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Twin Particles

Researchers from UFRJ Renew quantum physics experiment with photons

RODRIGO QUEIROZIn the quantum optics experiment, light seems to go back in timeRODRIGO QUEIROZ

Physicists from Rio de Janeiro have created a new method of carrying out an experiment in which a beam of light appears to travel into the past. To disorganize the trajectory of the light, instead of paper and special gases – adopted since the experiment was created, in the 60’s -, they used two laser sources and one of crystal. Made public in September last year, in an article for the Physical Review Letters, the new experiment marks the coming on stage of a group created three years ago and which last year published two more articles in first class magazines.

More than recreating the experiment, the group from the quantum optics laboratory of the Institute of Physics of the Federal University of Rio de Janeiro (UFRJ) showed how to deal with entangled photons, two particles of light that keep a strange connection, as if they were twins with a telepathic capacity: what happens to one happens to the other, even if they are a million kilometers apart.

The work has potential practical applications. A thorough knowledge of the entanglement of photons is fundamental for making progress on the quantum computers of the future, capable of doing in seconds, tasks that would take years on the most rapid of present-day computers. It is also decisive for quantum cryptography, which creates an unbreakable code, and even for the fantastic of teleportation – imagined in the TV Star Trek series, which makes it possible to take the crew of the spaceship Enterprise to distant places and bring them back, by means of atomic decomposition and recomposition.

Reversed Trajectory
The impression of a journey into the past, given by the experiment of the 60’s and by the one carried out by the group from UFRJ, takes place when a beam of laser light falls on a diffusive material – a sheet of glazed paper, for example -, which disorganizes it and scatters it in different directions. On passing through this screen, the laser light loses its main characteristic – particles (photons) organized in single line – and becomes similar to common light, like that of a domestic lamp.

What is unexpected is that, on meeting a non-linear medium – a recipient with a special gas, for example, the rays of light are reflected in such a way as to retrace the trajectories back, crossing the greaseproof paper once again and resuming their original organization. They appear to have gone back in time. “Actually, for this reflected beam, time continues to pass normally”, explains Paulo Henrique Souto Ribeiro, the coordinator of the laboratory. “There is merely a reversal of the trajectory”.

Ribeiro repeated the experiment with another non-linear medium. The basis of the experimentwas two sources of laser light. The first one, pumping, passes through a crystal, which gives the photons special properties and splits them into two beams. The second source, auxiliary, crosses the trajectory of the first laser inside the crystal and strengthens the intensity of the two bifurcated beams.

The interaction that takes place inside the crystal makes one of the bifurcated beams have exactly the same characteristics as the auxiliary one, but with the trajectory reversed, which puts it in a state of temporal reversion, as if the auxiliary beam were reflected in a mirror. To demonstrate this, the group put a screen to block half of the auxiliary beam. The connected beam then had the other half blocked. “Exactly as if they were images in a mirror”, says the coordinator.

Crystal power
Ribeiro is an electrical engineer who dumped his job in aircraft maintenance and took off for postgraduate studies at the Federal University of Minas Gerais (UFMG). He took his doctorate in 1995, spent a some time at the École Normale Supérieure in Paris, and settled in 1998 in Rio, with the objective of taking his experimental vision to a group of theoretical physicists. Afterwards, he set up the laboratory – with funds from the Ministry of Science and Technology, from the State of Rio de Janeiro Research Support Foundation (Faperj), and from the José Bonifácio University Foundation (FUJB) – and intensified collaborative relations, one of them with Professor Paulo Nussenweig, from the Institute of Physics of the University of São Paulo (IF-USP).

The team’s work is based on the special properties that light acquires in the inside of a special cube-shaped crystal, with a one centimeter edge. It is there that the twin particles are formed – or entangled photons, one of the most bizarre phenomena of the world of particles. Once the pairs of photons are created, any measure taken in one of them instantly affects the partner particle, even if it is millions of kilometers away. That is why they have already been compared, jokingly, to voodoo, the sorcery which makes the victim feel instantly the effects of actions carried out at a distance – needles stuck into a rag doll to cause pain in a person, in the spots where the doll is pricked.

Like the pinpricks, the phenomenon of the spliced pairs is fleeting: the laser of helium and cadmium used at UFRJ emits violet light, and, every second, shoots off some 10,000 trillion photons. These, after penetrating the crystal, collide with mirrors that hardly absorb any light and are captured by detectors capable of indicating the coincident arrival of each one of the pairs. Every second, the apparatuses – installed on a table with a system of shock absorbers that prevents any vibration – produce about a thousand pairs of twin photons.

Strange world
The phenomena of the atomic and molecular world inhabited by the twin particles are governed by the laws of quantum mechanics, which seem to be a nonsense, for those who stick to the dimensions of the visible world. Particles like electrons and photons behave like corpuscles and waves at the same time, and only pick one of these behaviors when watched. There are some other strange things. The particles can occupy two positions in space at the same time, or shatter into countless fragments – or waves – and even so maintain their properties. Unlike macroscopic bodies, they never reveal their position and speed simultaneously. These peculiarities led to an intense debate among the greatest scientistsof the 20th century.

The behavior of these entangled pairs used to disturb the German physicist Albert Einstein (1879-1955), who classified the instant connection between them as “ghostly remote action”. In an article from 1935, he tried to show what he regarded as the incoherence of the quantum theory, and foresaw that this would be resolved when the theory reached an adult stage. In reply, the Dane, Niels Bohr (1885-1962), with whom Einstein kept up a debate over three decades, showed that the atomic micro-universe really does affront common sense, and that everyone who adventures to study it should accept this. History seems to put the Kopenhagen School at a certain advantage: against Einstein’s wish, the smallest entities of the universe continue to behave strangely.

The machines that may be built using entangled photons are still in gestation. For the time being, quantum computers have no similarity at all with the computers we know. There have already been tests with quantum cryptography, though nothing that can be used for secure data transmission.Last September, the team led by Eugene Polzik, of Arhus University, in Denmark, told Nature magazine that they had succeeded in splicing two clouds of cesium. Each cloud contained about one trillion atoms, and splicing them is regarded as the first step towards the teleportation of massive particles.

“Quantum entanglement”, says Ribeiro, “is the physical property behind making teleportation a reality. The teleportation of the state of polarization of a photon has already been carried out experimentally, with the use of spliced or entangled photons”. Whereas the teleportation of larger objects would be far more complex: the human body, for example, has about 1025 (the number 1 followed by 25 zeroes) atoms, or ten trillion times the size of the twin clouds.