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metrology

Time keeping

Researchers build three atomic clocks capable of splitting one second into billionths

The measurement of time has always been one of man’s obsessions, ever since the time when sticking a pole in the ground and making some marks around it made the first indicator of the time in the world, the sundial, probably among the peoples of ancient Mesopotamia, over 3,000 years ago. From then until now, there have been plenty of changes. Clocks using water and earth, the famous hourglasses, were adopted, until the arrival, in the 16th century, in Europe, of the clocks using a pendulum, with Galileo Galilei. A little over 60 years ago, electronics brought about important innovations, to make these devices sharper.

Quartz watches, such as the ones we wear on our wrists, became popular. Next, it was the turn of the atomic clock, the most precise of all, which is only one second slow or fast in billions of years. It is this equipment that measures world time and serves to measure the time in space and telecommunication activities that researchers from two physics institutes, at the University of São Paulo, in São Carlos, and Campinas State University (Unicamp), are poring over. They want to acquire knowledge in this area, still scanty in Brazil, to train specialized personnel, and to implement innovations in these clocks that look nothing like any kind of timekeeper that is part of our daily lives.

The two research groups are part of the Optics and Photonics Research Center (Cepof), financed by FAPESP. In São Carlos, physicist and postdoctoral student Monica Santos Dahmouche is making the last adjustments to the first atomic clock built in Brazil. Her intention, and that of Cepof’s coordinator in São Carlos, Professor Vanderlei Bagnato, is to call for this equipment to take part in the measurement of the world’s right time, made up of 200 atomic clocks from 30 countries. They comprise what is called International Atomic Time (TAI, the acronym in French), instituted in 1972. Brazil takes part in this group with the atomic clock from the National Observatory (ON), in Rio de Janeiro, equipment that was purchased abroad. In South America, besides the ON, there are only another two in Argentina.

“Ours was entirely developed here”, says Monica. The request for including this clock in world time will be made to the Bureau International des Poids et Mesures (BIPM), located in Sèvre, in France. This institute also controls Universal Coordinated Time (UTC), the official world time, which has differences with regard to TAI of about 0.9 seconds a year, faster or slower, due to the Earth’s irregular rotation. UTC – today’s successor to Greenwich Mean Time (GMT), which measured world time from 1884 onwards – follows the timing of this rotation and every few years is adjusted with TAI.

Gathering together several clocks from all over the world to mark the right time may seem unnecessary, given the precision of these instruments. However, aspects like temperature, atmospheric pressure and noises may influence the frequency emitted by these clocks, which are today based on the pulsation of cesium 133, an element devoid of any harmful radiation. Cesium was the standard adopted in 1997, during the 13th World Conference on Weights and Measures. It is defined by the duration of 9,192,631,770 periods of oscillation of the radiation needed for the transition between two levels of the fundamental state of the cesium 133 atom. This means that there has to be a perturbation in the electrons of this element for the standard frequency to be emitted.

Cesium works as a point of reference not only to indicate the time, but also as a standard for electrical pulses. It has a stable frequency and generates a repetitive electrical signal. Accordingly, the unit of time is also used to determine, for example, the meter. Today, the meter is the length of the course covered by light in a vacuum during the interval of time of 1 second, divided into 299,792,458 parts. Such precision needs equally precise equipment that today – in commercial models – are to be found in telephone companies, in the transmission of television signals, and in satellites.

Quick connections
In telecommunications, for example, the atomic clock has been indispensable since the implantation of a new technique for data transmission, SDH (for Synchronous Digital Hierarchy), used in communications via optical fiber. This technique works with very fast transmission flows and directs the calls in fractions of a second, over a large network. Navigation has also gained a lot with atomic clocks. Ships, boats, aircraft, groups of people in remote places are adopting the GPS global positioning system for locating.

It is made up of 24 satellites in orbit around the planet, three of which are sufficient for the receiver on Earth to decode and advise the coordinates (latitude and longitude). Knowing the time that each signal came in and where it came from, the receiver determines the position, by calculating how long ago the signal was emitted by the satellite and how long it took to reach the ground”, explains Professor Flávio Caldas da Cruz, from the Physics Institute (IF) at Unicamp. “This time is informed by the atomic clock installed on board the satellites”.

