Researchers simplify design of optical atomic clocks without sacrificing performance

“Over the last two decades, many major advances have been made in the performance of next-generation atomic clocks,” said research team leader Jason Jones of the University of Arizona. “However, most of these systems are not suitable for use in real-world applications. To bring this advanced technology out of the laboratory, we are using a simplified design in which a single-frequency comb laser acts as both the clock’s pendulum, or ticking mechanism, and the gear mechanism that keeps track of time.”

Frequency combs — a type of laser that emits thousands of regularly spaced colors, or frequencies — have revolutionized atomic clocks and timekeeping. In Optica Publishing Group magazine Optical LettersJones and colleagues describe an optical atomic clock that uses a frequency comb to directly excite two-photon transitions in rubidium-87 atoms. They show that this new design achieves the same performance as a conventional optical atomic clock with two lasers.

“This advance could also help improve the performance of the GPS network, which relies on satellite-based atomic clocks, by improving that network’s performance and making backup or alternative clocks more accessible,” said Seth Erickson, first author of the paper. “It’s also the first step toward bringing high-performance atomic clocks into everyday applications and even into people’s homes, which could, for example, allow telecommunications networks to switch between different conversations very quickly. This could make it possible for many people to communicate simultaneously over the same telecom channels and increase their data speeds.”

Simplifying advanced timekeeping

In an optical clock, stimulating atomic energy levels with a laser causes atoms to transition between specific energy levels. The precise frequency of these transitions acts as the “tick” of the clock, allowing time to be measured with high precision. Although portable chip-scale optical atomic clocks have been developed, the most accurate and stable optical clocks use atoms trapped at temperatures near absolute zero to minimize atomic motion, which can alter the frequencies of laser light experienced by the atoms.

To avoid the need for such supercooling, Jones and his colleagues used atomic energy levels that require the absorption of two photons instead of one to move to a higher energy level. When photons are sent from opposite directions through an atom, the effects of motion on one of these photons cancel out any effects of motion on the other photon. This allows the use of hot (100°C) atoms and a significantly simpler clock design.

“A key innovation of this work is that instead of using a single-color laser to send photons at the atom from all directions, we send a wide range of colors from a frequency comb,” Jones said. “Using the right pairs of photons with different colors from the frequency comb causes them to come together in the same way that two photons from a single-color laser would, thus exciting the atom in a similar way. This eliminates the need for a single-color laser and further simplifies the atomic clock.”

The researchers say the widespread availability of commercial frequency combs at telecommunications wavelengths and robust fiber components such as Bragg gratings greatly facilitated the development of this new design. They used fiber Bragg gratings to narrow the broadband frequency comb spectrum to less than 100 GHz, centered on the atomic transition of rubidium-87. This narrowly filtered spectrum increased the overlap between the frequency comb output and the excitation spectrum for rubidium-87 atoms.

Clock comparison

To test the new approach, the researchers compared two nearly identical versions of the new direct-frequency comb clock with a conventional clock that included the use of an additional single-frequency laser. The new clocks showed consistent performance with instabilities of 1.9×10.⁻¹³ in 1 second and up to 7.8(38)×10 on average⁻¹⁵ at 2600 seconds. This performance was similar to that of the conventional clock and other published results using a single-frequency laser architecture.

The researchers are now working to improve their optical atomic clock design by making it smaller and more stable in the long term, and by incorporating new developments in laser technology. The direct frequency comb approach can also be used with other two-photon atom transitions, including transitions for which low-noise single-frequency lasers are not currently available.