Journal Club

Accompanying the lectures we will work on original literature in the field that has been published in scientific journals such as Science, Nature, Physical Review Letters, etc. This will allow you to become acquainted not only with the original work, but also should help you in writing your own articles in the future.

If you have trouble downloading the papers, please follow the tips and links below. If it still does not work, please let us know.

1. The first article in the journal club is the first part of the Nobel lecture of William D. ("Bill") Philips, who received the Nobel Prize of 1997 for the demonstration of laser cooling. His lecture describes what he discovered - and how. It is only partially a scientific paper and partially a story.

   We are interested, for now, in the beginning of the paper which describe the slowing of atoms by light -until page 209:

   • Why can the momentum of the reemitted photon in Figure 1 be neglected in calculating the slowing force?
   • What is optical pumping and why can it be a problem in laser cooling? How can it be circumvented?
   • What is the Doppler effect and what is its effect on the cooling? Which effect allows one to use magnetic fields in order to circumvent the Doppler shifts?
    
2. The second paper (2. Nov. 2011) we will discuss is: 
   Three-Dimensional Viscous Confinement and Cooling of Atoms by Resonance Radiation Pressure, Phys. Rev. Lett. 55,1 (1985)
   written by Steven Chu, L. Hollberg, J. E. Bjorkholm, Alex Cable, and A. Ashkin.
    
   • with all papers, please try to answer for yourself the main questions: what has been done, why has it been done, how was it accomplished, and what can be concluded from the results.
   • Obviously, sometimes it can be easier to understand some part of the paper by looking into the books, papers and articles that are referenced in the text. 
   • There are also a few specific questions for this particular paper in this week's problem sheet.
    
 
3. The third paper is a newer paper on the the 2D cooling of Rubidium atoms from the group of J.T.M. Walraven in the Netherlands: PRA58,2891 (1998).
    It implements and analyzes three methods for the generation of a cold atomic beam. While reading the article, try two answer the following questions:
    
   • What are the differences between the three methods?
    
   • What are the figures of merit, i.e. what are the important characteristics of the produced beams?
    
   • What are the typical applications of these cold beams?
    
   • What limits the efficiency of the discussed methods?
    
4. The paper on 16th of November will be "Evaporative Cooling of Sodium Atoms" by Kendall Davis et al., Phys. Rev. Lett. 76, 5202 (1995). Not that shortly afterwards an erratum paper with corrections was added.  Apart from the general method, motivation, and results aspects, please consider: 
    
   • What is limiting the evaporation process that the authors observe - why can't they go all the way to the BEC?
    
   • What causes the funny shape of the graph in figure 2? 
5. The fifth paper of the journal club reports on one of the first realizations of BEC in dilute gases: PRL75, 3969 (1995) It is again by the group of W. Ketterle at the MIT and uses similar techniques as the last paper: 
   • What is the major advancement compared to last weeks paper that allowed them to reach colder temperatures? 
   •  What is the main observable in this paper? 
   • What are the two experimental signatures of the BEC?
6. The paper for Nov. 30 is: Oscillations and interactions of dark and dark–bright solitons in Bose–Einstein condensates: Nature Physics 4, 496 - 501 (2008) Some questions to consider are: 
    
   • What is a dark-bright soliton in comparison to a "normal" soliton?   
   • Why is there special interest in the collision of solitons? What is different from colliding "normal" waves?  
7. In the journal club on December 7th, the topic will be the creation of vortices in a weakly interacting BEC: PRL84, 806 (2000). 
   
