Coherent Spectroscopy on a Single Atomic System at Rest in Free Space III

Reinhard Iffländer, Guenter Werth

HANS G. DEHMELT

An individual atomic ion localized in the center of a small Paul rf quadrupole trap is a promising object for ultimate resolution laser spectroscopy because, broadly speaking, the ion may be brought to "A State of Complete Rest in Free Space" by side band cooling. As a consequence, all Doppler shifts vanish. "Free Space" is approximated insofar as the electric trapping field vanishes in the center of the trap and there is no Stark effect. Neither need there be a Zeeman Effect as magnetic fields may be controlled down to the micro-Gauss range. Naturally, there is no transit time broadening. Minute laser powers provided by harmonic generators suffice for saturation of optical transitions as well focused beams may be used. Millionfold atomic amplification of the single-ion fluorescence from a metastable level may bring resolutions of 1 part in 1018 within reach. Furthermore, the close localization of an atomic particle in space is one of the most fundamental problems in physics and worthy of study on its own merit.

W.R.S. Carton

Benjamin K. Malia, Julián Martínez-Rincón, Yunfan Wu +2 more

We demonstrate the utility of optical cavity generated spin-squeezed states in free space atomic fountain clocks in ensembles of 390 000 ^{87}Rb atoms. Fluorescence imaging, correlated to an initial quantum nondemolition measurement, is used for population spectroscopy after the atoms are released from a confining lattice. For a free fall time of 4 milliseconds, we resolve a single-shot phase sensitivity of 814(61) microradians, which is 5.8(0.6) decibels (dB) below the quantum projection limit. We observe that this squeezing is preserved as the cloud expands to a roughly 200 μm radius and falls roughly 300 μm in free space. Ramsey spectroscopy with 240 000 atoms at a 3.6 ms Ramsey time results in a single-shot fractional frequency stability of 8.4(0.2)×10^{-12}, 3.8(0.2) dB below the quantum projection limit. The sensitivity and stability are limited by the technical noise in the fluorescence detection protocol and the microwave system, respectively.

Z. Ficek, R. Tanaś

Mikhail D. Lukin, Michael Fleischhauer, Alexander S. Zibrov +4 more