As part of the JST Targeted Basic Research Program, a team of researchers at the University of Tokyo and RIKEN have developed a pair of atomic clocks that agree with respect to each other at a fractional uncertainty of 2×10-18. This means they will be out of synchronization by less than a second during the entire age of our universe of 13.8 billion years. The clocks are based on optical lattice clock that performs the superb high-precision spectroscopy of thousands of atoms in a cold environment. This record-high agreement of the two clocks demonstrates an important leap in fundamental technology that facilitates optical lattice clocks towards a future redefinition of the second.
Currently, optical lattice clocks are being developed all over the world aiming at nearly 1000 fold improvement in uncertainty over the international atomic time (TAI) based on microwave cesium clocks. A major challenge in improving the uncertainty of optical lattice clocks is to eliminate the influence of electromagnetic waves radiated from the surrounding walls at room temperature (blackbody radiation), which changes the eigen-frequency of atoms.
To address this issue, researchers have developed cryogenic optical lattice clocks where strontium atoms are interrogated in a cryogenically-cooled environment at -180 degrees Celsius, reducing the influence of blackbody radiation by 1/100. A comparison between two such cryo-clocks over a period of one month has yielded a statistical agreement of 2×10-18.
Realization of such a high-precision atomic clock is a step forward towards the redefinition of the second, and holds potential for new roles that are radically different from conventional clocks. It is possible to detect relativistic time delay caused by gravitational potential difference between two clocks of a few centimeter height difference, which leads to “relativistic geodesy technology” to measure the height difference between two remote places. Furthermore, such high-precision clocks can be probes for New Physics, by investigating, such as, the constancy of physical constants comparing two clocks consisted of different atomic elements.
- a. Statistical uncertainty of frequency difference between two cryogenic optical lattice clocks (Allan standard deviation). The statistical uncertainty reaches 2×10-18 for an averaging time of 2 hours.
- b. Observed frequency differences between the two cryo-clocks. The individual clock frequencies (about 429×1012 Hz) do not deviate from each other by more than several milli-hertz. After averaging 11 measurements performed over a month, the two clocks agree to within Δν/ν0=(-1.1±2.0(statistical uncertainty))×10-18. Gray shaded region shows the systematic uncertainty of 4.4×10-18, which is experimentally evaluated for the two clocks.
The Exploratory Research for Advanced Technology (ERATO)
“Katori Innovative Space-Time Project”
Ichiro Ushijima, Masao Takamoto, Manoj Das, Takuya Ohkubo, & Hidetoshi Katori. “Cryogenic optical lattice clocks”. Nature Photonics, Published online 09 February 2015, doi: 10.1038/nphoton.2015.5.
Hidetoshi Katori Ph.D.
Professor, Department of Applied Physics, Graduate School of Engineering, The University of Tokyo, Chief Scientist, Quantum Metrology Laboratory, Advanced Science Institute, RIKEN
Department of Research Project, JST