ICTS

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.

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.

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.

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.

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.

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.

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.

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.

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 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.

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)

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)