A sketch of the internal circuitry that may take the place of silicon chips and become the heart of a quantum computer has gained a more clearly defined outline, thanks to an experiment carried out for the first time at the Physics Institute of the University of São Paulo (IF-USP). An all-Brazilian team, coordinated by the physicists Paulo Nussenzveig and Marcelo Martinelli, from IF-USP, has assembled a system in which it has been possible to create – and up to a certain point control – the phenomenon of the quantum entanglement between three beams of light of different wave-lengths. One beam was green and was in the visible part of the spectrum, whereas the other two were in the nearby infrared field, invisible to the naked eye. Until then the maximum that other groups of scientists had managed was to quantically entangle two light beams of different colors or various beams of the same frequency. “The entangling of bands of three colors may be useful for building quantum computers in the future,” comments Nussenzveig. “In theory, we could put together a network of quantum components operating on different frequencies.” The work was published on 17th of last month in Science Express, the on-line version Science, the American scientific journal.
In addition to showing the feasibility of tricolor entanglement, the pioneering study of the Brazilians brought another piece of good news. The physicists saw that this type of interlacing might give rise to a relatively robust optical system, which does not fade as easily as other simpler entanglement models. At the end of the day, no one wants to have a quantum PC that is naturally unstable. By slightly altering the intensity of the bands used in the experiment, they managed to modulate the degree of interlacing between the system’s photons (light particles). They also observed that the phenomenon that is described technically as the sudden death of entanglement, which has so far only been reported in more elementary systems, also occurred when they reduced the intensity of the light beams to below a certain level. The energy of the green laser beam used to start the experiment at USP is small, but not negligible: around 50,000 milliwatts, ten times bigger than that used in some pointers.
In addition to Nussenzveig and Martinelli, the team of researchers involved with the work included the graduate students Antônio Sales Coelho and Felippe A. Silva Barbosa, from IF-USP, and the physicists Katiúscia Cassemiro and Alessandro Villar, currently at the Max Planck Institute for Light Science in Germany. The study that led to the tricolor entanglement forms part of the research being carried out by the National Institute of Science and Technology in Quantum Informatics (INCT-IQ), coordinated by Amir Ordacgi Caldeira, from the State University of Campinas (Unicamp). The institute is an initiative of the Ministry of Science and Technology (MCT), through the National Council for Scientific and Technological Development (CNPq), in partnership with FAPESP.
Like all quantum phenomena, entanglement cannot be explained by the classic laws of physics. It belongs to a world that has its own rules, which are foreign to the understanding of macroscopic reality and that flirt with what the man in the street calls telepathy. Predicted in the 1930s and experimentally proven decades later, quantum entanglement stamps its own typical mark on a system. If two or more particles – atoms, electrons or photons, as in the USP experiment – are so closely connected that the modifications undergone by some of them are reflected in the properties of the others, regardless of whether they are separated by nanometers or by the Atlantic Ocean, they form a system that has the characteristics of quantum entanglement. In forecasting the possibility of entanglement, Albert Einstein said that the mysterious phenomenon was endowed with “spooky action at a distance.”
From the applied point of view, these correlations between entangled particles may be explored in such a way as to create the so-called quantum bytes or cubytes, which could theoretically expand enormously the capacity of computers for storing, processing, encrypting and transmitting information. The problem is that entanglement is a fragile phenomenon, the effects of which may disappear as a result of infinitesimal interference from the environment. Scientists normally choose photons for constructing entangled systems, instead of atoms or other elementary particles, because light can be transmitted by optical fibers or through the air without losing the entanglement effects.
To create the system described in Science, the researchers assembled what is known as an optical parametric oscillator (OPO). This is a device that makes it possible to bombard a system comprising a special crystal, located between two mirrors, with a beam of green light (laser). Entanglement occurs when the beam of green light crosses the crystal. At this moment, the green photons are converted into pairs of infrared photons with two different frequencies (see illustration on page 50). “It’s the crystal that ‘ties together’ the three light beams that creates the entanglement,” explains Martinelli. Finally, the new light beams produced, plus the remaining beam of green light, are redirected to subsystems of mirrors used to measure their properties. “In our experiment there are so many entangled photons that it’s impossible to count them,” says Nussenzveig.
Four years ago, the group from USP was successful in creating quantum entanglement using just two light beams. The following year, the physicists published an article forecasting the possibility of entangling three beams, which they have now proved experimentally to be possible. However, it was not an easy process. When they started their attempts to create three-color entanglement they came up against a frequent problem in science: the practical results did not coincide with the theoretical projections. There was a source of contamination that made it difficult to record the entanglement. “There was a noise from the light that was intrinsic to the system and that was of a quantic nature,” says Martinelli. “But there was another type of noise that upset the measurements.” They needed to understand the origin of the interference and eliminate it from the system.
In experiments with the optical parametric oscillator physicists usually work at room temperature. However, the strategy did not work when attempting to entangle three different light beams. The researchers discovered that, in this case, it was necessary to cool the crystal below a certain temperature to eliminate the unwanted noise from the system. The heat of the environment, above 20o, caused the crystal vibrate and produced interference. The answer was to keep the crystal at -23°C, thus establishing the conditions required for the entanglement to be measured satisfactorily.
It is interesting to note that this Brazilian study has made an important contribution to the study of quantum entanglement. In addition to Nussenzveig and Martinelli’s team, other research groups have published articles in periodicals of international renown. In April 2006, Luiz Davidovich’s group from the Federal University of Rio de Janeiro (UFRJ) carried out the first direct measurement of quantum entanglement of particles and published their findings in Nature, the British scientific journal. In April 2007, in Science, the same team showed how the phenomenon of sudden entanglement death occurred. “There are several Brazilian theoretical and experimental groups working at the very frontiers of knowledge,” says Davidovich. “They have an interesting characteristic: they’re scattered over various states in Brazil but they all interact one with another.”
National Science and Technology Institute for Quantum Information (INCT-IQ) (nº 08/57856-6); Type MCT/CNPq/FAPESP Program of National Science and Technology Institutes; Coordinator Amir Ordacgi Caldeira – Unicamp; Investment R$ 1.5 million (FAPESP) – for research groups from São Paulo
COELHO, A. S. et al. Three-color entanglement. Science Express. Published online on 17 September 2009.