Quantum Transport with ultracold atoms

How fast can a quantum system evolve between two states? This question is not only important for its basic nature, but it also has far-reaching implications on future quantum technologies. In this talk, I will report on an experimental study [1] testing two well-known 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 two-level 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 two-level and a multi-level system—excitations of a multi-level 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 Mandelstam-Tamm and Margolus-Levitin 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 two-level 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)

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 out-of-equilibrium dynamics of open many-body 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 blackbody-radiation-induced dissipation and engineered dissipation using a controllable decay channel.

We theoretically investigate the sound propagation in two-dimensional (2D) systems of ultracold fermionic and bosonic atoms. For superfluid fermions, we calculate the first and second sound velocities across the whole BCS-BEC crossover. In the low-temperature 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 Berezinskii-Kosterlitz-Thouless (BKT) superfluid transition. A mixing between the modes occurs only in the finite-temperature BEC regime, where our theory converges to the purely bosonic results [2]. In the case of weakly-interacting bosons, to model the recent measurements of the sound velocities of $^{39}$K atoms in 2D obtained in the weakly-interacting 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 Quantum-Limited Damping in a Two-Dimensional 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 two-dimensional 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).

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 interference-related phenomena to the many-body domain, considering ultracold bosonic atoms in optical lattices of finite extent as decribed by Bose-Hubbard 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 Bose-Hubbard 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 many-body scars [3]. Quantum interference can also induce nearly full revivals of the initial state within two-site Bose-Hubbard 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 many-body 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

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 time-dependent disorder, and transport is induced. We experimentally study the transport processes of ultracold, spin-polarized 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 static-disorder 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.

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 time-periodically driven optical lattice in contact with an interacting Bose condensate and microscopically model them using Floquet-Born-Markov theory. The interplay of driving and dissipation will guide these systems into non-equilibrium steady states. Compared to the usual adiabatic state preparation, suffering from non-adiabatic 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 Floquet-topological band structure of a hexagonal lattice created by high-frequency 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 one-dimensional 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 non-equilibrium 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 bath-induced 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.: Floquet-heating-induced non-equilibrium 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).

Numerical simulations are an important ingredient of modern research. In the field of Bose-Einstein 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 mean-field Bogoliubov-de 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 many-body Fermi systems in a unified framework, while keeping the numerical cost at the same level as the mean-field approach. I will present the latest developments of the DFT dedicated to ultracold atomic gases across BCS-BEC crossover, together with its (open-source) 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 spin-imbalanced 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] W-SLDA 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 spin-imbalanced unitary Fermi gas, Phys. Rev. A 104, 053322 (2021)

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 BCS-BEC crossover through a universal phase-imprinting technique. High-fidelity read-out 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 long-lived 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.

In our cold-atom experiments, we study the dynamics of strongly interacting impurity atoms embedded in a one-dimensional (1D) strongly interacting host gas. For accelerated impurity motion we have found some while ago the emergence of Bloch-type oscillations even though no imprinted lattice structure is present [1]. Such motion can be seen as repeated Bragg scattering off the host atoms’ two-point correlation function. The situation is more intricate for the case of initially constant (i.e. non-accelerated) impurity motion. An oscillatory motion of the impurity atoms has been proposed also in this case, most likely due to a beating between excitonic and polaronic excitations [2]. While our sensitivity to detect this “quantum flutter” effect is marginal, we find that the impurity wave packet upon interacting with the strongly interacting host gas is split into two distinct components, one that is stopped on the time-scale of the Fermi time, and another one that is transmitted as if it had not interacted. We are presently not able to interpret this striking observation. References [1] F. Meinert et al., Science 356, 945 (2017). [2] C. M. Mathy et al., Nature Physics 8, 881 (2012).

The fate of topological transport in the strongly correlated regime raises fundamental questions on the role of geometry in quantum many-body physics. Recently, different platforms have emerged as possible probes for many-body transport beyond the traditional Hall response; these include ultracold quantum gases, Rydberg atoms, and photonics. A paradigm of quantised transport is the topological Thouless pump, which represents the one-dimensional, dynamic analogue of the quantum Hall effect. A few experiments have explored the effects of interactions on Thouless pumping in two-body and optical mean-field systems, but the strongly correlated regime has so far remained out of reach. I this talk, I will present our recent experimental work on the breakdown of topological transport due to strong Hubbard interactions. The fermionic Thouless pump is facilitated by a dynamical optical superlattice. We observe the deviation from quantised pumping for repulsive Hubbard $U$, which we attribute to pinned atoms in a Mott insulator. On the attractive side another mechanism is reducing the pumping efficiency: a smaller energy gap for pair pumping makes the evolution less adiabatic. The dynamical superlattice operated at a single frequency establishes a novel platform for studying topology in the presence of strong correlations, including avenues to fractional transport.

