29 October 2012
Experiment could reveal mechanism behind quantum entanglement
by Will Parker
Physicists have devised an experiment that could reveal the precise workings of Einstein's "spooky action at a distance." The results, say the scientists, would show that either faster-than-light communication is possible, or, that the Universe is fundamentally nonlocal, in the sense that every bit of the Universe is connected to every other bit. The proposal comes from an international team of researchers from Switzerland, Belgium, Spain and Singapore, and is published in Nature Physics. Excitingly, there is a real prospect of performing the experiment, which would explore what the researchers call "hidden influence inequality."
The implications of quantum theory have been troubling physicists since the theory was first mooted nearly a hundred years ago. The problem is that quantum theory predicts bizarre behavior for particles - such as two entangled particles behaving as one even when separated by a vast distance.
It was Einstein who first drew attention to the worrying implications of what he termed the "spooky action at a distance" predicted by quantum mechanics. Common sense tells us that any such coordinated behavior must result from one of two arrangements. First, it could be arranged in advance. The second option is that it could be synchronized by some kind of signal sent between the particles.
Previous experiments have shown that if you want to invoke signals to explain things, the signals would have to be travelling faster than light - more than 10,000 times the speed of light, in fact. Einstein's theory of relativity sets the speed of light as a universal speed limit, so the idea of signals travelling 10,000 times as fast sets physics alarm bells ringing.
However, physicists have a getout: such signals might stay as "hidden influences" - useable for nothing, and thus not violating relativity. Only if the signals can be harnessed for faster-than-light communication do they openly contradict relativity.
According to the researchers, hidden influence inequality shows that the getout won't work when it comes to quantum predictions. To derive their inequality, which sets up a measurement of entanglement between four particles, the researchers considered what behaviors are possible for four particles that are connected by influences that stay hidden and that travel at some arbitrary finite speed.
Mathematically, these constraints define an 80-dimensional object. The testable hidden influence inequality is the boundary of the shadow this 80-dimensional shape casts in 44 dimensions. The researchers showed that quantum predictions can lie outside this boundary, which means they are going against one of the assumptions. Outside the boundary, either the influences can't stay hidden, or they must have infinite speed.
Experimental groups can already entangle four particles, so a test is feasible in the near future although the researchers say that the precision of experiments will need to improve to make the difference measurable. Such a test will boil down to measuring a single number. In a Universe following the standard relativistic laws we are used to, 7 is the limit. If nature behaves as quantum physics predicts, the result can go up to 7.3.
So, explains team member Nicolas Gisin, at the University of Geneva, if the result is greater than 7 (in other words, if the quantum nature of the world is confirmed) what will it mean? Here, Gisin says there are two choices. On the one hand, there is the option to defy relativity and "unhide" the influences, which means accepting faster-than-light communication. For many physicists this is seen as the most extreme possibility.
The alternative is that there exists some process that has an equivalent effect when viewed in our space-time. This would mean that the Universe is fundamentally nonlocal, in the sense that every bit of the Universe can be connected to any other bit anywhere, instantly. That such connections are possible defies our everyday intuition and represents another extreme solution, but arguably preferable to faster-than-light communication.
"We are interested in whether we can explain the funky phenomena we observe without sacrificing our sense of things happening smoothly in space and time," said Jean-Daniel Bancal, one of the researchers involved at the Centre for Quantum Technologies at the National University of Singapore.
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