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Physics

Fifth state of matter

Team from São Carlos believes it has created a Bose-Einstein Condensation, with almost stationary atoms

In the midst of a gaseous cloud with 10,000 elementary particles of sodium, there they were: roughly a thousand atoms, piled on top of each other, at a temperature of 70 billionth of a degree above absolute zero, (equivalent to -273,15 °C). This group of a thousand ultra cold atoms is the first indication that the fifth state of matter may have been created in a Brazilian laboratory. Physicists from the University of São Paulo (USP) believe they have produced a Bose-Einstein Condensation, the name given to a group of atoms (or molecules) that, when cooled down intensely, start to behave as a single entity. It is as if they were so close together that the atoms in this stage of matter actually formed just a single superatom, becoming practically immobile and occupying the same physical space.

“We have not yet detected the condensation directly”, explains Vanderlei Bagnato, from USP’s São Carlos Physics Institute (IFSC), the coordinator of the experiment, carried out under a thematic project financed by FAPESP. “But the indirect evidence is convincing.”A state of matter foreseen in the 1920s by Indian physicist Satyendra Bose and by Albert Einstein (hence its name), the condensation opens the gates to a world that is still not very well understood. In it, all the atoms move at one and the same speed, the lowest possible – or, in a more technical definition, they occupy the same basal level of quantum energy.

This property is not found in other states of matter (solid, liquid, gas, or plasma), in which the atoms show varied levels of energy. The physicists speculate that a characteristic like this may be useful for future applications in fields like quantum computing or new forms of lasers. During seven decades, this state of matter was merely a concept. In 1995, two independent groups, one from the University of Colorado, and the other from the Massachusetts Institute of Technology, created the first condensations, from rubidium and sodium. This feat led them to split the 2001 Nobel Prize in Physics.

For the time being, the most reliable sign that a part of the cold cloud of sodium created in São Carlos left classic physics and penetrated the quantum world is the space occupied by a fraction of its atoms – the fraction that the scientist believe to make up the condensation. The measurement of the so-called phase space density is a parameter used by physics to classify quantum matter. “According to this parameter, our sample shows the condensation”, says Bagnato. The smaller the size of a confined gaseous cloud, the lower its quantity of energy and, therefore, the lower its temperature.

So did the researchers then take a sort of digital photograph of the atoms of the condensation and measure its size? Not exactly. Actually, they lit up with a laser the cloud of sodium atoms and observed the formation of penumbras. Where there were atoms, the absorption of light and the creation of the respective shadow occurred. Next, they got a record of this shadow on electronic sensors similar to those in a digital camera. In this indirect way, they measured the size of the cloud of atoms and of a possible condensation that could be there.

After carrying out the procedures described above, the team from the IFSC concluded that the size of all the 10,000 atoms of the cloud of sodium produced in their laboratory amounted to an average of 6 micrometers (one micrometer is a meter split into a million pieces). The specific size of the thousand atoms that form the apparent condensation was around 2 micrometers. According to the measurements made by the researchers, a group of sodium atoms of such a magnitude is at a temperature of 70 nanoKelvin, those 70 billionths of a degree above absolute zero. Under the conditions of the experiment that was carried out, atoms at this temperature and with the density measured now would attain quantum degenerescence, forming a Bose-Einstein condensation.

They do not know for sure how many atoms reached this state of matter. They figure that around a thousand did so. The problem is that this kind of evidence is not sufficient to prove that there was a condensation there. “It is necessary to see explicitly the fraction of condensed atoms”, Bagnato explains.Due to the small number of atoms used in the experiment (today, there are groups abroad making condensations with billions of atoms) and to limitations proper to the machines used by the researchers from São Paulo, it was not possible to observe in a direct way the atoms of the condensation, a measurement that proves their existence unmistakably. It remained to do the so-called flight time test of the atoms, which, inside a gaseous cloud, makes it possible to separate the particles that have attained quantum degenerescence – and form a condensation – from those that have not reached this point. “We went up to the limit of the equipment, but we failed to do the flight time”, says Luis Gustavo Marcassa, another researcher from the IFSC.

What does this test consist of? The scientists switch off all the paraphernalia that cools down the clouds of sodium atoms and have between 5 and 15 milliseconds to record the kinetic energy (speed) of the particles present in the diluted gas. Following this measurement, they infer the temperature. When there is a condensation in the midst of a gaseous cloud, the flight time test results in a figure that recalls a mountain with a very sharp peak. A figure like this has not yet been generated. “Some kind of contamination in the external medium must have interfered in their experiment”, is the opinion of theoretical physicist Mahir Saleh Hussein, from USP, in the city of São Paulo. Bagnato believes that limitations are due to external magnetic fields that dislocate the atoms. The problems should be overcome if the researchers manage to make a condensation with more atoms, probably with another kind of equipment, now under construction.

The Project
Cold Atoms in the Quantum and Non-Quantum Regime: Atomic Collisions and Other Experiments (nº 99/11963-5); Modality Thematic Project; Coordinator Vanderlei Bagnato – São Carlos Physics Institute/USP; Investment R$ 1,188,917.18

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