Phones and other GPS-enabled devices pinpoint your location on Earth by contacting at least four satellites bearing atomic clocks. Each of these satellites provides a time stamp, and the system calculates your location based on the relative differences among those times. The accuracy of GPS Navigation is primary degraded due to Earth’s ionosphere, which interferes with the timing signals as they commute from a satellite to your GPS receiver. But the second-biggest contribution of error comes from the stability of the clocks onboard the GPS satellites.
An atomic clock is a clock whose timekeeping mechanism is based on the interaction of electromagnetic radiation such as microwaves or light with the excited states of certain atoms. Atomic clocks are the most accurate time and frequency standards known and are used as primary standards for international time distribution services, to control the carrier frequency of television broadcasts, and in global navigation satellite systems such as the American GPS and the European Union’s Galileo Program.
The atomic clocks used on today’s satellites are based on natural oscillation of the cesium atom — a frequency in the microwave region of the electromagnetic spectrum. Even the best atomic microwave clocks can still accumulate an error of about 1 nanosecond over a month.
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, greatly improving the precision of GPS.
A major obstacle to developing an optical clock is the difficulty of directly measuring optical frequencies. This problem has been solved with the development of self-referenced mode-locked lasers, commonly referred to as femtosecond frequency combs. Before the demonstration of the frequency comb in 2000, terahertz techniques were needed to bridge the gap between radio and optical frequencies, and the systems for doing so were cumbersome and complicated. With the refinement of the frequency comb, these measurements have become much more accessible and numerous optical clock systems are now being developed around the world
Most research focuses on the often conflicting goals of making the clocks smaller, cheaper, more portable, more energy efficient, more accurate, more stable and more reliable. Efforts at NIST, NPL and QML are also under way to develop miniaturized and more robust and practical versions of the optical clocks, for real-world applications. At NIST, that work is supported by NASA and the US Defense Advanced Research Projects Agency (DARPA), while in the UK a project named “Kairos” – referencing the Ancient Greek word for “the critical moment” – features NPL as part of a new consortium aiming to build a miniature atomic clock based around laser-cooled cesium atoms.
Optical atomic clock advances
The accuracy and the stability of optical clocks are mainly based on the fact that the frequency of the optical radiation used is higher (by several orders of magnitude) than that of the microwave radiation which is used in cesium atomic clocks, which makes optical clocks much more precise than cesium clocks.
In a strontium clock, laser cooling is used to slow an atomic gas down to temperatures near absolute zero. Then, an extremely narrow transition between long-lived eigenstates of the atoms is excited in order to stabilize the frequency of the excitation laser to that of the atoms. The simultaneous interrogation of numerous atoms leads to a particularly high signal-to-noise ratio and, thus, to high stability.
However, optical clocks do experience significant downtimes because of their higher technical complexity.
To deal with the downtimes that plague today’s optical clocks, the researchers combined a commercially available maser with a strontium optical lattice clock at PTB, Germany’s national metrology institute. The maser, which is like a laser except that it operates in the microwave spectral range, can be used as a type of reliable pendulum with limited accuracy to bridge the downtime of the optical clock. The researchers spanned the large spectral gap between the optical clock’s optical frequency and the maser’s microwave frequency with an optical frequency comb, which effectively divides the slower microwave-based “ticks” to match the faster “ticks” of the optical clock.
“We compared the continuously running maser with our optical clock and corrected the maser frequency as long as we had data available from the optical clock,” said Grebing. “During the optical clock’s downtimes, the maser runs on its own stably.”The researchers operated the maser and optical clock for 25 days, during which the optical clock ran about 50 percent of the time. Even with optical clock downtimes ranging from minutes to two days, the researchers calculated a time error of less than 0.20 nanoseconds over the 25 days.
The scientists from PTB have therefore developed a resonator whose frequency is among the most stable worldwide: with a length of 48 cm and ingenious thermal and mechanical isolation from its environment, it reaches a fractional frequency instability of 8 10-17.
Researchers converted the precise timekeeping of optical atomic clocks into microwave signals for electronics in reported in May 2020
But newer optical atomic clocks, based on atoms such as ytterbium and strontium, vibrate much faster at higher frequencies and generate optical signals. Such signals must be converted to microwave signals before electronic systems can readily make use of them.
