It’s easy with a quantum ball (Image: Rubberball/Corbis)

Conjurers frequently appear to make balls jump between upturned cups. In quantum systems, where the properties of an object, including its location, can vary depending on how you observe them, such feats should be possible without sleight of hand. Now this startling characteristic has been demonstrated experimentally, using a single photon that exists in three locations at once.

Despite quantum theory’s knack for explaining experimental results, some physicists have found its weirdness too much to swallow. Albert Einstein mocked entanglement, a notion at the heart of quantum theory in which the properties of one particle can immediately affect those of another regardless of the distance between them. He argued that some invisible classical physics, known as “hidden-variable theories”, must be creating the illusion of what he called “spooky action at a distance”.

A series of painstakingly designed experiments has since shown that Einstein was wrong: entanglement is real and no hidden-variable theories can explain its weird effects.


But entanglement is not the only phenomenon separating the quantum from the classical. “There is another shocking fact about quantum reality which is often overlooked,” says Aephraim Steinberg of the University of Toronto in Canada.

No absolute reality

In 1967, Simon Kochen and Ernst Specker proved mathematically that even for a single quantum object, where entanglement is not possible, the values that you obtain when you measure its properties depend on the context. So the value of property A, say, depends on whether you chose to measure it with property B, or with property C. In other words, there is no reality independent of the choice of measurement.

It wasn’t until 2008, however, that Alexander Klyachko of Bilkent University in Ankara, Turkey, and colleagues devised a feasible test for this prediction. They calculated that if you repeatedly measured five different pairs of properties of a quantum particle that was in a superposition of three states, the results would differ for the quantum system compared with a classical system with hidden variables.

That’s because quantum properties are not fixed, but vary depending on the choice of measurements, which skews the statistics. “This was a very clever idea,” says Anton Zeilinger of the Institute for Quantum Optics, Quantum Nanophysics and Quantum Information in Vienna, Austria. “The question was how to realise this in an experiment.”

Now he, Radek Lapkiewicz and colleagues have realised the idea experimentally. They used photons, each in a superposition in which they simultaneously took three paths. Then they repeated a sequence of five pairs of measurements on various properties of the photons, such as their polarisations, tens of thousands of times.

A beautiful experiment

They found that the resulting statistics could only be explained if the combination of properties that was tested was affecting the value of the property being measured. “There is no sense in assuming that what we do not measure about a system has [an independent] reality,” Zeilinger concludes.

Steinberg is impressed: “This is a beautiful experiment.” If previous experiments testing entanglement shut the door on hidden variables theories, the latest work seals it tight. “It appears that you can’t even conceive of a theory where specific observables would have definite values that are independent of the other things you measure,” adds Steinberg.

Kochen, now at Princeton University in New Jersey, is also happy. “Almost a half century after Specker and I proved our theorem, which was based on a [thought] experiment, real experiments now confirm our result,” he says.

Niels Bohr, a giant of quantum physics, was a great proponent of the idea that the nature of quantum reality depends on what we choose to measure, a notion that came to be called the Copenhagen interpretation. “This experiment lends more support to the Copenhagen interpretation,” says Zeilinger.

Journal reference: Nature, DOI: 10.1038/nature10119