Quantum theory survives latest challenge
Dec 15, 2010 43 comments
Since quantum mechanics was first formulated, a string of physicists including Albert Einstein have been uncomfortable with the idea of entanglement – whereby a group of quantum particles have a closer relationship than allowed by classical physics. As a result, some physicists have proposed alternative theories that allow such close relationships without the need for quantum mechanics. While it has been difficult to test these theories, researchers in the UK have used "twisted light" to make an important measurement that backs up quantum theory.
Quantum theory seems foreign to our everyday experience because it defies our idea of "realism" – the expectation that objects have definite properties whether we’re looking at them or not. Quantum theory also seems to call for entities that can instantly react to an event occurring elsewhere – apparently defying the principle of locality, which forbids communication faster than the speed of light.
These oddities were expressed mathematically by the physicist John Bell in his famous inequality. Bell showed that a particular combination of measurements performed on identically prepared pairs of particles would produce a numerical bound (or inequality) that is satisfied by all physical theories that obey realism and locality. He also showed, however, that this bound is violated by the predictions of quantum physics for entangled particle pairs.
In Bell experiments two distant observers measure, for example, the polarization of entangled particles along different directions and calculate the correlations between them. This was done in the 1970s by Stuart Freedman and John Clauser and in the 1980s by Alain Aspect, who used entangled photons to confirm quantum theory.
Sacrificing locality for realism
Physics has generally accepted that the quantum world flouts "local realism", but in 2003, Anthony Leggett of the University of Illinois at Urbana-Champaign tried to restore realism by sacrificing locality. If two entities can arrange their correlations through instantaneous communication, then perhaps it is still possible that they each have definite properties. Leggett’s real but non-local scenario passes the Bell test, but could it really describe the quantum world?
Four years later, physicists in Austria, Switzerland and Singapore answered with data. Instead of measuring the linear polarization states used to violate Bell’s inequality they looked for correlations between elliptical polarizations – combinations of linear and circular states. Even assuming that entangled photons could respond to one another instantly, the correlations between polarization states still violated Leggett’s inequality. The conclusion being that instantaneous communication is not enough to explain entanglement and realism must also be abandoned.
This conclusion is now backed up by Sonja Franke-Arnold and collegues at the University of Glasgow and University of Strathclyde who have performed another experiment showing that entangled photons exhibit entangled photons show stronger correlations than allowed for particles with individually defined properties – even if they would be allowed to communicate constantly. But rather than polarization, they studied the properties of each photon’s orbital angular momentum.
In photons, orbital angular momentum can be understood by imagining that the wave twists around the beam axis. It can draw a simple corkscrew pattern, a double helix or more complex helices with increasing angular momentum. Franke-Arnold and her team focused on the double-helix pattern.
Glasgow student Jacquie Romero did the experiment by firing an ultraviolet laser into an optical crystal designed to split the high-energy photons into pairs of entangled infrared photons. These went on to computer-controlled holograms, which were set to filter out roughly complementary orbital angular momentum states. Photons that passed the holograms were then counted by a single-photon detector.
The correlation between two entangled photons, one with a clockwise orbital-angular momentum while the other twists anticlockwise, is predicted by Bell's and Leggett's proposals as well as quantum theory. "We deliberately misalign our holograms from the complementary states and measure the resulting correlations," explained Franke-Arnold. The coincidence counts in the detector occured too often to agree with Leggett’s theory. They did, however, match quantum predictions.
'A philosophical result'
"The main outcome is really a philosophical result," says Franke-Arnold. Entangled particles can't be described as individual entities, not even with a telepathic connection to their partners.
Simon Gröblacher of the University of Vienna points out that these experiments rule out realism only for a large class of nonlocal theories – still others aren't described by Leggett’s inequality. His team first showed the violation of Leggett's inequality through photon polarisation, and he says that it's nice to see the violation verified with another property of photons. "The experiments seem to be simpler," he adds, noting that orbital-angular momentum offers options to test superpositions of more than two states.
The work is described in New Journal of Physics 12 123007.
About the author
Kate McAlpine is a science writer based in the US