Magnetic Field-Induced Spectroscopy of Forbidden Optical Transitions with Application to Lattice-Based Optical Atomic Clocks

Abstract

We develop a method of spectroscopy that uses a weak static magnetic field to enable direct optical excitation of forbidden electric-dipole transitions that are otherwise prohibitively weak. The power of this scheme is demonstrated using the important application of optical atomic clocks based on neutral atoms confined to an optical lattice. The simple experimental implementation of this method--a single clock laser combined with a dc magnetic field--relaxes stringent requirements in current lattice-based clocks (e.g., magnetic field shielding and light polarization), and could therefore expedite the realization of the extraordinary performance level predicted for these clocks. We estimate that a clock using alkaline-earth-like atoms such as Yb could achieve a fractional frequency uncertainty of well below 10(-17) for the metrologically preferred even isotopes.

A transportable strontium optical lattice clock

“…The installation process took about one day, mainly due to re-thermalization of the transportable cavity, which was not actively temperature stabilized during transportation. The beat note shows a linewidth of the order of 1 Hz compatible with the laser frequency stability of 2À3 Â 10 À15 at 1 s. The comparison with the stationary clock has also been used to Considering that the single photon 88 Sr clock transition 1 S 0 À 3 P 0 is forbidden at any order, the magnetic fieldinduced spectroscopy method is used to induce the clock transition, by means of an external magnetic field coupling the 3 P 0 to the 3 P 1 state [30]. The search-scan is performed by using a mixing magnetic field of B j j ¼ 19 mT and a clock probe beam intensity of I ¼ 5:7 W=cm 2 , leading to a Rabi frequency of 275 Hz, given by…”

“…where aðSrÞ ¼ 198 Hz=ðT ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi mW cm À2 p Þ [30]. In order to have a high-contrast spectrum, the excitation pulse length is Dt ¼ 100 ms, thus overdriving the clock transition having an estimated p pulse duration of Dt p ¼ 1:8 ms.…”

“…They are calibrated through the known coefficient b and k [30] by measuring the clock frequency shift as a function of the magnetic field (2.2-4.1 mT range, also inverting the direction of the field) and probe intensity (0.9-3.2 W cm À2 range), as shown in Fig. 9.…”

Frequency Metrology with Optical Lattice Clocks

“…J ¼ 0 transition between long-lived states. Alternatively, a multiphoton excitation of the clock transition, 81,82) or the mixing of the 3 P 0 state with the 3 P 1 state using a magnetic field 63) or an elliptically polarized light 83) may allow the use of even isotopes that exhibit purely scalar nature of the J ¼ 0 state. The optical lattice clock with the best performance needs to be experimentally explored among possible candidates because of difficulties in predicting some of the uncertainties associated with higher-order light field perturbations; such as resonant contribution to the 4th-order light shifts and multiphoton ionization processes.…”

Doppler spectroscopy of an ytterbium Bose-Einstein condensate on the clock transition

“…Although electric dipole-forbidden, the transition can be weakly enabled by hyperfine interactions or an applied magnetic field [6,7]. Mastering the laser technology required to observe such transitions with a spectroscopic resolution at or below the Hertz level enabled the development of optical atomic clocks (see [8,9] for reviews of this field, and [6,7,[10][11][12][13] for specific realizations using ytterbium atoms). Although optical atomic clocks are typically operated using dilute samples far from quantum degeneracy, many-body effects are nevertheless measurable because of the extreme spectroscopic sensitivity [14].…”