BECs).<p>Photo: AG Schmelcher</p>NAGR-fakmin-23422552-production2020-09-10T22:00:00ZPump-Probe Spectroscopy of Ultracold Bose Polarons<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/23422995/aim-pump-probe-polarons-bf14265f205af49f19973a487670386594e9cf63.png" />Researchers in the University of Hamburg and the Okinawa Institute of Science and Technology have devised and employed a novel pump-probe spectroscopy technique to interrogate the formation, decay processes and coherence properties of polaronic excitations in an ultracold environment serving as a prototype quantum simulator of the respective condensed matter setup. They claim that this result paves the way for a fruitful transfer of knowledge between the realms of ultrafast dynamics, ultracold atoms and condensed matter physics.<p>Photo: AG Schmelcher</p>NAGR-fakmin-22678041-production2020-06-22T22:00:00ZPlaying billiards with excitons – a milestone towards quantum simulators<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/22679127/electron-exciton-19213b65735290f2e56930c99749310bf22e8b2f.png" />Semiconductors play an important role in electronics, optoelectronics or photovoltaics. But like other solids they are quantum many-particle systems. Therefore, calculating their properties with computers is very challenging and sometimes impossible. However, this task could be performed by a quantum system with comparable properties that is fully controllable from outside: a quantum simulator. In a collaboration of the Max Planck Institute of Quantum Optics in Munich with colleagues at Universität Hamburg and ETH Zürich, scientists were able to explain certain properties of an ultra-flat semiconductor. Researchers now have a new toolbox at their disposal to describe these two-dimensional semiconductors in theory, which is an important milestone in the development of corresponding quantum simulators.<p>Photo: AG Schmelcher</p>NAGR-fakmin-19743207-production2019-10-21T22:00:00ZUnderstanding electron scattering with ultra-long-range Rydberg molecules<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/19790621/figure-1-d2b643428b5bb71e2336d125d331220967e35a28.png" />Ultra-long-range Rydberg molecules consist of a highly excited Rydberg atom with a loosely bound electron and at least one atom in its ground state located within the orbit of the Rydberg electron. Due to the molecule's size and the large extent of the electronic orbit, they form with huge internuclear distances up to a thousand times larger than in usual molecules. Accordingly, they posses giant dipole moments and are specifically useful to study fundamental properties of atomic interactions.<p>Photo: AG Schmelcher</p>NAGR-fakmin-19498754-production2019-09-24T22:00:00ZQuench Dynamics and Orthogonality Catastrophe of Bose Polarons<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/19499778/quench-dynamics-9a0cbedefd3eba0e5bfbd35262730bda9b137f7b.jpg" />Mobile impurities immersed in a surrounding quantum many-body environment are dressed by the collective excitation of the latter forming quasiparticles such as polarons (see the left panel of Fig. 1). This dressing mechanism results to alterations of the fundamental impurity properties, such as their effective mass and induced interactions as well as dramatic changes during their nonequilibrium dynamics.<p>Photo: AG Schmelcher</p>NAGR-fakmin-19427390-production2019-09-18T22:00:00ZQuantum Network Transfer and Storage with Compact Localized States Induced by Local Symmetries<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/19441567/quantum-network-bb2082b5506f4ca7d90309a907a74ad9479ad8e9.png" />Quantum computers have the potential to drastically speed up several important computational tasks. The computational strength of quantum computers stems from the fact that they operate on quantum-mechanical states, so-called qubits.<p>Photo: AG Schmelcher</p>NAGR-fakmin-18381307-production2019-01-24T23:00:00ZTaming polar active matter with moving substrates: directed transport and counterpropagating macrobands<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/18381144/taming-polar-active-matter-58799fefd2957fd965592fca4513cb3f83bc8fae.jpg" />Active matter consists of self-propelled units generating an intrinsically nonequilibrium situation, allowing for the spontaneous emergence of structures such as clusters, swirls and microflock patterns.<p>Photo: AG Schmelcher</p>NAGR-fakmin-18381331-production2018-07-25T22:00:00ZEntanglement Induced Interactions in Binary Mixtures<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/18381155/entanglement-induced-interactions-c9405434623b416a74787b4b819bc8b43d244d42.jpg" />Induced interaction has become a major concept to understand and simplify the physics of systems with two types of particles, so-called binary mixtures. An important example is the formation of Cooper pairs in a crystalline solid.<p>Photo: AG Schmelcher</p>