Monday, 10 May 2021

Dipolar interactions are fundamentally different from the usual van der Waals forces in real gases. Besides the anisotropy the dipolar interaction is nonlocal and as such allows for self organized structure formation [1]. Similar to the Rosensweig instability in classical magnetic ferrofluids self- organized structure formation was expected. However on the meanfield level such a transition is instable due to the diluteness of the gaseous sample. In contrast to these predictions in 2015 we could observe the formation of a stable droplet crystal and found that this unexpected stability is due to beyond mean-field quantum corrections of the Lee-Huang-Yang type. When arranged in a 1D array also phase coherence between the droplets was observed, which was first evidence for a supersolid state of matter. Upon crossing the transition to the dipolar supersolid a Goldstone mode appears, which we have observed. The existence of this mode proofs the superfluid stiffness or the so-called phase rigidity of this new state of matter. Recently we have also measured the static structure factor across the transition which allows to show that the characteristic fluctuations correspond to elementary excitations such as the roton modes, and that the supersolid state supports both superfluid as well as crystal phonons. A recent review on the discovery of quantum droplets and dipolar supersolids can be found in ref. [2].

References

[1] review: T. Lahaye, et al., Rep. Prog. Phys. 72, 126401 (2009)

[2] review: F. Böttcher et al. arXiv:2007.06391

I will give an overview over our efforts to control and study ultracold few-body quantum systems. After a general introduction to ultracold scattering and Feshbach resonances, I will turn to two applications of Feshbach resonances that have been at the focus of our studies over many years: The observation of Efimov states and the formation of ultracold samples of ground-states molecules. As I will detail, our present efforts are aimed at creating dense samples of RbCs and KCs molecules with the goal to form dipolar Bose-Einstein condensates and degenerate Fermi gases of molecules.

Trapped atomic ions can represent elementary quantum systems that are well isolated from the environment. They can be brought nearly to rest by laser cooling and both their internal electronic states and external motion can be coupled to and manipulated by light fields. This makes them ideally suited for studies in quantum optics, quantum dynamics and quantum information processing. This lecture covers the physics of confinement in ion traps, the coupling of ions to laser fields, laser cooling of single ions and ion crystals, sympathetic cooling between different ion species and near ground-state transport, separation and recombination of ions.

List of reviews:

D. J. Wineland, C. Monroe, W. M. Itano, D. Leibfried, B. E. King, and D. M. Meekhof, J. Res. Nat. Inst. Stand. Tech. 103, 259 (1998)

M. Sasura and V. Buzek, J. Mod. Opt. 49, 1593 (2002)

D. Leibfried, R. Blatt, C. Monroe, and D. Wineland, Rev. Mod. Phys. 75, 281 (2003)

H. Häffner, C. F. Roos, and R. Blatt, Physics Reports 469, 155 (2008)

D. Kielpinski, Front. Phys. China 3, 365 (2008)

D.J. Wineland, and D. Leibfried, Laser Phys. Lett. 8, No. 3, 175–188 (2011)

C. D. Bruzewicz, J. Chiaverini, R. McConnell, and J. M. Sage, Appl. Phys. Rev. 6, 021314 (2019).

Tuesday, 11 May 2021

The recent progress in the preparation of neutral molecules and ions at temperatures close to the absolute zero point has paved the way for a range of new research directions at the interface between chemistry and physics. Ensembles of cold, spatially localized ions in traps, often referred to as Coulomb crystals [1,2,3], are particularly attractive systems in this context in which it is possible to observe, manipulate and control single isolated particles under precisely controlled conditions.

In the lecture, we will give an overview over experimental methods used in this field and discuss a range of applications including molecular-ion quantum technologies, precision spectroscopic measurements, cold chemistry and collisions and ion-atom hybrid systems.

[1] S. Willitsch, Int. Rev. Phys. Chem. 31, 175 (2012)

[2] B. R. Heazlewood and T. P. Softley, Annu. Rev. Phys. Chem. 66, 475 (2015)

[3] S. Willitsch, Adv. Chem. Phys. 162 (2017), 307

I will give an overview over our efforts to control and study ultracold few-body quantum systems. After a general introduction to ultracold scattering and Feshbach resonances, I will turn to two applications of Feshbach resonances that have been at the focus of our studies over many years: The observation of Efimov states and the formation of ultracold samples of ground-states molecules. As I will detail, our present efforts are aimed at creating dense samples of RbCs and KCs molecules with the goal to form dipolar Bose-Einstein condensates and degenerate Fermi gases of molecules.

