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Rebellious particles

Atom behavior may harm the working of the quantum computer

MIGUEL BOYAYANA recent discovery by Luis Gustavo Marcassa’s team dampened the enthusiasm of those researchers who were busy working on the theoretical and experimental development of a quantum computer, produced with atoms kept under very special conditions. This equipment takes advantage of particle properties to carry out calculations and the physicists believe that in future it may replace current computers and be advantageous under certain circumstances. In experiments carried out at the Institute of Physics of the University of São Paulo (USP) in São Carlos, Marcassa’s group identified a subtle variation in the behavior of the system considered ideal for functioning as a quantum computer processor. Though small, this oscillation is sufficient to jeopardize the performance of these computers in the future. The physicists from São Carlos, however, did not give up after coming across the problem and have already proposed ways of solving it.

In an apparently simple test, Marcassa, Valter Aragão do Nascimento and Lucas Caliri entrapped a cloud of just 10,000 atoms of the chemical element rubidium, using lasers and magnetic fields, at extremely low temperatures – almost 10 microkelvin or 10 millionths of a degree above absolute zero (-273.15 degrees Celsius), when the particles are at their very lowest possible energy level. Then they lit up the atoms with infrared and blue lasers, thus stimulating them. This procedure transfers energy to the outermost of their 37 electrons – the lightest fundamental particles known, which have a negative charge and which orbit the nucleus formed by positively charged (protons) and neutral particles (neutrons).

When energized, the outermost rubidium electron jumps to a peripheral region that is much further away from the nucleus. This distancing of the electron makes the atom increase in size almost 10,000 times. It now measures almost 1 millionth of a millimeter (micrometer) and becomes almost the size of a bacteria. The new atom, blown up like a party balloon, is called the Rydberg atom – a tribute to the Swedish physicist, Johannes Rydberg, who predicted it – and it starts behaving in a very special way. It becomes more sensitive to electrical and magnetic fields, which allows it to interact with distant atoms.

“These atoms interact at very great distances [some micrometers], enabling one to distinguish each one of them and to elect the one we want to codify certain information in,” explains Marcassa. When duly manipulated, sets of two Rydberg atoms can form the information unit of the quantum computer – the quantum byte or cubyte. According to Marcassa, this selectivity is advantageous because it allows work to be done simultaneusly with different coded information in the pairs of atoms, thus giving the computer great processing power.

The problem is that in the real world not everything works as the theory predicts. When they lit up the 10,000 atoms with the laser, the physicists from São Carlos found that a minority – 2% to 3% – reached higher or lower energy levels than required. “If you want to build a quantum computer using Rydberg atoms you have to be careful,” says Marcassa, who described these results in an article published in May in Physical Review Letters. According to the physicist, the few atoms that have energy levels different from the ones required would be enough to upset the functioning of a quantum computer. “To carry out various operations you need to have precise control over the energy level of the atoms,” explains the researcher from São Carlos.

Fortunately, this problem seems to have a solution. By adding an electrical field that is additional to the trap, the USP group managed to reduce the proportion of atoms that do not reach the desired energy level. Nevertheless, the physicists are still not fully satisfied. “We’re trying other solutions,” says Marcassa. One of them, which is still being developed, demands the application of microwaves at a frequency that is four times greater than that used in domestic microwave ovens.

The project
Bosonic and fermionic gases in optical traps (nº 07/03758-0); Type Regular Line of Aid for Research Projects; Coordinator Luis Gustavo Marcassa – IFSC-USP; Investment R$ 404,233.90 (FAPESP)

Scientific article
Nascimento, V.A. et al. Electric field effects in the excitation of cold Rydberg-atom pairsPhysical Review Letters. v. 102, p. 213.201-1- 213.201-4. 29 May 2009.