Atomic Clocks Get Smaller, Lighter, More Precise

Atomic clocks were first built and used in the mid 1960s with the specific aim of redefining a split second, a definition that continues to stand the test of time.

Atomic clocks work by counting the flipping frequency of the electron spins of the cesium atom. Hence, they are incredibly accurate, making frequency and time by far the most precisely measured of all physical quantities.

Atomic resonance is so sharp it can tell if a standard quartz crystal clock deviates from the correct time by less than 1 millionth of a billion, or 1015.

To achieve such high timing resolution, atomic clocks make use of ultra-narrow transitions in strontium atoms, providing orders of magnitude better performance than their rubidium counterparts due to the narrower atomic features.

Over-dependence on GPS and other navigation aids along with growing vulnerabilities including jamming, spoofing and other forms of interference highlight the need for terrestrial backups to current GNSS systems. We take a hard look at the scope of the problem and possible remedies in our upcoming GPS Special Project.

In simple terms, the narrower the atomic transition, the more accurate the atomic clock.

That’s one key reason why today’s satellite navigation systems (such as GPS) are so incredibly useful. Indeed, some posit that without atomic clocks, we would not enjoy the benefits (and occasional frustrations) of GPS.

Some liken a GNSS satellite to a precise atomic clock hooked to a radio transmitting a time signal. The timing data gets translated into accurate three-dimensional location information—latitude, longitude and altitude–as well as direction and speed.

Since the mid 1960’s, scientists and engineers have improved the accuracy each decade by an order of magnitude.

That work continues apace.

While there are ongoing efforts to make atomic clocks more accurate—and importantly, smaller and lighter—most are not focused on satellite navigation applications. For instance, there is increasing emphasis on designing atomic clocks capable of measuring the vibration of atoms, providing sufficient accuracy to detect phenomena such as dark matter and gravitational waves.

Bearing that in mind, we note that physicists at Massachusetts Institute of Technology recently revealed they have built an atomic clock that measures not a cloud of randomly oscillating atoms, as the best designs now measure, but instead atoms that have been quantumly entangled.  This opens the door to a whole new world of quantum physics.

As for GNSS location technology, several U.K. companies and universities have teamed to make atomic clocks more accessible and practical via improved accuracy and miniaturization.

Leading the effort is Kelvin Nanotechnology of Glasgow, Scotland, a specialist in advanced photonics and quantum components. The effort also includes WideBlue, another Glasgow-based design specialist, along with researchers from the universities of Birmingham and Strathclyde.

Kelvin Nanotechnology will make the grating MOTs (magneto-optical traps) and compact collimation optics designed by WideBlue. The University of Strathclyde will design the gMOT chip, and its counterpart in Birmingham is responsible for testing the prototype optical system.

The collaboration focuses on the scaling an atomic clock by “reducing the optical constraints into scalable micro-fabricated components as a critical step to bringing laboratory performance out into real-world applications,” said James McGilligan, senior research associate for the project at Kelvin Nanotechnology.

The project is scheduled to last about 18 months. “While atomic clocks are already remarkably accurate, we are focusing on advanced micro-fabrication techniques and improvements in laser cooling optics to bring about significant reductions in the size and weight of the next generation of portable atomic clocks,” McGilliagan told EE Times.

David Burt, business development manager at Kelvin Nanotechnology, said weight considerations remain the focus atomic clocks used in satellite navigation systems, Still, “we see many commercial opportunities in other sectors, including defense, undersea oil and mineral exploration.”

According to Paul Griffin of Strathclyde’s physics department and the university’s lead researcher, the project is “tackling head-on the difficult problem of taking research-grade technology from the laboratory and into practical and scalable quantum devices.”

Griffin said atoms with complex internal structures such as strontium and ytterbium enable advances in the sensitivity of quantum-enabled measurements of time and gravity. “Over the last decade, our team at Strathclyde has shown how the technology for laser cooling alkali atoms can be reduced down to a simple hand-held device powered from a single battery.”

The project also aims to develop new tools for laser cooling and manipulation of strontium atoms. “Our goal is that in five years’ time, the core hardware for ultra-cold strontium atoms will be an off-the-shelf component, which would be transformative for not only timing but also applications such as quantum computing,” Griffin said.

