Atomic ions laser cooling

The next stage in the development of atomic clocks that utilize the control of atoms will be characterized by the use of optical transitions in atoms, specifically laser-cooled trapped atoms or ions, and the laser synthesis of optical and microwave radiation (Chapter 6). 

These ion lasers are very inefficient, partly because energy is required first to ionize the atom and then to produce the population inversion. This inefficiency leads to a serious problem of heat dissipation, which is partly solved by using a plasma tube, in which a low-voltage high-current discharge is created in the Ar or Kr gas, made from beryllium oxide, BeO, which is an efficient heat conductor. Water cooling of the tube is also necessary. 

Single atomic ions confined in radio frequency traps and cooled by laser beams (Figure 7.4a) formed the basis for the first proposal of a CNOT quantum gate with an explicit physical system [14]. The first experimental realization of a CNOT quantum gate was in fact demonstrated on a system inspired by this scheme [37]. In this proposal, two internal electronic states of alkaline-earth or transition metal ions (e.g. Ba2+ or Yb3+) define the qubit basis. These states have excellent coherence properties, with T2 and T2 in the range of seconds [15]. Each qubit can be... 

A mercury atom that was ionized by a weak electron beam was captured in a miniature Paul (radio frequency) trap that has internal dimensions of rQ s 466 pm and zQ s 330 pm. The rf trapping frequency was 21.07 MHz with a peak voltage amplitude of about 730 V. The ion was laser cooled by a few microwatts of cw laser radiation that was frequency tuned below the 6s Si -6p Pi electric dipole transition near 194 nm. When the Hg+ ion was cold and the 194 nm radiation had sufficient intensity to saturate the strongly allowed S-P transition, 2 x 10 photons/s were scattered. With our collection efficiency, this corresponded to an observed peak count rate of about 10 s-1 against a background of less than 50 s— -. 

This works only for a dilute concentrations of the atoms, to prevent the absorption of the photons into the gas in the form of heat due to atom-atom collisions. Only certain atoms and ions have optical transitions amenable to laser cooling, since it is extremely difficult to generate the amounts of laser power needed at wavelengths much shorter than 300 nm. The following is a partial list of atoms that have been laser-cooled H, Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Cr, Er, Fe, Cd, Ag, Hg (plus metastable Al, Yb, He, Ne, Ar, Kr), and some ions. 

The familiar 632.8 nm output of the He-Ne laser is available in several integrated Raman spectrometers of analytical interest. Unlike ion lasers, the lasing transition in the He-Ne is an atom (neon), so power need not be consumed to create excited ions. Helium ions and electrons carry the current in the He-Ne laser tube, but energy is transferred to Ne atoms before lasing, and the process is much more efficient than that of ion lasers. Ordinary 110 V electrical power and air cooling are sufficient, and He-Ne lasers are generally much smaller (and less expensive) than ion lasers. However, the output optical power is much lower for the He-Ne laser compared to Ar+ lasers, with the common... 

DOPPLER COOLING The first ideas for laser cooling of atoms [6] and ions [7] were published in 1975. The basic idea of what is now called Doppler cooling can be understood by referring to the famous picture. Here we see a typical atomic resonance curve, which we can take to show the rate at which an atom... 

Thus the ideal experiment requires a single atom to be investigated. Since some time it is known that ion traps allow to study the fluorescence from a single laser cooled particle practically at rest, thus providing the ideal case for the... 

S. Trajmar and J. C. Nickel The Dissociative Ionization of Simple, Molecules by Fast Ions, Colin J. Latimer Theory of Collisions between Laser Cooled Atoms, P S. Julienne, A. M. Smith, and K. Burnett... 

Itano, W.M. Bergquist, J.C. Bollinger, J.J. Wineland, D.J., Laser cooling of trapped ions in Laser Manipulation of Atoms and Ions, Proc. Enrico Fermi Summer School, Course CXVIII, Varenna, Italy, July, 1991, edited by E. Arimondo, W.D. Phillips, and F. 

A schematic of the SCSI-MS technique is presented in Figure 10.1. The technique relies on the measurement of the resonant excitation frequency of one of the two oscillatory modes of a trapped and crystallized linear two-ion system consisting of one laser-cooled atomic ion of known mass and the a priori unknown atomic or molecular ion, whose mass is to be determined. From this measured frequency, the mass of the unknown ion can be deduced from a simple relation between the frequency and the relative masses of the two ions (see Section 10.3). 

It is interesting first to note that in contrast to standard mass measurement techniques, a strong Coulomb coupling between ions (the ion of interest and the laser-cooled atomic ion) is essential, rather than being problematical. However, as is discussed in Section 12.5, the non-linear nature of the Coulomb interaction between the two ions can lead to unwanted systematic errors in the mass measurements. 

A limitation of the SCSI-MS technique in its present linear RF trap version is the requirement that the mass of the unknown singly-charged ion be within the range (ca 0.3-3)ma, ion, where is the mass of the laser-cooled atomic ion, in order to achieve stable operating conditions with respect to the dynamical confining potential (see Section 10.3.1). With the availability of more atomic ion species that can be laser cooled, it will be possible to extend the range of masses that can be measured with the present complex experimental arrangement beyond the Mg and Ca ions that are... 

Recently, the axial alignment of two ions in a Penning trap has been demonstrated [13]. When a sufficiently general procedure can be realized such that any two simultaneously-trapped ions can be so aligned, the above-mentioned mass range for a single laser-cooled atomic ion species may be increased also. 

Not only can the radiation pressure force lead to ion cooling through the Doppler-effect, but it can be used further to exert a periodic force on the atomic ions. By modulating the intensity of one of the two laser-cooling beams propagating along... 

In Figure 10.4c and d, images of two laser-cooled atomic ions and one atomic and one molecular ion (CaO +) are presented when applying a driving force of a fre-qnency close to co +. The smearing of the fluorescence as compared to Figure 10.4a and b, shows clearly that the ions are excited motionally. 

---