Understanding this weird property of particles is ushering in a new era of technology.
This year’s Nobel Prize for physics has been awarded to a trio of scientists whose insights into the fundamentals of quantum mechanics have set the stage for a new era of technology. Quantum computers, quantum networks and secure quantum cryptography are all rooted in experiments carried out, over several decades, by Alain Aspect, John Clauser and Anton Zeilinger.
Their work revolves around a phenomenon called quantum entanglement, in which two or more particles become correlated with each other so that they behave as if they were single units. This leads to counterintuitive effects—changing the properties of one particle in an entangled pair, for example, will immediately change the other, no matter how far apart the particles are. They could be right next to each other, or at opposite ends of the galaxy. Albert Einstein—who was no fan of the probabilistic nature of quantum mechanics—described entanglement as “spooky action at a distance”. He and others were concerned that it seemed to break the rules of special relativity, which stipulated that nothing could travel faster than the speed of light. If entangled particles were separated by a great distance, how could information apparently travel between them instantaneously?
In 1935 Einstein and two colleagues, Boris Podolsky and Nathan Rosen, proposed a thought experiment (subsequently known as the “epr paradox”) to probe whether the weird behaviour seen in entanglement implied that quantum mechanics was not a complete description of reality. Perhaps particles also carried with them hidden information, not described by quantum mechanics, about how they might behave during experiments or when they were measured. In 1964 John Stewart Bell, a physicist at cern, in Geneva, developed the epr paradox further and came up with testable predictions to determine whether or not the “hidden variables” really existed.
Almost a decade later, John Clauser built the first experiment to test Bell’s idea and his results agreed with the predictions of quantum mechanics and showed that Einstein’s hidden variables probably do not exist. The experiments left a few loopholes, however, which were closed in the early 1980s by Alain Aspect, then a graduate student at Paris-Sud University in Orsay, France. By fine-tuning and improving upon Dr Clauser’s experiments, Dr Aspect put the final nail in the coffin of Einstein’s hidden variables.
The third laureate, Anton Zeilinger of the University of Vienna, has spent decades looking for ways to put quantum entanglement to use. In 1997 he showed that it was possible to transfer information between particles, a process called “quantum teleportation”. He also demonstrated that two pairs of entangled particles can interact in interesting ways—bring together one particle from each entangled pair and the two remaining particles (which have never been in contact) will themsleves become entangled.
Manipulating the quantum states of systems of entangled particles has become the basis of technologies such as quantum computing and quantum encryption. Building on the work of this year’s physics laureates, signals composed of entangled photons (particles of light) have been sent through optical fibres several kilometres long and even been transmitted between the ground and a satellite orbiting hundreds of kilometres above Earth.
“Quantum information science is a vibrant and rapidly developing field, it has broad and potential implications in areas such as secure information transfer, quantum computing and sensing technology,” said Eva Olsson, a member of the physics-prize awarding committee of Sweden’s Royal Academy of Science. “Its predictions have opened doors to another world.”
Speaking after the announcement was made, Dr Zeilinger said that he had been surprised to receive the call from the academy an hour earlier. “I’m still kind of shocked,” he said, “but it’s a very positive shock.”