Laser systems used in atomic clock development. (Source: NIST, Boulder, Colo.)

Physicists at the University of Colorado recently reported advances in optical clocks, connecting the devices via laser beam between two buildings. The clocks are based on strontium, ytterbium and aluminium atoms. The strontium-based clock was located on the campus while the other two were housed 1.5 km distant at the National Institute of Standards and Technology Boulder Laboratories.

The clocks agreed with each another within 1 part in 1018, sufficiently precise to detect distortions in space-time continuum and gravitational waves.

Mobile atomic clocks

Another research project seeks to eliminate the need for satellites altogether. University of Sussex researchers are developing what they describe as portable atomic clocks that could one day be integrated into mobile phones, driverless cars or drones.

Using laser beam technology, the researchers claimed an advance in the efficiency of a crucial element of an atomic clock, the lancet. That is, the component responsible for counting, analogous to the pendulum in a mechanical clock.

According to Alessia Pasquazi, a lead investigator in the university’s Emergent Photonics Laboratory, a portable atomic clock would enable access to mapping data when a user is driving through a tunnel or an urban area where satellite signal strength is weak.

Pasquazi said portable atomic clocks would rely on an extremely accurate form of geo-mapping, enabling access to a location and planned route without the need for a satellite signal. “This breakthrough improves the efficiency of the lancet by 80 percent,” she said in an interview.

The reference, the equivalent of a pendulum in a traditional clock, is derived from the quantum property of a single atom confined in a chamber—the electromagnetic field of a light beam oscillating hundreds-of-trillions of times per second. The clock counting element needed at such speeds is an optical frequency comb, a highly specialized laser simultaneously emitting many colors, evenly spaced in frequency.

Micro-combs are very efficient in lowering the dimensions of frequency combs by exploiting tiny optical micro-resonators, but Pasquazi noted they are also extremely delicate devices that are complex to operate and therefore are difficult to use in practical atomic clocks.

The Sussex researchers claimed a breakthrough with development of a high-efficiency micro-comb that exploits a special type of wave called a “laser cavity soliton.” Solitons are particularly robust and can travel unperturbed over very long distances.

The Sussex team used pulses of light, confined in a tiny cavity on a chip. “The soliton that travels in this combination has the benefit of fully exploiting the microcavities’ capabilities of generating many colors, whilst also offering the robustness and versatility of control of pulsed lasers,” Pasquazi said.

The next step is transferring the chip-based technology to fiber technology. The device will eventually be integrated with an “ultra-compact” atomic reference currently under development.

Researchers hope to produce practical atomic clocks within five years, working with partners in the British aerospace industry. They will then focus on integrating the technologies into portable atomic clocks and, eventually, consumer devices such as mobile phones.

Consumer applications remain aspirational, however. “It could take another 20 years to get there,” Pasquazi acknowledged. “While my part of the project is pretty advanced, and we are now seeking funds to fabricate the chips, my colleagues looking at the geo-location and navigation aspect[s] and developing a really efficient atomic reference and integrating that with the soliton device certainly have the bigger challenge.”

Terrestrial GNSS

Obviating the need for GNSS satellites remains a challenge, and integrating everything at chip scale could prove equally difficult. “We have produced prototypes that are perhaps the size of a shoe box. But that is already a big improvement on the size and weight of the current generation of atomic clocks being carried by all the navigation satellites,” she noted.

Either way, “We can definitely improve the performance and security of next generation GNSS systems, with much better time reference and navigation characteristics, which could act at times of need as a kind of independent backup,” said Pasquazi.

The growing need for a terrestrial GNSS alternative is highlighted in the latest in a series of reports by the U.K.’s Royal Academy of Engineering. The report warns that “society may already be dangerously over-reliant on satellite radio navigation systems.”

The study notes a significant failure of GPS could cause numerous services to fail simultaneously, including many assumed to be independent of each other. “The use of non-GNSS backups is important across all critical uses of GNSS,” the report notes.

Vulnerabilities range from signal jamming to interference from solar flares.

The Academy recommends deployment of widely available positioning, navigation and timing services as an alternative to GNSS. That step would help secure national infrastructure.

A terrestrial radio navigation system called eLORAN is among the terrestrial backup options, it noted.

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