Sympathetic cooling of trapped Cd + isotopes

A collection of cold trapped ions offers one of the most promising avenues towards realizing a quantum computer @1‐4 #. Quantum information is stored in the internal states of individual trapped ions, while entangling quantum logic gates are implemented via a collective quantized mode of motion of the ion crystal. The internal qubit states can have extremely long coherence times @5#, but decoherence of the motion of the ion crystal may limit quantum logic gate fidelity @6#. Furthermore, when ions are nonadiabatically shuttled between different trapping regions for large-scale quantum computer schemes @4,7#, their motion must be recooled for subsequent logic operations. Direct laser cooling of the qubit ions is not generally possible without disturbing coherence of the internal qubit states. Instead, additional ‘‘refrigerator’’ ions in the crystal can be directly laser cooled, with the qubit ions cooled in sympathy by virtue of their Coulomb-coupled motion @8#. The laser cooling of the refrigerator ions can quench unwanted motion of the ion crystal, while not affecting the internal states of the qubit ions @4,9,10#. Sympathetic cooling has been observed in large ensembles of ions in Penning traps @8,11#, impurities in small collections of ion crystals, and in small ion crystals consisting of a single species, where strong laser focusing was required to access a particular ion without affecting the others @12#. Here, we report the first demonstration of sympathetic cooling in a small ion crystal with two different species where both species are independently optically addressed. We study sympathetic cooling of individual Cd 1 isotopes confined in a rf trap. One ion isotope~the refrigerator ion! is continuously Doppler-cooled by a laser beam red detuned from its D2 line ( S1/2-P3/2), while the other isotope ~the probe ion! is either Doppler cooled or Doppler heated by another beam, whose frequency is scanned around its D2 resonance line. The effect of the sympathetic cooling is to enable measuring fluorescence on the blue side of the probe ion’s resonance. Ordinarily, when the probe laser beam is tuned to the blue of the probe ion’s resonance, the ion ceases fluorescing due to Doppler heating, but the sympathetic cooling from the refrigerator ion keeps the probe ion cold and fluorescing regardless of the probe tuning. In the experiment, the probe ion is 112 Cd 1 , while the refrigerator ion is 114 Cd 1 ~both isotopes have zero nuclear spin!. The respective D2 lines of these two neighboring isotopes are separated by about 680 MHz, with the heavier ion at lower frequency, and the natural linewidth of each ion’s excited P3/2 state is g/2p.47 MHz. The experimental apparatus is schematically shown in Fig. 1. The Cd 1 D2 line resonant light near 214.5 nm is generated by quadrupling a Ti:sapphire laser. The laser is stabilized to a molecular tellurium feature near 429 nm to better than 1 MHz. The quadrupled UV output is split into two parts; one part is upshifted by ;420 MHz, while the