Apparent power-dependent frequency shift due to collisions in a cesium fountain.

We describe a situation where a varying collisional frequency shift in cesium fountain primary frequency standards can be misinterpreted as a power-dependent shift. This misinterpretation may affect analyses of fountains test-operated at multiple pi/2 microwave pulse areas. Such tests are typically performed in the search for microwave- and cavity-related systematic frequency biases.

Cancellation of the collisional frequency shift in caesium fountain clocks.

We have observed that the collisional frequency shift in primary caesium fountain clocks varies with the clock state population composition and, in particular, is zero for a given fraction of the |F=4,mF=0) atoms, depending on the initial cloud parameters. We present a theoretical model explaining our observations. The possibility of the collisional shift cancellation implies an improvement in the performance of caesium fountain standards and a simplification in their operation.

Dynamical MCG mapping with an atomic vapor magnetometer.

We have developed a magnetometer based on magnetic resonance in cesium vapor optically pumped by resonant laser radiation, which has the sensitivity and bandwidth to record human magnetocardiograms. The device is operated as a first-order gradiometer in a weakly shielded environment and allows us to record 2-dimensional field maps by shifting the subject with respect to a single sensor and making time-sequential measurements. We discuss the magnetometer technique and its performance as well as obtained results, which include a comparison of MCG data recorded with our single channel optical magnetometer with SQUID measurements from a commercial multi-channel device as well as from SQUID reference data. The results obtained so far in the detection of cardiomagnetic signals using the optical magnetometer make us confident that the technique has a high potential to serve as an alternative to SQUID detection.

Dynamical mapping of the human cardiomagnetic field with a room-temperature, laser-optical sensor.

The magnetic field produced by the human heart carries valuable information for medical research, as well as for diagnostics and screening for disease. We have developed an optical method that allows us to produce movies of the temporal dynamics of the human cardiomagnetic field map. While such movies have been generated before with the help of SQUIDmagnetometers, our technique operates at room temperature and promises substantial economic advantages.

Coherent population trapping resonances in thermal (85)Rb vapor: D(1) versus D(2) line excitation.

We have compared coherent population trapping (CPT) resonances, both experimentally and theoretically, for excitation of the D(1) and D(2) transitions of thermal (85)Rb vapor. Excitation of the D(1) line results in greater resonance contrast than excitation of the D(2) line and in a reduction in the resonance width, in agreement with theoretical expectations. These results translate into a nearly tenfold improvement in performance for the application of CPT resonances to a frequency standard or a sensitive magnetometer when the D(1) line, rather than the D(2) line, is used.

Coherent population trapping resonances in thermal (85)Rb vapor: D(1) versus D(2) line excitation: errata.

In our recent Letter,(1) Ref. 12 was printed incorrectly. The correct reference is M. Zhu, Coherent population trapping-based frequency standard and method for generating a frequency standard incorporating a quantum absorber that genrates the CPT state with high frequency, U.S. patent 6,359,916 (March 19, 2002).

Frequency-dependent optical pumping in atomic ?-systems.

We consider the effects of optical pumping on the conversion of laser-frequency modulation into intensity modulation by an atomic absorption line in a vapor of alkali atoms driven in a ?-configuration. It is found that, due to optical pumping in combination with the excited-state hyperfine structure, the absorption line shape is distorted substantially as the Fourier frequency of the FM is changed. The most significant effect of the distortion is a shift of the apparent line center, which depends on how the frequency of the modulation compares with the optical pumping rate. This shift has implications for locking lasers to atomic transitions and also for FM-AM noise conversion in atomic vapors.

How accurate is optical second-harmonic generation?

We have checked whether the frequency ratio between fundamental and second-harmonic optical frequencies is exactly 1:2 by measuring the frequency difference between the second harmonics of two diode lasers and comparing it with the frequency difference of the fundamentals. We have improved the 30-year-old experimental limit on the accuracy of the 1:2 frequency ratio by 7 orders of magnitude for frequency-dependent shifts to 2.3 parts in 10(13) for a KNbO(3) crystal.

Coherent bisection of optical frequency intervals as large as 530 THz.

We demonstrate the coherent bisection of a 530-THz frequency interval between two lasers by phase locking a third laser to the midfrequency point of the first two by using the nonlinear phase-locking scheme. This is done by comparing the sum frequency of the first two lasers with the second harmonic of the third laser. We also examine the properties of such a frequency divider for the important practical case of a few-terahertz intervals in the 850-nm diode-laser regime.