Trapped atomic ions can represent elementary quantum systems that are well isolated from the environment. They can be brought nearly to rest by laser cooling and both their internal electronic states and external motion can be coupled to and manipulated by light fields. This makes them ideally suited for studies in quantum optics, quantum dynamics and quantum information processing. This lecture covers the physics of confinement in ion traps, the coupling of ions to laser fields, laser cooling of single ions and ion crystals, sympathetic cooling between different ion species and near ground-state transport, separation and recombination of ions.

List of reviews:

D. J. Wineland, C. Monroe, W. M. Itano, D. Leibfried, B. E. King, and D. M. Meekhof, J. Res. Nat. Inst. Stand. Tech. 103, 259 (1998)

M. Sasura and V. Buzek, J. Mod. Opt. 49, 1593 (2002)

D. Leibfried, R. Blatt, C. Monroe, and D. Wineland, Rev. Mod. Phys. 75, 281 (2003)

H. Häffner, C. F. Roos, and R. Blatt, Physics Reports 469, 155 (2008)

D. Kielpinski, Front. Phys. China 3, 365 (2008)

D.J. Wineland, and D. Leibfried, Laser Phys. Lett. 8, No. 3, 175–188 (2011)

C. D. Bruzewicz, J. Chiaverini, R. McConnell, and J. M. Sage, Appl. Phys. Rev. 6, 021314 (2019).

Wednesday, 12 May 2021

We give an introduction to Rydberg atoms and their mutual interaction. This strong interaction is the basis for the Rydberg blockade that enables quantum gate operations and can even be observed in thermal vapour cells. We report on the realization of a single photon source based on the Rydberg blockade in a microscopic vapour cell. The Rydberg blockade can also be used to prepare isolated single ions in a quantum gas. We report on the study of the transport of a single ion in a quantum gas. We will then discuss the interaction of Rydberg atoms with a neutral background gas. This lead to the prediction and the observation of ultra-long range molecules. We will summarize some of the key features of those.

Trapped ions are one of the leading platforms for realizing quantum processors. Their quantum state can be precisely manipulated by laser light, and their internal electronic state can be coupled to optical photons either via spontaneous emission in free space [1] or via their interaction to an optical cavity. Quantum interfaces between trapped ions and optical photons have been realised in this way forming the building blocks of future quantum networks.

In this lecture, I will introduce different approaches for coupling trapped ions to optical photons and talk about applications in quantum networks [2] . I will discuss technology developed for generating atom-photon entanglement [3] , and for linking such systems to telecommunication channels [4] . I will summarize methods for entangling distant ion qubits via photons [5] and in this way couple separate quantum registers via photonic links.

[1] L. Slodička, G. Hétet, M. Hennrich, and R. Blatt, Free Space Interference Experiments with Single Photons and Single Ions, in Engineering the Atom-Photon Interaction: Controlling Fundamental Processes with Photons, Atoms and Solids, edited by M. W. M. Ana Predojevic (Springer International Publishing, 2015).

[2] L.-M. Duan and C. Monroe, Colloquium: Quantum Networks with Trapped Ions, Reviews of Modern Physics 82, 1209 (2010).

[3] A. Stute, B. Casabone, P. Schindler, T. Monz, P. O. Schmidt, B. Brandstätter, T. E. Northup, and R. Blatt, Tunable Ion–Photon Entanglement in an Optical Cavity, Nature 485, 7399 (2012).

[4] V. Krutyanskiy, M. Meraner, J. Schupp, V. Krcmarsky, H. Hainzer, and B. P. Lanyon, Light-Matter Entanglement over 50 Km of Optical Fibre, Npj Quantum Information 5, 1 (2019).

[5] L. J. Stephenson, D. P. Nadlinger, B. C. Nichol, S. An, P. Drmota, T. G. Ballance, K. Thirumalai, J. F. Goodwin, D. M. Lucas, and C. J. Ballance, High-Rate, High-Fidelity Entanglement of Qubits Across an Elementary Quantum Network, Physical Review Letters 124, 1 (2020).

Thursday, 13 May 2021

The recent progress in the preparation of neutral molecules and ions at temperatures close to the absolute zero point has paved the way for a range of new research directions at the interface between chemistry and physics. Ensembles of cold, spatially localized ions in traps, often referred to as Coulomb crystals [1,2,3], are particularly attractive systems in this context in which it is possible to observe, manipulate and control single isolated particles under precisely controlled conditions.

In the lecture, we will give an overview over experimental methods used in this field and discuss a range of applications including molecular-ion quantum technologies, precision spectroscopic measurements, cold chemistry and collisions and ion-atom hybrid systems.

