Quarterly Report IV/2022

Author: Sebastian Hofferberth


Power of a Quasispin Quantum Otto Engine at Negative Effective Spin Temperature (A4) 
Jens Nettersheim, Sabrina Burgardt, Quentin Bouton, Daniel Adam, Eric Lutz, and Artur Widera 

PRX Quantum 3, 040334 (2022)


Heat engines have been an intensely studied topics of physics since the 19th century, playing a central role in the development of thermodynamics and statistical physics. Conventionally, these devices are macroscopic and follow the laws of classical physics. In recent years, though, multiple implementations of heat engine cycles have been demonstrated in quantum systems, based even on single ions or atoms. This unites the coherent quantum dynamics of atomic systems with the coupling to thermal environments as a prime example of open quantum systems. In particular, these quantum heat engines provide very high control over the reservoir the engine is coupled to, enabling for example squeezed or negative-temperature reservoirs through the precise preparation of a large, but finite-size environment. In this study, the authors implement these concepts using individual Cesium atoms as heat engine embedded in a cloud of ultracold Rubidium atoms which emulate the bath. More specifically, the seven magnetic states of one hyperfine ground-state level of the Cs atom are used to implement the system, on which work is exerted by a modulated external magnetic field. Heat is exchanged with the bath via collisions of the Cs atoms with the Rb background gas. The spin-temperature of the engine is given by the population of the magnetic states, which in the experiment can also be externally controlled via microwave pulses. This allows for the preparation of the engine e.g. in a negative-temperature state. Finally, the effect of the heating & cooling cycles can be precisely monitored via single-atom spin-readout of the Cs atoms. This time-resolved observation of the spin-dynamics during work-cycles of this quantum Otto engine enables the authors to establish connections between the quantum fluctuations of the engine, quantified by the Shannon entropy, and the performed work. The study thus provides fascinating insights into the connection between the dynamics of quantum spin populations of a multilevel heat engine and its thermodynamic properties at the level of individual quanta.





Microscopic dynamics and an effective Landau-Zener transition in the quasiadiabatic preparation of spatially ordered states of Rydberg excitations (B2) 
Andreas F. Tzortzakakis, David Petrosyan, Michael Fleischhauer, and Klaus Mølmer: 

Phys. Rev. A 106, 063302 (2022)


Arrays of single neutral atoms trapped in configurable optical tweezers have emerged as a powerful platform for simulating spin dynamics in 2d-systems. Effective spins are encoded in atomic levels and interaction between atoms is realized via excitation to high-lying Rydberg states. A very interesting question is how such a spin array can be prepared in a desired initial state, with one approach being adiabatic transfer from a fully “spin-down” non-excited system to a spatially ordered configuration of Rydberg atoms. This protocol has found wide-spread used in Rydberg-tweezer quantum simulators. In this collaborative work between researchers from OSCAR and external partners, the adiabatic preparation protocol is specifically studied for finite-size one-dimensional systems. Investigating such a moderately sized system allows for full numerical studies, which in turn enable the authors to deduce an effective two-state description explaining the adiabatic transfer as a Landau-Zener transition from initial ground to target excited many-body state. The detailed study presented here as well as the simple intuitive explanation extracted from it are of relevance to on-going quantum simulation experiments, In particular for optimization problems studied in Rydberg arrays.


Photonic quadrupole topological insulator using orbital-induced synthetic flux 
Julian Schulz, Jiho Noh, Wladimir A. Benalcazar, Gaurav Bahl, Georg von Freymann:

Nat Commun 13, 6597 (2022)


The properties of crystalline solid-state materials can often be traced back to symmetry properties of the constituting atomic structure of the crystal. In particular, the richness of macroscopic properties in crystalline structures as well as multiatomic molecules is closely related to the way in which orbitals of individual atoms connect. Topological quantum chemistry explores this intimate relationship between electronic orbitals and topological phases in crystalline structures and has led to the realization that topologically nontrivial materials are much more common than previously thought. At the same time, direct investigation of the role of microscopic symmetries and properties is often difficult in actual materials. Thus, it is of great interest to realize synthetic systems that can replicate the properties of real materials. Among various different simulation platforms, also including ultracold atoms and polaritons, photonic systems using custom-tailored waveguide structures have emerged as a powerful platform to study topological effects.
In this work, the authors present the experimental realization of a quadrupole topological insulator using a waveguide lattice fabricated via direct laser printing. This allows for precise control of the phase accumulation of light travelling through the waveguide structure which mimics the symmetry properties of atomic orbitals of the simulated atomic wavefunction. The time evolution of this wavefunction becomes directly observable in this photonic material by imaging the out-of-plane scattered light. This study highlights (again) the flexibility and unique opportunities of laser-printed photonic simulators and paves the way for further investigations of non-trivial topological lattice structures.