Indirect laser cooling of elementary particles
Final Report Abstract
The antihydrogen atom is the system of choice for tests of matter–antimatter symmetry (CPT/Lorentz invariance) and of antimatter gravity. While high-precision tests of CPT have been carried out both on leptons and on baryons, antihydrogen spectroscopy holds the prospect of extending direct CPT tests to composite antimatter. Furthermore, neutral antihydrogen would allow the first-ever direct study of the gravitational attraction on antimatter. All of these experiments will require an antimatter sample at very low temperatures – well below 1 K. In 2002, the production of cold antihydrogen from positrons stored at 4…15 K and antiprotons was demonstrated for the first time by two experiments installed at the CERN Antiproton Decelerator. The subject of the research project “Indirect laser cooling of elementary particles” was the investigation of a technique which will allow the production of anti-atoms well below this temperature range. It is based on the laser cooling of a negative atomic ion, which in turn may be used to cool any negatively charged particle (such as the antiproton) by collisions. There is currently only one known element whose negative ion exhibits a strong transition between bound states, a requirement for laser cooling. Few atomic anions have bound excited states because the excess electron is only loosely bound to the neutral atomic core. When it is excited, it can easily become detached. In 2000, a research group at McMaster University discovered an optical transition potentially suitable for laser cooling in the negative osmium ion. The experimental work in this project therefore concentrated on the study of the osmium anion by laser spectroscopy. A sophisticated apparatus consisting of a negative-ion source, a mass separator, a narrow-bandwidth laser and a Penning trap contained in a superconducting magnet (for subsequent confinement of the ions) was constructed. A novel technique which combines laser excitation with electric-field neutralization was developed and applied for the first time. First, the relevant transition in the most abundant osmium isotope 192Os− was studied. The prior measurement of the transition frequency was confirmed and its precision improved by a factor 100. With a relative precision of 10−7 this is the most precise measurement ever made of any feature in a negative ion. The transition rate was also measured and found to be lower than previous measurements had indicated. The laser spectroscopy was then repeated on two less abundant stable isotopes which exhibit a nuclear spin (187Os− and 189Os−), resulting in a hyperfine structure splitting of the spectral lines. The number of observed transition lines yielded the total angular momentum quantum number of the excited state. With this information, it was possible to calculate the expected energy level diagram in the magnetic field of the Penning trap and to identify a transition between magnetic sub-levels suitable for laser cooling. It was found that the excited state may also decay to one other sub-state of the ground state, indicating the need for an additional laser to bring ions back into a state from which they can be excited by the cooling laser. Overall, the experimental data gathered in the course of this research project has provided significant insight into the structure of the negative osmium ion. The bound excited state has been fully characterized, yielding valuable information on the feasibility of Os− laser cooling. The lowerthan-expected transition rate means that the ions will have to be pre-cooled to 4 K by other techniques prior to laser cooling to reduce the number of required photon absorption and emission processes and hence the likelihood of ion loss by two-photon detachment. The need for an additional repumping laser increases the cost of the experimental setup, but is not a fundamental obstacle.
Publications
- “A novel cooling scheme for antiprotons”. New J. Phys. 8 (2006) 45
A. Kellerbauer, J. Walz
- “Dynamics of antihydrogen production in Penning traps and indirect laser cooling of antiprotons”. Habilitation thesis, University of Heidelberg (2009)
A. Kellerbauer
- “High-resolution laser spectroscopy on the negative osmium ion”. Phys. Rev. Lett. 102 (2009) 043001
U. Warring, M. Amoretti, C. Canali, A. Fischer, R. Heyne, J. O. Meier, Ch. Morhard, A. Kellerbauer
- “Laser spectroscopy on Os−: A prerequisite for the laser cooling of atomic anions”. Ph.D. thesis, University of Heidelberg (2009)
U. Warring
- “Ultracold antiprotons by indirect laser cooling”. In: 9th International Conference on Low-Energy Antiproton Physics, Vienna, Austria, 16–19 September 2008. / Ed. by B. Juhász et al. – Hyp. Int. 194 (2009) 77
A. Kellerbauer, C. Canali, A. Fischer, U. Warring
- “First optical hyperfine structure measurement in an atomic anion”. Phys. Rev. Lett. 104 (2010) 073004
A. Fischer, C. Canali, U. Warring, A. Kellerbauer, S. Fritzsche
- “Frequency control of a 1163 nm singly resonant OPO based on MgO:PPLN”. Opt. Lett. 35 (2010) 820
P. Groß, I. D. Lindsay, C. J. Lee, M. Nittmann, T. Bauer, J. Bartschke, U. Warring, A. Fischer, A. Kellerbauer, K.-J. Boller