[1] S. Willitsch, Int. Rev. Phys. Chem. 31, 175 (2012)

[2] B. R. Heazlewood and T. P. Softley, Annu. Rev. Phys. Chem. 66, 475 (2015)

[3] S. Willitsch, Adv. Chem. Phys. 162 (2017), 307

Trapped Rydberg ions are a novel approach for quantum information processing [1] . This idea joins the advanced quantum control of trapped ions with the strong dipolar interaction between Rydberg atoms. For trapped ions, this method promises to speed up entangling interactions and to enable such operations in larger ion crystals.

In this lecture, I will introduce the novel experimental platform of trapped Rydberg ions [2] . I will introduce the specific physics involved when exciting ions into Rydberg states, the effects on the trapping potential due to the strong polarizability of Rydberg ions, and the controllable strong interaction between ion and motion. Moreover, I will summarize methods and results in speeding up trapped ion entanglement operations via state-dependent forces on Rydberg ions and via strong dipolar Rydberg interaction [3] .

[1] M. Müller, L. Liang, I. Lesanovsky, and P. Zoller, Trapped Rydberg Ions: From Spin Chains to Fast Quantum Gates, New J. Phys. 10, 093009 (2008).

[2] A. Mokhberi, M. Hennrich, and F. Schmidt-Kaler, Trapped Rydberg Ions: A New Platform for Quantum Information Processing, in Advances In Atomic, Molecular, and Optical Physics, Vol. 69 (Elsevier, 2020), pp. 233–306.

[3] C. Zhang, F. Pokorny, W. Li, G. Higgins, A. Pöschl, I. Lesanovsky, and M. Hennrich, Submicrosecond Entangling Gate between Trapped Ions via Rydberg Interaction, Nature 580, 345 (2020).

Friday, 14 May 2021

Coupling a quantum gas to an optical cavity creates a many-body system that sheds light on fundamental concepts of physics. Uniquely, long-range interactions can be induced by irradiating atoms with a coherent light field. This leads to phenomena such as self-organization, supersolidity, and the Hepp-Lieb phase transition in the Dicke model. Most resent research works explore spin-full ensembles coupled to cavities and study driven dissipative many-body phases. This lecture will introduce the basic toolset to experimentally and theoretically discover quantum gases in cavities.

**Literature**

- Serge Haroche and Jean-Michel Raimond, Exploring the Quantum: Atoms, Cavities, and Photons (Oxford Graduate Texts, 2006)
- H. Ritsch, P. Domokos, F. Brennecke, and T. Esslinger: Cold atoms in cavity-generated dynamical optical potentials , Reviews of Modern Physics 85, 553-601 (2013). arXiv:1210.0013
- K. Baumann, C. Guerlin, F. Brennecke, and T. Esslinger: Dicke quantum phase transition with a superfluid gas in an optical cavity Nature 464, 1301-1306 (2010). arXiv:0912.3261
- J. Léonard, A. Morales, P. Zupancic, T. Esslinger, and T. Donner: Supersolid formation in a quantum gas breaking continuous translational symmetry Nature 543, 87-90 (2017). arXiv:1609.09053
- N. Dogra, M. Landini, K. Kroeger, L. Hruby, T. Donner and T. Esslinger: Dissipation-induced structural instability and chiral dynamics in a quantum gas Science 366, 1496 (2019). arXiv: 1901.05974

I will present an extended picture of recent developments in the control of motional states of trapped ions. The flexibility of ion-motion coupling realised through control of both the laser-ion interaction and of trapping potentials has led to a wide range of quantum states of oscillator systems, which in this case is the mechanical motion of the ion. These states give rise to opportunities for quantum enhanced sensing, quantum simulation and quantum computation. I will review the core physics behind this, before giving an overview of the current state of the art and challenges.

Fermions are the building blocks of matter, from nuclei to neutron stars. Understanding strongly interacting fermions is one of the great challenges of many-body physics, relevant for high-temperature superconductors and the quark-gluon plasma of the early universe. Ultracold Fermi gases of atoms have emerged as a paradigmatic form of fermionic matter, where interactions can be made as strong as quantum mechanics allows. The systems allow to investigate the smooth crossover from a Bose-Einstein Condensate of Molecules to a Bardeen-Cooper-Schrieffer (BCS) state of long-range Cooper pairs. For resonant interactions – in the middle of this “BEC-BCS Crossover”, the superfluid critical temperature is 17% of the Fermi temperature. Scaled to the density of electrons, superfluidity would occur far above room temperature. Transport properties of these systems are often the most difficult to understand. I will present recent measurements of density and heat transport in strongly interacting Fermi gases, displaying universal diffusivities given just by Planck’s constant and the particle mass.

