Throughout history, man has invented many kinds of clocks to mark the passing of time. The technological journey started with the sundial, went on to the hourglass, the spring coil mechanisms, and by the digital markers, until arriving at the more advanced and precise models, which are today the atomic devices. This equipment works with lasers and is based on the oscillation of the natural radiations from cesium-133 atoms, without being harmful to living beings. The most recent model of these clocks was designed and constructed at the São Carlos Physics Institute (IFSC) of the University of São Paulo, in São Carlos. It is of the type called fountain, a name relating to the synchronized movements of cooled atoms, inside the equipment, from top to bottom, and it represents an evolution over the commercial atomic clocks that use heated atoms and magnets. Only France, United States, Germany and the United Kingdom have already made similar clocks.
“The philosophy is the same as the researchers’ from other countries, but we have managed our own configurations for this equipment, which in future should act as a new time and frequency standard all over the world”, says Professor Vanderlei Salvador Bagnato, the coordinator of the project, who is part of the Optics and Photonics Research Center (Cepof) of São Carlos, one of the 11 the research, innovation and diffusion centers financed by FAPESP. The conclusions and the results obtained by the Brazilian researchers will be shown at a symposium on time and frequency metrology of the Institute of Electrical and Electronics Engineers, an organization know by the acronym IEEE, in Miami, in the United States, this month of June.
Atomic clocks are markers of time that go slow one second in over 100 million years. A delay that certainly does not interfere with people’s daily lives, like the time to wake up, to start work, in varied commitments, or in the departure times of buses or planes. But it has fundamental importance in many other areas. They are, for example, responsible for marking world time. Over 300 atomic clocks scattered over 50 countries, including Brazil – the National Observatory, in Rio de Janeiro –, indicate the official time all over the planet. They include Universal Time Coordinated , with its acronym UTC, based on the so called International Atomic Time (or TAI, from the French, Temps Atomique International), instituted in 1972 to replace Greenwich Mean Time (GMT), based on the observation of the Sun and the stars.
Atomic clocks are indispensable in telecommunications. They control the traffic of optical fiber communications, measure the flow of data and the duration of the transmissions, and help to direct the calls. In the exchange of data and voice, synchronism guarantees that the system works well. Nowadays, without a precise time, equivalent between two or more points in the telecommunication systems, the risk would be run of impairing the calls. In geographical location via satellite, the fractions of seconds are also indispensable. Made up of 24 satellites that orbit the earth, the GPS (standing for global positioning system) identifies precisely a point on the ground, facilitating the navigation of aircraft, ships, boats and, more recently, sophisticated automobiles and jeeps. A mere three signals are sufficient for the receiver on the ground to decode the transmission and inform the coordinates (latitude, longitude and altitude). “These satellites emit microwave signals that are synchronized amongst themselves, hit the ground and return. The difference in the arrival time of the signal from each satellite determines for the ground-based receiver the exact location on the surface of the planet. The distance between the satellites is also marked in fractions of a second and is important for determining the coordinates. All this information in time comes from the atomic clock installed inside the satellites”, Bagnato explains.
Cold atoms, imprisoned
A fountain atomic clock starts to work with a vapor of cesium-133 atoms, cooled and imprisoned by an optical trap of laser beams, in the lower part of the equipment. The force of the laser makes the cohesive group of atoms rise up inside a metal tube that has a chamber (cavity). This is the place where the atoms are given a bath of microwaves of a frequency identical to the oscillation of the radiation of the cesium. As they are cold, a disturbance in the frequency occurs, which corresponds to a second. Afterwards, the first lasers and other systems are switched off and the atoms descend and receive beams from another laser, detecting the modifications caused by the microwaves.
