The principle of timekeeping has been to count the repetitions or oscillations of repeating natural phenomena such as rotation of earth or movement of stars. Over the millennia a myriad of devices has been invented for timekeeping including pendulum or crystals, each with a period far shorter than the daily rotation of the Earth. Since the time could be subdivided into much smaller intervals, makes it possible to measure seconds or even fractions of a second. For example in Quartz clocks, an electric current causes a quartz crystal to resonate at a specific frequency that is far higher than a pendulum’s oscillations.
Atomic clocks are one of the most accurate time devices available today that track time by measuring the resonance frequency of atoms. The ticking of time is measured through microwaves emitted by the electrons around those atoms jumping from a lower to higher orbit as they absorb and then lose energy from a laser. Most atomic clocks use atoms of the isotope caesium-133. Caesium atoms emit microwaves precisely 9,192,631,770 times per second.
Atomic clocks are extremely accurate because they are based on natural and universal atom vibrations. However, even the best atomic microwave clocks can still accumulate an error of about 1 nanosecond over a month due to the stability of clocks. Precision timing is the underlying technology for Mobile phones, financial transactions, the Internet, electric power and Global Positioning System (GPS), all of which rely on accurate time and frequency standards. An atomic clock aboard a GPS satellite can develop an error of up to 10 nanoseconds every 24 hours. Without regularly communicating with each other, clocks onboard GPS satellites would gradually desynchronize, resulting in a decrease of precision when triangulating someone’s position.
For the atomic clocks aboard GPS satellites, making frequent resynchronization efforts across thousands of miles can be logistically costly. That’s why computer scientists and physicists are exploring ways to improve the efficiency of and reduce the error rate for transferring time information.
Atomic devices are limited by the relatively low frequency of microwave radiation. To keep time even more accurately, and eventually introduce a new definition of the second, physicists are developing clocks based on higher-frequency optical transitions. Optical clocks use atoms or ions that oscillate about 100,000 times higher than microwave frequencies, in the optical, or visible, part of the electromagnetic spectrum. The higher frequencies mean that optical clocks “tick” faster than microwave atomic clocks and could thus provide time-stamps that are 100 to 1,000 times more accurate.
All other things being equal, the stability of an atomic clock is proportional to its operating frequency and inversely proportional to the width of the electronic transition. In an optical atomic clock, an ultra-stable laser is locked to a spectrally narrow electronic transition in the optical region of the spectrum – the so-called “clock transition”.
Although quantum technology has proven valuable for highly precise timekeeping, making these technologies practical for use in a variety of environments is still a key challenge. In an important step toward portable quantum devices, researchers have developed a new high-flux and compact cold-atom source with low power consumption that can be a key component of many quantum technologies.
“The use of quantum technologies based on laser-cooled atoms has already led to the development of atomic clocks that are used for timekeeping on a national level,” said research team leader Christopher Foot from Oxford University in the U.K. “Precise clocks have many applications in the synchronization of electronic communications and navigation systems such as GPS. Compact atomic clocks that can be deployed more widely, including in space, provide resilience in communications networks because local clocks can maintain accurate timekeeping even if there is a network disruption.”
Exploiting quantum effects can improve clock precision by several orders of magnitude, with an advance to TRL 6 feasible within 10 years. However, utilizing these better clocks will require major reductions in system size, weight, and power. The same report noted that quantum magnetometers, which enable quantum navigation, could be “an important part of achieving GPS-denied advantage,” including because quantum inertial sensing is not susceptible to jamming.
Many defence-critical applications require exceptionally precise time and frequency standards enabled only by atomic clocks. The U.S. Air Force Scientific Advisory Board noted in a recent report that quantum clocks and quantum sensors would merit further investment since enhanced timing precision could enhance Air Force missions and capabilities, including SIGINT, counter-DRFM, electronic warfare (EW), and also more robust communications. Clock synchronization plays a crucial role in enhancing the bandwidth for communication satellites, and the new approach may help future satellites to be more efficient — and therefore process data more quickly — as quantum communication technology matures over the next few decades.
