Frequency-based metrology is important in a wide range of measurement physics and quantum physics applications. Improvements in frequency standards and dissemination at the primary level are anticipated to lead to significant advances in the following key application areas:

Provision of the SI units of time and length
Optical frequency standards are already used for the practical realisation of the metre in a laboratory environment. The SI second is currently defined in terms of a microwave transition in caesium. However optical frequency standards offer the prospect of stabilities and reproducibilities which surpass that of the caesium primary standard and can be compared to this standard with high accuracy. They can already be used as secondary representations of the second, and there is the potential for an optical re-definition of the second in the longer term.

Optical atomic clocks for space
The emergence of a new generation of high performance optical atomic clocks opens up new possibilities for space missions employing highly stable and accurate on-board frequency references. Future mission scenarios which would benefit from optical atomic clock technology range from unique science experiments designed to test the fundamental laws of physics to applications in Earth observation and navigation.

Within the area of fundamental physics, optical clocks could be used to search for violations of the Einstein Equivalence Principle (the equivalence of inertial and gravitational mass) by making measurements of the gravitational redshift with unprecedented precision, or to discriminate between different theories of gravity through measurements of the Shapiro time delay of highly stable optical signals to and from a spacecraft as it passes close by the sun at conjunction. For Earth observation, the sensitivity of highly stable and accurate clocks means that they can be used for direct measurement of the Earth’s geoid (gravitational equipotential surface) at the few centimetre level with very high spatial resolution. Finally, optical atomic clocks are expected to find application in future evolutions of satellite navigation systems, where they offer the possibility of up to two to three orders of magnitude improvement in satellite clock prediction accuracy.

NPL has recently delivered a report to the European Space Agency that proposes a technology development plan targeted at space deployment of an optical atomic clock by 2020. This plan considers alternative clock technologies based on trapped ions and cold atoms as well as the necessary optical local oscillators and systems for laser cooling. It aims to develop these capabilities in a co-ordinated and efficient way through collaboration between a number of European partners. The full report is available here ( PDF 3.22 MB).

Fundamental physics
Frequency stands at the apex of measurement technology as the most precisely measurable physical quantity, and for this reason frequency metrology has long played an important role in testing fundamental physical theories. Advances in optical frequency standards and metrology can be expected to open up new horizons in precision atomic spectroscopy, leading to:

• Improved measurements of fundamental constants such as the Rydberg constant and the fine structure constant;
• Laboratory searches for possible slow time-variations of fundamental constants;
• Tests of fundamental physical theories such as relativity, quantum electrodynamics and CPT invariance.

Satellite navigation and ranging
Frequency standards are an enabling technology for satellite navigation systems, whilst at the same time satellite systems are the dominant mechanism for distributing standards of time and frequency worldwide. The combination of high-accuracy primary frequency standards, commercial atomic frequency standards and stable communications links allows accurate time and frequency to be disseminated to a mass market. In global positioning systems, accuracies of a few tens of metres in one-way mode already require frequency standard uncertainties better than parts in 10¹³. Satellite ranging for deep space networks will require ground station time transfer accuracies of parts in 10¹⁵ to 10¹⁷.

Telecommunications
The rapid expansion of optical communication bands for fibre and space-based communications is leading to new requirements for frequency standards and metrology in the 1500 nm region of the spectrum. The relentless increase in transmitted data volume and speed pace is leading to the development of dense wavelength division multiplexing (DWDM) systems with close channel spacings and high bit rates. Techniques for spectral characterisation of wavelength-division multiplexed channels are at the heart of efficient data distribution and recovery. A range of suitable frequency standards spanning the infra-red optical communications bands is therefore required, together with simple techniques for measuring absolute frequencies and/or frequency differences in this spectral region.

Astronomy and survey
Optical frequency standards and high-stability optical flywheel oscillators also have a number of applications in astronomy and survey, including:

• Observation of gravity waves using frequency-stabilised lasers;
• Survey of distant star planetary systems using swept or multi-frequency laser sources;
• Very-long-baseline interferometry for geodesy and earthbound distance measurement;
• Large structure measurement by laser ranging;
• Airborne and ocean-based oil survey by measurement of gradiometric variations in g.