Monday, 17 May 2021

Coupling a quantum gas to an optical cavity creates a many-body system that sheds light on fundamental concepts of physics. Uniquely, long-range interactions can be induced by irradiating atoms with a coherent light field. This leads to phenomena such as self-organization, supersolidity, and the Hepp-Lieb phase transition in the Dicke model. Most resent research works explore spin-full ensembles coupled to cavities and study driven dissipative many-body phases. This lecture will introduce the basic toolset to experimentally and theoretically discover quantum gases in cavities.

**Literature**

- Serge Haroche and Jean-Michel Raimond, Exploring the Quantum: Atoms, Cavities, and Photons (Oxford Graduate Texts, 2006)
- H. Ritsch, P. Domokos, F. Brennecke, and T. Esslinger: Cold atoms in cavity-generated dynamical optical potentials , Reviews of Modern Physics 85, 553-601 (2013). arXiv:1210.0013
- K. Baumann, C. Guerlin, F. Brennecke, and T. Esslinger: Dicke quantum phase transition with a superfluid gas in an optical cavity Nature 464, 1301-1306 (2010). arXiv:0912.3261
- J. Léonard, A. Morales, P. Zupancic, T. Esslinger, and T. Donner: Supersolid formation in a quantum gas breaking continuous translational symmetry Nature 543, 87-90 (2017). arXiv:1609.09053
- N. Dogra, M. Landini, K. Kroeger, L. Hruby, T. Donner and T. Esslinger: Dissipation-induced structural instability and chiral dynamics in a quantum gas Science 366, 1496 (2019). arXiv: 1901.05974

Laser-cooled trapped ions constitute a quantum system that can be prepared, controlled and measured by suitably tailored pulses of laser light. In my talk, I will discuss the techniques that underpin the approach of using trapped ions as an artificial quantum system for the study of quantum many-body physics and discuss how to characterize the complex quantum states that result from subjecting a system of trapped ions to entangling interactions.

Strongly interacting Fermi gases pose a difficult challenge for many-body theory. A prime example is the Fermi-Hubbard model, of fermions hopping and interacting on a lattice, which may hold the key to our understanding of high-temperature superconductivity. Quantum simulators based on fermions trapped in optical lattices and imaged with single-atom resolution enable precision measurements of spin and charge correlations as well as of transport properties. I will discuss such measurements.

Bridging quantum simulation and quantum computation, it is intriguing to ask how the Pauli principle can be used to implement robust qubits in a digital quantum computer.

I will present a robust quantum register composed of hundreds of fermionic atom pairs trapped in an optical lattice. With each fermion pair forming a spin-singlet, the qubit is realized as a set of near-degenerate, symmetry-protected two-particle wavefunctions describing common and relative motion. We observe quantum coherence beyond ten seconds. Universal control is provided by modulating interactions between the atoms. The methods open the door towards fully programmable quantum simulation and digital quantum computation based on fermions.

Tuesday, 18 May 2021

More than 30 years ago, Richard Feynman outlined his vision of a quantum simulator for carrying out complex calculations on physical problems. Today, his dream has become a reality in laboratories around the world. Ultracold atoms trapped in optical lattices provide a particular intriguing setting for realising such quantum simulators with the possibility to control and detect the systems down to the level of single atoms on single lattice sites. In my talk, I will discuss select applications for such neutral atom quantum simulators to probe quantum phases of strongly interacting electronic systems, including hidden magnetic order, topological phases as well as non-equilibrium dynamics that provide new paradigms for statistical physics. I will discuss the status of the field and give an outlook on future scalability of the systems.

Laser-cooled trapped ions constitute a quantum system that can be prepared, controlled and measured by suitably tailored pulses of laser light. In my talk, I will discuss the techniques that underpin the approach of using trapped ions as an artificial quantum system for the study of quantum many-body physics and discuss how to characterize the complex quantum states that result from subjecting a system of trapped ions to entangling interactions.

Recent years have seen a remarkable development in our ability to manipulate individual atoms and individual photons, including the ability to design strong and controlled interactions. The interaction range between highly excited atomic states (Rydberg states) can substantially exceed an optical wavelength, which allows one to trap and manipulate individual atoms optically, yet induce controlled interactions on demand to implement quantum gates, create highly entangled states, or study quantum phase transitions. Alternatively, the Rydberg interaction can be used to create effective strong interactions between individual photons traveling in an atomic medium. I will discuss recent results on the experimental realization of strong repulsive interactions between individual photons, the creation of maximally entangled GHZ states with up to 20 atoms, implementations of quantum gates in arrays of individually trapped atoms, and quantum simulations of antiferromagnetic quantum phase transitions in different lattice geometries.

