Thermal noise and mechanical loss of SiO2/Ta2O5 optical coatings at cryogenic temperatures.

Mechanical loss of dielectric mirror coatings sets fundamental limits for both gravitational wave detectors and cavity-stabilized optical local oscillators for atomic clocks. Two approaches are used to determine the mechanical loss: ringdown measurements of the coating quality factor and direct measurement of the coating thermal noise. Here we report a systematic study of the mirror thermal noise at 4, 16, 124, and 300 K by operating reference cavities at these temperatures. The directly measured thermal noise is used to extract the mechanical loss for SiO2/Ta2O5 coatings, which are compared with previously reported values.

Demonstration of a Timescale Based on a Stable Optical Carrier.

We report on the first timescale based entirely on optical technology. Existing timescales, including those incorporating optical frequency standards, rely exclusively on microwave local oscillators owing to the lack of an optical oscillator with the required frequency predictability and stability for reliable steering. We combine a cryogenic silicon cavity exhibiting improved long-term stability and an accurate ^{87}Sr lattice clock to form a timescale that outperforms them all. Our timescale accumulates an estimated time error of only 48±94 ps over 34 days of operation. Our analysis indicates that this timescale is capable of reaching a stability below 1×10^{-17} after a few months of averaging, making timekeeping at the 10^{-18} level a realistic prospect.

Ultrastable laser with average fractional frequency drift rate below 5 × 10⁻¹9/s.

Cryogenic single-crystal optical cavities have the potential to provide high dimensional stability. We have investigated the long-term performance of an ultrastable laser system that is stabilized to a single-crystal silicon cavity operated at 124 K. Utilizing a frequency comb, the laser is compared to a hydrogen maser that is referenced to a primary caesium fountain standard and to the 87Sr optical lattice clock at Physikalisch-Technische Bundesanstalt (PTB). With fractional frequency instabilities of σ(y)(τ)≤2×10(-16) for averaging times of τ=60 s to 1000 s and σ(y)(1 d)≤2×10(-15) the stability of this laser, without any aid from an atomic reference, surpasses the best known microwave standards for short averaging times and is competitive with the best known hydrogen masers for longer times of 1 day. The comparison of modeled thermal response of the cavity with measured data indicates an average fractional frequency drift below 5×10(-19)/s, which we do not expect to be a fundamental limit.

Bose-Einstein condensation of alkaline earth atoms: ;{40}Ca.

We have achieved Bose-Einstein condensation of ;{40}Ca, the first for an alkaline earth element. The influence of elastic and inelastic collisions associated with the large ground-state s-wave scattering length of ;{40}Ca was measured. From these findings, an optimized loading and cooling scheme was developed that allowed us to condense about 2 x 10;{4} atoms after laser cooling in a two-stage magneto-optical trap and subsequent forced evaporation in a crossed dipole trap within less than 3 s. The condensation of an alkaline earth element opens novel opportunities for precision measurements on the narrow intercombination lines as well as investigations of molecular states at the ;{1}S-;{3}P asymptotes.

Calibration of a Shack-Hartmann sensor for absolute measurements of wavefronts.

We demonstrate a method with which to calibrate a Shack-Hartmann sensor for absolute wavefront measurement of collimated laser beams. Nearly perfect spherical wavefronts originating from a single-mode fiber were used as references. After the calibration, the uncertainty of the wavefront was less than lambda/100 peak to valley across a diameter of 6 mm. For example, this method allowed us to balance aberrations and prepare collimated beams with wavefronts that are plane to lambda/500 across 1 mm.