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Physics

Atoms trap

Team creates shortcut to obtain a special state of matter

There are four groups in the world striving to obtain from calcium atoms a strange state of matter – Bose-Einstein condensations -, and among them there is the one from the State University of Campinas (Unicamp), led by Artemio Scalabrin. The other groups use elements from the first column of the periodic table, like sodium, lithium and rubidium. “The advantage of calcium and the others that make up the second column is their peculiar electronic structure that may make it possible to simplify the method for obtaining condensation”, says Scalabrin.

Flávio Caldas da Cruz, also from the Lasers Group of the Institute of Physics at Unicamp that is carrying out the project, recalls: “When the first condensation was produced, the expectation was created that there would be the same explosion in the sector as was caused by the creation of the laser, in the sixties. However, almost seven years later, only a limited number of experimental groups have achieved condensations. The main reason is the great technical difficulty to obtain these objects, in particular the one that is associated with the magnetic trap”.

“Hence the interest in calcium condensation”, Cruz explains. “Thanks to its structure of levels, this element has specific properties that offer prospects for reaching this state of matter using just optical methods. To do so, a second kind of trap, like optical tweezers, would replace the magnetic trap. This would mean an important shortcut to get condensations.”

Imprisoned atoms
With their second thematic project in its final stage, 60 works in international magazines, seven theses for doctor’s degrees and six for master’s degrees concluded over the last ten years, the Lasers Group believes it has the clout for the venture. “We always develop the equipment needed for the research”, reveals Daniel Pereira, a member of the team. “Today, we are the only Brazilian group with a presence in the select circle of international laboratories capable of carrying out laser frequency measures in the region of terahertz (which corresponds to 1 trillion oscillations per second)”.

In the battle for condensation, the group already does the most difficult part: imprisoning the calcium atoms. To do so, it adopts the following procedure: initially, an oven heats up the metallic calcium, transforming it into vapor and raising it to 600 degrees Celsius. With the heating, the atoms reach an average speed of 700 meters a second. In a vacuum, the atoms are collimated – that is to say, their trajectories become parallel and they make up a beam.

Next, passing through a tube, the jet of atoms is decelerated by the pressure of radiation from a laser that is pointing in the opposite direction – photons, particles of light, have the capacity to put pressure on matter, which produces a phenomenon like the tail of a comet. Deceleration by laser – about a million times greater, in absolute value, than acceleration by gravity – makes the speed of the atoms fall to around 0.5 m/s. Decelerated, the atoms can be imprisoned through a device made up of six lasers – facing each other in pairs and arranged according to three orthogonal directions, each one corresponding to one Cartesian coordinates of space – and a magnetic field.

With this technique, about 10 million atoms are confined in a sphere 1 millimeter in diameter. Their low mobility makes their temperature plummet to the level of 1 milliKelvin, one thousandth of a degree above absolute zero (zero Kelvin corresponds to – 273,15 degrees Celsius). The time that the atoms remain in the trap is around 20 milliseconds – a length of time that seems ridiculously short, but is long for the time scale for atomic phenomena. “Even at this extremely low temperature, collisions between atoms still occur. They are called ultracold collisions, which have been studied very little so far for elements of the second column of the periodic table”, comments Pereira.

Atomic clocks
Obtaining condensation is not the only purpose of this experiment. “One of its most important practical applications is to set universal standards for frequency, time and length”, Scalabrin informs. “A transition of calcium is currently used as a frequency standard. And it also makes it possible to define standards for time and length, essential for metrology, telecommunications and electricity networks and air and sea transport. When the atoms have higher velocities, this standard suffers from variations and is disturbed by a larger number of collisions. At temperatures close to absolute zero, the frequency reaches an almost ideal stability. The prospects, from this point on, are the construction of portable atomic clocks of the very highest accuracy.”

The researcher talks of transition of calcium. To understand this, you have to recall the quantum model of the atom, drawn up by a Dane, Niels Bohr, in 1912. When it receives an external charge of electricity, the electron leaps from a level relatively close to the nucleus to another further away. When an indeterminate interval of time has gone by, the particle leaves this excited condition and returns to the fundamental state, sending the excess energy back to external medium. Bohr drew up his model for the hydrogen atom, the simplest of them all, made up by just one proton and one electron. But it may be generalized for more complex atoms. In the case of calcium, which has 20 electrons distributed through several layers, it is the two particles from the last layer that pass on to different levels when they are excited.

