A single quantum particle is a wave that spreads out over arbitrarily — or unspecifiably — large distances. Yet it can never be detected in more than one place at a time. This is because under any attempt to observe or measure the particle, the quantum particle wave collapses to become one particle at one point.
Incredibly, a groundbreaking experiment has beamed one quantum particle projected in two streams, spread out as quantum wave functions over a vast distance, that has had a measurement taken on it at one location, which has made the wave collapse at another remotely situated location. The particle was observed at two places at one time, and an action in one place produced a result in quite another place.
Where and With What
With a team of researchers located at the University of Tokyo in Japan, headed by Maria Fuwa, and at Griffith University in Australia, there, headed by Howard M. Wiseman, a homodyne detector was used to measure — or perform an action upon — the particle in Tokyo while tomography was used to build a tomographic 3-D reconstruction of the resultant change to the quantum state of the particle as seen in Brisbane. Maria Fuwa, as the lead author of their paper in Nature Communications, gave the following astounding explanation:
Here, we demonstrate this single-particle spooky action [at a distance] … by splitting a single photon between two laboratories and experimentally testing whether the choice of measurement in one laboratory really causes a change in the local quantum state [of the wavefunction] in the other laboratory. … Our experiment also verifies the [quantum] entanglement of the split single photon….
Homodyne and Tomograph
A homodyne detector is a device that combines a split quantum particle stream with laser sending the split particle in two directions in separate beams. The beams coming out the beam splitter hit photobodies that generate electrical current corresponding to the electromagnetic field incident (alighting) on it. As Wiseman stated in a PhysOrg press release:
[R]ather 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.
A tomographer is a device employing a penetrating wave to reconstruct three-dimensional images built up from composites of sectioned images representing a distribution of different points of view.
What the Experiment Proves
The work of Tokyo-Brisbane team proves two very important things. The first is that they have proven non-local collapse of a particle’s wavefunction. “Non-local” refers to an action taken not locally (i.e., not where Wiseman and the tomography device were) but taken at any distant other location (i.e, in Tokyo where the homodyne device was).
“Collapse of wave function” means that the wavefunction of the split particle, which had been extending over an arbitrarily large area, was suddenly, at the moment of the non-local action, collapsed to a single particle: What had been a vast wave suddenly became a single particle with a specific location. They proved that a quantum wave could be acted upon in location A and that the results could be seen in location B thus proving quantum entanglement.
Einstein: “no spooky action at a distance”
The second thing they proved is that Einstein was wrong. Weird spooky action at a distance, properly called quantum entanglement, is not the result of a property of quantum particles. He was wrong to say that the apparent entanglement was the misinterpretation of properties inherent to quantum particles that only became noticeable under conditions that created the appearance that entanglement had been causative.
Einstein was wrong to hypothesize no-spooky-action-at-a-distance and to blame weirdness on unidentified quantum properties. The argument that Einstein used to disprove spooky action posited a single particle undergoing non-local collapse of wavefunction. This is why it was important for the Tokyo-Brisbane team to conduct their experiment with a single particle: They used Einstein to disprove Einstein. Professor Wiseman, who is also one of two authors of the many interacting worlds interpretation of cosmology, commented about Einstein:
Einstein never accepted orthodox quantum mechanics and the original basis of his contention was this single-particle argument. This is why it is important to demonstrate non-local wave function collapse with a single particle, … 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, … Through these different measurements, you see the wave function collapse in different ways, thus proving its existence and showing that Einstein was wrong.
On top of the scientific wonder of the accomplishment, as Factor-Tech points out, single-particle quantum entanglement, though a rare form of entanglement, holds out interesting potential for quantum computing and quantum communications. The University of Tokyo-Griffith University study was published in Nature Communications under the title “Experimental proof of nonlocal wavefunction collapse for a single particle using homodyne measurements.”
Please note that the very useful and amusing video explaining particle spin that accompanies articles at ScienceAlert and Factor-Tech discusses quantum entanglement between two quantum particles. While very handy information, two-particle entanglement does not related directly to the Fuwa-Wiseman study herein explained since Fuwa, Wiseman and team very deliberately used one single quantum particle to illustrate single-particle entanglement in order to directly address Einstein’s “no-spooky-action-at-a-distance” discussion.