In this talk, I will review how quantum fluctuations in mixtures of bosonic atoms can lead to the formation of self-bound droplets. Emphasis will be on persistent currents in ring-trapped binary mixtures as well as dipolar supersolids. For binary condensates with equal intra-component interactions but an unequal number of atoms in the two components, there is an excess part that cannot bind. A droplet then becomes amalgamated with a residual condensate. This results in particular rotational behavior that sheds new light on the coexistence of localization and superfluidity.

Quantum gases constitute a versatile testbed for exploring the behavior of quantum matter subjected to electric and magnetic fields. While most experiments consider classical gauge fields that act as a static background for the atoms, gauge fields appearing in nature are instead quantum dynamical entities that are influenced by the spatial configuration and motion of matter, and that fulfill local symmetry constraints. In this talk, I will discuss our recent realization of the chiral BF theory: a topological field theory for linear anyons that corresponds to a possible one-dimensional reduction of the Chern-Simons gauge theory effectively describing fractional quantum Hall systems. By using the local symmetry constraint of the theory, we encode the gauge field in terms of the matter field. The result is a system with chiral interactions, which we engineer in a potassium Bose-Einstein condensate by synthesizing optically dressed atomic states with a momentum-dependent scattering length. Theoretically, we show that this system realizes the chiral BF Hamiltonian at the quantum level. Experimentally, we observe the chirality of the interactions, the formation of chiral bright solitons – self-bound states of the matter field that only exist when propagating in one direction -- and exploit the local symmetry constraint of the theory to reveal the BF electric field.

We study the coupled dynamics of a two-species Bose-Einstein condensate with tunable interspecies interaction. With a degenerate mixture of 41K-87Rb in an optical trap we excite the dipole oscillations of both atomic species in the linear response regime. Varying the interspecies interaction from the weakly to the strongly attractive side, we measure the frequencies and the composition of the two dipole eigenmodes. We compare experimental results with numerical simulations performed by solving two coupled Gross-Pitaevskii equations, to obtain a detailed picture of the dipole excitations in asymmetric bosonic mixtures.

Ultracold atoms in optical lattices are powerful quantum simulators for strongly correlated systems. With synthetic gauge fields, induced by time-periodic driving, they can simulate interacting topological states of matter. I will address open challenges: Quantum gases are mesoscopic and have soft boundaries. We propose an interacting topological interface that allows to experimentally observe edge modes via quantum gas microscopy. We also show that the topology of an interacting Chern insulator is approximately encoded in the single-particle density matrix, which can be measured in time-of-flight experiments. Disorder usually impedes coherent transport. Together with gauge fields it can remarkably induce a topological phase with quantized transport. We show that strong correlations can even enhance this effect.

Manipulating the flow of cold atoms can realize novel types of devices for practical applications of quantum technologies. We study atomic interferometers with multiple terminals and a synthetic magnetic flux to control atomic transport. We demonstrate the setup with a Bose-Einstein condensate in a light-shaped optical potential [1]. Various modes of transport can be studied, from weakly perturbed to highly non-equilibrium regimes, as well as topological protected transport [2]. These interferometers integrates multiple practical functionalities into a single device. The Aharonov-Bohm effect can be used to direct the current into specific output ports to realize flexible non-reciprocal switches [1,3]. Further, one can measure the flux in-situ via a flux-induced transition of the reflections from negative Andreev-like to positive [1,4]. By changing the flux linearly in time, one can convert constant matter wave currents into alternating currents [1]. This effect can be used to realize an atomic frequency generator and to investigate fundamental problems of the Aharonov-Bohm effect. Our work establishes integrated cold atom devices for quantum sensing, enhancing quantum simulators and studying many-body quantum transport. [1] JWZ Lau, KS Gan, R Dumke, L Amico, LC Kwek, T Haug, arXiv:2205.01636 [2] T Haug, R Dumke, LC Kwek, L Amico, Communications Physics 2 (1), 1-9 [3] T Haug, H Heimonen, R Dumke, LC Kwek, L Amico, Physical Review A 100 (4), 041601 [4] T Haug, R Dumke, LC Kwek, L Amico, Quantum Science and Technology 4 (4), 045001

