Marcus Zimmermann

New Limits on the Drift of Fundamental Constants from Laboratory Measurements

We have remeasured the absolute 1S-2S transition frequency nu(H) in atomic hydrogen. A comparison with the result of the previous measurement performed in 1999 sets a limit of (-29+/-57) Hz for the drift of nu(H) with respect to the ground state hyperfine splitting nu(Cs) in 133Cs. Combining this result with the recently published optical transition frequency in 199Hg+ against nu(Cs) and a microwave 87Rb and 133Cs clock comparison, we deduce separate limits on alpha/alpha=(-0.9+/-2.9) x 10(-15) yr(-1) and the fractional time variation of the ratio of Rb and Cs nuclear magnetic moments mu(Rb)/mu(Cs) equal to (-0.5+/-1.7) x 10(-15) yr(-1). The latter provides information on the temporal behavior of the constant of strong interaction.

Optical clockwork with an offset-free difference-frequency comb: accuracy of sum- and difference-frequency generation

We demonstrate a simple optical clockwork mechanism based on the broadened frequency comb of a femtosecond laser and on difference-frequency generation (DFG) in a nonlinear crystal. The DFG comb possesses a vanishing carrier envelope offset frequency that permits the construction of a simple and thus potentially more stable optical clockwork. In addition it offers the possibility of extending the frequency comb into the infrared spectral region. The overall accuracy and stability of the DFG comb relative to the initial frequency comb were measured to be 6.6 x 10(-21) and 10(-18) tau(-1), respectively, where tau is the averaging time in seconds. Assuming that sum- and difference-frequency generation are independent processes, our measurements suggest a <10(-20) accuracy for them.

Optical clockworks and the measurement of laser frequencies with a mode-locked frequency comb

Femtosecond laser-frequency comb techniques are vastly simplifying the measurement and synthesis of optical frequencies. A single mode-locked femtosecond laser, with its spectrum broadened by self-phase modulation in a microstructured or tapered nonlinear fiber, can produce millions of sharp laser lines in a precise evenly spaced grid spanning much of the visible and near-infrared spectrum. The absolute frequency of each line is determined by two observable radio-frequency signals. The pulse repetition rate gives the spacing of the comb lines and the rate at which the phase of the lightwave slips, relative to the intensity envelope from pulse to pulse determines the offset frequency by which each line is displaced from a precise integral multiple of the repetition frequency. This offset frequency can be measured most easily if the comb spans more than an optical octave so that one can observe a radio frequency beat note between the second harmonic of the infrared comb lines with the corresponding comb lines at the blue end. Such an optical-frequency synthesizer makes optical oscillations readily countable and provides the long-awaited compact optical clockwork for an all-optical clock.

White-light frequency comb generation with a diode-pumped Cr:LiSAF laser

We have created a broad spectrum spanning more than an optical octave by launching femtosecond pulses from a battery operated Cr:LiSAF laser into a photonic crystal fiber. Despite the massive broadening in the fiber, the comb structure of the spectrum is preserved, and this frequency comb is perfectly suited for applications in optical frequency metrology.

Precision spectroscopy of hydrogen and femtosecond laser frequency combs

Precision spectroscopy of the simple hydrogen atom has inspired dramatic advances in optical frequency metrology: femtosecond laser optical frequency comb synthesizers have revolutionized the precise measurement of optical frequencies, and they provide a reliable clock mechanism for optical atomic clocks. Precision spectroscopy of the hydrogen 1S–2S two-photon resonance has reached an accuracy of 1.4 parts in 1014, and considerable future improvements are envisioned. Such laboratory experiments are setting new limits for possible slow variations of the fine structure constant α and the magnetic moment of the caesium nucleus μCs in units of the Bohr magneton μB.

Oxygen-ordering superstructures in underdoped YBa 2 Cu 3 O 6 + x studied by hard x-ray diffraction

High-energy x-ray diffraction is used to investigate the bulk oxygen-ordering properties of

Oxygen superstructures throughout the phase diagram of (Y,Ca)Ba2Cu3O6+x.

{\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}.

Precision spectroscopy of atomic hydrogen and variations of fundamental constants

Four different superstructures of Cu-O chains aligned along the b axis and ordered with periodicity

Measuring the frequency of light with a mode-locked laser

ma,

Optical Frequency Measurement

along the a axis have been observed. For