08:30  08:45

Frank Jülicher (director of the MPIPKS) and scientific coordinators
Opening

08:45  09:45

Andrea Alberti
(Universität Bonn)
Atoms at their quantum speed limit
How fast can a quantum system evolve between two states? This question is not only important for its basic nature, but it also has farreaching implications on future quantum technologies. In this talk, I will report on an experimental study [1] testing two wellknown limits on the maximum evolution rate, named after their discoverers—Mandelstam–Tamm and Margolus–Levitin. Despite their fundamental character, only the Mandelstam–Tamm limit has been so far investigated in experiments and exclusively in effective twolevel systems. In our experiment, we follow the motion of an atom trapped in an optical lattice using fast matter wave interferometry. A geometric analysis of the matter wave evolution reveals striking difference between a twolevel and a multilevel system—excitations of a multilevel system do not saturate the speed limit but, unexpectedly, produce a small, universal deviation from it.
In the second part of my talk, I will address the related question of what is the fastest route — the quantum brachistochrone — to transport an atom between distant states. We demonstrate [2] coherent transport of an atomic wave packet over a distance of 15 times its size in the shortest possible time. Because of the large separation between the two sites, ours is a paradigmatic example of a quantum process where the MandelstamTamm and MargolusLevitin speed limits fail to capture the relevant time scale. In contrast, we show that quantum optimal control provides us with solutions to the quantum brachistochrone problem.
Our results, establishing quantum speed limits beyond the simple twolevel system, are important to understand the ultimate performance of quantum computing devices, quantum simulators, and related advanced quantum technologies such as atomtronics.
[1] G. Ness, M. R. Lam, W. Alt, D. Meschede, Y. Sagi, and A. Alberti, “Observing crossover between quantum speed limits,” Sci. Adv. 7, eabj9119 (2021)
[2] M. R. Lam, N. Peter, T. Groh, W. Alt, C. Robens, D. Meschede, A. Negretti, S. Montangero, T. Calarco, and A. Alberti, “Demonstration of Quantum Brachistochrones between Distant States of an Atom,” Phys. Rev. X 11, 011035 (2021)

09:45  10:30

Oliver Morsch
(INOCNR and Dipartimento di Fisica Pisa)
Engineered dissipation for Rydberg atomtronics
Rydberg atoms are widely used for quantum simulation applications. One particular kind of application is Rydberg atomtronics, in which arrays of Rydberg atoms are used to study transport phenomena. Such systems also offer the intriguing possibility of using engineered dissipation in order to study outofequilibrium dynamics of open manybody systems. In this talk I will review the different dissipative channels present in Rydberg atoms and how to either suppress or enhance them. In particular, I will present recent results on the control of blackbodyradiationinduced dissipation and engineered dissipation using a controllable decay channel.

10:30  11:00

coffee break

11:00  11:45

Luca Salasnich
(Padova University)
First and second sound in twodimensional bosonic and fermionic systems (virtual)
We theoretically investigate the sound propagation in twodimensional (2D) systems of ultracold fermionic and bosonic atoms. For superfluid fermions, we calculate the first and second sound velocities across the whole BCSBEC crossover. In the lowtemperature regime we reproduce the recent measurements of the first sound velocity with $^{6}$Li atoms [1], which, due to the decoupling of density and entropy fluctuations, is the sole mode excited by a density probe. Conversely, a heat perturbation excites only the second sound, which, being sensitive to the superfluid depletion, vanishes in the deep BCS regime, and jumps discontinuously to zero at the BerezinskiiKosterlitzThouless (BKT) superfluid transition. A mixing between the modes occurs only in the finitetemperature BEC regime, where our theory converges to the purely bosonic results [2]. In the case of weaklyinteracting bosons, to model the recent measurements of the sound velocities of $^{39}$K atoms in 2D obtained in the weaklyinteracting regime and around the BKT transition temperature [3], we derive a refined calculation of the superfluid density, finding a fair agreement with the experiment. Our calculations also suggest the hybridization of the first and second sound modes at low temperatures [4].
[1] M. Bohlen, L. Sobirey, N. Luick, H. Biss, T. Enss, T. Lompe, and H. Moritz,
Sound Propagation and QuantumLimited Damping in a TwoDimensional Fermi Gas,
Phys Rev. Lett. {\bf 24}, 240403 (2020).
[2] A. Tononi, A. Cappellaro, G. Bighin, L. Salasnich, Propagation of first and second sound in a twodimensional Fermi superfluid, Phys. Rev. A {\bf 103}, L061303 (2021).
[3] P. Christodoulou, M. Galka, N. Dogra, R. Lopes, J. Schmitt, and Z. Hadzibabic, Observation of first and second sound in a BKT superfluid, Nature {\bf 594}, 191 (2021).
[4] K. Furutani, A. Tononi, and L. Salasnich, Sound modes in collisional superfluid Bose gases, New J. Phys. {\bf 23}, 043043 (2021).