Quantum phase slips are a dual process of particle tunneling in coherent networks. Besides being of central interest to condensed matter physics, quantum phase slips are resources that are sought to be manipulated in quantum circuits. Here, we devise a specific matter-wave circuit enlightening quantum phase slips. Specifically, we investigate the quantum many-body dynamics of two side-by-side ring-shaped neutral bosonic systems coupled through a weak link. By imparting a suitable magnetic flux, persistent currents flow in each ring with given winding numbers. We demonstrate that coherent phase slips occur as winding number transfer among the two rings, with the populations in each ring remaining nearly constant. Such a phenomenon occurs as a result of a specific entanglement of circulating states, that, as such, cannot be captured by a mean-field treatment of the system. Our work can be relevant for the observation of quantum phase slips in cold-atom experiments and their manipulation in matter-wave circuits. To make contact with the field, we show that the phenomenon has clear signatures in the momentum distribution of the system providing the time-of-flight image of the condensate.

Bosonic systems guided in a ring-shaped atomtronic circuit present quantized values of the winding number. In this talk, I will present cases for which fractional values of winding number quantization occur. First I will consider attracting bosonic systems. Then, I will discuss fermionic systems with $N$ components that present SU($N$) symmetry. For repulsive interactions, a specific emerging phenomenon of attraction from repulsion appears, providing a quantization with similar properties to the attracting boson case. For attractive interactions, the quantization is determined by the number of components $N$.

We analytically study the localized running waves in the discrete Josephson transmission lines (JTL), constructed from Josephson junctions (JJ) and capacitors. The quasi-continuum approximation reduces calculation of the running wave properties to the problem of equilibrium of an elastic rod in the potential field. Making additional approximation, we reduce the problem to the motion of the fictitious Newtonian particle in the potential well. We show that there exist running waves in the form of supersonic kinks and solitons and calculate their velocities and profiles. We show that the nonstationary smooth waves which are small perturbations on the homogeneous non-zero background are described by Korteweg-de Vries equation, and those on zero background – by modified Korteweg-de Vries equation. We also study the effect of dissipation on the running waves in JTL and find that in the presence of the resistors, shunting the JJ and/or in series with the ground capacitors, the only possible stationary running waves are the shock waves, whose profiles are also found. Finally we study the nonlinear dispersion and modulation stability in the discrete JTL.

“Kicking” a quantum system by subjecting it to a pulsed time-dependent Hamiltonian can give rise to a rich array of transport behaviors, from localization to multifractality. I will describe two recent experiments on kicked quantum matter using ultracold neutral atoms. The first is an experimental observation of the “quantum boomerang effect,” a fundamental dynamical transport phenomenon unique to Anderson-localized matter which was only recently theoretically predicted and has not to our knowledge been previously observed. The second is an experimental exploration of an extended multifractal state of matter and anomalous re-entrant localization, in a kicked cold-atom quasicrystal. The results illuminate a variety of phenomena ranging from dynamics in localized systems to fractal wavefunctions to new techniques of quantum control.

We present a series of experiments designed to study Anderson localisation in two dimensions (2D). A Bose-Einstein condensate of Rb-87 atoms is confined to a red-detuned light sheet, yielding an effective 2D system: atoms can only move in a plane. A repulsive dumbbell-shaped potential, generated by a spatial light modulator is focused onto this plane. The atoms start in one reservoir and transit through a channel where point-like scatterers are positioned randomly to arrive at the second reservoir, where the incoming atomic current is measured. The amount of noise, quantified by the fill factor, and the channel size are methodically varied. Localisation landscape theory can be used to compute the localisation length and the fraction of delocalised atoms for all the configurations studied experimentally, as well as provide considerable qualitative insights. While a basic exponential fall off of the current with channel length is expected, the dependence on the channel width is not analytically known. In addition to localisation landscape theory, we compare our experimental results to time-dependent Gross-Pitaevskii simulations to fully capture the observed dynamics.

