Strontium Tweezer Experiment

In our most recent cold-atom experiment, we aim to trap and cool single strontium atoms in arrays of small optical dipole traps.
Strontium's unique properties have attracted interest in the scientific community over the past years. As an alkaline earth element, it offers a richer level structure compared to the alkali metals (in our group e.g. Lithium, Sodium and Potassium). Its isotopes can be cooled to degenerate quantum gases (not least thanks to Simon Stellmer's efforts). With a strong, blue transition and weaker, but very precise red transitions it is tailored for trapping in small dipole traps (optical tweezers) and optical atomic clocks.


By combining a liquid-crystal based spatial light modulator (LCOS-SLM) with high NA microscope objectives (NA > 0.65), it is possible to trap hundreds of single atoms in arbitrary (well-defined) one-, two- or even three-dimensional configurations. This allows us to combine aspects from our quantum gas experiments and our single ion experiments. For example, optical lattice geometries not accessible in a standing wave configuration can easily be created with optical tweezers.
In the mid term, our goal (well, one of them) is to trap multiple atoms in one of our millimeter-sized small optical cavities. Due to strontium's large dipole moment, this can allow for even larger light-atominteractions than we have seen here.


The experiment passed its design phase and is currently under heavy construction, so there are no publications yet. Please see the setup section or the pictures to get an idea.


Some of the things we already have in our lab:

  • The laser system (1S0 1P1 @ 461 nm , 1S03P1 @ 689 nm, repumpers @ 679 and 707 nm)
  • A home-built strontium source (oven and a two-dimensional MOT) used to create a cold atomic beam directed towards our main chamber(s)
  • Our main physics chamber, featuring: a custom non-magnetic titanium body, optical access from twelve directions, two high-NA objectives, options to extend to multiple science chambers.
  • Fast-switching magnetic field coils for gradients up to 150 G/cm at the atom position
  • Commercial FPGA-based experiment control system
strongly simplified level scheme of strontium, showing only transitions addressed by our lasers
© Jonas Schmitz

strongly simplified level scheme of strontium, showing only transitions addressed by our lasers

Our blue laser system © Jonas Schmitz
Frequency doubling cavity to create 461nm light (blue) from 922nm light (infrared) © Jonas Schmitz
One of our high-NA objectives, focused on a tiny SNOM fiber tip (blue dot) © Jonas Schmitz
Our two-dimensional magneto-optical trap uses stacks of strong permanent magnets © Jonas Schmitz
The main vacuum chamber is surrounded by various magnetic field coils, some of which are watercooled © Jonas Schmitz
Fluorescence of the atomic beam emerging from the strontium oven (below) © AG Quantum


Avatar Rönchen

Felix Rönchen


Wegelerstraße 8

53115 Bonn

Avatar Kree

Tobias Kree

Gruppe Köhl, Doktorand*in


Wegelerstraße 8

53115 Bonn

Avatar Berner

Ralf Berner


Wegelerstraße 8

53115 Bonn

Avatar Grundmann

Tobias Grundmann


Wegelerstraße 8

53115 Bonn

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