Wednesday, 19 May 2021

More than 30 years ago, Richard Feynman outlined his vision of a quantum simulator for carrying out complex calculations on physical problems. Today, his dream has become a reality in laboratories around the world. Ultracold atoms trapped in optical lattices provide a particular intriguing setting for realising such quantum simulators with the possibility to control and detect the systems down to the level of single atoms on single lattice sites. In my talk, I will discuss select applications for such neutral atom quantum simulators to probe quantum phases of strongly interacting electronic systems, including hidden magnetic order, topological phases as well as non-equilibrium dynamics that provide new paradigms for statistical physics. I will discuss the status of the field and give an outlook on future scalability of the systems.

The primary tool for quantum information studies with trapped ions is the radio-frequency Paul trap, which uses a combination of static and dynamic electric fields to trap the charged atoms. Penning traps, which substitute the dynamic fields with a large static magnetic field, provide a different trapping toolbox which offers different opportunities. In this talk, I will describe the basic physics of the Penning trap, and then discuss how large Penning traps have been used to perform precision measurements as well as to confine 2-dimensional crystals which have been a fruitful area for quantum simulation and sensing. I will then how micro fabricated Penning traps might offer opportunities for scaling up quantum computing, removing some of the major challenges which have been encountered using the radio-frequency approach.

Recent years have seen a remarkable development in our ability to manipulate individual atoms and individual photons, including the ability to design strong and controlled interactions. The interaction range between highly excited atomic states (Rydberg states) can substantially exceed an optical wavelength, which allows one to trap and manipulate individual atoms optically, yet induce controlled interactions on demand to implement quantum gates, create highly entangled states, or study quantum phase transitions. Alternatively, the Rydberg interaction can be used to create effective strong interactions between individual photons traveling in an atomic medium. I will discuss recent results on the experimental realization of strong repulsive interactions between individual photons, the creation of maximally entangled GHZ states with up to 20 atoms, implementations of quantum gates in arrays of individually trapped atoms, and quantum simulations of antiferromagnetic quantum phase transitions in different lattice geometries.

Thursday, 20 May 2021

Mixed-species fermion systems with tunable interactions offer a rich playground for quantum many-body physics in the strongly interacting regime. In a first experiment, we study a strongly imbalanced mixture of bosonic K-41 impurities immersed in a Fermi sea of 6-Li atoms. We investigate the spectrum of polarons in the vicinity of a Feshbach resonance, and explore different density regimes. When a partial BEC is formed in the center of the trap, a situation is realized where Fermi polarons coexist with Bose polarons. In our second experiment, motivated by prospects to realize novel superfluids in fermion systems with mass imbalance, we explore a new Fermi-Fermi mixture of Dy-161 and K-40. For interaction tuning we have identified a broad Feshbach resonance, at the center of which the system is found to be collisionally stable. As a first signature of strong interactions, we have studied hydrodynamic effects in the expansion of the resonant mixture.

We consider a model describing Bose-Josephson junction (BJJ) coupled to a single bosonic mode exhibiting quantum phase transition (QPT). Onset of chaos above QPT is observed from semiclassical dynamics as well from spectral statistics. We identify the imprint of unstable "\Pi-oscillation" as quantum scar, which leads to the deviation from ergodicity and quantify the degree of scarring. Persistence of phase coherence in non-equilibrium dynamics is an observable signature of scarring.

Rydberg Atoms in highly excited electronic states with n=30-120 can be excited within Bose-Einstein condensates (BECs), and while lifetimes are shorter than in vacuum [1,2], they live long enough to interestingly interact with the BEC [3]. We theoretically study this interaction in the mean-field limit and beyond.

For multiple Rydberg atoms in a single electronic state, we show that the phase coherence of the condensate allows the tracking of mobile Rydberg impurities akin to bubble chambers in particle physics [4]. For a single Rydberg atom with multiple electronic states, we provide spectral densities of the BEC as a decohering environment [5], and show that the BEC can image a signature of the entangling evolution that causes Rydberg qu-bit decoherence [6].

[1] Schlagmüller et al. PRX 6 (2016) 031020.

[2] Kanungo et al. PRA 102 (2020) 063317.

[3] Balweski et al. Nature 502 (2013) 664.

[4] Tiwari et al. PRA 99 (2019) 043616.

[5] Rammohan et al. arxiv:2006.15376 (2020).

[6] Rammohan et al. arxiv:2011.11022 (2020).