Although in current use, atomic clocks, like wristwatches, are constantly evolving. The first one built in Brazil, in São Carlos, is a thermal beam atomic clock. It started to be idealized in 1997, inside a special room, fully shielded, with its wall filled with polystyrene and other noise insulating materials. It maintains a stability of resonance, in relation to cesium, at 10-11, that is, besides measuring a fraction of a section up to 11 decimal places, in the orders of picoseconds (billionths of a second), it would take 31 billion years for be one second slow. This mark is very close to the commercial atomic clock, which is 10-12. “Scientific research is looking for clocks that determine the frequency of the resonance of cesium (those 9,192,631,770 oscillations) with greater precision”, says Monica.

The stability of the frequency of the cesium in the thermal beam clock starts with the atoms being launched from an oven into a cavity (chamber), where they are given infrared laser beams, below, therefore, the visible spectrum. There, they interact with the radiation of 9,192,631,770 gigahertz (GHz) generated by a microwave synthesizer and a quartz oscillator. It is in this chamber that the rearrangement of the atom’s electrons occurs. This leads them to absorb the energy from the laser and to start emitting photons, which are transformed into an electric current, and the frequency is measured by software. In this measurement, there is a check that the frequency generated in the microwave synthesizer is correct, in resonance with the cesium, or if it needs to be corrected.

The other atomic clock that is under development in São Carlos, called a fountain type atomic clock, which uses cold atoms, is regarded as an evolution of the first. Unlike the earlier one, which works horizontally, in this one the atoms are thrown upwards, as it they were a fountain, inside a metal cylinder. When they go up, the atoms pass through a cavity, reach the apex, and come down.

On their way down, they pass once again through the same cavity, which is fed by a microwave generator which provides the radiation that resonates with the transition of the cesium, and finally they interact with the detecting laser beams in the lower region of the clock. In this case, the interaction of the atoms inside the cavity is greater, making it possible to determine the frequency of the resonance of the cesium with greater precision. “This clock was totally produced in Brazil, including the project. This was how our group mastered all the stages, from idealization to the construction technique”, says Monica.

Neither the fountain clock nor the thermal beam one is produced commercially. In those that are on the market, magnets play the role of selecting and detecting the cesium atoms instead of the laser. For this reason, this equipment too needs shielded rooms, far from any kind of interference.

Red laser
In Campinas, researchers from Cepof are working on what may be the new generation of atomic clocks. Unlike the equipment based on transitions of cesium 133 atoms, they use calcium. This chemical element oscillates when it is detected and maintained by a specific laser, red in color, at 456,000 GHz, much quicker than cesium, at 9 GHz.

What it does is to transfer the pulsation from the red laser to another laser, now a polychromatic one, transferring the repetition tones to an electronic circuit that will be counting the oscillations and pulses. With the oscillation of the calcium atoms excited by the laser, there are gains of several orders of magnitude in precision. “It is very quick, and we get a repetitive and stable time count”, says Cruz. Its precision reaches 10-17, in the order of femtoseconds (one second divided into trillions), or a number that has another 15 digits after the 2 digits shown in the display of digital wristwatches.

When it is ready, in the middle of this year, the calcium atomic clock of Unicamp’s IF will be the third in the world. At the moment, there are two of them working, one produced in 2001 at the National Bureau of Standards (NIST), the American institute of metrology, and the other in 2002, at the Physikalisch-Technische Bundesanstalt (PTB), Germany’s institute of metrology.

The projects
1. Thermal Beam Atomic Clock and Cold Atom Atomic Clock; Modality
Optics and Photonics Research Center (Cepof), São Carlos; Coordinator
Vanderlei Salvador Bagnato – Physics Institute at USP, São Carlos; Investment R$ 50,000.00 and US$ 70,000.00
2. Calcium Optical Atomic Clock (nº 01/11144-6); Modalities Optics and Photonics Research Center (Cepof), São Carlos, and Regular Research Benefit Line; Coordinator of the project Flávio Caldas da Cruz – Unicamp’s Physics Institute; Investment R$ 140,311.18 and US$ 145.778,74

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