# QUANTUM

Welcome! We are carrying out research in the field of quantum optics and quantum many-body systems using ultracold atomic and molecular quantum gases at the Max Planck Institute for Quantum Optics and the Ludwig-Maximilians University. Furthermore, our group is part of the Munich Quantum Center.

Long-range interacting many-body systems differ fundamentally from their short-range counterparts. Creating a laser-controlled superposition with a highly excited and strongly interacting Rydberg state, we recently succeeded in implementing a novel kind of long-range interaction between microscopic atomic magnets. Control over the range, isotropy and sign of the interaction was demonstrated by an interferometric measurement technique, laying the basis for future studies of many-body spin systems.

Almost all quantum systems thermalize after being brought out of equilibrium initially. Many-body localization is a prominent counterexample in which localization hinders thermalization, even at high energies. Recently, we observed the localization transition in two dimensions with bosonic atoms in a disordered optical lattice. Through a single atom resolved study of the dynamics, we identified a thermalized and a localized phase and inferred a critical disorder for the phase transition.

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The Mott metal-to-insulator transition is a prominent phenomenon in condensed matter physics. We have used ytterbium atoms in an optical lattice to realize an extended-symmetry SU(N) Mott insulator, taking a direct look into this transition to better understand fermionic many-body systems.

The geometric and topological structure of energy bands play an important role in modern condensed matter physics but are difficult to experimentally access. Using ultracold bosons in a honeycomb lattice, we demonstrate a straight-forward method that can be used to both reconstruct the Bloch states at every quasimomentum and determine the topological invariants of the bands. Our method is based on using strong-force dynamics to realize a system that can be described by Wilson lines.

The phenomenon of Many-Body Localization (MBL) presents a generic alternative to thermalization in isolated quantum systems. Using ultracold fermions we study the effect of coupling identically disordered MBL systems with each other and ask - "Can these localized systems collectively serve as a bath for one-another and delocalize the entire system?" We find that MBL is indeed *unstable* to such a coupling and generically delocalizes. Further, we find that the behavior is strikingly different from Anderson Localization, which remains *stable* to such a coupling.

The Pauli exclusion principle is one of the most fundamental manifestations of quantum statistics. Here, we report on its local observation in a spin-polarized degenerate gas of fermions in an optical lattice. We probe the gas with single-site resolution using a new generation quantum gas microscope avoiding the common problem of light induced losses. In the band insulating regime, we measure a strong local suppression of particle number fluctuations and a low local entropy per atom. Our work opens a new avenue for studying quantum correlations in fermionic quantum matter both in and out of equilibrium.

Phys. Rev. Lett. **115**, 263001 (2015)

Press release: English (.pdf), Deutsch (.pdf)

Topological charge pumping, a dynamic version of the quantum Hall effect, enables a robust and quantized transport of charge through an adiabatic cyclic evolution of the underlying Hamiltonian. We have realized such a pump with ultracold bosonic atoms forming a Mott insulator in a dynamically controlled optical superlattice. We observed a quantized deflection per pump cycle for groundstate particles as well as a counterintuitive reversed deflection for atoms in the first excited band, illustrating the pump’s genuine quantum nature.

Nature Physics (advance online publication), DOI 10.1038/nphys3584

Press release: English (.pdf), Deutsch (.pdf)

Long-range coherence is typically restricted to equilibrium situations at low temperatures. Here we have, in stark contrast, for the first time managed to observe the dynamic emergence of coherence in a system far from equilibrium following a strong quantum quench. Furthermore, the emerging order is different from the ground-state one and cannot be found in the equilibrium phase diagram.

Using the strong and long range interacting Rydberg states, we were able to realize a superatom, a collective system of more than 100 normal atoms. Due to the many constituents, these systems are very robust and could be used as quantum memories. We demonstrate microscopic control and coherent manipulation of the superatoms, laying the fundament to future applications.

We have recently observed a novel state of matter that, despite being interacting, never thermalizes. This * Many-Body Localized States* represent a new class of systems that fail to be described by standard thermodynamics and statistical physics and require new theoretical and experimental approaches to characterize them.

