Atomic clocks date back to the mid-1950s when Louis Essen developed the first caesium atomic clock at the UK’s National Physical Laboratory (NPL). Accuracy has since improved by six orders of magnitude to around 10-16.
Since the CW seminar on quantum technologies, the US stock market “flash crash” has led to an increase in the requirement for time accuracy. The response has been new types of clock and the development of the chip scale atomic clock (CSAC). With a two orders of magnitude reduction in size, weight and power, it has led to new applications.
These have brought an increased focus on the accuracy and reliability of timing in critical applications, ranging from mobile phone infrastructure, national power grid to financial trading.
There has been an increase in the need for clocks of different form, function and cost to different market applications.
Clocks can be split into three different classes:
- national time standards,
- global navigation satellite system (GNSS) replacement, and
- measurement synchronisation.
Development of national time standards pushes the boundaries of physics to create new clocks capable of increased accuracy. A variety of clocks are under international consideration. Optical clocks are the current state of the art with demonstrated accuracies in the region of 10-18. These are developed by the national measurement institutions and adopted when reliability and accuracy have been proven.
The vulnerability of GNSS signals to accident, deliberate jamming and spoofing, together with the reliance on timing signals is resulting in a reconsideration of their reliability. Implementors must decide whether the free-running time is adequate or if an alternative traceable clock reference is needed (as can be found in financial trading). QT clock designs are being created for commercial markets. For example, an original clock design by NPL is in the process of being commercialised by Teledyne e2v as the Minac miniature atomic clock. NPL also has other clock designs under development.
Atomic clocks provide capabilities in the synchronisation of measurements. One example is the Square Kilometre Array radio telescope with a proposed measurement synchronisation to femtoseconds at each antenna.
The advent of the CSAC has also led to the possibility of using synchronised measurements from sensors placed on multiple lightweight and mobile platforms, such as drones. In the synchronisation of 5G increased density of base stations may need beter clocks.
Accurate timing sources are also accurate frequency references. The greater the accuracy of the source, the lower the noise of the frequency reference. QT has the potential to offer levels of phase noise which will be truly disruptive. Optical clocks, for example, have significant potential to improve performance in radar and other signal detection systems.
One challenge is the development of technologies to produce a compact version of an optical frequency comb to convert a precision optical signal to RF. Various projects around the world have focused on this and, in the UK, NPL is working on a microresonator-based comb to measure and read-out the frequencies of optical clocks. Timing and frequency references underpin vital systems that we rely on every day and increasing their accuracy also allows us to see further out into the universe. References based on quantum principles have been in use for decades but the need for size, weight and power improvements are always there.
Developments using a range of different quantum properties are underway which aim to meet these new requirements.