Precision and controlled study of the interaction between an ion and atom, at ultra-cold temperatures, has become possible with the development of hybrid trap experiments. These experiments allow the controlled overlap of trapped ions and atoms, so that their interactions can be investigated systematically. Under appropriate conditions, various interesting possibilities manifest, including the low partial wave collision of an ion and an atom, the study of a charge ion as an impurity in a cloud of atoms, spin and/or charge exchange processes as examples. These are accessible goals at the limit of present day experimental develops.

Of the numerous ion-atom systems that can be studied, one of particular interest is the interaction between a homonuclear ion-atom pair. The usual system in this situation is that of an alkali atom and its singly charged ion. At low temperatures, when this pair interact, the possibility of resonant charge exchange is manifest along with direct elastic collisions. However, whether the charge exchange has occurred or not in a particular ion-atom collision is fundamentally impossible to determine as the elastic and charge exchange amplitudes interfere. We present the consistent collision formalism and assert that there exists a single total cross section at ultra-cold temperatures. This implies that the general practice of separation of the direct elastic and resonant charge exchange channels for ultra-cold collisions is incorrect for these systems.

Beyond this, we relate the low energy cross section to the collisional diffusion of the ion in a cloud of its parent atoms. In addition, because of the existence of exchange symmetry for a homonuclear ion atom system, the phenomenon of charge hopping manifests. At ultra-cold temperatures, where the ion and the atoms are sufficiently delocalized in space and in thermal equilibrium, charge hopping becomes possible and can occur with any of the atoms overlapping with the wavepacket of the ion. This leads to charge hopping based diffusion. The collisional and hopping diffusion are both quantum phenomena and they compete to result in unique consequences, which shall be discussed.

Techniques for controlling the internal quantum states and motion of atoms have led to extremely precise clocks and state-of-the-art studies of degenerate gases. Extending such techniques to various types of molecules further enriches the understanding of fundamental physics, basic chemical processes, and many-body science. Here we explore high-precision measurements with molecules, and focus on a molecular lattice clock that features very long-lasting quantum-state superpositions, and can potentially lead to constraints on new physics.

Trapped ensembles of neutral atoms at nanoKelvin temperatures form pristine material with which to model complex quantum systems and build new ones for fundamental physics and applications. For example, fermion pairing can be explored using an ultracold gas magnetically tuned to a collisional Feshbach resonance. By combining atomic gases of two different elements, we realize a mixture of bosonic and paired fermionic superfluids, a system out of reach with liquid helium mixtures. In another set of studies, we observe and elucidate an exotic collisional resonance between fermions with closed and open electronic shell-structure. This latter result may enable the creation of ultracold di-atomic molecules with an unpaired electron, promising candidates for ultracold chemistry, quantum simulation, and information processing. By tuning external confinement, we also observe interaction-driven dynamical delocalization in a trapped bosonic gas.

Friday, 21 May 2021

Due to their exaggerated properties, highly excited (Rydberg) atoms have attracted physicists for more than a century, and the study of these atomic giants is intimately connected to major advances in modern quantum theory. In the last years, a new aspect was added to this everlasting fascination. Forces between Rydberg atoms are huge causing measurable effects over macroscopic distances in the micrometer range. As an important feature, these dipolar interactions can not only be tuned in strength, but also in their characteristic dependence on the distance between the atoms. Such control, in combination with modern methods of laser cooling and trapping, opens exciting perspectives for using Rydberg atoms as simulators for quantum many-body systems in order to address fundamental problems as, e.g., the emergence of magnetism in condensed matter or energy transport in photosynthetic complexes. In fact, it was first thought that the fragility of Rydberg atoms (the electron’s binding energy is only a few millielectronvolts or below) would impede meaningful applications. Yet, recent advances in quantum engineering have promoted Rydberg atoms to one of the hottest candidates for large-scale quantum simulation.

In my talk, I will provide an introduction into this rapidly growing new field of modern physics with special emphasis on the quantum simulation of Heisenberg spin systems.

We study the evolution of a cold single BaRb+ molecule while it continuously collides with ultracold Rb atoms. The initially weakly bound molecule can undergo a sequence of elastic, inelastic, reactive, and radiative processes. We investigate these processes by developing methods for discriminating between different ion species, electronic states, and kinetic ion energy ranges. By analyzing the experimental data while taking into account theoretical insights, we obtain a consistent description of the typical trajectory through the manifold of available atomic and molecular states. Monte Carlo simulations describe the measured dynamics well.

In recent years, a novel field of physics and chemistry has developed in which trapped ions and ultracold atomic gases are made to interact with each other. These systems find applications in studying quantum chemistry and collisions [1], and a number of quantum applications are envisioned such as ultracold buffergas cooling of trapped ions and quantum simulation of fermion-phonon coupling [2].

