Researchers simplify design of optical atomic clocks without compromising performance

"Over the last two decades, many great advances have been made in the performance of next generation atomic clocks," said research team leader Jason Jones from the University of Arizona. "However, many of these systems are not suitable for use in real world applications. To take this advanced technology out of the lab, we use a simplified design in which a single frequency comb laser acts as both the clock's pendulum, or ticking mechanism, and as the gearwork that tracks time."

Frequency combs -- a type of laser that emits thousands of regularly spaced colors, or frequencies -- have been revolutionary for atomic clocks and timekeeping. In the Optica Publishing Group journal Optics Letters, Jones and colleagues describe an optical atomic clock that uses a frequency comb to directly excite a two-photon transition in rubidium-87 atoms. They show that this new design achieves the same performance as a traditional optical atomic clock with two lasers.

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

Simplifying advanced timekeeping

In an optical clock, exciting atomic energy levels with a laser causes atoms to transition between specific energy levels. The precise frequency of these transitions serves as the "tick" of the clock, allowing the measurement of time 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 change the laser light frequencies experienced by the atoms.

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

"A major innovation of this work is that instead of using a single-color laser to send photons at the atom from each direction, we send a broad range of colors from a frequency comb," said Jones. "Using the correct pairs of photons with different colors from the frequency comb allows them to add together in the same way as two photons from a single-color laser would, thus exciting the atom in similar way. This eliminates the need for a single-color laser, further simplifying the atomic clock."

The researchers say that the widespread availability of commercial frequency combs and robust fiber components, such as Bragg gratings, at telecommunication wavelengths 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 at the atomic transition of rubidium-87. This narrowly filtered spectrum increased the overlap between the frequency comb output and the excitation spectrum for the rubidium-87 atoms.

Clock comparison

To test the new approach, the researchers compared two almost identical versions of the new direct frequency comb clock with a traditional clock that included the use of an additional single frequency laser. The new clocks showed consistent performance with instabilities of 1.9×10⁻¹³ at 1 second and averaging down to 7.8(38)×10⁻¹⁵ at 2600 seconds. This performance was similar to that of the traditional 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 over the long term as well as incorporating new advances in laser technology. The direct frequency comb approach could also be used with other two-photon atomic transitions, including ones for which low-noise single frequency lasers are not currently available.