Compact Atomic Clock Design Uses Cold Atoms to Boost Precision

The heart of the prototype clock (the vacuum chamber containing the atoms) is about the size of a coffee mug, 150 cubic centimeters, set in a small table of lasers and electronics. This is about 10 times larger than NIST’s chip-scale atomic clock packages—for now. But when miniaturized and improved, NIST’s new clock design has the potential to be about the same size and 1,000 times more precise and stable than chip-scale atomic clocks over crucial timespans of a day or more.
By achieving this goal, the cold-atom clock could also match the performance of commercial cesium-beam atomic clocks, common laboratory instruments, but in a smaller package.

NIST pioneered the development of chip-sized atomic clocks in 2004.

Miniature atomic clocks based on coherent population trapping (CPT) states in thermal atoms are an important component in many field applications, particularly where satellite frequency standards are not accessible. Cold-atom CPT clocks promise improved accuracy and stability over existing commercial technologies. Here we demonstrate a cold-atom CPT clock based on 87Rb using a high-contrast double-Λ configuration. Doppler frequency shifts are explained using a simple model and canceled by interrogating the atoms with counterpropagating light beams. We realize a compact cold-atom CPT clock with a fractional frequency stability of 4×10^−11τ−1/2, thus demonstrating the potential of these devices. We also show that the long-term stability is currently limited by the second-order Zeeman shift to 2×10^−12 at 1000 s.

Physical Review A – cold-atom double-Λ coherent population trapping clock

NIST’s cold-atom clock relies on about 1 million rubidium atoms held in a small glass vacuum chamber. The atoms are cooled with lasers and trapped with magnetic fields at very cold, microkelvin temperatures. Two near-infrared lasers excite the atoms symmetrically from above and below. Each laser generates two frequencies of light, which are tuned until the atoms oscillate between two energy states and stop absorbing light. This sets the clock ticking rate at a specific microwave frequency.

By aiming at the atoms from opposite directions simultaneously, the laser arrangement cancels a major source of measurement error—the Doppler shift, or the change in the atoms’ apparent resonant frequency as they interact and move with the laser light. The clock also has special quantum features unique to rubidium atoms that boost the signal contrast and make the detection of the clock ticks more precise.

NIST researchers are already working on the next version of the cold-atom clock. In addition to reducing its size, researchers expect to improve its performance by adding magnetic shielding and antireflection coating.

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