Jun Ye group has developed world’s most precise and accurate optical atomic clock based on neutral atoms. The Sr-lattice clock is also the most stable optical atomic clock in the world. The Sr-lattice clock is the most stable optical atomic clock in the world. Its precision timekeeping mechanism is based on a narrow electronic transition in Sr atoms trapped inside an optical lattice, which is designed to separate the atomic internal and external degrees of freedom during clock measurement.
Ye’s group has built a strontium clock that is so precise, out of every 10 quintillion ticks only 3.5 would be out of sync – the first atomic clock ever to reach that level of precision. “In 2014, the world’s most accurate optical clock wouldn’t lose or gain one second in the entire age of the universe,” says Jun Ye at the University of Colorado at Boulder. Previous caesium clocks kept time accurately to within a second over the course of 300 million years.
To build a more precise clock, Ye and his team designed a 3D structure that let them measure signals from more atoms at once within the width of the laser beam. The latticework enabled the researchers to survey atoms that were much more densely packed together – 10 trillion atoms per cubic centimetre compared with previous clocks with 10 billion atoms per cubic centimetre – and have better control of those atoms’ interactions, minimising how often they crash into each other.
They cooled the atoms to -273˚C and trapped each one in its own spot to control the interactions between them. “Imagine a scenario where you have single-person housing in a city block. One person lives in each house and neighbours are never allowed in,” says Ye. “Each atom fits in one particular site.”
Ye says the ultracold temperatures turn the atoms into what is known as a quantum gas. “When atoms in the gaseous phase are very hot, they’re moving apart and colliding with each other,” he says. “This changes when you lower the temperature of the gas so much that these particles start to move like waves – they start to avoid each other.”
“To help reach this exciting regime, we have developed the world’s most stable laser that can maintain optical coherence over 10 s. This laser has permitted us to approach (within a factor of 2) the fundamental quantum projection noise limit for the clock operation with a thousand atoms. The resulting stability of the Sr-lattice clock is currently at 2 x 10-16 at 1 s. This improved stability has greatly facilitated our efforts to characterize the overall uncertainty of the Sr-lattice clock.”
Entangled aluminium ion is world’s best timekeeper
The latest work at NIST features what is known as a quantum-logic clock. Built by Samuel Brewer and colleagues, it uses a positive ion of aluminium-27 as its timekeeper. When exposed to ultraviolet laser light at wavelength 267 nm, the ion undergoes a transition with a very narrow linewidth – making its frequency very well defined. What is more, that transition is largely immune to sources of external noise – such as blackbody radiation – that in other types of optical clock shift the frequency away from its true value.
A magnesium-25 ion is used to cool the aluminium down to the very low temperatures needed to minimize thermal noise. Cooling involves the absorption of photons at another specific frequency, but practical limitations mean that this cannot be done using the aluminium itself. This is because the required frequency in is too high for any practical laser. By entangling the two ions, the magnesium cools the aluminium via Coulomb interactions. This process also allows the quantum state of the aluminium ion to be read-out following exposure to the clock laser.
After fine-tuning some other aspects of the clock mechanism, the researchers totted up all the estimated systematic uncertainties. Their tally came to 9.4×10−19, which just pips the 1.4×10−18 achieved last year by Andrew Ludlow and colleagues, also at NIST, with an optical clock made from a “lattice” containing thousands of ytterbium atoms.
In their research, Brewer and team have been addressing the clock’s remaining Achilles’ heel – the fact that aluminium has a relatively low mass. Held in a trap using oscillating electric fields, the aluminium can move around the trap more easily than a heavier ion. This creates a problem of relativistic time dilation, a slight shift in the ion’s transition frequency due to its finite speed. An uncertainty of one part in 1018 corresponds to a mere 40 cm/s – “a slow walking speed”, as Brewer puts it. “That has dominated the [clock’s] uncertainty for the last ten years or so,” he says.
