Shrinking quantum clocks fill growing need for accurate timekeeping

In today’s interconnected world, precise timekeeping is everything.

From banks stamping financial transactions to communication network operators, corporations all over the world need to have the exact same time. 

The US Global Positioning System (GPS), with its 31 satellites fitted with atomic clocks, serves as the world’s main precise time distributor. But the GPS signal is vulnerable to disruptions caused by space weather or deliberate jamming. To survive GPS outages, infrastructure operators rely on devices such as the matchbox-sized chip-scale atomic clocks. These clocks, however, are not stable enough, and within a few hours the times of different users might diverge. 

Physicists from the UK National Physical Laboratory (NPL) in cooperation with UK company Teledyne e2v are now developing novel quantum clocks that would fill the growing need for accurate timekeeping. “We are aiming at a higher level of stability,” said Richard Murray, business development manager at Teledyne e2v. “Our quantum clock would be several orders of magnitude more stable than those chip-scale atomic clocks and as a result of that it would have to be bigger. It would provide customers longer holdover times in case of GPS outage than they can get from the chip-scale clocks today.”

Decades of research

The new devices, which Teledyne said could enter the market within two years, would allow companies to go without GPS for days and still have the same time as their counterparts in other parts of the world. Funded through the UK National Quantum Technologies Programme, the work is a continuation of two decades of quantum clock research at NPL. According to Rhys Lewis, the head of the NPL’s Quantum Metrology Institute, the team is shrinking sophisticated timekeeping equipment, which would originally fill an entire room, into the size of a shoebox lid. 

“This will be a very small package, about 50cm wide and 5cm high,” Lewis said. “It uses different techniques from the large-scale clocks, so it has lower performance than the state-of-the-art atomic clock but it’s much smaller, cheaper, lighter, and has lower power consumption.”

NPL scientist Louis Essen built the world’s first atomic clock in the 1950s. The clock used microwaves to measure the absorption frequency of caesium atoms, which is naturally constant. While originally a second used to be defined as a fraction of the Earth’s rotation period, the precise microwave frequency absorbed by caesium atoms now determines its duration. 

Lasers oust microwaves

Since the 1950s, the technology has been improved. Lasers replaced microwaves as the preferred tool for measuring absorption frequencies. Having larger frequencies than the microwaves, the lasers allowed the scientists to subdivide the second with an even greater resolution, further improving accuracy, but also enabling miniaturisation.

“In the small clock being commercialised by Teledyne e2v, we have improved on a technique called coherent population trapping, which provides a very sharp signal,” said Lewis. “We shine a laser beam into the caesium vapour and on top of the laser frequency we add microwave modulation, which helps us find precisely the absorption of the caesium atoms. Once we are on the right frequency with our microwave modulation, we can lock the microwave signal to the atoms and then take that frequency, which is 4.6GHz, and divide it down to 10MHz and that becomes the output of the clock.” 

Miniaturisation is possible thanks to semiconductor micro-fabricated laser systems and low-noise electronics. After creating a prototype design, a cycle of testing and redesigning follows. Lewis said: “Teledyne takes the prototype and turns it into something that is easier to manufacture and more reliable. It’s a team effort between a science organisation and a manufacturing organisation to build a new product line.”