In May 2020, it was reported that researchers in the United States have figured out how to convert high-performance signals from optical clocks into a microwave signal that can more easily find practical use in modern electronic systems.
“How do we preserve that timing from this optical to electronic interface?” says Franklyn Quinlan, a lead researcher in the optical frequency measurements group at the U.S. National Institute of Standards and Technology (NIST). “That has been the big piece that really made this new research work.”
By comparing two optical-to-electronic signal generators based on the output of two optical clocks, Quinlan and his colleagues created a 10-gigahertz microwave signal that synchronizes with the ticking of an optical clock. Their highly precise method has an error of just one part in a quintillion (a one followed by 18 zeros). The new development and its implications for scientific research and engineering are described in the 22 May issue of the journal Science.
Optical clocks can already be linked together physically through fiber-optic networks, but this approach still limits their usage in many electronic systems. The new achievement by the U.S. research team—with members from NIST, the University of Colorado-Boulder, and the University of Virginia in Charlottesville—could remove such limitations by combining the performance of optical clocks with microwave signals that can travel in areas without a fiber-optic network.
The improvement comes as many researchers expect the international standard that defines a second in time—the Système International (SI)—to switch over to optical clocks. Today’s cesium-based atomic clocks require a month-long averaging process to achieve the same frequency stability that an optical clock can achieve in seconds.
Curtis describes the improved capability to synchronize microwave signals with optical clock signals as a “paradigm shift” that will impact “fundamental physics, communication, navigation, and microwave engineering.” One of the most immediate applications could involve higher-accuracy Doppler radar systems used in navigation and tracking. A more stable microwave signal can help radar detect even smaller frequency shifts that could, for example, better distinguish slow-moving objects from the background noise of stationary objects.
Future space telescopes based on very-long-baseline interferometry (VLBI) could also benefit from the highly stable microwave signals synchronized with optical clocks. Today’s ground-based VLBI telescopes use receiver devices spread across the globe to detect microwave and millimeter-wave signals and combine them into high-resolution images of cosmic objects such as black holes. A similar VLBI telescope located in space could boost the imaging resolution while avoiding the Earth’s atmospheric distortions that interfere with astronomers’ observations. In that scenario, having optical-clock-level stability to synchronize all the signals received by the VLBI telescope could improve observation time from seconds to hours.
There is still more work to be done before more electronic systems can take advantage of such optical-to-microwave conversion. For one thing, the sheer size of optical clocks means that nobody should expect a mobile device to have a tiny optical clock inside anytime soon. In the team’s latest research, their optical atomic clock setup occupied a lab table about 32 square feet in size (almost 3 square meters).
“Some of my coauthors on this effort led by Andrew Ludlow at NIST, as well as other folks around the world, are working to make this much more compact and mobile so that we can kind of have optical-clock-level performance on mobile platforms,” Quinlan says.
NIST Team Compares 3 Top Atomic Clocks With Record Accuracy Over Both Fiber and Air, reported in March 2021
In a significant advance toward the future redefinition of the international unit of time, the second, a research team led by the National Institute of Standards and Technology (NIST) has compared three of the world’s leading atomic clocks with record accuracy over both air and optical fiber links, reported in March 2021.
The study compared the aluminum-ion clock and ytterbium lattice clock, located in different laboratories at NIST Boulder, with the strontium lattice clock located 1.5 kilometers away at JILA, a joint institute of NIST and the University of Colorado Boulder. The team’s measurements were so accurate that uncertainties were only 6 to 8 parts in 1018 — that is, errors never exceeded 0.000000000000000008 — for both fiber and wireless links.
NIST researchers previously described in detail how they transferred time signals over the air link between two of the clocks, the NIST ytterbium and JILA strontium clocks, and found the process worked as well as the fiber-based method and 1,000 times more precisely than conventional wireless transfer schemes. This work shows how the best atomic clocks might be synchronized across remote sites on Earth and, as time signals are transferred over longer distances, even between spacecraft.
