"Over the last two decades, many great advances have been made in the performance of next generation atomic clocks," said Jason Jones, research team leader 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 comb lasers - which emit thousands of evenly spaced frequencies - have transformed atomic clock designs. In their work published in the journal 'Optics Letters', Jones and colleagues describe using a frequency comb to trigger a two-photon transition in rubidium-87 atoms. This new method matches the performance of traditional optical atomic clocks, which rely on two separate lasers.

Research Report:"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 Seth Erickson, the paper's first author. "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."

Optical clocks work by using lasers to excite atomic energy levels, causing atoms to transition between precise states that serve as the "tick" of the clock, measuring time with extraordinary precision. While some portable optical atomic clocks exist, the most accurate systems require cryogenic cooling to reduce atomic motion, which otherwise interferes with the laser's accuracy.

To circumvent the need for ultra-cold environments, Jones's team utilized atomic energy levels requiring two photons to transition to higher energy levels. By sending photons from opposite directions, the team effectively canceled out atomic motion effects, allowing the use of hot atoms at 100C and simplifying the 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," Jones explained. "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."

Research Report:The research team benefited from the availability of commercial frequency combs and robust fiber components, such as fiber Bragg gratings, at telecommunications wavelengths. They employed fiber Bragg gratings to narrow the broad frequency comb spectrum to less than 100 GHz, ensuring a strong overlap with the rubidium-87 atomic transition frequencies.

The researchers tested their new clock design by comparing two nearly identical versions of the direct frequency comb clock with a traditional atomic clock that used an additional single frequency laser. The new clocks demonstrated excellent performance, with instabilities of 1.9+ 10? at 1 second and averaging down to 7.8(38)+ 10?5 at 2600 seconds, comparable to traditional clocks and other systems based on single-frequency lasers.

The team is now focused on improving the clock's stability and compactness, while incorporating advances in laser technology. This approach could also be applied to other atomic transitions requiring two-photon excitation, potentially using atoms where low-noise single frequency lasers are currently unavailable.

Research Report:Atomic frequency standard based on direct frequency comb spectroscopy

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