## Monika Aidelsburger receives PhD prize

Monika Aidelsburger receives the PhD prize of the "Münchener Universitätsgesellschaft" for her thesis "Artificial gauge fields with ultracold atoms in optical lattices".

In a recent experiment, we locally observed an entanglement wave in quantum magnets made out of ultracold rubidium atoms. In contrast to ion systems, local atom number fluctuations influence the propagation of the magnetic excitation and we developed a novel in-situ Stern-Gerlach imaging technique to measure their impact on the detected entanglement.

Phys. Rev. Lett. 115, 035302 (2015)

Viewpoint on our work.

We succeeded to prepare magnetic quantum crystals based on laser-controlled long-range interactions between Rydberg atoms. These experiments critically relied on our local manipulation techniques that allow to control the atomic density of many-body systems at the single atom level. The crystals have been identified by a characteristic staircase in the magnetization that emerges due to the incompressibility of the system.

Quantum phase transitions are characterized by a dramatic change of the ground-state behavior; famous examples include the appearance of magnetic order or superconductivity as a function of doping in cuprates.

In this work, we explore how a system dynamically crosses such a transition and investigate in detail how coherence emerges when an initially incoherent Mott insulating system enters the superfluid regime.

One of the leading experts in theoretical condensed matter physics, **Professor Eugene Demler from Harvard University **(Cambridge, USA), has joined the LMU and MPQ as a winner of a Humboldt Research Award. This Award is granted by the Alexander von Humboldt Foundation to out-standing foreign academics in order to promote cooperation with excellent German researchers. Prof. Demler was nominated by Prof. Immanuel Bloch and Prof. Wilhelm Zwerger (TU München) who will host him during his stay in Germany. Having started his visit on January 20th Demler will work in Munich until March 20th, and then again from May to July next year.

Chern numbers are topological invariants characterizing Bloch bands. A striking manifestation of non-zero Chern numbers is the quantization of the Hall conductivity revealed by the quantum Hall effect. Here, we report on the first non-electronic Chern-number measurement with ultracold bosonic atoms that were loaded into an optical lattice potential subjected to artificial gauge fields. By applying a linear force to the atoms they experience a transverse motion proportional to the Chern number of the occupied band. By analyzing the in-situ evolution of the cloud we determined an experimental value of the Chern number ν_{exp}=0.99(5) in agreement with theory.

Nature Physics **11**, 162-166 (2015), AOP 3171 (2014)

See also: Commentary by Wolfgang Ketterle

The geometric structure of a single-particle energy band in a solid is fundamental for a wide range of many-body phenomena and is uniquely characterized by the distribution of Berry curvature over the Brillouin zone. We have demonstrated a matter-wave interferometer that precisely measures Berry curvature in an graphene-like optical honeycomb lattice and could demonstrate the highly singular nature of the Dirac point.

Science **347**, 288 (2015), Science Express (2014)

See also: Science perspective by A. Lamacroft

Press release:English (.pdf), Deutsch (.pdf)

Check out our article for the general reader in 'Spektrum der Wissenschaft' (the german issue of Scientific American) in the SdW, Nov. 2014 issue on 'Simulated Quantum Worlds'.

We studied far-from equilibrium spin transport in Heisenberg quantum magnets. For 1D systems we explain the observed diffusion like transport microscopically by the spectral properties of the Hamiltonian. In contrast, 2D Heisenberg magnets show anomalous superdiffusion.

Using the two stable electronic states of ytterbium, we were able to observe an orbital spin-exchange interaction - the building block of orbital quantum magnetism - in a SU(*N*)-symmetric fermionic quantum gas. Spin-exchanging interactions and SU(*N*) spin symmetry in ytterbium were so far only predicted theoretically, and their experimental observation paves the way for the experimental study of previously inaccessible quantum many-body phenomena.

