Individual tracer atoms in an ultracold quantum gas

Combining individually controlled, ultracold atoms – so-called tracers – with a bosonic quantum bath realizes a paradigm of quantum physics. Employing tracers as quantum probes or quantum impurities for the many-body system opens a large number of fascinating possibilities to probe and manipulate local properties of a quantum many-body system. We exploit the dependence of the tracer-bath interaction on position, number or spin-state of the tracers; likewise, these quantities contain information about the ultracold gas. In our project, single Caesium atoms form the tracers which we immerse into a Bose-Einstein condensate of Rubidium atoms.

Individual Tracer Atoms in an Ultracold Dilute Gas

Diffusion is a phenomenon with fundamental implications for our daily life, mixing sugar and milk into our coffee, distributing the smell of a delicious meal through our homes, and unfortunately also making our nose itch in summer, when pollen are spreading everywhere. In science, observing diffusion dynamics allows to obtain information about microscopic processes by observing the motion of macroscopic objects. A good reason to address the topic in our experiment and to explore so far unexplored regimes. We have studied individual particles (Caesium atoms) diffusing in through a very dilute ultracold cloud (of Rubidium atoms), as it occurs in nature when particles are diffusion in the upper atmosphere for example. Since in our case the diffusing particles – so-called tracers - and the bath atoms are distinguishable, the influence of collisions on the diffusion can be identified and understood in great detail – down to the level a single collision events of two atoms. Thereby this work has helped to create a more comprehensive physical description of diffusion processes This project is linked to our investigation of diffusion in a light bath.

Single-atom thermometer for ultracold gases.

Measuring the temperature of ultracold atom clouds is very important for almost any experiment, but normally requires the measurement of the whole cloud at once. The measured temperature is thus an average of the system, which might not contain the full information, if the system is not in equilibrium. We approach this problem by inserting individual Cs atoms into the Rubidium cloud, which quickly adapt the temperature of their environment, requiring just a couple of milliseconds. Afterwards, we measure the momentum distribution of the Cs atoms, which yields the temperature of the bath. In fact, we can pin the position of Cs atoms during or after interaction, which potentially allows to locally address the region of interest, where temperature is measured. With that perspective our work opens many possibilities for local probing of a many-body quantum system.

Single-atom quantum probe for ultracold gases

By immersing individual cesium atoms in an ultracold rubidium bath, collisions between the two species are possible. We distinguish between elastic collisions and spin-exchange collisions.
Based on these collisions, the single cesium atom can be used as a quantum probe in two different ways. Measurement method A can extract the temperature of the rubidium bath, the externally applied magnetic field, or a combination of these two parameters. Measurement method B focuses on determining the temperature and density of the rubidium bath.

Measurement Mehod A

Measurement method A:

Single spin-exchange collisions between cesium atoms and rubidium atoms enable the connection of the internal state dynamics (spin dynamics) with the motional degree offreedom, whereby the probability of these processes occurring depends on the competition between the externally applied magnetic field and the collision energy (i.e., the bath temperature). This connection enables the external parameters bath temperature, magnetic field, and combinations of these to be projected onto the internal spin states and thus extract the respective parameter from the spin distribution of the single atom. The single-atom quantum probe shows increased sensitivity in the non-equilibrium state, which results in a low-perturbative measurement method.

Measurement Method B:

The single cesium atom is prepared in a coherent superposition state. We observe the dephasing that occurs when the single atom interacts with the rubidium bath. The interaction strength and, thus, the dephasing mechanism depends on the environment and changes if ultracold rubidium gases of different temperatures or densities provide the environment. By measuring the dephasing, the bath temperature or bath density can then be determined based on this dependency.

The quantum heat engine, an engine made of individual atoms

In addition to applying a single-atom quantum probe, this project describes the realization and characterization of a single-atom heat engine. Individual cesium atoms act as the machine's working medium, powered by ultra-cold spin baths of rubidium atoms. Spin exchange collisions between the two atomic species lead to the energy transfer of individual quanta. This quantized heat exchange was applied cyclically to realize an Otto cycle. High power output, constant high efficiency, and high stability characterize the Otto cycle.