In our experiment, we overlap a cloud of ultracold 6Li atoms in a dipole trap with a 171Yb+ ion in a Paul trap. The large mass ratio of this combination allows us to suppress trap-induced heating [3]. For the first time, we buffer gas-cooled a single Yb+ ion to temperatures close to the quantum (or s-wave) limit for 6Li-Yb+ collisions. We study the temperature dependence of the spin exchange rates in these collisions and compare to theory to find estimates for the atom-ion scattering lengths. Our results open up the possibility to study trapped atom-ion mixtures in the quantum regime and to study ions interacting with weakly bound atomic Feshbach dimers [4]. Moreover, Feshbach resonances are predicted to exist between the atoms and ions that can be explored at the ultracold temperatures acquired in our lab. Finally, I will discuss strategies and prospects for reaching deeper into the quantum regime.

[1] M Tomza et al., Rev. Mod. Phys. 91, 035001 (2019).

[2] U. Bissbort et al., Phys. Rev. Lett. 111, 080501 (2013).

[3] M. Cetina et al., Phys. Rev. Lett. 109, 253201 (2012).

[4] H. Hirzler et al., Phys. Rev. Research 2, 033232 (2020).

[5] T. Secker et al., Phys. Rev. Lett. 118, 263201 (2017).

[6] N. Ewald et al., Phys. Rev. Lett. 122, 253401 (2019).

In this talk, we present a few-body approach to the physics of a charged impurity in an ultracold bath. We study how the nature of the bath affects the dynamics and evolution of the charged impurity due to cold chemical reactions: atom-atom-ion three-body recombination and molecular ion formation after ion-molecule collisions. In particular, we find that the nature of the charged impurity is readily controlled by tuning the binding energy of the ultracold molecular bath. Consequently, one finds the first principle explanation to some of the relevant parameters required to characterize the impurity's many-body evolution. In addition, we present our findings on the formation of van der Waals molecules on a buffer gas cell through three-body recombination, showing that almost any atom X in a buffer gas cell in the presence of He will lead to the formation of HeX van der Waals molecules.

The quest for Anderson localization of light is at the center of many experimental and theoretical activities. Cold atoms have emerged as an interesting quantum system to study coherent transport properties of light. Initial experiments have established that dilute samples with large optical thickness allow studying weak localization of light, which has been well described by a mesoscopic model. Recent experiments on light scattering with cold atoms have shown that Dicke super- or subradiance occurs in the same samples, a feature not captured by the traditional mesoscopic models. The use of a long range microscopic coupled dipole model allows to capture both the mesoscopic features of light scattering and Dicke super- and subradiance in the single photon limit. I will review experimental and theoretical state of the art on the possibility of Anderson localization of light by cold atoms.

I will review and describe our experiments studying and controlling spin-dependent quantum chemistry and quantum transport in an atomic ( 87 Rb) Bose-Einstein condensate (BEC), where different spin states can be addressed and coupled to induce “synthetic” spin-orbit coupling (SOC) and/or “synthetic dimensions”. We have demonstrated a new approach of quantum control of (photo) chemical reactions (photoassociation of molecules from atoms) --- which can be thought of a “quantum chemistry interferometry” --- by preparing reactants in (spin) quantum superposition states and interfering multiple reaction pathways [1]. By performing a “quantum quench” in a SOC BEC, we induce head-on collisions between two spinor BECs (realizing a “condensate collider”) and study spin transport and how it is affected by SOC, revealing rich phenomena arising from the interplay between quantum interference and many-body interactions [2]. By creating a “synthetic” cylinder with also synthetic magnetic fluxes, the BEC acquires an emergent crystalline order (in absence of an optical lattice) with a topological band structure featuring band crossings protected by nonsymmorphic symmetry, that we reveal by Bloch oscillations and can further control and manipulate [3]. Our experimental system can be a rich playground to study physics of interests to AMO physics, quantum chemistry and condensed matter physics.

