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Physics

The oscillation of particles

Improved understanding of the electrons that are essential for the creation of quantum computers

EDUARDO CESARKnowing the obstacles that lie along the course is as important as reaching the finishing line in the race to design an efficient quantum computer, a machine that is able to use the properties of  fundamental particles of matter to perform calculations far faster than conventional computers. This is what is indicated by the work of some Brazilian physicists who have been studying spin (a characteristic of atomic particles), which in the case of particles with negative charges (electrons) can be described, although only roughly, as the sense in which they rotate around their own axis.

Investigating this property of electrons, the team under physicist José Carlos Egues, from the University of São Paulo (USP) in São Carlos, recently discovered an interaction between this particle and the path that it travels along, which may affect the control of the spin, one of the technologies – spintronics – imagined for the development of useful non-conventional electronics for quantum computing. Along the same lines, electric engineer Gilberto Medeiros-Ribeiro’s group, which is based at the Synchroton Light National Laboratory (LNLS) in Campinas, has been experimentally testing the control of the spin of electrons in different components which, who knows, may one day be used to build these computers of the future.

In current computers – both those used at home as well as at work – the information is coded in units called bits, represented by the numbers 0 and 1, which stand for the absence or presence of electric current. By analogy, the basic unit in quantum computing is the quantum bit or qubit, which can be coded in the spin of electrons. Although electrons are not spheres, being more like points, with no spatial dimension, it is assumed that they rotate around their own axis like a spinning top. Depending on the direction of rotation, it is said that the spin is up or down, which is the equivalent to the 0 and 1 of traditional computers.

This is where the similarities end. According to quantum mechanics, which is the part of physics that explains the behavior of subatomic particles, each electron can, at any particular time, rotate both down and up as well as in every other direction, as if they had simultaneously assumed the values 0 and 1 and every value in between, such as 0.23 or 0.65, among others. This is what physicists refer to as the overlapping of quantum states. The direction of rotation and the value of the spin are only known at the moment they are measured. This characteristic makes the spin of electrons an interesting basis for quantum computing, given that, if it is controlled, the overlapping of states exponentially increases the capacity to perform calculations – each qubit would be able to handle far more information than the so-called classic bits. Under ideal conditions, a few hundred qubits could code more information than the entire number of elementary particles in the Universe.

Despite this astounding potential, the practical value of quantum computing is yet to be fully demonstrated. “Applications are still a long way off. For the time being it is a conceptual mistake to say that the quantum computer will be good at everything, outperforming traditional computers at any task,” explains Egues. Up until now, the advantages are limited to very specific problems such as the understanding of quantum phenomena in physics and biology or the development of safer ways to code information (cryptography).

Spin under control
The challenge is to make these potential applications come true. In the case of spintronics, the difficulty involves inducing the desired spin in the electrons of a specific material, as well as keeping the electron from oscillating, like a spinning top that starts staggering, and falling into the opposite spin. According to Egues, the first concrete demonstration that it was possible to deal with spin instability appeared in 1999, in a paper written by David Awschalom, from the University of California in Santa Barbara, in the United States.

Based on this result, a race got underway to increase this control and master the inversion of the spin – to make it go from up to down, which would be equal to an operation with a single qubit. One of the factors that hinders control is the subtle variation in the temperature of the material in which the electrons are located – in the atomic world, temperature corresponds to the level of agitation of the particles, which means that even in a solid substance they are never at a total standstill.

This agitation interferes with the electron’s path around the atoms and can modify its spin – it is the so-called spin-orbit, a two-way path, given that the path affects the spin and vice-versa. In a paper published in 2007 in the journal Physical Review Letters, Egues and his collaborators identified a new type of interaction between the spin of the electron and the orbit of the electron itself – an effect that has a small, but significant influence over the oscillation of the electrons. “These interactions between spin and orbit can also be good, because they make it possible to manipulate the spin as wanted, controlling the path of the electrons,” says the USP physicist.

In another paper, first published in 2007 in the journal Physical Review B, Egues’ group, in partnership with Fabricio Souza, who is currently a researcher at the University of Brasília, showed that it is theoretically possible to build a device that can select the electrons according to their spin at any given moment – the so-called spin current diode; this would be useful for carrying out computational operations that involve controlling the rotation of those particles. The spin diodes, experimental versions of which were built this year by researchers at Johns Hopkins University in the United States, only allow through electrons with the same rotation direction, functioning like a filter, like conventional electronics diodes which only allow electricity to flow in one direction.

The era of diamonds
Although many papers deal with the control of electrons in materials such as silicon, which is widely used in traditional computers, a number of studies evaluate using more exotic alternatives, such as diamonds. Generally considered an insulating material, because they allow neither movement nor manipulation of electrons, under certain circumstances diamonds can function as an excellent semiconductor. For this to work, all we need are some impurities in the carbon atoms, such as nitrogen atoms. Each nitrogen atom takes the place of two of the six carbon atoms that make up the diamond’s tetrahedral internal structure. This replacement leaves one carbon atom space empty – this is the so-called center of vacancy – which enables one to manipulate the nitrogen electrons.

Using microwaves, Gilberto Medeiros-Ribeiro and Thiago Alegre, from the State University of Campinas, managed to experimentally control the spin of electrons in these vacancy centers, as they described in an article published in 2007 in Physical Review B. “If we can spatially determine the impurity in the diamond, then it is possible to determine the change of the state of spin,” says the researcher from LNLS. In the future, control of this phenomenon should allow us to use those structures to carry out computing operations.

However, Medeiros-Ribeiro recommends caution, as little is known about the basic mechanisms of quantum computing. “Every time we solve one problem, another one appears. The obstacles are hard to overcome and there are fundamental limits as to what is possible that we do not really understand very well yet,” he says. For Egues, the unknown elements illustrate how scientifically rich this field is: “If we knew where this would lead, there would be fewer groups studying these issues.”

The Projects
1. Magnetic interactions and spin polarized electronic transportation in magnetic quantum points (nº 07/05783-2); Type: Regular Research Awards; Coordinator: José Carlos Egues de Menezes – IFSC; Investment: R$ 93,343.20 (FAPESP).
2. Nanostructured materials investigated by tunneling microscopy and atomic force by means of transportation measures (nº 98/14757-4); Type: Young Researcher Program; Coordinator: Gilberto Medeiros Ribeiro – LNLS; Investment: R$ 587,417.83 (FAPESP)

Scientific articles
BERNARDES, E. et al. Spin-Orbit Interaction in Symmetric Wells with Two Subbands. Physical Review Letters, v. 99, 2007.
ALEGRE, T. P. M. et al. Polarization-selective excitation of nitrogen vacancy centers in diamonds. Physical Review B, v. 76, 2007.

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