In quantum systems with a classical limit, advanced semiclassical methods provide the crucial link between classically chaotic dynamics and corresponding features at the quantum level. Recently, single-particle techniques dealing with ergodic wave interference in the usual semiclassical limit of small Planck constant, have begun to be transformed into the field theoretical domain of large-N particle systems thereby accounting for genuine many-body quantum interference. This semiclassical theory thereby lays the dynamical foundations for many-body qantum chaos. After introducing this approach I will discuss two complementary many-body interference phenomena (i) Scarring, naturally requiring a classical limit, is intriguing because it indicates deviations from ergodicity for a fully chaotic system that globally shows eigenstate thermalization. I will demonstrate such scarring for Bose-Hubbard systems. (ii) Vice versa I will show scrambling of quantum information in an integrable Bose-Hubbard system.

We propose a novel framework to characterize the thermalization of many-body dynamical systems close to integrable limits using the scaling properties of the full Lyapunov spectrum. We investigate macroscopic weakly nonintegrable lattice dynamics beyond the limits set by the KAM regime. We perform our analysis in two fundamentally distinct long-range and short-range integrable limits which stem from the type of nonintegrable perturbations - weak two-body interactions (nonlinearities) versus weak lattice coupling (hopping). Long-range limits result in a single parameter scaling of the Lyapunov spectrum, with the inverse largest Lyapunov exponent being the only diverging time control parameter and the rescaled spectrum approaching an analytical function. Short-range limits result in a dramatic slowing down of thermalization which manifests through the rescaled Lyapunov spectrum approaching a non-analytic function. An additional diverging length scale controls the exponential suppression of all Lyapunov exponents relative to the largest one.

We investigate a spin ensemble strongly coupled to light in a high-finesse cavity, in the presence of a controlled random field. The system is described by the disordered Tavis-Cummings model, showing a breakdown of the collective coupling and the disappearance of semi-localized dark states with increasing disorder. In the large cavity detuning regime, the system realizes a disordered Lipkin-Meshkov-Glick model, with a ferromagnetic ground state evolving towards paramagnetic for large disorder. The new level of control we demonstrate over light-matter coupling opens the perspective of programming cavity-induced long-range interactions in cold-atoms sytems.

Many robust physical phenomena in quantum physics are based on topological invariants, which are intriguing geometrical properties of quantum states. A prime example is the 2D quantum Hall effect with its quantized Hall conductance by the respective 2D topology as shown by Thouless. Interactions can dramatically change the picture and cause fractional quantisation that again can be traced to a topological origin. More intriguingly, one can envision higher dimensional topology in more than three dimensions. This was recently beautifully illustrated experimentally in the form of the 4D quantum Hall effect. In this work, we propose superconducting nanosystems as a promising platform for realising higher-dimensional topological phenomena and fractional charge transport. We do this by proposing three systems that make use of the recent observation that multi-terminal superconducting structure with N controlled phase differences constitute an N-dimensional synthetic space and enable a realisation of higher-dimensional topologies. We show the following cases - An Andreev dot system with a non-trivial second Chern number and a non-Abelian Berry phase [1]. - A superconducting realisation of a tensor monopole [2]. - A Josephson junction circuit working as fractional Thouless pump for Cooper pairs. All systems can be realised with present days technology in superconducting nanoelectronics and pave the way to observe higher dimensional topology in superconducting system. [1] H. Weisbrich, R. L. Klees, G. Rastelli, and W. Belzig, PRX Quantum 2, (2021). [2] H. Weisbrich, M. Bestler, and W. Belzig, Quantum 5, 601 (2021). [3] H. Weisbrich, R. Klees, O. Zilberberg, W. Belzig (unpublished)

We investigate the static properties and the non equilibrium dynamics of dipolar Bose-Einstein condensates of Dysprosium atoms subjected a rotating external magnetic field. The application of the latter modifies the magnitude and sign of the dipole-dipole potential due to its anisotropic nature. Utilizing the three-dimensional extended Gross-Pitaevskii framework, we account for quantum fluctuations and map out the underlying phase diagram with respect to atom number and strength of contact interaction for various field orientations. A decreasing contact interaction leads to a transition from a superfluid to a supersolid and then to arrays of dipolar droplets characterized by a non-vanishing global phase coherence. These phases occur both in quasi-1D and quasi-2D regimes while the anisotropy of the magnetic controls their regions of existence. Following interaction quenches across the aforementioned phase transitions, we find the dynamical nucleation of supersolids or droplet lattices. The inclusion of three-body losses leads to the self-evaporation of these structures in the long-time dynamics, while tuning the dipolar anisotropy leads to enhancement of the droplet lifetimes.