Nature Physics AOP 3061 (2014)

Prof. Cheng Chin, winner of a **Humboldt Research Award**, will join our group for his research sabbatical in the beginning of August 2014. Prof. Chin studies quantum many-body phenomena based on ultracold atoms and molecules at the University of Chicago, including phenomena from different branches of physics such as nuclear, condensed matter, gravitational and astro-physics. During his stay in Munich he will work in close cooperation with our group at LMU and MPQ, as well as the group of Prof. Wilhelm Zwerger at TUM.

We implemented a ladder system with uniform magnetic field using ultracold atoms in optical lattices. By measuring the currents along the legs of the ladder we were able to observe a transition from a Meissner-like phase to a vortex phase.

By using a combination of Ramsey interferometry and Bloch oscillations we implement a protocol to extract the geometric phase of a one-dimensional dimerized optical lattice modelling polyacetylene. This one-dimensional Berry phase, also known as Zak phase, can be viewed as an invariant characterizing the topological properties of the energy bands.

We implemented large uniform effective magnetic fields with ultracold atoms using laser-assisted tunneling in a tilted optical lattice. We also show that for two atomic spin states with opposite magnetic moments, our system naturally realizes the time-reversal-symmetric Hamiltonian underlying the quantum spin Hall effect. Phys. Rev. Lett. **111**, 185301 (2013)

Using our quantum gas microscope we succeeded to observe magnon bound states in one-dimensional quantum magnets. These two-body bound states have been predicted to exist in Heisenberg chains by Hans Bethe 80 years ago. Using a novel microscopic preparation and detection method we identify the states by their characteristic correlations and, furthermore, we observe their dynamics. Nature 502, 76–79 (2013), Press Release MPQ (english, deutsch).

See also: News and views by Sougato Bose

Manuel Endres receives the PhD prize of the "Münchner Universitätsgesellschaft" for his thesis "Probing correlated quantum many-body systems at the single-particle level".

We observed coherent motion of a single spin down atom embedded in an environment of spin up atoms. Our measurements revealed coherent superexchange dynamics over large distances in the Mott insulating regime. In the superfluid regime we observed polaronic physics which lead to a reduced spreading speed due to the strong impurity-bath interactions. Nature Physics 9, 235-241 (2013), Press Release MPQ (english, deutsch).

See also: News and views by Patrick Windpassinger

We could for the first time observe a negative absolute temperature for mobile particles. By using an intermediate bosonic Mott insulator together with a Feshbach resonance in bosonic Potassium we were able to create a stable attractive Bose gas at negative absolute temperature.

Science 339, 52 (2013).

See also:

New: Frequently asked questions

Nature News article

Science perspective by Lincoln Carr

Press Release

Pressemitteilung

Media Gallery

Prof. Nigel Cooper, winner of a** Humboldt Research Award**, has joined our group in January 2013 as a guest scientist for three months. Nigel Cooper is an internationally renowned Professor of Theoretical Physics at the University of Cambridge where he leads a group working on *Theory of Condensed Matter,* and he is Fellow of Pembroke College. During his stay in Munich he will work in the field of artificial gauge fields and topological phases with our group at MPQ and LMU, as well as the group of Prof. Wilhelm Zwerger at TUM.

# EVENTS

**Open Lab Day - 28 June 2016**

You are a Master student and considering doing your research thesis in experimental quantum physics?

# SEMINARS

**Group Seminar MPQ: Energy landscape in training neural networks with discrete and continuous synapses**

Thursday, 25 August, 2016, 10.00 a.m. (s.t.) in Herbert-Walther-lecture room, MPQ, Ivan...

**Group Seminar LMU: Rotation of quantum impurities in the presence of a many-body environment**

Tuesday, 23 August, 2016, 10 a.m. (s.t.) in H 107, LMU, Misha Lemeshko, IST Austria, We present...

# CONTACT

**Max-Planck-Institut für Quantenoptik **

Hans-Kopfermann-Str. 1

85748 Garching, Germany

Phone: +49 (0)89 32905 - 138

Fax: +49 (0)89 32905 - 313

**Ludwig-Maximilians-University**

Quantum Optics Chair/

Fakultät für Physik

Schellingstr. 4

80799 Munich, Germany

Phone: +49 (0)89 2180 - 6131

Fax: +49 (0)89 2180 - 63850