M. Eichenseer

Absolute frequency measurement of the In + clock transition with a mode-locked laser

The absolute frequency of the In(+) 5s(2) (1)S(0)5s5p (3)P(0) clock transition at 237 nm was measured with an accuracy of 1.8 parts in 10(13). Using a phase-coherent frequency chain, we compared the (1)S(0)(3)P(0) transition with a methane-stabilized HeNe laser at 3.39 microm, which was calibrated against an atomic cesium fountain clock. A frequency gap of 37 THz at the fourth harmonic of the HeNe standard was bridged by a frequency comb generated by a mode-locked femtosecond laser. The frequency of the In(+) clock transition was found to be 1,267,402,452,899.92 (0.23) kHz, the accuracy being limited by the uncertainty of the HeNe laser reference. This result represents an improvement in accuracy of more than 2 orders of magnitude over previous measurements of the line and now stands as what is to our knowledge the most accurate measurement of an optical transition in a single ion.s.

A Nd:YAG Laser with short-term frequency stability at the Hertz-level

Abstract We report on the frequency stabilisation of a Nd:YAG laser at 946 nm to the Hertz-level. The laser will be used for ultra-high resolution spectroscopy of the 5 s 2 1 S 0 −5 s5p 3 P 0 transition in In+ and will ultimately serve as a local oscillator of an optical frequency standard based on a single trapped indium ion. To resolve the extremely narrow 1 S 0 − 3 P 0 resonance (natural linewidth 0.82 Hz) at 237 nm, the frequency-quadrupled Nd:YAG laser radiation has to be frequency stable at the Hertz-scale for measurement times up to several tens of seconds. We obtain the frequency stability of the laser by locking it onto an external reference cavity of high finesse, placed on an active vibration isolation platform.

Towards an indium single-ion optical frequency standard

A narrow transition of a single laser-cooled ion, stored in a radio-frequency trap, can serve as a reference for a frequency standard of very high accuracy and stability. For the implementation of such a system, we study the 5s2 1S0–5s5p 3P0 transition in In+, at a wavelength of 237 nm. This resonance has a linewidth of 0.8 Hz, with systematic frequency shifts expected to be at the mHz level. A fractional resolution of 1.3 × 10−13 has been so far achieved, limited by the frequency instability of the clock laser used to excite the line. The absolute frequency of the 1S 0–3P 0 transition was measured using the frequency comb of a mode-locked fs laser, and as a reference a methane-stabilized He–Ne laser at 3.4 μm, which was calibrated against an atomic caesium fountain clock. The transition frequency was determined as 1267 402 452 899.92 (0.23) kHz, the uncertainty being limited by the uncertainty of the He–Ne standard. The short-term frequency stability of the clock laser was recently greatly improved. With a new laser set-up a linewidth < 4 Hz (FWHM) for integration times up to 26 s was achieved, using laser platforms which actively isolate from external mechanical vibrations.

Common-mode-free frequency comparison of lasers with relative frequency stability at the millihertz level.

We report on a frequency comparison of frequency-stabilized lasers located in remote laboratories resting on different foundations. By locating the lasers in this way correlated frequency excursions of the lasers are suppressed to a high degree. The beat signal between them shows a linewidth at the hertz level for averaging times of 1 s. The optical link is established by a 100 m single-mode optical fiber where the frequency noise induced by the fiber is reduced to the level of a few tens of millihertz. One laser is stabilized onto a Fabry-Perot resonator with a long-term precision of 25 mHz (fractional frequency instability, sigma(y) = 1.2 x 10(-16)), the highest lock fidelity obtained so far to our knowledge.

