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Optical cavities are essential elements of optical atomic clocks, as they serve to pre-stabilize the clock laser. Our Group is currently developing state-of-the-art cavities to reduce the instability of optical clocks at NIST.
Since a free-running laser typically has a fast linewidth way too broad for high resolution spectroscopy of optical clock transitions, it is necessary to pre-stabilize its frequency relative to a narrow resonance of a carefully designed optical cavity. An optical Fabry-Perot cavity consists of high reflectivity mirrors separated by a spacer, usually made of a low expansion material such as ULE glass. When the frequency of the laser is tuned such that an integral number of wavelengths fits in the gap between the two mirrors, constructive interference leads to the build-up of an intense standing wave inside the cavity and light is transmitted through the cavity. These cavity resonances are spaced by c/2L (where c is the speed of light and L is the length of the cavity), termed the free spectral range, which has a typical value of around 500 MHz.
To stabilize the clock laser we tune its frequency near one of the resonances of the cavity and use a feedback loop to keep the frequency of laser locked tightly to that of the resonance. A well-designed lock can reduce the residual frequency noise of the laser to that of the thermal noise floor of the optical cavity. This technique can lead to laser linewidths of 100 mHz and below. To reach such performance levels it is necessary to isolate the reference cavity from vibrations and thermal fluctuations. Under such conditions we can attain the long interaction time (0.1 - 1 s) required to resolve extremely narrow atomic spectra. With the Yb lattice clock we have used such pre-stabilized lasers to achieve spectroscopic linewidths of 1 Hz and fractional instabilities as low as 3 x 10-16τ-1/2, where τ is the averaging time.
Since the stability of an optical atomic clock based on large numbers of trapped neutral atoms can be limited by the thermal noise of the clock laser reference cavity, we continue to strive to advance cavity performance. Presently we are designing new cavities with lower thermal noise limits through optimal choice of the materials used for the cavity spacer and the cavity mirror substrates and coatings. Cryogenically-cooled cavities may provide another path to reduced cavity thermal noise and improved laser performance.
Apply for an NRC postdoc in optical atomic clock or fs-laser frequency comb research. Application deadlines are February 1 and August 1 annually (but inquire well in advance).
Summer program at NIST-Boulder for undergraduates in science, engineering, and mathematics. The application deadline is February 15 annually.
Postdoctoral, Visiting Scientist, and Graduate Positions
Our Group has periodic openings for Postdoctoral Fellows, Visiting Scientists, and Graduate Students. Please contact us for more information.