TBC

Overcoming the detrimental effect of disorder at the nanoscale is very hard since disorder induces localisation and an exponential suppression of transport efficiency. Here we unveil novel and robust quantum transport regimes achievable in nano-systems by exploiting long-range hopping. We demonstrate that in a 1D disordered nanostructure in the presence of long-range hopping, transport efficiency, after decreasing exponentially with disorder at first, is then enhanced by disorder [disorder-enhanced transport (DET) regime] until, counterintuitively, it reaches a disorder-independent transport (DIT) regime, persisting over several orders of disorder magnitude in realistic systems. To enlighten the relevance of our results, we demonstrate that an ensemble of emitters in a cavity can be described by an effective long-range Hamiltonian. The specific case of a disordered molecular wire placed in an optical cavity is discussed, showing that the DIT and DET regimes can be reached with state-of-the-art experimental setups.

The recently developed ideas for nanoscale potentials, where the potential barriers are formed by kinetic energy changes over very short distances, allow one to create strongly coupled lattice systems [1]. Since in such systems the barriers are almost point-like, they can be very well described by generalised Kronig-Penney type models, for which analytical single particle solutions exist [2]. Here we will show that these systems can allow for studies of the static and dynamics of topological states in strongly correlated few-body systems and, in particular, discuss these system’s non-classical properties as a function of topology and interparticle interaction. Since the generalised Kronig-Penney model we use possesses a large number of degrees of freedom that can also be changed time-dependently, we will also present results related to quenches between the topologically trivial and non-trivial regime. [1] M. Lącki, M. A. Baranov, H. Pichler, and P. Zoller, Nanoscale “dark state” optical potentials for cold atoms, Phys. Rev. Lett. 117, 233001 (2016). [2] I. Reshodko, A. Benseny, J. Romhányi, and T. Busch, Topological states in the Kronig–Penney model with arbitrary scattering potentials, New J. Phys. 21, 013010 (2019).

I consider the situation where an atom chirally coupled to a running wave cavity can produce effective chiral couplings for non-ideal emitters (room temperature quantum dots, etc) coupled to the same cavity. Abstractly, the idea is to engineer a situation where chirality (directional coupling available to spin-polarized emitters) can be transferred as a resource from ideal emitters to non-ideal emitters which cannot be chirally coupled by themselves. The purpose of this is ultimately control photon transport in networks, where directionality of transport is required to ensure deterministic (i.e. non-lossy) transfer of quantum information.

Topological phases of matter exhibit remarkable electronic properties. A prominent example is the robust quantization of the Hall conductivity in quantum Hall insulators. A widespread technique for generating topological band structures in synthetic quantum systems, such as ultracold atoms in optical lattices, is Floquet engineering. This method relies on the periodic modulation of the system’s parameters to emulate the properties of a non-trivial static system. The rich properties of Floquet systems, however, transcend those of their static counterparts. The associated quasienergy spectrum can exhibit a non-trivial winding number, which leads to the appearance of anomalous chiral edge modes even in situations where the bulk bands have zero Chern numbers. Here, I report on the realization of such an anomalous Floquet topological system in a periodically-modulated hexagonal optical lattice and show how wavepacket dynamics can be used to study the bulk [1] and edge topological properties of the system. The novel properties of topological Floquet phases open the door to exciting new many-body topological phases without any static analog. [1] K. Wintersperger, C. Braun, F. N. Ünal, A. Eckardt, M. Di Liberto, N. Goldman, I. Bloch, M. Aidelsburger, Realization of anomalous Floquet topological systems with ultracold atoms, Nature Physics 16, 1058-1063 (2020).

