Tweezer-Lab
In our lab, research focuses on the interaction in small ensembles of a few or even single 87Rb atoms. The atoms are trapped in so-called optical tweezers, which can be layed out in an arbitrary geometric arrangement. In order to initiate the interaction among atoms within an between different traps despite the, seen on the atomic level, large distances of up to a few micrometers, we excite some of the atoms to Rydberg states. A Rydberg state is a state with large principal quantum number n, giving the valence electron a high probability to be located at large distanced of a few 100 Bohr radii from the nucleus. This results, for example, in an enormous polarizability, which allows to Rydberg atoms to even polarize each other.
On small length scales, the 1/R3-dependent resonant dipole-dipole coupling, which scales with the fourth power of the principal quantum number, provides decisive contributions to the interaction of two Rydberg atoms. On larger length scales, the Van der Waals interaction due to the mutually induced dipole moments is dominant. It decreases on a 1/R6 scale but scales with the twelfth power of the principal quantum number. These different interaction mechanisms of two Rydberg states and their interplay give rise to a variety of interesting effects, which are already applied in the field of quantum computing and quantum simulation as well as in studies of criticality and universality.
In Rydberg blockade, for example, an atom excited to a Rydberg state in an ensemble shifts the energy levels of other atoms in its vicinity to such an extent that no further resonant excitation of additional Rydberg states is possible within a specific blockade radius. As a result, the entire ensemble consisting of many atoms behaves similarly to a single atom in which only a single excitation is possible at any given time. Therefor such systems are also called superatoms. The investigation of the interaction of chains of such superatoms and their behavior is part of our research. The coaction of the blockade and the angular dependence of the dipolar exchange interaction under certain boundary conditions causes the ground state of the quantum many-body system to be in a topological phase. The study of these phases and their robustness to external perturbations is also the focus of research in the tweezer lab.
Technical
The heart of our experiment is a high-vacuum glass cell in which 87Rb atoms are captured and cooled down by a magneto-optical trap directly from the background gas with a pressure of approximately 10-9 mbar. The atoms are then trapped in the optical tweezers, each with a radius of about 1 µm, in the focal plane of a high-resolution objective by irradiation of a Nd:YAG laser. The geometric arrangement of our trap pattern can be arbitrarily adjusted in advance of an experimental cycle by manipulation of the phase front of the trap beam in front of the objective using a spatial light modulator. Each dipole trap created in this way subsequently contains single or a few 10 atoms, depending on the exact preparation protocol.
The interaction of the atoms is afterwards initialized by exaction to Rydberg states mentioned previously. Here, we use a two-photon excitation, allowing us to prepare Rydberg S and D states. The required laser system consists of a 420nm laser, exciting atoms to the intermediate 62P3/2 state and a 1010-1030 nm laser for the final excitation to the desired Rydberg state. Both lasers are stabilized by a modified Pound-Drever-Hall scheme to a ULE resonator. This allows quasi-continuous tunability of the laser frequency whilst maintaining very narrow linewidths of less than 50 Hz.
To selectively transfer part of the Rydberg states back to the atomic ground state in state-sensitive manner, another laser with 480nm wavelength is available in the experiment. The detection of the atomic states as a measurement signal is done using the natural fluorescence of the atoms, which occurs during excitation by a laser on one of the two D lines. A fraction of the isotropically emitted photons upon decay to the ground state is directed to a highly sensitive sCMOS camera with low readout noise and high quantum efficiency. For this purpose, we use the same objective that is also used to generate the traps. Thus, we can detect the atoms with diffraction-limited spatial resolution.