Refs (study materials):

[1] D. Blasing et al., “Observation of Quantum Interference and Coherent Control in a Photo-Chemical

Reaction”, Phys. Rev. Lett. 121, 073202 (2018)

[2] C. Li et al., “Spin Current Generation and Relaxation in a Quenched Spin-Orbit Coupled Bose-

Einstein Condensate”, Nature Communications 10, 375 (2019)

[3] C. Li et al., “A Bose-Einstein Condensate on a Synthetic Hall Cylinder”, arXiv: 1809.02122

[4] A. Olson et al., “Tunable Landau-Zener transitions in a spin-orbit coupled Bose-Einstein condensate”,

Phys. Rev. A. 90, 013616 (2014); “Stueckelberg interferometry using periodically driven spin-orbit-

coupled Bose-Einstein condensates”, Phys. Rev. A. 95, 043623 (2017)

Trapped ions are among the most advanced technology platforms for quantum information processing. When laser-cooled close to absolute zero temperature, atomic ions form a Coulomb crystal with micron-scale spacings in a radio-frequency ion trap. Qubit or spin-1/2 levels, encoded in hyperfine energy states of each ion, can be initialized, manipulated, and detected optically with high precision. Laser fields can also couple the qubit states of arbitrary pairs of ions through (virtual) excitation of collective phonon modes, creating programmable quantum logic operations and spin Hamiltonians. In this talk, I will focus on programmable trapped-ion quantum spin simulators for analog, digital, or analog-digital hybrid quantum simulation protocols. These devices can be beneficial to solve hard problems in areas as diverse as condensed matter physics and high-energy physics.

Study materials:

- Monroe et al, Programmable quantum simulations of spin systems with trapped ions Reviews of Modern Physics 93, 2, (2021)
- Rajabi et al, Dynamical Hamiltonian engineering of 2D rectangular lattices in a one-dimensional ion chain, npj Quantum Information 5, 32, (2019)
- Blatt R. and Roos C.F., Quantum simulations with trapped ions, https://www.nature.com/articles/nphys2252
- Teoh et al, Machine learning design of a trapped-ion quantum spin simulator Quantum Sci. Technol. 5, 024001, (2020)

Saturday, 22 May 2021

The tunability of interactions and the ability to manipulate quantum systems at the single particle level have made ultracold atoms a fantastic playground to obtain a deeper understanding of many body physics.

In my lecture, I will explain how we are able to prepare systems of up to twenty particles with full control over all degrees of freedom. I will explain how we can control interactions between the particles, how we can manipulate and detect them, how to determine correlations between them, and interpret them. In the end I will show you how in this way, we can observe cooper pairs directly.

Quantum simulation provides new methods for studies on many-body quantum systems and is applied for various unsolved interesting problems in physics. We will mainly report about two topics using ultracold Yb atoms in an optical lattice. Both of the topics were performed with many-body systems in 1D, 2D, and 3D systems. In both topics, we investigated the validity of our quantum simulator using the 1D systems by quantitative comparisons between experimental results and the up-to-date exact numerical calculations. The experimental results in higher dimensions are useful as references for numerical calculations.

The first is the non-equilibrium dynamics of the bosonic Hubbard model [1]. After we prepare a Mott state in a deep optical lattice, we suddenly decrease the lattice depth and atoms start hopping. We developed a new technique to measure nonlocal atom correlations of atoms and internal energies and the measured experimental results are consistent with theoretical results.

The second one is the study of spin correlation of fermionic atoms with SU(N) symmetry in an optical lattice [2]. One of the advantages of Yb atoms is the fact that the Yb atom is a two electron system and has high spin degrees of freedom, namely SU(N), in the ground state. This SU(N) symmetry dramatically affects the nature of many-body physics. The spin degrees of freedom “absorb” the entropy of the system and “cool” the system. By directly measuring the spin correlations of neighbor sites, we confirm this Pomeranchuk cooling effect and the result is consistent with theoretical results. The estimated temperature of atoms in the 1D system is [0.096 ± 0.054(theory) ± 0.030(experiment)]/kB times the tunneling amplitude, where kB is the Boltzmann constant, which is the record low temperature of cold-atom Fermi Hubbard model.

[1] Y. Takasu et al., Science Advances 6, eaba9255 (2020)

[2] S. Taie et al., Arxiv: 2010.07730 (2021)

Study of disorder induced phase transitions is one of the most exciting field of research in condensed matter. The rapid experimental progress to simulate such phenomena using the systems of ultracold atoms in optical lattices have attracted a great deal of attention recently. Systems with quasiperiodic disorder are known to exhibit a localization transition in low dimensions. After a critical strength of disorder, all the states of the system become localized, thereby ceasing the particle motion in the system. However, in our analysis, we show that in a one-dimensional double-well lattice with quasiperiodic disorder, after the localization transition, some of the localized eigenstates become extended for a range of intermediate disorder strengths. Eventually, the system undergoes a second localization transition at a higher disorder strength, leading to all states being localized. We also show that the two localization transitions are associated with the mobility regions hosting the single-particle mobility edges. We further analyse the role of topology exhibited by the double-well lattice on this re-entrant phenomenon.