Interferometry with cold
Magnesium Atoms
 
 

Welcome !

This is the website of the Magnesium atom interferometry team.

Read more about

Atom interferometry

In atom optics, the roles of light and matter are exchanged. The deBroglie waves associated with quanta play the role of the light rays of classical optics, while laser light fields serve as optical elements. Effects associated with classical light waves can be observed in the matter wave regime, eg. interference and diffraction. Atom optics, in contrary to classical optics, exploits the potential of the inner structure of the atoms. Unlike classical particles, atoms exhibit a complex and discrete spectrum of inner states. These can be probed and manipulated by interaction with external fields. The extreme sensitivity of the involved effects facilitates a rich diversity of high-precision measurement methods at the quantum limit. Such experiments, in turn, require ensembles of atoms with well-defined properties. Preparing such ensembles is one of the main goals of laser-cooling and trapping.

Atom interferometry extends the classical concept of interference to matter waves. Elements such as beam splitters are implemented by resonant atom-light interaction and can be combined to form interferometers for matter waves. Due to the inner states of the atom being involved, such devices intrinsically are very important tools for precision spectroscopy.

We perform high-resolution spectroscopy on laser-cooled Magnesium atoms. Presently, our Ramsey-Borde type interferometer allows us to probe the 31S0-33P1 intercombination line in Magnesium at 457 nm at resolutions below 300 Hz, making it one of the most precisely determined atomic transitions.

Our work is motivated by the quest for precise time and frequency standards both in technology and fundamental research. Applications are global positioning systems, fiber-optical communication networks and the verification of relativistig effects.>

Team

Current members of
our research team are

Dr. Tanja Mehlstäubler (PostDoc),
Dr. Ernst Rasel
(PostDoc),
Karsten Moldenhauer
(PhD student),
Jan Friebe
(PhD student),
Matthias Riedmann
(PhD student),
Nils Rehbein
(PhD student).



Research

Matter-Wave Interferometry With Magnesium Atoms

Alkaline-earth atoms have a rich spectrum of optical transitions reaching from very fast cycling transitions to extremly narrow lines between the two different spin systems (see figure 1).These intercombination lines can be used for high resolution spectroscopy in the opticaldomaine. In 24Mg we use the fast 31S0-31P1 transition for cooling and trapping the atoms,which are then probed on the 31S0-33P1 transition at 457 nm with a linewidth of only 31 Hz.

figure 1: Simplified level scheme of 24Mg
with singlet and triplet spin states.

In order to take advantage of such a extreme narrow spectroscopic line very long interaction times are required. With a standard spectroscopic aproach on an atomic beam this would imply laser diameters of several meters. The Ramsey method of seperated fields solves this problem by splitting the interaction regions into two spatially separted parts each exciting the atom only half way. The resolution is then only determined by the time between the two interrogation pulses.
figure 2: The Ramsey-Bordé interferometer.
The resolution is determined be the dark time TD between two beam splitter pulses.

We use a modified scheme of seperated field spectroscopy with two pairs of counter propagating laser beams to perform Doppler free spectroscopy (see figure 2). In this so called „Ramsey-Bordé-interferometer“ each laser beam acts as a 50-50 beam splitter for the atomic wave function leaving one part in the ground state, the other in the excited state. With the 4th laser pulse the two possible paths of the atomic wave function are overlapped making them indistinguishable. The interference due to different phases collected on both paths gives a periodically modulated ground state population dependend on the laser frequency (see figure 3).

figure 3: interference fringes in the fluorescence signal probing the ground state population


New experimental results

Atom interferometry

high resolution and stability

Recently we improved the performance of our running atomic interferometer system. With magnesium atoms at Doppler temperature (T =2 mK) a principle limitation can be found in the residual motion of the atoms. Nevertheless high resolution and stability of the frequency measurement can be pushed quite far. The resolution is defined as the FWHM (full width half maximum) of the cosine-shaped interference pattern. Look at some examples of fine frequency resolution in the graphs. The laser frequency scale (detuning) is relative to one central part of the interference pattern (lower frequency recoil component) at an absolute frequency of n0=655.66 THz. Each point is integrated over 1.05 seconds. At the highest resolution (dnres=290 Hz) the noise still is quite dominant. This is mainly due to the fact that with the required long interrogation time the signal contrast strongly decreases. There are many effects responsible for this contrast decay e.g. laser frequency stability, lifetime of the upper atomic state (=”damping of the pendulum”) and the expansion of the atomic cloud. The relative frequency stability is measured in terms of the Allan standard deviation (sy(t)). It depends on the noise to signal ratio of the patterns and the quality of the resolution (Q=n0/dnres). We measured up to 0.5 % of noise to signal ratio. This was achieved by implementing active stabilization systems for the trapping light intensity and beam pointing.