   • Which methods are there to create vortices in a BEC and which of these methods was used here? 
   • Are the interactions within the gas important for this experiment? 
   • How does an acousto-optical modulator work? (not in the paper)
   In later experments with Na it was possible to create many more vortices, some nice images can be found here.
8.  In the journal club on Dec. 14, we will look at one of the first papers where a BEC is loaded into an optical lattice. These measurements were performed in Schellingstrasse 4: "Exploring Phase Coherence in a 2D Lattice of BECs", Phys. Rev. Lett. 87, 160405 (2001)
   • How does this experiment compare with diffraction of light from a grating? What is the ``slit width''?
   • The authors describe a ``2D periodic array of [many] quantum gases''. What is the shape of an individual one of these quantum gases (try to find an approximate formula for the density or wave function; without exact constants of course).
   • In all data shown the atoms were subjected to some sort of optical lattice. Also, all experiments ended by imaging the atoms after free expansion. In between there is three different ways of how the lattice is switched off. What are the three different variants?
   • In the diffracted patterns shown, why is the number of ``peaks'' so low if this is supposed to be like grating diffraction patterns? For example the upper row of figure two: Only three peaks are visible - where are the other diffraction orders?
   • Why do the rings show up? 
   • Where does the square come from? 
 
9. In the last article of this year (Dec. 21) the focus is on interacting bosons in optical lattices and we will look at rather recent work on Mott insulators in 2D that has been performed in the group of Cheng Chin in Chicago: Nature 460, (2009)
    
   • What was the experimental procedure to create the 2D system? 
   • What was the observable used to study the Mott insulator, and why is this observable especially useful in 2D? 
   • How did they extract the density fluctuations shown in figure 4 and what is the signature of the Mott insulator in this measurement?
    
10.  On Jan. 11, we will discuss a paper by M. Gustavsson et al., Phys. Rev. Lett. 100, 080404 (2008). They demonstrate Bloch oscillations which go on for an extremely long time. The authors use an effect called "Feshbach resonances", which will be discussed later in the lecture, for now you only need to know that this makes it possible to freely choose, and vary,  the scattering length a for Cesium atoms.
     
    • How do the Bloch oscillations typically "end": Do they just become slower and stop?
    • Give at least three effects that could in principle limit the lifetime of the Bloch oscillation dynamics.
    • What do the authors do with the interactions to make the Bloch oscillations survive much longer?
    • Why does a vertical force gradient result in a "collapse and revival" of the oscillations, and what determines the revival time?
       
     
11.  On January 25th we will look at scattering experiments done with cold atoms. It is published by a group from new Zealand in PRL, 93, 173201 (2004). Related articles can be found here and here.
     
    • What is being done in order to have not only s-wave scattering present?
    • What is the Abel transformation? Why is it used prominently here?
    • How are the different scattering channel signals distinguished?
     
12. This time we will look at the first creation of Feshbach molecules, that is weakly bound molecules that are created with the help of a Feshbach resonance. This experiment uses again fermionic 40K and was performed in the group of D. Jin in Boulder, Colorado: Nature, 424, 47 (2003)  
    
    •  How do they create the Feshbach molecules?
    • Why don't the molecules show up in absorption imaging?
    • How do they measure the binding energy of the molecules, and why is this important?
    • What would be alternative ways to create ultracold molecules?
13. The last paper we will discuss in the Journal club is the following article:
    Vortices and superfluidity in a strongly interacting Fermi gas}, M. Zwierlein et al. Nature 435, 1047 (2005). As the title says, here the superfluid properties of a gas around the BEC-BCS crossover are analyzed. In this case this is done by looking at vortices that form in the gas. 
    
    •  Why do the authors argue are vortices so important for the fermionic fluids?
    • What is the behaviour of lifetime (according to this paper) of these vortices and what are the reasons for it?

You can of course also browse papers yourself in the various journals. For the journal club the most relevant journals will be Physical Review Letters (PRL), Physical Review A (PRA), Reviews of Modern Physics (Rev. Mod. Phys), Nature and Science.

  For the most convenient direct access to journals from within the university network, you need to set the proxy server of your browser to the university library or to the physics institute's server. Here you can read how to do it. This seems to include CIP computers, as apparently not all new LMU accounts are pre-configured accordingly.

  To access papers from outside the university, you need to use VPN into the university network, or alternatively there is a (kind of slow) web login interface which you can access here.