Laser metrology with subHertz lasers

Summary form only given. We describe a subHertz laser and its use in an optical frequency standard that is based on the narrow /sup 2/S/sub 1/2/-/sup 2/D/sub 5/2/ electric quadrupole transition (/spl lambda//spl sime/282 nm) of a single trapped /sup 199/Hg/sup +/ ion. The ion is suspended in vacuum by well-controlled electric and magnetic fields and laser-cooled to low kinetic temperatures. A small linear trap has been constructed that gives Lamb-Dicke confinement for the cold ion even for the S-D optical transition. The trap will be housed in a vacuum enclosure that is fixed to the bottom plate of a liquid helium dewar. Operation of the trap at cryogenic temperatures gives long ion storage times and significantly suppresses frequency shifts due to collisions and blackbody radiation. The S-D quadrupole transition has a natural linewidth less than 2 Hz and, in a room-temperature trap, we have already demonstrated that transitions into the D state can be detected with nearly 100% efficiency by means of electron shelving. If the transition is probed by the time-domain Ramsey method with a free precession time of 30 ms, the fractional frequency stability is predicted to be about 1/spl times/10/sup -15/ /spl tau//sup -1/2/, provided that the linewidth of the laser is less than 1 Hz for times from 30 ms to several seconds. A major step toward the completion of this standard has been the construction of a well-isolated, high-finesse, Fabry-Perot cavity to stabilize the frequency of the laser. We have recently demonstrated a linewidth of about 0.5 Hz from 100 ms to 20 s between two 563 nm laser beams locked to independent stable cavities. We will report our cavity results and our progress toward locking the frequency of the laser to the cryogenically stored ion.