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The spinning of light

A simple technique measures the property of a beam of light that is useful for quantum computing

UVA COSTRIUBAIf there is one thing that physicists appreciate it is simplicity. For most of them, the notion that everything – even the Universe – can be summarized into a handful of equations is too appealing to be ignored. Sometimes, attempts to translate nature into numbers requires the construction of complex and expensive particle accelerators. But this was not the case with the work done by Jandir Hickmann’s team from the Federal University of Alagoas (Ufal), which was published at the end of July in Physical Review Letters. With much more modest tools (a laser source, some lenses and strips of insulation tape), he and three colleagues created a very simple strategy for measuring a property of light that perhaps can be used to manipulate information in a quantum computer.

The measured property is the so-called orbital angular momentum. Mentally visualizing these properties, like virtually everything in the quantum world, is quite complicated. But the physicist from Ufal uses a metaphor from classical physics to explain it. Imagine that the Earth is a beam of light. With the Sun as a reference, our planet carries out two distinct movements. One of them, around its own axis, is its rotation, which induces the occurrence of days and nights. The equivalent of this movement in the beam of light would be the angular momentum of spin. The other movement of the Earth is around the Sun, the translation, which produces the years. Its quantum equivalent would be the orbital angular momentum. Combined, the two movements make light energy become concentrated in certain regions of the beam. As the light moves in one direction, this area of energy concentration moves in spirals along the axis of the light beam; if it were visible it would form an image that resembles a corkscrew.

For a very long time scientists have known how to measure the properties of spin but the orbital angular momentum is much more difficult to measure. Using an optical table, Hickmann and his colleagues fired an argon laser towards a detector. On the way, the light had to pass through a hologram, a kind of filter in which it acquired orbital angular momentum, before crossing through an aperture in the shape of an equilateral triangle made of a strip of insulation tape. Why this material? “Just to show how easy it is to do,” says the Brazilian physicist.

When crossing the aperture, the laser interacts with the edges of the triangle and deviates (diffraction). What is seen in the detector is a different triangle, formed by a set of luminous disks. All that is needed is to count the number of these disks that form one of the sides of the triangle to know the value of the orbital angular momentum; it will be proportional to the number of disks.

JANDIR HICKMANN / UFALColored disks: the result of light deviationJANDIR HICKMANN / UFAL

This finding was encouraging for the physicists. “It was surprising, at least for me, that there was a relationship that’s so simple and beautiful,” said Miles Padgett, from the University of Glasgow, in Scotland, in the Physical Review Focus, a vehicle for scientific disclosure from the American Physical Society, which ran a prominent feature on the Brazilian research. The reason for the excitement is that it is hoped that orbital angular momentum can be used as the basis for the so-called quantum computing.

The idea behind this fledgling technology is to use the interactions and properties of light and of atomic particles to perform calculations that would be difficult to do otherwise. This is because one of the strangest characteristics of the quantum world is the fact that a particle, until it is observed, can contain all possible configurations. For example, a light particle can simultaneously have -1 or 1 spin, before being detected. As a result, it could, in principle, be used to carry out two operations simultaneously.
Experiments that use spin for quantum processing have had some success, but Hickmann sees even greater potential for the use of orbital angular momentum. “That’s because it has no limit for the number of states it can assume. While the angular momentum of spin can only be 1 or -1, orbital angular momentum can take any value, provided it is a whole positive or a negative number,” explains the physicist.

As a result, at least in principle, the number of simultaneous calculations that can be done by a quantum processor based on orbital angular momentum becomes very much greater. As efficient measurement techniques are developed, this idea can get a little closer to reality. But only a little. “The biggest problem in quantum computing is decoherence,” says Hickmann. This is the name given to the loss of that delicate condition in which particles are maintained, presenting all states at the same time and, therefore, useful for processing information. “The difficult part is generating quantum states that are robust enough not to be lost,” he says. According to the physicist, so far the only quantum processors tested have very low processing capacity. “It’s as if I had a processor capable of counting only up to 16 quantum bits. It has few fingers so cannot count much,” he explains, “which is why it still has no practical application.”

Scientific article
HICKMANN, J. et al. Unveiling a truncated optical lattice associated with a triangular aperture using light’s orbital angular momentum. PRL. v. 105. 30 jul. 2010.