11:45  12:30

Peter Schlagheck
(University of Liège)
Manybody quantum interference in the dynamics of ultracold bosonic atoms
Interference in the propagation of matter waves is a generic quantum phenomenon, not describable with classical methods, that affects transport properties in various ways and generally gives rise to strong or weak localization phenomena. Here we discuss the generalization of interferencerelated phenomena to the manybody domain, considering ultracold bosonic atoms in optical lattices of finite extent as decribed by BoseHubbard models, whose classical counterpart is given by a discrete nonlinear Schrödinger equation. The time evolution resulting from the preparation of an initial coherent or Fock state in such a BoseHubbard system can give rise to an enhanced return probability to this initial quantum state, and thereby compromise quantum ergodicity, due to coherent backscattering [1], the presence of discrete symmetries [2], as well as manybody scars [3]. Quantum interference can also induce nearly full revivals of the initial state within twosite BoseHubbard systems [4]. All those interference effects can be evidenced using quasiclassical methods based on the Truncated Wigner approximation, whose comparison with exact numerical simulations allows one to unambiguously identify the impact of genuine quantum phenomena in propagation and transport processes of bosonic manybody systems.
[1] T. Engl et al., Phys. Rev. Lett. 112, 140403 (2014).
[2] P. Schlagheck et al., Phys. Rev. Lett. 123, 215302 (2019).
[3] Q. Hummel et al., in preparation
[4] P. Schlagheck et al., arXiv:2203.17130

12:30  14:00

lunch break

14:00  15:00

Artur Widera
(TU Kaiserslautern)
Localization and transport of spinpolarized Fermi gases in timecontrolled disorder
Disorder can profoundly modify the transport properties of quantum systems resulting in, e.g., localization of noninteracting systems. However, the mechanism underlying localization fails for a timedependent disorder, and transport is induced.
We experimentally study the transport processes of ultracold, spinpolarized fermionic Li clouds in optical speckle potentials. Here, the disorder is not only characterized by the correlation length, which determines the transport properties in the staticdisorder case, but also by a tunable correlation time. I will report on the transport properties of initially localized Fermi gases when the disorder correlation length reduces.

15:00  15:45

André Eckardt
(Technical University of Berlin)
Nonequilbrium states of drivendissipative quantum gases
A system of ultracold atoms can be brought in contact with a thermal bath by letting it interact weakly with a large cloud of another atomic species. We consider atoms in a timeperiodically driven optical lattice in contact with an interacting Bose condensate and microscopically model them using FloquetBornMarkov theory. The interplay of driving and dissipation will guide these systems into nonequilibrium steady states. Compared to the usual adiabatic state preparation, suffering from nonadiabatic excitation processes, this scenario can have two advantages; it is robust, since energy (and entropy) can be dumped into the bath, and it allows for the preparation of interesting states beyond the strict constraints of thermal equilibrium. I will present two examples in rather different regimes: (i) In a system of fermions loaded into the Floquettopological band structure of a hexagonal lattice created by highfrequency driving, the coupling to the environment allows to “cool” almost all particles into a single band so that a topological insulator giving rise to a quantized Hall response is prepared [1]. (ii) Subjecting a onedimensional bosonic system to a spatially local drive of intermediate frequency that resonantly excites (heats) the system, the interplay of driving and dissipation is found to give rise to the formation of a nonequilibrium Bose condensate in a subspace that approximately decouples from the drive [2]. Finally, if time permits, I will mention the (experimental and numerical) observation of a dynamical phase transition occurring at a critical time during the bathinduced relaxation dynamics of an open system [3].
[1] A. Schnell and A. Eckardt.: Stabilizing a Floquet topological insulator in a driven optical lattice by bath engineering (in preparation).
[2] A. Schnell, L.N. Wu, A. Widera, A. Eckardt.: Floquetheatinginduced nonequilibrium Bose condensation in an open optical lattice (preprint, arXiv:2204.07147).
[3] L.N. Wu, J. Nettersheim, J. Feß, A. Schnell, S. Burgardt, S. Hiebel, D. Adam, A.E., A. Widera, A. Eckardt: Dynamical phase transition in an open quantum system (in preparation).

