Page 10 - EngineerIt March 2021

MEASUREMENT

A caesium clock operates energy level, and they in turn release additional photons. In this manner, a self-sustaining
by exposing caesium atoms to microwave field builds up in the bulb. The tuned cavity around the bulb helps to redirect
microwaves until they vibrate at one of photons back into the system to keep the oscillation going. The result is a microwave
their resonant frequencies, and then signal that is locked to the resonance frequency of the hydrogen atom and that is
counting the corresponding cycles continually emitted as long as new atoms are fed into the system. This signal keeps a
as a measure of time. The frequency quartz crystal oscillator in step with the resonance frequency of hydrogen.
involved is that of the energy absorbed “We employ both clocks for their difference in performance,” Matthee explained. “The
from the incident photons when they caesium clock is the most accurate but generates high levels of RF noise, while the
excite the outermost electron in a hydrogen clock is RF-wise, quiet.
caesium atom to jump (“transition”) While we generate very accurate local time, we also need to compare our time with
from a lower to a higher orbit. other laboratories. We monitor errors with any GPS or GLONASS satellite that is above
The hydrogen maser is the most our horizon. The International Bureau of Weights and Measures (BIPM) collects data from
elaborate and expensive commercially approximately 80 different time centres in the world and examines scenarios where a
available frequency standard. The number of time laboratories have a common view of a number of satellites at the same
word maser is an acronym that time. The differences are then calculated, and averages computed. The BIPM distributes
stands for microwave amplification a bulletin once a month, allowing us to review our accuracy. We analyse the data and
by stimulated emission of radiation. compare it with our own data and if there is a variation, we update the model that then
Masers operate at the resonance drives the correction to the clocks. It is done continually as we receive this comparative
frequency of the hydrogen atom, which information. Our aim is to stay with a ten nanosecond accuracy and constantly achieve
is 1 20 405 752 Hz. better,” Matthee said.
A hydrogen maser works by
sending hydrogen gas through a Leap seconds
magnetic gate that allows only atoms in Leap seconds were added periodically to adjust to irregularities in the earth’s rotation
certain energy states to pass through. in relation to Coordinated Universal Time (UTC), in order to remain close to mean solar
The atoms that make it through the time (UT1). At the 2015 World Radiocommunication Conference (WRC-15), it was
gate enter a storage bulb surrounded proposed to suppress the leap second. The decision by WRC-15 calls for further studies
by a tuned, resonant cavity. Once regarding current and potential future reference time scales, including their impact and
inside the bulb, some atoms drop to a applications.
lower energy level, releasing photons A report will be considered by the World Radiocommunication Conference in 2023. Until
of microwave frequency. These photons then, UTC shall continue to be applied as described in Recommendation ITUR TF.4606 and
stimulate other atoms to drop their as maintained by the International Bureau of Weights and Measures (BIPM). n

South Africa unveils world class

photonics prototyping facility

outh Africa’s photonics industry optical equipment ranging from electronic, mechanical and diagnostic equipment, for a
is set to benefit from the state- variety of wave lengths.
Sof-the-art photonics prototyping The PPF seeks to develop ideas and identify needs for the current South
facility (PPF) that was unveiled at the African market. The national facility aims to address the current innovation chasm
CSIR on Friday 5 March 2021. experienced in the South African photonics industry by providing the necessary skills
The facility will enhance the and facilities to support the industrialisation and commercialisation of photonics-based
development of photonics-based technologies. This will help develop South Africa’s expertise in the area of photonics
products, specifically the prototype product development and prototyping, thus stimulating the growth of the country’s
development phase, to test the market photonics industry by forming small, medium and micro-sized enterprises (SMMEs) and
for acceptance of the planned product in sustainable jobs.
the country. The facility offers class The PPF, which is funded by the Department of Science and Innovation (DSI), will
1 1000 cleanrooms, technical and fast-track product development of photonics technologies and devices, in line with

EngineerIT | March 2021 | 8