Quantum mechanics ‘spooky action at a distance’ confirmed experimentally

Scientists have proven experimentally, and for the first time, what Einstein called ‘spooky action at a distance’. This is the concept – which Einstein himself believed rather paradoxical and hence somewhat troublesome – that the information for entangled objects can travel faster than the speed of light.

A single photon is incident on a beam splitter of reflectivity R and then subjected to homodyne measurements at two spatially separated locations

A single photon is incident on a beam splitter of reflectivity R and then subjected to homodyne measurements at two spatially separated locations (credit: Nature Communications)

Entanglement is a phenomenon of quantum mechanics in which pairs or group of particles are generated/interact in ways such that their quantum state (e.g. having a spin ‘up’ or ‘down’) is interdependent on one another. To put it in simple terms, imagine you can ‘prepare’ two entangled particles and send them in separate ways at an arbitrarily long distance. Now – and here is where the ‘paradoxical’ aspect of all this comes in – if you measure the state of particle 1, to be ‘spin up’ for instance, you’ll know ‘instantaneously’ that particle 2 will have the opposite state, or ‘spin down’ in this example (because the total angular momentum of the universe has to be conserved). Some see this as problematic because it would be like sending information faster than the speed of light. For quantum physicists this is not too much of an issue because whoever measures particle 2 still has to make the measurement, and also whoever measures particle 1 has to tell the other (via a classical channel, e.g. by phone and hence without violating special relativity) how they performed the measurement.

However, according to a competitive hypothesis the state of the two particles is not necessarily defined at the moment of creation of entanglement. When the particles are sent apart, they are still completely undetermined (in our example they will be both ‘spin up’ and ‘spin down’, like the famous Schroedinger cat – both dead and alive) and at the very instant when one is measured the other one ‘acquires, right then’ its opposite state. This would be indeed somewhat paradoxical (it would be in contrast with Einstein’s special relativity) and it is what was recently demonstrated in the study conducted at Griffith University, Brisbane (AU) and the University of Tokyo (JP), and published in the journal Nature Communications earlier this year.

In the experiment in question, the researchers managed to split a single photon between two laboratories. This is possible because according to quantum mechanics a particle can be described as a wave that spreads out in space over great distances (you can think at this wave as related to the probability of locating the particle over that distance in space). The experiment involved firing a beam of photons (unitary particle of light) into a beam splitter that would ‘cut’ (entangle) each photon in two, and then sending half of the light to one lab and half to the other.

Using a finely tuned homodyne detector, scientists in lab A tried to detect each photon and measure its phase, and so did their colleagues in lab B. They found that if researchers in lab A had detected the photon, the researchers in lab B could not, and vice versa. In addition, they found that the state of each detected photon in lab A or B depended on the state of the counterpart photon measured in the other lab. This demonstrates the so called ‘non-local wave function collapse of a single particle’; it is what you’d expect if the single split photons were entangled and shows that the idea of a “spooky action at a distance” disparaged by Einstein in the 1920’s and 30’s, is indeed real.

“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.” says professor Howard Wiseman, co-author of the study. “However, 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. Through these different measurements, you see the wave function collapse in different ways, thus proving its existence and showing that Einstein was wrong,” He concluded.

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Carlo Bradac

Carlo Bradac

Dr Carlo Bradac is a Research Fellow at the University of Technology, Sydney (UTS). He studied physics and engineering at the Polytechnic of Milan (Italy) where he achieved his Bachelor of Science (2004) and Master of Science (2006) in Engineering for Physics and Mathematics. During his employment experience, he worked as Application Engineer and Process Automation & Control Engineer. In 2012 he completed his PhD in Physics at Macquarie University, Sydney (Australia). He worked as a Postdoctoral Research Fellow at Sydney University and Macquarie University, before moving to UTS upon receiving the Chancellor Postdoctoral Research and DECRA Fellowships.

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