Multiple wave atomic interferometry

Laser cooled atoms give the opportunity to realize a more advanced scheme of an atom interferometer. In wave optics multiple beam interference like in fabry-pérot type interferometers give sharp resonances which are widely used for spectroscopy or stabilization of laser frequency. We achieved high resolution with our multiple wave atom interferometry scheme [xx]. With up to 12 interfering partial atomic waves we improved the resolution to below 2 kHz.

Developing new cooling and trapping methods for Magnesium


Presently we are setting up a new experiment to improve the properties of our cold ensemble of Magnesium atoms for the interferometry. 24Mg such as other alkaline-earth atoms has a non-magnetic ground state setting the limit at the Doppler temperature of a few mK. To push this limit into the µK-regime we have worked on a new cooling scheme, where the narrow intercombination lines shall be used to allow orders of magnitude colder temperatures.

As the weak light forces on our narrow intercombination line would not even be able to sustain the atoms against gravity, we plan to artificially broaden this transition by a second laser. In a two step excitation (shown in figure 4) the linewidth of transition 1 2 is determined by the coupling strength of laser2 mixing state 2 and 3. If state 3 is a fast decaying state to the singlet ground state, we can efficiently cool the atoms on such a cycle process. For using the 41S0 state (see figure 1) figure 4 shows the obtainable scattering rate as a function of the quench laser intensity.
figure 4: scheme of the quench process
figure 5: scattering rate as a function of quench
laser intensity
In collaboration with the PTB this „quench-cooling“ could be tested in Calcium and the experimental results compared to our numerical simulations of the 3D-cooling and trapping process in a MOT [XX]. For Magnesium we are currently setting up a frequency doubled Ti:Sapph system to produce enough power at 462 nm (> 100mW) for the quench cooling. This shall be tested soon in our new chamber (see picture below) with improved optical access and interferometry zones. The quench cooled ensemble of atoms, only limited by the recoil at the 457 nm transition of a few µK, should there be transfered into a far detuned crossed dipole trap allowing storage times of many seconds.


our new chamber for the Magnesium interferometer


Laser development Nd:YVO4 thin disk laser at 914.5 nm

The laser transitions 4F3/2-4I9/2 of Nd-doped materials are of great interest for generating blue laser radiation, using frequency doubling. In collaboration with the Institut für Strahlwerkzeuge (Prof. Dr. A. Giesen), Stuttgart, Germany, we have developed a Nd:YVO4 laser emitting light at 914.5 nm. The second harmonic at 457 nm represents the ideal powerfull replacement of our Dye laser, used to probe the intercombination line of Mg-atoms.

With a 0,3 at. % doped crystal an output power as high as 5.8 W has been obtained, at a cooling fluid temperature of -35 °C. Monomode operation has been achieved, with an output power of 1.3 W. The blue radiation will be generated by frequency doubling in a non linear crystal (PP-KTP), using the quasi-phase matching technique. The single-pass conversion efficiency is better by a factor 3 compared with other crystals usually used.

References :

[1] A. Giesen, H. Hügel, A. Voss, K. Wittig, U. Brauch, H. Opower, „Scalable Concept for Diode-Pumped High-Power Solid-State Lasers“, Appl. Phys. B 58, 363-372 (1994)

Publications

J. Friebe, A. Pape, M. Riedmann, et al.,"Absolute frequency measurement of the magnesium
intercombination transition 1S0-->3P1" Phys. Rev. A 78, 033830 (2008)

S. Falke, H. Knoeckel, J. Friebe, et al.,"Potassium ground-state scattering parameters and Born-Oppenheimer potentials from molecular spectroscopy", Phys. Rev. A 78,012503 (2008)

T. E. Mehlstäubler, K. Moldenhauer, M. Riedmann, N. Rehbein, J. Friebe, E. M. Rasel, and W. Ertmer, "Observation of sub-Doppler temperatures in bosonic magnesium ", Phys. Rev. A 77, 021402(R) (2008)

N. Rehbein, T. E. Mehlstäubler, J. Keupp, et al., Optical quenching of metastable magnesium Phys. Rev. A, 76, 043406 (2008)

H. Stoehr, N. Rehbein, A. Douillet, et al.,"Frequency-stabilized Nd : YVO4 thin-disk laser", Appl. Phys. B. 91,29-33 (2008)

J. Friebe, K. Moldenhauer, E.M.Rasel, et al.,"beta-BaB2O4 deep UV monolithic walk-off
compensating tandem", Opt. Comm., 261,300-309 (2006)

T. E. Mehlstäubler, „Neuartige Kühlmethoden für einen Magnesium-Frequenzstandard“,
Dissertation, Universität Hannover (2005)