The energy for this is provided by the photons, particles linked to the electromagnetic interaction, which make up the emission of a laser. To produce these transitions, the laser needs to be highly monochromatic, with one very well defined color. The frequency associated with each transition is that of the laser – identical to the light emitted by the atom when its electrons return to its fundamental state. “Among the many possible transitions of an atom, the really interesting ones are those that involve the so-called meta-stable levels, in which the electron is able to remain for a long time, before decaying”, explains Cruz. “This is due to the fact that the longer the time that the electron remains in a level, the more defined is the energy necessary for producing this stimulation – which is a direct result of the Principle of Uncertainty, one of the pillars of quantum physics.”

Three transitions are particularly interesting in calcium. Two are stimulated by the electromagnetic radiation in the range of far infrared, at 1.6 and 3.2 terahertz (THz). Another, with even more energy, is produced by visible light at 456 THz. These transitions make calcium extremely promising for the development of atomic clocks of very high accuracy. “The reason is that, the higher the frequency of the oscillator, the greater the stability of the tick-tacks of the clock”, Cruz explains.

“Based on cesium and rubidium, conventional atomic clocks make use of the oscillations generated in the microwave band of the electromagnetic spectrum, at the gigahertz level (1 billion oscillations per second). The possible calcium clock, fed by oscillations from the electromagnetic field of lasers, would operate at frequencies thousands of times higher”.

The idea of producing a far infrared calcium clock, replacing the microwave generators with lasers as the stimulation source, has been around for 30 years and has seen been growing. It has, for example, been found that by cooling down the atoms it was possible to achieve more stable frequencies. And that calcium shows another of those transitions, in the visible band of the spectrum, with an even higher frequency. All this generated great expectations, but there was a problem: until a couple of years ago, it seemed impossible to measure frequencies in the terahertz band. It would not be much use to produce a transition in such a high frequency if there were no way of measuring it. That was when it was discovered that a laser with ultrashort pulses could be used to measure the frequency of another laser.

Unlike the laser used to stimulate the atom, the emission from which must be continuous and highly monochromatic, this measuring laser issues interrupted and polychromatic pulses – that is, made up of radiation of different frequencies. And its polychromatic emission is the so-called “frequency comb” – something that works like a ruler to measure other frequencies. That by itself already looked very good. But what was impressive was the period of each pulse: something in the order of FEM to seconds (1 quadrillionths of a second). With such fast pulsation, this laser can easily set the parameters for frequencies of hundreds of terahertz.

This pulsating laser has changed the scenario and provided something that was missing for the calcium atom clock. This is because, in principle, all that is needed to make the clock is a stable oscillator and a measurer of oscillations. The oscillator, in this case, is the monochromatic laser stabilized with the transition of the calcium atom. And the measurer, the ultrashort pulse laser. From this point to the actual clock is a question of overcoming technical obstacles, something that may also be said of obtaining the condensations. But the way has now opened up.

Very close to absolute zero
Research by the Unicamp Laser Group is setting out for a new field: the experimental production of a Bose-Einstein condensation was achieved for the first time in 1995, and the authors of this – Americans Eric Cornell and Carl Wieman, from Colorado University, in Boulder, and the German, Wolfgang Ketterle, from the Massachusetts Institute of Technology (MIT) – received the 2001 Nobel prize for Physics.

Foreseen theoretically in 1924 by the Indian, Satyendra Nath Bose (1894-1974) and by the German Jew, Albert Einstein (1879-1955), the condensation had been sought for decades, until being obtained, independently, by the Cornell and Wieman and by Ketterle. The promptness in awarding the prize indicates the importance of the achievement. The condensation is a state of matter in which the atoms lose their individual properties and start to behave as a single unit. It occurs when the corpuscles are at an extremely low state of energy, at a temperature a few billionths of a degree above absolute zero. The situation probably does not exist in nature, since not even intergalactic space is so cold, but it can be obtained in laboratories.

The technique for this consists of drastically reducing the thermal agitation of the atoms, imprisoning them in a very small volume, by means of a trap with lasers and a magnetic field. And, afterwards, by the use of a magnetic trap manipulated by radiofrequency, so as to expel the more energetic atoms, allowing only the impeccably quiet ones to remain. The coherence of the behavior of this ultracold atom gas is such that it compares with a gas at room temperature like, to take a trivial example, the laser compares with the light from a lantern.

One of the possible applications for condensation is precisely the creation of atomic lasers. There are teams that are working in this direction, but the process is at a very preliminary stage. At the moment, research is concentrated on the study of the physical properties of condensations, still largely unknown.

The Project
Atomic and Molecular Spectroscopy with Lasers (nº 97/05257-5); Modality Thematic project; Coordinator Artêmio Scalabrin – Physics Institute of Unicamp; Investment R$ 199,624.98 and US$ 336,494.00

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