278 Ion traps
Frequency of laser radiation,Signal (arbitrary units)(a)(b)Fig. 12.10(a) The vibration levels of an ion in a harmonic trap have the same spacing for both the ground and excited
electronic states. Excitation by light of frequencyωL=ω 0 −ωv, followed by spontaneous emission, leads to a decrease in the
vibrational quantum number, until the ion reaches the lowest level withv= 0, from whence there is no transition, as indicated
by the dotted line at this frequency (but light at frequencyω 0 andω 0 +ωvwould excite the ion inv= 0). (b) The spectrum
of a trapped ion shows sidebands on either side of the main transition. The inset figure shows the spectrum before sideband
cooling. The enlarged part of the figure shows that after sideband cooling the signal on the upper sideband SUat angular
frequencyω 0 +ωvis stronger than the lower one SLatω 0 −ωv—this asymmetry indicates that the ion is mainly in the lowest
vibrational state (i.e. vibrational quantum number much less than 1). The vertical axis gives the transition probability, or the
probability of a quantum jump during the excitation of the narrow transition. After Diedrichet al. (1989). Part (b), copyright
1989 by the American Physical Society.