A team of scientists have for the first time successfully demonstrated the non-local collapse of a particle’s wave function in an experiment using a single particle.

Using homodyne detectors to measure the particle, and quantum tomography to map the effect of those measurements, the scientists, from Griffith University and the University of Tokyo, were able to verify single-particle quantum entanglement an unusual form of entanglement that could prove invaluable for quantum computing and communications.

While quantum entanglement usually refers to two particles that are bound by opposing spins, the directions of which will only be set when they are observed, single particles can also be entangled, meaning their wave function – ie the equation that defines their likely location and behaviour – can cover any distance.

In other words, a single entangled particle can only be in one place at a given time, but it can be located over a very large distance. When the particle is measured, the wave function will instantly collapse to a set location.

This was demonstrated by the scientists, who split a single photon between their labs in Japan and Australia, but was previously regarded as an unlikely phenomenon by Albert Einstein.

Almost 90 years ago, he used single-particle entanglement as evidence that quantum mechanics was incorrect, deriding non-local wave function collapse as “spooky action at a distance”.

“Einstein never accepted orthodox quantum mechanics and the original basis of his contention was this single-particle argument,” explained Professor Howard Wiseman, director of Griffith University’s Centre for Quantum Dynamics.

“This is why it is important to demonstrate non-local wave function collapse with a single particle.”

While taking issue with quantum mechanics, Einstein proposed an alternative hypothesis for the particle’s behaviour.

“Einstein’s view was that the detection of the particle only ever at one point could be much better explained by the hypothesis that the particle is only ever at one point, without invoking the instantaneous collapse of the wave function to nothing at all other points.”

Although this alternative theory seems more acceptable to the human brain, Wiseman and his colleagues have shown it to be incorrect.

“Rather than simply detecting the presence or absence of the particle, we used homodyne measurements enabling one party to make different measurements and the other, using quantum tomography, to test the effect of those choices,” he explained.

“Through these different measurements, you see the wave function collapse in different ways, thus proving its existence and showing that Einstein was wrong.”

The research was published today in Nature Communications.

Journal reference: Fuwa M, Takeda S, Zwierz M, Wiseman HM, Furusawa A. Experimental proof of nonlocal wavefunction collapse for a single particle using homodyne measurements. Nature Communications 06 March 2015. doi:10.1038/ncomms7665.