# 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 of Quantum Optics and the Ludwig Maximilians University. Furthermore, our group is part of the Munich Quantum Center.

Some isolated quantum systems with disordered potentials fail to ever reach thermal equilibrium, a phenomenon known as many-body localization (MBL). An outstanding question concerns the stability of this phenomenon. While it is well known that exposing the quantum system to the outside world destroys localization, it is not clear what happens when the MBL system is connected to another small quantum system known as a quantum bath. Does a quantum bath destroy the localization, and if so, how small can that bath be? We experimentally address these questions by observing a mixture of cold interacting atoms, some of which are in a laser-induced disordered (MBL) state and some acting as a quantum bath.

Strongly-interacting gauge theories are extremely challenging to access with conventional numerical techniques. Here, we take a first step towards quantum simulation of gauge theories by implementing a Floquet-based method with two-component ultracold bosons in a double-well potential. For resonant periodic driving at the on-site interaction strength and an appropriate choice of the modulation parameters, the effective Floquet Hamiltonian exhibits Z_{2} symmetry. We study the dynamics of the system for different initial states, reveal challenges that arise due to symmetry-breaking terms and outline potential pathways to overcome them.

Jayadev Vijayan won the 'Best talk award' for his talk on "Observing spin-charge separation in a quantum simulator" at IONS BCN'19, a conference held at ICFO, Barcelona and organized by the OSA (Optical Society of America).

For more information about the conference click here.

Typically, direct optical access to molecular constituents is out of reach due to their tiny size in the sub-nanometer regime. Because highly-excited Rydberg atoms feature strong interactions even at distances of micrometers, two Rydberg atoms can form huge molecules, comparable to the size of small bacteria and larger than optical wavelengths. Here, we studied these exotic molecules by exciting atom pairs in an initially prepared unity filled two-dimensional atom array and spectroscopically resolved the vibrational structure for the first time. Using a high-resolution objective, we microscopically detected the excited molecules by correlated atom loss. Furthermore, we find a striking alignment of the photoassociated molecules, which can be controlled by the polarization of the excitation light and the specific molecular state.

A single-particle mobility edge marks a critical energy separating extended from localized states and characterizes the single-particle intermediate phase of our one-dimensional non-interacting quantum system with a weak quasiperiodic potential. Here, we investigate the corresponding interacting system, where the existence of many-body localization (MBL) and a many-body intermediate phase (MBIP) are still open and heavily debated questions. We measure the time evolution of an initial charge density wave after a quench and analyze the corresponding relaxation exponents. We find clear signatures of MBL, when the corresponding noninteracting model is deep in the localized phase. We also critically compare and contrast our results with those from a tight-binding Aubry-André model, which does not exhibit a single-particle intermediate phase, in order to identify signatures of a potential MBIP.

Using spin-resolved quantum gas microscopy, we directly observed two fundamental predictions for Luttinger liquids, a theory which generically describes gapless 1d systems including the doped Fermi-Hubbard model studied here. We detected a linear variation of the spin-density wave vector as a function of doping in good agreement with quantum Monte-Carlo calculations at T/t=0.29. The microscopic origin of this phenomenon was attributed to the dilution of antiferromagnetic correlations by holes and doublons acting as domain walls. When studying spin-imbalanced clouds in squeezed space, we observed a linear increase of the spin-density wave vector with polarization in excellent agreement with exact diagonalization calculations of the Heisenberg model. This wavelength extension was attributed microscopically to pairs of parallel spins acting as domain walls for the antiferromagnetic order. Finally, when inducing interchain coupling to map out spin correlations in the crossover regime, we observed fundamentally different spin correlations in the direct vicinity of doublons in 2d, suggesting the formation of a magnetic polaron.

In this work we study ultracold polar molecules with long-range interactions over extended periods of time using a „supermagic“ trapping technique. To obtain the interacting molecular gas we create a superposition of the molecules’ ground and first excited rotational states. In general, these two states react differently to the optical trap holding the molecules. The couplings between nuclear spins, rotation and the trap light lead to a Gordian knot of transitions in the rotation spectra. This masks the observation of the collective molecular spin dynamics. In this work we disentangle the spectra using two tricks: A special, so-called „magic“ polarization of the trap beam and a small static electric field. In this supermagic trap we can then expose the dipolar interactions between the molecules. This paves the way towards the simulation of complex quantum models with long-range interactions, e.g. to explore superconducting materials.

To spread the word about the fascinating physics behind ultracold polar molecules, Frauke loves to travel to Science Slams all over Germany. There the goal is to present own research to a general audience in 10 minutes. Ideally in an understandable and entertaining way, because in the end the audience is also the jury of all performances. Now you can watch her performance from 19^{th} of September 2018 in Ulm on YouTube and learn about her molecular dating agency “alkaLiebe”

We experimentally and numerically investigate mass transport of fermions in a one-dimensional optical lattice by releasing an initially trapped gas suddenly into a homogeneous potential landscape. For initial states with an appreciable amount of doublons, we observe a dynamical phase separation between rapidly expanding singlons and slow doublons remaining in the trap center, realizing the key aspect of fermionic quantum distillation in the strongly-interacting limit. For initial states without doublons, we find a reduced interaction dependence of the asymptotic expansion speed compared to bosons, which is explained in terms of the interaction energy produced by dynamically generated doublons in the interaction quench.

Breaking down our research so that 99,999% of the worlds population would understand it, was the goal of Jayadev Vijayan with his article "Zooming into superconducters". With his *Leap* - a popular science article on quantum research written by scientists and reviewed by teenagers — published in Quantum Views, he explains his research to 13-14 year old High School students who reviewed the article and (hopefully!) understood why cold atoms is a research field worth studying.

See yourself if you understand Jayadevs explanation: HERE

Learn more about Quantum Leaps on the Quantum Journal website.

# SEMINARS

**Group Seminar at LMU: Microscopy and correlations across the many-body localization transition**

Tuesday, 9th April 2019, 9:15am, LMU, Bloch group seminar room Matthew Rispoli,Harvard...

**Special Seminar with Theory Division: Spatial entanglement patterns and Einstein-Podolsky-Rosen steering in a Bose-Einstein condensate**

Tuesday, 2nd April, 2019, 10:00am, MPQ Herbert-Walther Hoersaal (NEW!) Dr. Matteo Fadel, PhD and...