J. Arlt, G. Birkl, E. M. Rasel und W. Ertmer;
„Atom optics, guided atoms, and atom interferometry”, Advances in Atomic, Molecular and Optical Physics, Vol. 50 (2005)

J. Keupp, A. Douillet, T.E. Mehlstäubler, N. Rehbein, E.M. Rasel and W. Ertmer;
"A high-resolution Ramsey-Bordé spectrometer for optical clocks based on cold Mg atoms", Highlight Paper in EPJ D 36, 289-294 (2005)

J. Friebe, “Effiziente Frequenzverdopplung mit neuartigen Kristallstrukturen”, Diplomarbeit, Universität Hannover (2005)

A. Douillet, T.E. Mehlstäubler, J. Keupp, N. Rehbein, H. Wolff, E.M. Rasel and W. Ertmer
“Improved high resolution spectroscopy with cold magnesium atoms”, Proceedings (p.1092f), 2003 IEEE International Frequency Control Symposium, Tampa, Fl. USA (2003)

H. Wolff, “Aufbau und Charakterisierung eines frequenzverdopplerten Nd:YVO4 Scheibenlasers bei 457 nm”, Diplomarbeit, Universität Hannover (2003)

T.E. Mehlstäubler, J. Keupp, A. Douillet, N. Rehbein, E.M. Rasel, and W. Ertmer
Modelling three-dimensional-quench cooling for alkaline- earth atoms
J. Opt. B, 5, p.183 (2003)

J. Keupp, "Ein atominterferometrischer Frequenzdiskriminator hoher Stabilität für optische Magnesium-Atomuhren", Dissertation, Universität Hannover (2003)

T. Binnewies, G. Wilpers, U. Sterr, F. Riehle, J. Helmcke, T. E. Mehlstäubler, E. M. Rasel, W. Ertmer
Doppler cooling and trapping on forbidden transitions
Phys. Rev. Lett., 87 (2001)

H. Hinderthür, F. Ruschewitz, U. Sterr, K. Sengstock , W. Ertmer, F. Riehle, J. Helmcke
Atom Interferometry based on seperated Light Fields
in 'Atom Interferometry',ed.: P.Berman, Academic Press, 293 - 362 (1997) Other publications H. Hinderthür, F.

Ruschewitz, H.-J. Lohe, S.Lechte, K. Sengstock, W. Ertmer
Multiple-beam Atom Interferometry in the Time Domain
Phys. Rev. A 59, 2216-2219 (1999)

H. Hinderthür, A. Pautz, F. Ruschewitz, K. Sengstock, W. Ertmer
Atom interferometer with polarizing beamsplitters
Phys. Rev. A 57, 4730-4735 (1998)

J.H. Müller, D. Bettermann, V. Rieger, K. Sengstock , U. Sterr, W. Ertmer
Topological Phase Shift in a Cold-Atom Interferometer
Appl. Phys.B60, 199-204 (1995)

U. Sterr, K. Sengstock , J. H. Müller, D. Bettermann, W. Ertmer
The Magnesium Ramsey - Interferometer: Applications and Prospects
Appl. Phys. B 54, 341 (1992)

F.E. Dingler, V. Rieger, K. Sengstock , U. Sterr, W. Ertmer
Excitation of Only a Single Recoil Component in Optical Ramsey Interferometry Using Cross-Over Resonances
Optics Comm. 110, 99-104 (1994)

K. Sengstock , U. Sterr, G. Hennig, D. Bettermann, J.-H. Müller, W. Ertmer
Optical Ramsey Interferences on Laser Cooled and Trapped Atoms, Detected by Electron Shelving
Optics Comm. 103, 73-78(1993)

V. Rieger, K. Sengstock , U. Sterr, J.-H. Müller, W. Ertmer
Atom-Interferometric Determination of the DC-Stark Shift of the Mg-Intercombinationline
Optics Comm. 99, 172-176 (1993)

U. Sterr, K. Sengstock , J.-H. Müller, W. Ertmer
High Resolution Isotope Shift Measurement of the Mg I1So-3P Intercombination Transition
Appl. Phys. B 56, 62-64 (1993)


Options for students (PhD and Diploma students)

We offer various research options for PhD and Diploma students in our experimental team. If You want to learn more about possibilities to earn Your PhD or Diploma degree in physics on one of our research projects, please do not hesitate to get in touch with us.

Also we are always on the lookout for students with a keen interest in experimental physics and/or electronics to support our experiments as paid student helpers („HiWis“).

If You just wish to give atom optics a try for a shorter period of time (from some days to some weeks), You might also consider an internship in our group.

Feel free to contact us by phone or email: Dr. Ernst M. Rasel, Phone +49-(0)511-762-19203

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Collaboration Partners 

We are members of the SFB 407 of the Deutsche Forschungsgemeinschaft and the European training networks CAUAC and PROCOPE.

Funding

Our research is supported by