The National Institute of Information and Communications Technology in Japan is proceeding with development of an optical frequency standard using an electric quadrupole transition of Ca+ ions in a small radio-frequency trap. The single, trapped 40Ca+ ions are laser cooled to a temperature of <10 mK using extended-cavity diode lasers at 397 nm and 866 nm. In order to measure the electric quadrupole transition, the linewidth of the clock laser diode at 729 nm is reduced to ~66 Hz using an ultra-low-expansion (ULE) cavity. We observed the electric quadrupole transition with a linewidth of ~600 kHz, together with the ionic motional sidebands. We are also developing optical frequency combs without photonic fibers broadening the spectrum. The experimental is now improved in order to observe the narrower linewidth of the quadrupole transition.
We have laser cooled a small cloud of
40Ca+ ions stored in a Penning trap. The large Zeeman splittings that result from the presence of the imposed magnetic field necessitate the use of two cooling lasers tuned to the ${}^{2}{S}_{1/2}{\ensuremath{-}}^{2}{P}_{1/2}$ transition near 397 nm (whereas only a single blue laser frequency is required in an rf trap). The 397 nm radiation is provided by a pair of blue diode lasers operated in extended cavities. Ions can escape from the cooling cycle by falling into a
2D3/2 state. There is also a small probability that ions can be pumped into a
2D5/2 state. The presence of large Zeeman splittings complicates the provision of repumper radiation to empty the D states. We describe two repumping schemes. The first scheme employs five infrared extended cavity diode lasers (ECDL's). The second scheme employs three infrared ECDL's, two of which have their injection current modulated to produce sidebands. An upper bound to the temperature of 1 K is inferred from the linewidth of the 397-nm fluorescence for a small cloud of
40Ca+ ions in our Penning trap. This work is part of a program aimed at using atomic ions in a Penning trap for decoherence studies and quantum information processing.
Doppler cooling of a single 40 Ca + ion in a spherical Paul trap is systematically investigated. Temperatures comparable to the Doppler limit are achieved by optimizing the cooling parameters. These parameters include the laser detunings, the intensities of the cooling beams, and the magnitude of the applied magnetic fields. The temperature of the Doppler-cooled ion is estimated to be 570 µK from the asymmetry of the vibrational sidebands on the quadrupole transition, which is close to the Doppler limit of 540 µK.