Exploring Gravity with Optical Clocks

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The Düsseldorf Optical Clock


(A. Görlitz, C. Abou-Jaoudeh, C. Bruni, A. Nevsky, I. Ernsting, S. Schiller)

This development was started in 2006 in the framework of the ESA-DLR project “Space Optical Clocks (SOC)”.
ELIPS - 3 Space Optical Clocks

The optical clock developed at the Universität Düsseldorf is a lattice optical clock using neutral Ytterbium (Yb) atoms. It consists of a cold atom apparatus on a 1m x 2 m optical table, containing three laser systems (399 nm, 556 nm, 759 nm) for cooling and trapping the atoms. The clock laser is located on a 0.9 m x 0.6 m table in another laboratory room, and its light is delivered via fiber to the cold atom apparatus. A frequency comb serves to measure the clock laser frequency with respect to a Hydrogen maser that is long-term stabilized to GPS time signals. The cold atom source table and the clock laser table are transportable. Currently, Yb atoms can be reliably cooled and trapped in the second stage (556 nm) magnetooptical trap. The 3D lattice has been set up and storage in this lattice is being studied.

Figure 1 The transportable Yb clock system (2 m2), developed in the SOC project [Abou-Jaoudeh et al, Nevsky et al.]. Middle: the transportable compact Yb clock laser system (90 x 60 cm) with 2 Hz linewidth. The silvery box contains the reference cavity of the clock laser. Right: Yb atoms trapped in the 1st stage MOT (399 nm). In the apparatus, stable trapping of Yb in the 2nd stage has been obtained.

Figure 2: Schematic of the transportable ultracold Yb lattice clock system. All components for the ultracold source and the optical lattice are included on one optical table.

Figure 3: Top: Schematic of the Yb clock laser system. Two independent reference cavities for 578 nm are employed to characterize the frequency stability of the clock laser which is locked to one of the two. Bottom: beat note between the stabilized laser and the second cavity.

In 2008, we have made our first observation of the clock transition in an ensemble of cold Yb atoms, trapped in a second-stage (post-cooling) MOT in another cold atom apparatus.

The interrogation of the 171Yb clock transition at 578 nm was performed in a directly loaded postcooling MOT. This means that the MOT was operating on the 556 nm 1S03P1 intercombination transition of Yb. The Yb atoms were decelerated in the Zeeman slower using the allowed 399 nm 1S01P1 transition and trapped in the MOT. The MOT temperature as measured by a time-of-flight technique was around 100 μK. If the MOT is not exposed to the clock laser, the number of atoms in the MOT reaches a steady-state value within roughly 10 s, which is determined by a dynamic equilibrium between the loading rate into the MOT and the loss rate which is mainly due to collisions (light-assisted and background). The number of atoms in the MOT can be easily monitored by observing the fluorescence of the atoms due to the interaction with the cooling light.

Figure 3: Fluorescence from a 171Yb MOT operating on the intercombination transition at 556 nm. Atom number: ~ 2 x 107, density: ~ 1011 cm-3, loading time: ~ 3 -10 s, temperature: ~ 100 μK.

Once clock laser light resonant with the 1S03P0 clock transition at 578 nm is introduced, additional losses occur since atoms can then be transferred into the long-lived metastable 3P0 state which is no longer trapped in the MOT. This process in turn leads to a reduction of the steady-state value of the fluorescence which depends on the clock-laser frequency. The fluorescence intensity is observed by a photomultiplier and converted into a voltage signal. Scanning of the laser frequency occurred by means of an AOM in the clock laser setup and the frequency was measured by a frequency comb stabilized to a H-maser referenced to GPS. The clock laser is based on a resonantly doubled quantum dot diode laser (A. Nevsky et al. 2008).

The result of one scan is shown in Figure 4. The observed transition line is shifted by approx. 0.31 MHz to higher frequency due to light shift of the clock levels by the 556 nm cooling light and is broadened to 736 kHz FWHM, due to a combination of Doppler broadening and saturation broadening

Figure 4: The 1S03P0 clock transition at 578 nm. Shown is the fluorescence arising from the 556 nm 1S03P1 cooling light during interrogation of the clock transition. The observed transition frequency is shifted by approx 0.3 MHz and broadened to 736 kHz due to light shift in the MOT.

Further information:
Presentation on the Düsseldorf optical clock status (10/2009)*
C. Abou-Jaoudeh et al. “A Compact Source of Ultracold Ytterbium for an Optical Lattice Clock”, Proc. EFTF - IFCS, p. 756 (2009)
A. Nevsky et al., “A narrow-line-width external cavity quantum dot laser for high-resolution spectroscopy in the near-infrared and yellow spectral ranges”, Appl. Phys. 92, 501 (2008)