The researchers had to deal with two distinct sources of motion. One occurs at the frequency of the trap’s oscillating fields (about 40 MHz) due to a residual field causing the ion to move away from the centre of the trap. By making the trap slightly more symmetrical and reducing the drive frequency, Brewer says that the team has lowered the uncertainty in this source of motion by a factor of ten.
They also reduced time dilation due to “secular motion”, which is related to the temperature of the ions in the trap. Brewer explains that the trap functions as a 3D harmonic oscillator, and with a finite temperature the ion occupies a distribution of motional states. The idea, he says, is to try and put the ion in its ground state in all three dimensions. By getting close to this using what is known as “pulsed Raman sideband cooling”, he and his colleagues were able to reduce the uncertainty in the secular motion by about a factor of 15
With optical clocks evolving quickly, Brewer says it is not yet clear which technology – and hence transition – will be chosen to redefine the second. He points out that lattice clocks tend to be more stable and so can reach their stated accuracies more quickly. But he believes that the aluminium-ion clock is “as good a candidate as any other” being developed. He and his colleagues are currently working on a new version of their clock with a further improved trap and vacuum, which, he says, might reduce systematic uncertainty by a further factor of ten.
UK Researchers develop New cold atom source for portable quantum devices, reported in June 2021
In The Optical Society (OSA) journal Optics Express, S. Ravenhall, B. Yuen and Foot describe work carried out in Oxford, U.K. to demonstrate a completely new design for a cold atom source. The new device is suitable for a wide range of cold-atom technologies. “In this project we took a design we made for research purposes and developed it into a compact device,” said Foot. “In addition to timekeeping applications, compact cold-atom devices can also be used for instruments for gravity mapping, inertial navigation and communications and to study physical phenomena in research applications such as dark matter and gravitational waves.”
Although it may seem counterintuitive, laser light can be used to cool atoms to extremely low temperatures by exerting a force that slows the atoms down. This process can be used to create a cold-atom source that generates a beam of laser-cooled atoms directed toward a region where precision measurements for timekeeping or detecting gravitational waves, for example, are carried out.
Laser cooling usually requires a complicated arrangement of mirrors to shine light onto atoms in a vacuum from all directions. In the new work, the researchers created a completely different design that uses just four mirrors. These mirrors are arranged like a pyramid and placed in a way that allows them to slide past each other like the petals of a flower to create a hole at the top of the pyramid through which the cold atoms are pushed out. The size of this hole can be adjusted to optimize the flow of cold atoms for various applications. The pyramid arrangement reflects the light from a single incoming laser beam that enters the vacuum chamber through a single viewport, thus greatly simplifying the optics.
The mirrors, which are located inside the vacuum region of the cold-atom source, were created by polishing metal and applying a dielectric coating. “The adjustability of this design is an entirely new feature,” said Foot. “Creating a pyramid from four identical polished metal blocks simplifies the assembly, and it can be used without the adjustment mechanism.”
To test their new cold-atom source design, the researchers constructed laboratory equipment to fully characterize the flux of atoms emitted through a hole at the apex of the pyramid. “We demonstrated an exceptionally high flux of rubidium atoms,” said Foot. “Most cold-atom devices take measurements that improve with the number of atoms used. Sources with a higher flux can thus be used to improve measurement accuracy, boost the signal-to-noise ratio or help achieve larger measurement bandwidths.” The researchers say that the new source is suitable for commercial application. Because it features a small number of components and few assembly steps, scaling up production to produce multiple copies would be straightforward.
Commercially available atomic clock
The new company, such as Muquans in France, offers commercial devices based on new ideas around quantum sensing. It has launched new generation of ultra-high performance measurement instruments based on a unique and patented technology, which relies on the utilization of laser cooling, trapping and manipulation of neutral atoms.
It offers an absolute quantum atomic clock, which provides a time reference signal offering relative stability and accuracy close to 10 exp(-15) and dedicated to time metrology applications.
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