![]() ![]() Instead, the aluminum atom is coupled to the beryllium atom using a “quantum logic” operation that makes the beryllium atom emit easily detected light when the aluminum atom has been excited. The pump laser can be tuned to resonantly excite the aluminum ion to a higher state, but it’s hard to detect the aluminum state directly. Both have demonstrated linewidths within a factor of 10 of the mercury line.Ī closely related clock type is the “quantum logic” optical clock developed at NIST, in which two different ions-aluminum-27 and beryllium-9-are held 4 µm apart in an electromagnetic trap and laser-cooled. Other ionic optical clocks being studied include a strontium-88 line at 674 nm and ytterbium-171 ions at 435 nm. 2 The precise control possible with a single atom promises very high accuracy, but the use of a single atom at a time can limit the signal-to-noise ratio and measurement precision. After a series of refinements, the group showed its mercury clock could exceed the accuracy of the best cesium clocks (see Fig. The mercury ion oscillated on a transition near 282 nm with linewidth of only 7 Hz. Jim Bergquist and colleagues at NIST in 2000 demonstrated the first such optical clock, by electromagnetically confining mercury-199 ions between a pair of electrodes. This slows the atom’s motion to near zero and confines it in a volume just a few tens of nanometers across-an order of magnitude shorter than optical wavelengths-which allows long interaction times with the excitation laser beam, as needed for accurate measurement. One type of optical clock is based on trapping and isolating a single atom and laser-cooling it to about one millikelvin. With these tools in hand, developers began demonstrating impressive performance in optical clocks, which in theory might reach accuracy of 10 -17 to 10 -18. The final step was development of laser femtosecond frequency combs, which can directly measure the absolute optical frequency by dividing the frequency down to the microwave range. ![]() Advances in laser stabilization provided the required light source. The development of laser cooling and laser trapping techniques succeeded in slowing and trapping atoms, greatly reducing Doppler shifts and extending interaction times. Building blocks of optical clocksĪlthough the theoretical advantages of optical clocks have long been obvious, realizing that potential has required major innovations in all three fundamental building blocks of optical clocks: an atomic species with a narrow transition in the visible or ultraviolet, a laser with sub-hertz linewidth that can be tuned to match the atomic resonance, and a counter capable of measuring laser frequencies in the optical range.Īchieving high accuracy requires tools to limit the movement of the atoms being studied and holding them in place long enough for the measurement system to achieve high resolution. That’s a big plus for practical measurements. Time averaging improves atomic-clock precision by smoothing out instabilities, but the precision improves only as the square root of the measurement time-so the more stable optical clock needs only a few seconds to reach 10 -15 precision, compared to many hours for a microwave atomic clock. Optical clocks are also about 100 times more stable than typical cesium clocks, which is a big advantage in achieving precise timing. “It’s like adding five digits to your stop watch,” says Chris Oates, an optical-clock researcher at NIST in Boulder. Their 9.2 GHz microwave frequency was high 50 years ago, but optical frequencies are 100,000 times higher so they can slice time into much smaller intervals. 1īut cesium clocks are coming up against fundamental limits. Lasers cool and confine a fountain of cesium atoms, limiting the clock’s fractional frequency uncertainty to a remarkable 4 × 10 -16, corresponding to a drift of one second in more than 60 million years. Today, the primary time standard is an advanced cesium clock at the National Institute of Standards and Technology (NIST Boulder, CO) called NIST-F1 (see Fig. The first cesium clock in 1955 was accurate to one part in 10 10. Since then researchers have made steady improvements in atomic-clock accuracy. ![]() In 1967, metrologists formally defined the second as equal to 9,192,631,770 oscillations of a hyperfine transition of ground-state cesium-133. Atomic clocks based on a microwave transition of cesium have been the gold standard of timekeeping for decades. ![]()
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