Ultimate Frequency Stability Research Articles

A superradiant clock laser on a magic wavelength optical lattice.

An ideal superradiant laser on an optical clock transition of noninteracting cold atoms is predicted to exhibit an extreme frequency stability and accuracy far below mHz-linewidth. In any concrete setup sufficiently many atoms have to be confined and pumped within a finite cavity mode volume. Using a magic wavelength lattice minimizes light shifts and allows for almost uniform coupling to the cavity mode. Nevertheless, the atoms are subject to dipole-dipole interaction and collective spontaneous decay which compromises the ultimate frequency stability. In the high density limit the Dicke superradiant linewidth enhancement will broaden the laser line and nearest neighbor couplings will induce shifts and fluctuations of the laser frequency. We estimate the magnitude and scaling of these effects by direct numerical simulations of few atom systems for different geometries and densities. For Strontium in a regularly filled magic wavelength configuration atomic interactions induce small laser frequency shifts only and collective spontaneous emission weakly broadens the laser. These interactions generally enhance the laser sensitivity to cavity length fluctuations but for optimally chosen operating conditions can lead to an improved synchronization of the atomic dipoles.

New-generation of cryogenic sapphire microwave oscillators for space, metrology, and scientific applications

This article reports on the characterization of cryogenic sapphire oscillators (CSOs), and on the first test of a CSO in a real field installation, where ultimate frequency stability and continuous operation are critical issues, with no survey. Thanks to low-vibration liquid-He cryocooler design, Internet monitoring, and a significant effort of engineering, these oscillators could bridge the gap from an experiment to a fully reliable machine. The cryocooler needs scheduled maintenance every 2 years, which is usual for these devices. The direct comparison of two CSOs demonstrates a frequency stability of 5 × 10(-16) for 30 s ≤ τ ≤ 300 s integration time, and 4.5 × 10(-15) at 1 day (1 × 10(-14) typical). Two prototypes are fully operational, codenamed ELISA and ULISS. ELISA has been permanently installed the new deep space antenna station of the European Space Agency in Malargüe, Argentina, in May 2012. ULISS is a transportable version of ELISA, modified to fit in a small van (8.5 m(2) footprint). Installation requires a few hours manpower and 1 day of operation to attain full stability. ULISS, intended for off-site experiments and as a technology demonstrator, and has successfully completed two long-distance travels.

Superconducting-cavity-stabilized oscillators (SCSO) for precise frequency measurements

We report our progress in achieving precise frequency measurements by integrating high- Q superconducting-cavity-stabilized oscillators (SCSO) with high-resolution thermometry and phase-locked-loop (PLL) techniques. The relevant effects that influence the ultimate frequency stability of SCSO (10 −17–10 −18) are described, and applications of this technique to precise measurements of the liquid helium critical phenomena are discussed.

High-resolution line-Q measurements of velocity-selective nonlinear Faraday rotation in Rb vapor

Optical feedback from velocity-selective nonlinear Faraday rotation in alkali metal vapors has been shown to be an effective technique for simultaneously stabilizing the frequency and reducing the linewidth of AlxGa1−xAs/GaAs lasers. The ultimate frequency stability of such a laser is limited by the quality factor Q=ν0/Δν of this frequency reference. In this letter we report the measurement of Q=6×106 for optical feedback from velocity-selective nonlinear Faraday rotation in Rb vapor.

Atomic hydrogen spin-exchange collisions in a cryogenic maser

The results of an experiment in which the oscillation frequency of a cryogenic hydrogen (H) maser is studied as a function of the H density and the 1420-MHz resonator tuning are presented. The results are interpreted in terms of a theory which predicts that hyperfine interactions between colliding H atoms should include a frequency shift not predicted by classical spin-exchange theories. This effect has the potential to limit the ultimate frequency stability of H masers operating at cryogenic temperatures. The authors found qualitative agreement between the data and the magnitude of the predicted frequency shift. The sign of the effect which is observed however, is unmistakably opposite to that predicted by theory. >