A range of new applications will be enabled by ultra-precise optical clocks, some by using them in space, near or far distant from Earth. They cover the fields of fundamental physics (tests of General Relativity), time and frequency metrology (comparison of distant terrestrial clocks, operation of a master clock in space), geophysics (mapping of the gravitational potential of the Earth), and potential applications in astronomy (local oscillators for radio ranging and interferometry in space).
We propose to (1) develop two “engineering confidence“ ultra-precise transportable lattice optical clock demonstrators with relative frequency instability < 1×10-15/root(tau)1/2, inaccuracy < 5×10-17, one of which as a breadboard. They will be based on trapped neutral Ytterbium and Strontium atoms. Goal performance is about 1 and 2 orders better than today’s best transportable clocks, in inaccuracy and instability respectively. The two systems will be validated in a laboratory environment (TRL 4) and performance will be established by comparison with laboratory optical clocks and primary frequency standards.
(2) We will develop the necessary laser systems (adapted in terms of power, linewidth, frequency stability, long-term reliability, and accuracy), atomic packages with control of systematic (magnetic fields, black-body radiation, atom number), where novel solutions with reduced space, power and mass requirements will be implemented. Some of the laser systems will be developed towards particularly high compactness and robustness. Also, crucial laser components will be tested at TRL 5 level (validation in relevant environment).
The work will build on the expertise of the proposers with laboratory optical clocks, and the successful development of breadboard and transportable cold Sr and Yb atomic sources and ultrastable lasers during the ELIPS-3 ESA development project “Space Optical Clocks (SOC)”.
Atomic clocks prepare for the field and for space
EU-funded researchers are well on their way to delivering transportable optical clocks with frequency instability below 1x10-16 and fractional inaccuracy below 5x10-17. Their performance will eventually be improved to be about two orders of magnitude higher than today's most stable and accurate microwave clocks.
At the heart of any clock is an oscillatory phenomenon that occurs at a highly regular interval, whether this is a swinging pendulum or voltage-driven oscillations of a quartz crystal. Mechanical and electromechanical timepieces, however, tend to be susceptible to temperature changes and ageing, despite their ingenious designs. Moreover, the increasing need for more precise timing has demanded oscillators with higher frequencies.
Optical atomic clocks make use of the frequency of electron transitions from one atomic orbital to another. They represent a revolutionary step forward in time standards, enabled by advances in the field of laser technology and quantum optics. They make use of ultrahigh, optical, oscillation frequencies. Optical atomic clocks will thus supersede Cesium (Cs)-based clocks, which 'tick' at microwave frequency, about 10 billion times per second.
In so-called lattice optical atomic clocks, cold atoms are drawn into a laser wave in form of a standing wave (optical lattice). Herein, thousands of atoms are confined simultaneously. By tuning the lattice laser light to a carefully determined wavelength, its effects on the electron transitions can be minimised. Thus, optical atomic clocks are capable of unprecedented accuracy and stability.
With EU funding of the http://www.soc2.eu (SOC2) project, a team of researchers is developing and operating critical components and subsystems required for ultra-precise neutral-atom lattice optical clocks suitable for transport and eventually for use in space. The researchers work with ytterbium (Yb) and strontium (Sr) atoms.
SOC2 scientists have developed the needed laser subsystems and integrated them with atomics subsystems for strontium and ytterbium into complete clock systems. For example, for the Sr-based clock, they devised compact and robust frequency stabilisation subsystems based on optical cavities, a permanent-magnet atom slower, and a very compact atom chamber. Their compact, low-power-consumption system routinely produces ultracold Sr atoms.
For the Yb-based clock, scientists developed external cavity diode lasers using narrowband interference filters that promise improved stability, compared to commonly used grating-stabilised lasers. The first prototype of the modular apparatus is fully operational. It works automatically and stably for several continuous hours of use. It has recently been successfully transported by van from the Universität Düsseldorf to the Italian metrology institute in Torino, were it is undergoing in-depth characterisation.
The SOC2 optical atomic clocks, once finalised, will represent breadboard-type demonstrators for future clocks to be used in space-based experiments, in particular for performing a more precise test of one fundamental aspect of Einstein's theory of General Relativity, the time dilation. A space clock would also be useful for delivering ultrastable frequencies across Earth.