Strontium project

Dan Sadler, Danielle Boddy, Graham Lochead, and Matthew Jones

Outline:

Laser cooling and laser spectroscopy are powerful techniques for controlling the motion and quantum state of individual atoms. In this project we aim to extend this level of control to the interactions between the atoms. By exciting laser-cooled strontium atoms to a very high lying electronic energy state - known as a Rydberg state, we can switch on strong, long-range van der Waals interactions between the atoms, which completely dominate the behaviour of the atom cloud. Uniquely, our experiment uses strontium atoms, which also have a second valence elctron that can be used to probe and manipulate the Rydberg gas. So far, we have shown that this can provide information on collisions that can transfer atoms from one Rydberg state to another, with very high temporal resolution. Currently we are extending these techiques to provide spatial information, which will enable us to probe spatial correlations in this strongly interacting system.

We also have several fruitful collaborations with theiry. At Durham with Christoph Vaillant and Robert Potvliege we are carrying out detailed calculations of the interaction strength. With the group of Thomas Pohl at MPIPKS in Dresden, Germany we are investigating quantum many-body effects and entanglement.

Postdoc position available:
A postdoctoral position is available to come and join our team and get involved in our exciting experiments on spatial correlations in cold Rydberg gases. Click on the flyer below for more details.

Latest graph:

March 2011: A laser at 689nm is shone through a hot atomic beam of strontium, and excites atoms on the narrow singlet-triplet intercombination line to the 5s5p ³P₁ triplet state. The resultant fluorescence is recorded on a CCD camera. This image shows that image, and a contour plot. The laser is moving into the page, and the hot atomic beam right to left. The atoms fluoresce strongly where they interact with the laser, and the population of the triplet state is seen to decay as the atoms move right to left. From this image we can gather information about the thermal distribution of the atoms, and the lifetime of the triplet state. We can use this triplet state transition to cool strontium from milliKelvin to hundreds of nanoKelvin.

Progress so far:
	Two-electron excitation paper!
	Our work on studying interactions in a cold Rydberg gas using two electron excitation has been accepted for publication in Physical Review Letters! The graph shows the narrowing of the autoionization spectrum of the 56D state over time, a clear indication of transfer to a different angular momentum state.

Autoionization
The first two-electron excitation has been performed in the worlds first cold strontium Rydberg gas! After the first electron was excited to the 5s20s Rydberg state using a laser at 420nm another laser at 408nm was used to excite the second electron from the 5s to the 5p state. This causes the atom to ionize, a process called "autoionization". The graph to the right shows the ion spectrum from the Rydberg atoms in the 20s state spontaneously ionizing (black curve), and the autoionization ion spectrum (red curve). Autoionization is clearly a much more sensitive technique!

	Spectroscopy for laser stabilization
	Our strontium vapour cell has been used to perform high resolution spectroscopy, in particular sub-Doppler DAVLL (see figure) and polarization spectroscopy ("polspec"). The paper we wrote can be found here. We use polspec to stabilise the blue (461nm) laser in all our experiments.

Our new vapour cell design
We have been working on a new design for strontium vapour cells based on dispensers - commercially available sealed sources that emit strontium vapour when heated by an electrical current. The cells are compact, operate at room temperature and do not require a pump. The first generation was developed by Elizabeth Bridge during her final year project. Our paper on this design will appear in Review of Scientific Instruments. More recently Clementine Javaux (a summer student from the Institut d'Optique) and Graham have built a second generation cell that uses two dispensers to reach 100% absorption on the 461nm transition as shown in the graph on the right.

	Cold strontium arrives in Durham!
	At 17.30pm on the 15th August 2008 (a Friday!) w produced our first magneto-optical trap (MOT) of strontium atoms. In the image on the right, taken by James using an ordinary digital camera, the MOT is the diffuse cloud of atoms in the centre of the picture. They glow blue because they are scattering light from the blue cooling laser at 461nm. Stray light from this laser also illuminates the MOT coils used to trap the atoms visible at the top and bottom of the picture. For more pictures of the experiment click on the Strontium Gallery link.

Strontium Rydberg EIT
We have detected strontium Rydberg states in our atomic beam using electromagnetically induced transparency (EIT). In EIT, the absorption of laser light on an atomic "probe" transition is reduced by coupling the upper state to a third level with an additional laser. In our experiments this third level is a highly excited Rydberg state (n=18). The coupling to the Rydberg state gives narrow EIT resonances within the Doppler broadened absorption profile (see left). This technique could be very useful as a high resolution probe in future experiments with cold Rydberg gases and plasmas. Our paper on this work can be found here.


	Durham Atomic and Molecular Physics