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The phantoms of photons

Brazilian researchers measure the entanglement of light particles, an important step in research towards the quantum computer

hélio de almeidaA little bit of magic from another world, conceived and carried out by national physicists, occupied three pages of the British magazine Nature on the 20th of April last month. Headed by Luiz Davidovich and Paulo Henrique Souto Ribeiro, a research team from the Federal University of Rio Janeiro (UFRJ), with the theoretical assistance of colleagues from the Max-Planck Institute, in Dresden, published an article in the renowned scientific periodical describing of what they said was the first direct measurement of a strange property of the quantum universe: the entanglement or entanglement of atoms or particles. This property, which Albert Einstein described as having “phantasmagoric action at a distance”, could perhaps be the most characteristic signature of quantum mechanics when confronted by classical physics – understanding it and, if possible, dominating it is an essential step for the establishment of cryptography and of the quantum computer, an idea that has gained force since the decade of the 1990’s. This is because entanglement seems to be capable to process and transmit information with much greater efficiency than the conventional chip.

In the case of the experiment carried out at the Quantum Optic Laboratory of the Physics Institute of UFRJ, the scientists created a system in which two pairs of photons, light particles, were generated, from the emission of a laser beam upon a crystal. Next, they determined the quantity of entanglement in the system by way of a single measurement of the physical properties of the two particles, realized upon one of the two photons of each pair. Normally, the quantum physicists carry out various measurements and, afterwards, make calculations to determine the quantity of entanglement of a grouping of particles. But the Brazilian researchers believe they have developed a simpler and more efficient way of reaching this objective: by measuring the polarization (direction of the light’s electric field, for example, vertically or horizontally) and the moment (related to the direction of propagation, if to the right or the left) of the light corpuscles and they established an association between these parameters and the quantity of entanglement present in the photon duets. “To determine the quantity of entanglement and to understand the physical implications of this phenomenon is one of the major challenges of quantum physics”, says  Davidovich. From the practical point of view, elevated levels of entanglement would be necessary to put into operation futuristic quantum PCs.

But exactly what is such entanglement t? Theoretically formulated in the decade of the 1930’s and experimentally proven in the 1960’s, it is a phenomenon with something of a mystery for people accustomed to the laws of Newtonian physics, or that is to say, the majority of mortals. According  to the concept of entanglement, the properties of two or more entangling  particles (atoms, electrons, photons etc.) can only be known through the measurement in which they, the particles, form a grouping, in which measurements carried out upon one of the system’s components alters the state of another independently from its location in space, independently whether the particles are practically together or separated by thousands of kilometers. Hence such is the almost supernatural effect to which Einstein alluded on describing quantum entanglement.

Put into this form, the complicated concept of quantum entanglement puts a knot in people’s heads. In order to understand it, instead of thinking about photons and particles, it is more didactic to imagine a system composed of two dice. Since they are entangled, when thrown, whether one is in Brazil and the other in Japan, the dice always give the same result: the sum of their values is, for example, eight. This final parameter of this exotic system is known, easily measured, but what is not known is what number combination (four and four, five and three, six and two) led to this result. In this case, when finally the value of one of the die is discovered, the enigma in relation to the other also disappears.

Perpendicular polarization
With the photon pairs of the experiment carried out at the UFRJ, which are entangled in relation to two physical parameters (polarization and the direction of propagation), more or less the same thing happens. If the physicists determine that one of these light particles vibrates in the vertical direction, the other, its partner in quantum terms, can only oscillate in the horizontal direction. But, in the strange world of quantum physics, the polarization of the second photon can only be determined exactly after the measurement of this physical parameter on the first light particle. “In this system, we know beforehand that the polarization of one photon is perpendicular to that of the other, but that of each photon individually is completely undetermined”, says the physicist Stephen Patrick Walborn, who is completing his post-doctoral studies at the Rio de Janeiro  university and is the main conductor of the experiment carried out at UFRJ’s Quantum  Optic Laboratory, one of the three national research institutions that make up part of the Millennium Quantum Information Institute, an initiative sponsored by the Ministry of Science and Technology (MCT).