15:45  16:10

Gabriel Wlazłowski
(Warsaw University of Technology)
Towards generalpurpose simulation platform for superfluid fermions across BCSBEC crossover
Numerical simulations are an important ingredient of modern research. In the field of BoseEinstein condensates, the Gross–Pitaevskii equation (GPE) is a workhorse that facilitates the interpretation of experimental data for various setups. The counterpart of GPE for superfluid fermions are meanfield Bogoliubovde Gennes (BdG) equations. Formally, their applicability is limited to weak couplings, while the experiments operate typically for strong couplings (around the unitary limit). The density functional theory (DFT) can be a remedy for this disparity. It is a versatile method describing with very good accuracy the static, dynamic, and thermodynamic properties of manybody Fermi systems in a unified framework, while keeping the numerical cost at the same level as the meanfield approach. I will present the latest developments of the DFT dedicated to ultracold atomic gases across BCSBEC crossover, together with its (opensource) numerical implementation. Selected applications of the method to various experimental setups will be presented: dissipative dynamics of atomic Josephson junction, simulations of 2D vortex collider setup, properties of spinimbalanced and rotating unitary Fermi gas, and even quantum turbulence. Finally, I will discuss opportunities offered by DFT method in the context of the modeling atomtronical devices based on ultracold Fermi atoms.
[1] WSLDA Toolkit, https://wslda.fizyka.pw.edu.pl
[2] A. Boulet, G. Wlazłowski, P. Magierski, Local energy density functional for superfluid Fermi gases from effective field theory, Phys. Rev. A, in press (2022)[arXiv:2201.07626]
[3] K. Hossain, K. Kobuszewski, M. M. Forbes, P. Magierski, K. Sekizawa, G. Wlazłowski, Rotating quantum turbulence in the unitary Fermi gas,
Phys. Rev. A 105, 013304 (2022)
[4] J. Kopyciński, W. R. Pudelko, G. Wlazłowski, Vortex lattice in spinimbalanced unitary Fermi gas, Phys. Rev. A 104, 053322 (2021)

16:10  16:30

coffee break


colloquium chair: Andrè Eckardt (Technical University of Berlin)

16:30  17:30

Giacomo Roati
(INOCNR and LENS)
qtua22 colloquium: Quantum transport and dissipation of superfluid Fermi gases in structured optical potentials
I will report on the realization of supercurrents in homogeneous, tunable fermionic rings. We gain exquisite, rapid control over quantized persistent currents in all regimes of the BCSBEC crossover through a universal phaseimprinting technique. Highfidelity readout of the superfluid circulation state is achieved by exploiting an interferometric protocol, which also yields local information about the superfluid phase around the ring. In the absence of externally introduced perturbations, we find the induced metastable supercurrents to be as longlived as the atomic sample. We trigger and inspect the supercurrent decay by inserting a single small obstacle within the ring. For circulations higher than a critical value, the quantized currents dissipate via the emission of vortices. Our results demonstrate fast and accurate control of quantized collective excitations in a macroscopic quantum system, and establish strongly interacting fermionic superfluids as excellent candidates for atomtronic applications.

17:30  18:00

Informal discussions

18:00  19:30

Welcome dinner