We describe a condensate of ultra cold bosonic atoms in a linear optical cavity illuminated by a two-pump configuration where each pump is making different angles with the direction of the cavity axis. We show such configuration allows a smooth transition from a one-dimensional quantum optical lattice configuration to a two-dimensional quantum optical lattice configuration induced by the cavity-atom interaction. Using a Holstein-Primakoff transformation, we find out the atomic density profile of such self-organised ground state in the super-radiant phase as a function of the angular orientations of the pump in such dynamical quantum optical lattice and show how the corresponding results can also be understood in terms of an Extended Bose-Hubbard model in such quantum optical lattice potential. We shall also describe low energy collective excitations in such system. Ref: arXiv:2206.04518 [cond-mat.quant-gas] by Poornima Shakya, Amulya Ratnakar and Sankalpa Ghosh

Rings of interacting condensates present a very interesting problem for several reasons. First of all, such rings should exhibit chaotic dynamics, simply because these are many-dimensional systems. These dynamics, however, strongly depend on initial conditions and whether the system is in an eigenstate. We show that chaos onset in large rings has a dramatic delay, contrary to expectations from linear stability analysis. Secondly, one can also study topological phases in such rings. We show that such a phase differs drastically from a trivial phase and this shows in chaotic dynamics. We study these phenomena in dependence of interaction, system size and topological parameters.

In this talk, we will report on the efficient design of quantum optimal-control protocols to manipulate the motional states of an atomic Bose-Einstein condensate (BEC) in a one-dimensional optical lattice. Our protocols operate on the momentum comb associated with the lattice, and allow us to reach a wide variety of targets by varying a single control parameter. The perspectives of this work include the generation of arbitrary unitary transformations in a qudit and the implementation of standard quantum gates and algorithms, the design of robust or selective pulses for quantum sensing and metrology issues, to the application of control methods to transport problems in quantum simulation. Beyond the control of the one-body physics detailed above, we report on the engineering of spontaneous four-wave mixing in a Bose-Einstein condensate subjected to an appropriate time modulation of the confining optical lattice. Our technique generates a state with a tunable supersolid order breaking the lattice symmetry. Our work demonstrates the use of Floquet engineering for the production of exotic states of matter, and paves the way for further studies of dissipation in the resulting phase, and of similar phenomena in more complex geometries.

Quantum many-body scarring presents a novel mechanism that delays the onset of thermal equilibrium in non-integrable models. Recent experiments have demonstrated that periodic driving can lead to a significant enhancement of quantum many-body scarring [1]. Nevertheless, the mechanisms behind driving-induced scar enhancement remain poorly understood. Here we report a detailed study of the effect of periodic driving on the PXP model describing Rydberg atoms in the presence of a strong Rydberg blockade - the canonical static model of quantum many-body scarring. We show that periodic modulation of the chemical potential gives rise to a rich phase diagram, with at least two distinct types of scarring regimes that we distinguish by examining their Floquet spectra [2]. Using a large-scale Bose-Hubbard quantum simulator, our theoretical model was also experimentally realized and studied by emulating the PXP model with the tilted optical lattice [3]. [1] D. Bluvstein et al., Science 371, 1355 (2021). [2] A. Hudomal et al., arXiv:2204.13718 (2022). [3] G. Su et al., arXiv:2201.00821 (2022).

A Thouless pump transports an integer amount of charge when pumping adiabatically around a singularity. We study the splitting of such a critical point into two separate critical points by adding a Hubbard interaction. Furthermore, we consider extensions to a spinful Rice-Mele model, namely a staggered magnetic field or an Ising-type spin coupling, further reducing the spin symmetry. The resulting models additionally allow for the transport of a single charge in a two-component system of spinful fermions, whereas in the absence of interactions, zero or two charges are pumped. In the SU(2)-symmetric case, the ionic Hubbard model is visited once along pump cycles that enclose a single singularity. Adding a staggered magnetic field additionally transports an integer amount of spin while the Ising term realizes a pure charge pump. We employ real-time simulations in finite and infinite systems to calculate the adiabatic charge and spin transport, complemented by the analysis of gaps and the many-body polarization to confirm the adiabatic nature of the pump. The resulting charge pumps are expected to be measurable in finite-pumping speed experiments in ultra-cold atomic gases, for which the SU(2) invariant version is the most promising path. We discuss the implications of our results for a related quantum-gas experiment by Walter et al. [arXiv:2204.06561].

Heating to high-lying states strongly limits the experimental observation of driving induced non-equilibrium phenomena, particularly when the drive has a broad spectrum. Here we show that, for entire families of structured random drives known as random multipolar drives[1], particle excitation to higher bands can be well controlled even away from a high-frequency driving regime[2]. This opens a window for observing drive-induced phenomena in a long-lived prethermal regime in the lowest band[2,3]. [1] Phys. Rev. Lett. 126, 040601 (2021) [2] arxiv: 2201.08130 (2022) [3] Phys. Rev. B 105, 245119 (2022)