Optical Clock Technology Tested in Space for the First Time in 2016
For the first time, an optical clock has traveled to space, surviving harsh rocket launch conditions and successfully operating under the microgravity that would be experienced on a satellite. This demonstration brings optical clock technology much closer to implementation in space, where it could eventually allow GPS-based navigation with centimeter-level location precision. GPS has become ubiquitous technology that provides real-time positioning, navigation and timing (PNT) data in cars, boats, planes, trains, smartphones and wristwatches, and has enabled advances as wide-ranging as driverless cars, precision munitions, and automated supply chain management.
In The Optical Society’s journal for high impact research, Optica, researchers report on a new compact, robust and automated frequency comb laser system that was key to the operation of the space-borne optical clock. “Our device represents a cornerstone in the development of future space-based precision clocks and metrology,” said Matthias Lezius of Menlo Systems GmbH, first author of the paper. “The optical clock performed the same in space as it had on the ground, showing that our system engineering worked very well.”
Frequency combs are an important component of optical clocks because they act like gears, dividing the faster oscillations of optical clocks into lower frequencies to be counted and linked to a microwave-based reference atomic clock. In other words, frequency combs allow the optical oscillations to be precisely measured and used to tell time. Optical clocks and frequency combs have been improving rapidly because of advances in building more portable and powerful lasers, as well as andvances in laser spectroscopy, Lezius said. EPFL scientists have found a way to miniaturize frequency combs, realizing a new step toward miniaturization of such tools. Their device can measure light oscillations with a precision of 12 digits.
Until recently, frequency combs have been very large, complex set-ups only found in laboratories. Lezius and his team at Menlo Systems, a spin-off company of Nobel Laureate T.W. Hänsch’s group at the Max Plank Institute for Quantum Optics, developed a fully automated optical frequency comb that measures only 22 by 14.2 centimeters and weighs 22 kilograms.
The new frequency comb is based on optical fibers, making it rugged enough to travel through the extreme acceleration forces and temperature changes experienced when leaving Earth. Its power consumption is below 70 watts, well within the requirements for satellite-based devices.
The complete optical clock system
The researchers combined their new frequency comb with an atomic cesium clock for reference and a rubidium optical clock developed by research groups at Ferdinand Braun Institute Berlin and Humboldt University of Berlin as well as a group from the University of Hamburg that recently moved to Johannes Gutenberg University of Mainz (JGU). Airbus Defense & Space GmbH was involved in the construction, interfacing, and integration of the payload module that went into space and also provided support and equipment during the flight.
In April 2015, the entire system was flown on a research rocket for a 6-minute parabolic flight into space as part of the TEXUS program that launches from the Esrange Space Center in Sweden. Once microgravity was achieved, the system started measurements automatically and was controlled from the ground station via a low-bandwidth radio link.
The highly accurate measurements made possible with frequency combs could be useful for many applications. Frequency combs are the “gears” necessary to run clocks ticking at optical frequencies. For example, space-based frequency combs could improve the accuracy of global remote sensing of greenhouse gases from satellites and could be used for space-based gravitational wave detectors. “Applications based on frequency combs are quite important for future space-based optical clocks, precision metrology and earth observation techniques,” said Lezius. “The space technology readiness of frequency combs is developing at a fast pace.”
“The experiment demonstrated the comb’s functionality as a comparative frequency divider between the optical rubidium transition at 384 THz and the cesium clock providing a 10 MHz reference,” said Lezius. Although the optical clock used in the demonstration had about one tenth the accuracy of atomic clocks used on GPS satellites today, the researchers are already working on a new version that will improve accuracy by several orders of magnitude.
The researchers plan to fly an improved version of the optical clock into space at the end of 2017. In that experiment, the frequency comb module will not fly under a pressurized dome in order to test how well it works in the vacuum conditions that would be experienced on a satellite. The researchers also seek to further improve the system’s resistance to harsh cosmic radiation to ensure that it can operate for several years in orbit.
Within a few years, Lezius and his team aim to have a space-qualified frequency comb module that the space community can use in future missions and applications. They are aiming for a device with a volume of about 3 liters that weighs a few kilograms and has a power consumption of approximately 10 watts.