Such synchronism is equally important in banking transactions and even in prospecting for oil, when you have to measure in fractions of a second the time from sending a signal to the inside of the Earth and getting the signal back, to help to identify the existence of oil down below. “We can also use an atomic clock to gauge precision instruments that will be used in measures of electronic and magnetic magnitudes”, says Bagnato. In all the examples, the precision required is of picoseconds, or the fraction of a second(s) with up to eleven decimal places (10-11), equivalent to 1 s divided by 1 billion. This is the measurement shown in the commercial atomic clocks. But in the area of scientific and technological research, all over the world, yet greater precision is sought. The most advanced atomic clock, also on the fountain system, was constructed at the Observatoire de Paris, in France, and it has a precision of 10 -16, now in the order of femtoseconds, a measure that is equivalent to 1 second divided by 1 quadrillion. “We have not yet finalized the gauging of our apparatus, because we are waiting for a piece of equipment to complete this measure, but we believe we will reach at least 10-13, which represents for us scientific and technological maturity”, Bagnato says. “After all, it is the first fountain clock made in the Southern Hemisphere”, he commemorates.
“Constructing atomic clocks in Brazil is fundamental for basic research and for the development of technology. It is important to have mastery over this knowledge. The standard of the second is the most precise there is, and it serves for other measurement like the meter”, says physicist Humberto Siqueira Brandi, the director of scientific and industrial metrology at the National Institute of Metrology, Standardization and Industrial Quality (Inmetro). He refers to the fact that the standard used to identify the meter is no longer a bar of metal in a European institution, as in the past.
Today, a meter is the distance traveled by light in a vacuum in the interval of time of 1 s divided by 299,792,458 parts, or meters a second, which is the exact measure of the speed of light. “These measurements are possible with atomic clocks, and the more advanced they are, like the fountain clocks, the greater the guarantee of decision”, says Brandi. A more precise atomic clock may also serve to gauge other similar ones there are in Brazil, as well as to evaluate precision, taking into account the action of external agents like temperature, humidity, vibrations and magnetic fields.
The fountain model is the second atomic clock constructed by the team led by Bagnato, currently made up of doctoral students Aida Bebeachibuli, with a scholarship from the Council for Advanced Professional Training (CAPES), Stella Tavares Miller, with a scholarship from the National Council for Scientific and Technological Development (CNPq), and postdoctoral student Daniel Varela Magalhães, a researcher from USP who is currently working at the Observatoire de Paris. The first clock was of the horizontal thermal beam type, in which cesium atoms are thrown at high speed from a furnace to a chamber where they are given infrared laser beams and interact with the radiation (electromagnetic wave) of 9,192,631,770 gigahertz (GHz) produced by a microwave generator. This same frequency represents a second, which is defined by the duration of 9,192,631,770 periods of oscillation, between the fundamental state and lowest energy levels of the radiation of the atom of cesium-133.
The fountain also works with very precise energies in radiations that oscillate at a well-determined frequency. It works in a vertical manner and similar to a thermal clock, but the precision is greater because it works with cold atoms and without the speed of the other types of atomic clock. The function of the lasers is to join these cesium atoms and to paralyze them in a sort of optical trap. The force of the laser then makes the cohesive group of atoms rise up in a metal tube to a cavity, where it will be given a bath of microwaves with a frequency of 9,192,631,770 gigahertz (GHz), which is the same as the oscillation of the radiation of the cesium-133 atom. When entering the cavity, the atoms experience the frequency and leave there. As they are cold, the energy level is different. This difference between the two frequencies corresponds to one second.
The next step for the group from Cepof is to develop compact cold atom clocks, equipment as yet unprecedented in the world. Its name, the team already has. It will be the TAC (Tupiniquim Atomic Clock), or Brazilian atomic clock.
They are developing a small clock, the size of the commercial ones, which are a bit larger than a videocassette recorder. This will be a piece of equipment of the thermal beam kind. “We are also preparing the Super Tac, which should be a clock that does not have to have a replacement of the cesium atoms from time to time, like the other ones”, says Bagnato.
Atomic clocks; Modality Research, Innovation and Diffusion Centers (Cepids); Coordinator Vanderlei Salvador Bagnato — USP/Optics and Photonics Center of São Carlos; Investment US$ 70,000.00 a year (FAPESP)