A Nd:YAG laser with short term frequency stability at the Hertz level

The purpose of the frequency-stable laser described in this contribution is the use for an optical frequency standard on the basis of a single trapped indium ion, stored in a radio frequency trap. As laser source for excitation of the In/sup +/ clock transition , the fourth harmonic of a commercial diode-pumped quasi-monolithic Nd:YAG non-planar ring laser (miser) operating at 946 nm is used. The miser is locked to a Zerodur reference cavity (finesse 70000), which is placed inside a thermo-stabilized vacuum chamber. The whole setup is mounted on an optical breadboard placed on top of an AVI (active vibration isolation) support. In order to characterize the absolute frequency stability of the miser system we use a further independent ULE reference cavity with a finesse of 60 000, which is mounted on a separate and independent AVI platform. A superior way to characterize the frequency stability of the miser is the use of a second independent laser, stabilized to another independent ULE cavity with a finesse of 200000. This second laser system is assembled in another laboratory with a separate basement. The laser light is guided by a 100 m long optical fibre between these two laser systems. The power circulating inside the cavities is stabilized by detecting the transmitted light and applying the corresponding error signal to the AOMs in front of the cavities. The frequency stability of the miser is analyzed by superposing parts of the two independently stabilized lasers and evaluating the corresponding beat signal in both the frequency and time domain. A frequency stability on the Hertz level is then observed. The measured root Allan-variance reaches values around /spl Delta//spl nu///spl nu/ = 10/sup -15/ (/spl tau/ /spl ap/ 1 s). A detailed analysis of the performance of the AVI system as well as experimental studies of the different factors, contributing to the laser frequency instability, is presented.

Indium single ion optical frequency standard

A single laser cooled ion in a radiofrequency trap can serve as the basis for a highly stable optical frequency standard. We present recent results of our work on single indium ions, using the /sup 1/S/sub 0/ to /sup 3/P/sub 0/ transition at 236.5 nm wave-length as the clock transition. This resonance has a linewidth of only 0.82 Hz where systematic shifts should be controllable on the mHz level. A single In/sup +/ ion is stored in a miniature Paul-Straubel trap and laser cooled to a temperature of about 100 /spl mu/K. The clock transition is excited using a frequency quadrupled Nd:YAG laser. A fractional resolution of 1.3/spl middot/10/sup -13/ has been achieved so far (linewidth of 170 Hz at 1267 THz). The absolute frequency of the clock transition clock transition has been measured with an uncertainty of 2/spl middot/10/sup -13/ using a frequency chain and a methane-stabilized HeNe laser as a reference, that was calibrated with a cesium clock.

Single Indium Ion optical frequency Standard

A single indium ion stored in a radio frequency trap and laser-cooled to a temperature below 1 mK can serve as an optical frequency standard of exceptionally high accuracy and stability

High resolution spectroscopy of a single trapped ion

Excitation of the /sup 3/P/sub 0 /state is detected in optical-optical double resonance via the dark periods in the fluorescence on the fast /sup 1/S/sub 0/ /sup 3/P/sub 1/ transition. The dye laser formerly used to excite this line was recently replaced by a diode-based laser system: a grating stabilized diode at 922 nm, further frequency stabilized onto a Fabry-Perot interferometer, is seeded into a tapered amplifier. Subsequent frequency-quadrupling employing a periodically poled KTP and a BBO crystal, each placed in an external enhancement cavity, leads to the right frequency at 231 nm. For the /sup 1/S/sub 0/ /sup 3/P/sub 0/ transition a resolution of 1.3 /spl times/ 10/sup -13 /has been achieved so far, limited by the frequency instability of the Nd:YAG laser. The short-term frequency stability of this laser has been strongly improved recently. With a new laser setup a linewidth < 5 Hz (FWHM) for integration times /spl times/30 s has been achieved using active vibration isolation platforms for the reference cavity. Recent progress of that work is reported.

Ultra frequency-stable Nd-YAG laser for an indium optical frequency standard

We present experimental results of the frequency stabilization of a Nd:YAG laser at 946 nm to the Hertz-level. The laser will be used for ultra-high resolution spectroscopy of the 5s21S0-5s5p3P0 transition at 237 nm in In+ (natural linewidth 0.82 Hz) and will ultimately serve as a local oscillator of an optical frequency standard based on a single traped Indium ion. The frequency stability of the laser is obtained by locking it onto an external reference cavity of high finesse, placed on an active vibration isolation platform.

Narrow-linewidth Nd:YAG laser for the single indium ion optical frequency standard

Summary form only given. Using active vibration isolation of a high-finesse reference cavity, we demonstrate a Hz-level linewidth of a 946 nm laser, used for high-resolution spectroscopy of the indium ion clock transition. Recent experimental results are presented.