Optical clocks ‘could detect gravitational waves’
Einstein’s theory of relativity predicts that an atomic clock’s ticking, that is, the frequency of the atoms’ vibrations, is reduced—shifted toward the red end of the electromagnetic spectrum—when observed in stronger gravity. That is, time passes more slowly at lower elevations.
Researchers at the US National Institute of Standards and Technology (NIST) say that their latest optical clocks – based around ytterbium atoms trapped in a matrix of laser beams – appear good enough to detect gravitational waves and perhaps even dark matter. Other groups working on optical clock technology include a team at the UK’s National Physical Laboratory (NPL), where a similar system based on strontium is under development. The University of Tokyo’s Quantum Metrology Laboratory (QML) is another world-leading location, where Hidetoshi Katori and colleagues have been able to use the relativistic effect of a gravitational red-shift to measure the difference in height between two optical clocks.
Publishing the latest experimental results in Nature, Andrew Ludlow and colleagues at the Boulder lab reported record-breaking uncertainty, stability, and reproducibility with two of the clocks. Key figures of merit include a systematic uncertainty of 1.4 x 10-18, and a measurement instability of 3.2 x 10-19.
This means that the clocks are sufficiently sensitive for their time accuracy to be limited by changes in the strength of gravity with altitude on Earth, which according to Einstein’s theory of relativity sees time pass more slowly at lower elevations – something known as “gravitational red-shift”. “Here we demonstrate local optical clock measurements that surpass the current ability to account for the gravitational distortion of space-time across the surface of Earth,” wrote the NIST team in its paper. While this makes the ytterbium clocks less useful as timepieces, the flip side is the possibility of using them to test relativistic phenomena, including gravitational waves.
Ludlow said: “The agreement of the two clocks at this unprecedented level, which we call reproducibility, is perhaps the single most important result, because it essentially requires and substantiates the other two results.” He added: “This is especially true because the demonstrated reproducibility shows that the clocks’ total error drops below our general ability to account for gravity’s effect on time here on Earth. “Hence, as we envision clocks like these being used around the country or world, their relative performance would be, for the first time, limited by Earth’s gravitational effects.” The laser-based optical “lattice” used by the NIST team traps ytterbium atoms in such a way that a relatively large signal can be generated than with more conventional ion-based atomic clocks, improving their precision.
Military requirements of ultra-accurate timing sources
For military Precision time and time interval (PTTI) technology is critical to warfighting capability, for use in both navigation and communications. Precise time synchronization is needed to efficiently determine the start of a code sequence in secure communications, to perform navigation, and to locate the position of signal emitters. Precise frequency control is required in communications for spectrum utilization and frequency-hopped spread-spectrum techniques.
There are many examples of essential military operations that depend on PTTI and could benefit from improvements in PTTI technology. These include: GPS clocks and autonomous operations, Weapon system four-dimensional coordination, GPS anti jamming, Network-centric warfare, and Secure military communications. Advanced GPS satellite clocks would greatly improve ranging accuracy, reduce collateral damage, and enhance system survivability
The future architecture of DOD is moving towards distributed engagement and surveillance that requires synchronized timing of distributed platforms. The ability to distribute the precise time and frequency from an optical clock to remote platforms could enable future precise navigation and sensing systems. Researchers have recently demonstrated tight, real-time synchronization of a remote microwave clock to a master optical clock over a turbulent 4 km open-air path via optical two-way time–frequency transfer.
Once synchronized, the 10 GHz frequency signals generated at each site agree to 10^−14 at 1 s and below 10^−17 at 1000 seconds. In addition, the two clock times are synchronized to 13 femtosecond over an 8-hour period. The ability to phase-synchronize 10 GHz signals across platforms supports future distributed coherent sensing, while the ability to time-synchronize multiple microwave-based clocks to a high-performance master optical clock supports future precision navigation / timing systems.
