A team of physicists from the United States and Russia announced today* that it has developed a means for computing, with unprecedented accuracy, a tiny, temperature-dependent source of error in atomic clocks. Although small, the correction could represent a big step towards atomic timekeepers’ longstanding goal of a clock with a precision equivalent to one second of error every 32 billion years—longer than the age of the universe.
Precision timekeeping is one of the bedrock technologies of modern science and technology. It underpins precise navigation on Earth and in deep space, synchronization of broadband data streams, precision measurements of motion, forces and fields, and tests of the constancy of the laws of nature over time.
“Using our calculations, researchers can account for a subtle effect that is one of the largest contributors to error in modern atomic timekeeping,” says lead author Marianna Safronova of the University of Delaware, the first author of the presentation**. “We hope that our work will further improve upon what is already the most accurate measurement in science: the frequency of the aluminum quantum-logic clock,” adds co-author Charles Clark, a physicist at the Joint Quantum Institute, a collaboration of the National Institute of Standards and Technology (NIST) and the University of Maryland.
The paper was presented today at the 2011 Conference on Lasers and Electro-Optics in Baltimore, Md.
The team studied an effect that is familiar to anyone who has basked in the warmth of a campfire: heat radiation. Any object at any temperature, whether the walls of a room, a person, the Sun or a hypothetical perfect radiant heat source known as a “black body,” emits heat radiation. Even a completely isolated atom senses the temperature of its environment. Like heat swells the air in a hot-air balloon, so-called “blackbody radiation” (BBR) enlarges the size of the electron clouds within the atom, though to a much lesser degree—by one part in a hundred trillion, a size that poses a severe challenge to precision measurement.
This effect comes into play in the world’s most precise atomic clock, recently built by NIST researchers***. This quantum-logic clock, based on atomic energy levels in the aluminum ion, Al+, has an uncertainty of 1 second per 3.7 billion years, translating to 1 part in 8.6 x 10^-18, due to a number of small effects that shift the actual tick rate of the clock.
To correct for the BBR shift, the team used the quantum theory of atomic structure to calculate the BBR shift of the atomic energy levels of the aluminum ion. To gain confidence in their method, they successfully reproduced the energy levels of the aluminum ion, and also compared their results against a predicted BBR shift in a strontium ion clock recently built in the United Kingdom. Their calculation reduces the relative uncertainty due to room-temperature BBR in the aluminum ion to 4 x 10^-19 , or better than 18 decimal places, and a factor of 7 better than previous BBR calculations.
Current aluminum-ion clocks have larger sources of uncertainty than the BBR effect, but next-generation aluminum clocks are expected to greatly reduce those larger uncertainties and benefit substantially from better knowledge of the BBR shift.
We develop a nonstandard concept of atomic clocks where the blackbody radiation shift (BBRS) and its temperature fluctuations can be dramatically suppressed (by one to three orders of magnitude) independent of the environmental temperature. The suppression is based on the fact that in a system with two accessible clock transitions (with frequencies V1 and V2) which are exposed to the same thermal environment, there exists a “synthetic” frequency largely immune to the BBRS. As an example, it is shown that in the case of 171Yb+ it is possible to create a clock in which the BBRS can be suppressed to the fractional level of 10^−18 in a broad interval near room temperature (300±15 K). We also propose a realization of our method with the use of an optical frequency comb generator stabilized to both frequencies V1 and V2. Here the frequency Vsyn is generated as one of the components of the comb spectrum and can be used as an atomic standard
Atomic clocks and GPS
The Global Positioning System (GPS) provides very accurate timing and frequency signals. A GPS receiver works by measuring the relative time delay of signals from a minimum of three, but usually more GPS satellites, each of which has three or four onboard caesium or rubidium atomic clocks. The relative times are mathematically transformed into three absolute spatial coordinates and one absolute time coordinate. The time is accurate to within about 50 nanoseconds. However, inexpensive GPS receivers may not assign a high priority to updating the display, so the displayed time may differ perceptibly from the internal time. Precision time references that use GPS are marketed for use in computer networks, laboratories, and cellular communications networks, and do maintain accuracy to within about 50ns.
One of the most significant error sources is the GPS receiver’s clock. Because of the very large value of the speed of light, c, the estimated distances from the GPS receiver to the satellites, the pseudoranges, are very sensitive to errors in the GPS receiver clock; for example an error of one microsecond (0.000 001 second) corresponds to an error of 300 metres (980 ft). This suggests that an extremely accurate and expensive clock is required for the GPS receiver to work. Because manufacturers prefer to build inexpensive GPS receivers for mass markets, the solution for this dilemma is based on the way sphere surfaces intersect in the GPS problem.
Commercially Available Chip Scale Atomic Clock
The Symmetricom SA.45s CSAC is the world’s first commercially available chip scale atomic clock, providing the accuracy and stability of atomic clock technology while achieving true breakthroughs in reduced size, weight and power consumption.
Chip scale atomic clock applications
Prime candidates fitting the chip scale atomic clock application profile include:
• Undersea seismic sensing
• Dismounted (backpack) IED jammers
• Dismounted (backpack) military radios
• Enhanced military GPS receivers
• Tactical UAVs (unmanned aerial vehicles)
Oil and gas exploration firms place a grid of geophysical sensors on the ocean floor to help determine likely spots where petroleum deposits are located. Once the sensors are in place, a powerful air gun or array of air guns launches a sonic pulse from a ship. The ship moves in a pattern that allows the air gun to be fired from many different angles relative to the sensor grid. Some of the pulse’s energy reflects off the ocean floor and back to the surface, but the rest penetrates the ocean floor, travels through the layers of rock underneath and eventually reflects back to the sensors where it is time stamped. Once the ship has finished its predetermined pattern, the sensors are retrieved along with the time stamped data. Because the sonic pulse travels at different speeds in different materials, the “bounce back” times are different based on which materials the pulse traversed. When this timing data is post-processed, it creates a picture of the layers of rock and sediment beneath the ocean floor, showing which locations likely hold oil or gas deposits. Symmetricom’s SA.45s CSAC can greatly improve the accuracy, reduce the cost and reduce the effects of temperature on sensor systems.
Using the SA.45s as a time base, military GPS receivers can achieve greatly reduced Time To Subsequent Fix (TTSF) for 24 hours or more. It also becomes possible to operate with only three satellites in view (instead of the usual four), a distinct advantage in many urban settings.