One of the application is multistatic synthetic aperture radar where an array of microwave oscillators are synchronized to a single master optical oscillator; LO, local oscillator. The master site’s clock is based on a laser stabilized to an optical cavity (optical oscillator). The remote site’s clock is based on a combined quartz oscillator and DRO. This remote microwave clock is tightly synchronized to the optical clock over a folded 4 km long air path via O-TWTFT. The time and the frequency outputs from each clock are compared in a separate measurement to verify femtosecond time offsets and high phase coherence of the synchronized signals
DARPA’s Quantum-Assisted Sensing and Readout (QuASAR) and PULSE
Quantum-Assisted Sensing and Readout (QuASAR) intends to make the world’s most accurate atomic clocks—which currently reside in laboratories—both robust and portable. QuASAR researchers have developed optical atomic clocks in laboratories with a timing error of less than 1 second in 5 billion years. Making clocks this precise portable could improve upon existing military systems such as GPS, and potentially enable entirely new radar, LIDAR and metrology applications.
Recently the program demonstrated the world’s most accurate clock with a total uncertainty of 2 parts in 10^18 , or about 10,000 times better than GPS clocks. This means that if the clock began ticking at the Big Bang nearly 14 billion years ago it would be accurate to better than one second today.
Recent advances in optical atomic systems give promise to a new generation of optical atomic clocks and quantum metrology that stands to transform numerous DoD applications. The Quantum-Assisted Sensing and Readout (QuASAR) program is building on established control and readout techniques from atomic physics to develop a suite of measurement tools that will be broadly applicable across disciplines, helping to address outstanding challenges in physics, materials, biological sciences, inertial navigation and robust global positioning systems.
Typically, the performance of measurement devices is limited by deleterious effects such as thermal noise and vibration. Notable exceptions are atomic clocks, which operate very near their fundamental limits. Driving devices to their physical limits will open new application spaces critical to future DoD systems. Indeed, many defense-critical applications already require exceptionally precise time and frequency standards enabled only by atomic clocks. The Global Positioning System (GPS) and the internet are two key examples.
Measurement systems based on atomic physics benefit from the exquisite properties of the atom. Among these are (a) precise frequency transitions, (b) the ability to initialize, control, and readout the atomic state and (c) environmental isolation. In addition, atomic properties are absolute, and do not “drift” over time. In this sense, atoms are self-calibrated, making them ideal for precision sensing.
QuASAR will push toward fundamental operating limits by developing atom and atom-like sensors that operate near the standard quantum limit (SQL), constructing hybrid quantum sensors that combine the optimal sensing and readout capabilities of disparate quantum systems and entangling multiple sensors/devices to operate below the SQL. These types of devices will find broad application across the DoD, particularly in the areas of biological imaging, inertial navigation and robust global positioning systems.
DARPA’s Ultrafast Laser Science and Engineering (PULSE) program is developing the technological means for engineering improved spectral sources, such as ultra-fast optical lasers—advances that in turn could facilitate more efficient and agile use of the entire electromagnetic spectrum and generate improvements in existing capabilities such as geolocation, navigation, communication, coherent imaging and radar, and perhaps give rise to entirely new spectrum-dependent capabilities.
The Program in Ultrafast Laser Science and Engineering (PULSE) applies the latest in pulsed laser technology to significantly improve the precision and size of atomic clocks and microwave sources, enabling more accurate time and frequency synchronization over large distances. Recent PULSE demonstrations include synchronization of clocks with femtosecond precision across kilometers of turbulent atmosphere, corresponding to a 1,000-fold improvement over what is possible using conventional radio-frequency techniques.
These capabilities are essential to fully leverage super-accurate atomic clocks, as clocks such as those that QuASAR seeks to build are more precise than our current ability to synchronize between them. If successful, PULSE technology could enable global distribution of time precise enough to take advantage of the world’s most accurate optical atomic clocks.
Defense applications, such as geo-location, navigation, communication, coherent imaging and radar, depend on the generation and transmission of stable, agile electromagnetic radiation. Improved radiation sources—for example, lower noise microwaves or higher flux x-rays—could enhance existing capabilities and enable entirely new technologies.
PULSE will also aim to apply this technology to enable synchronization, metrology and communications applications spanning the electromagnetic spectrum, from radio frequencies to x-rays. By building on established ultrafast laser techniques, PULSE seeks to:
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