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Devangshu Datta: Terms of Entanglement

Cryptography and quantum computing could benefit from Entanglement experiments

Devangshu Datta  |  New Delhi 

Devangshu Datta

Albert Einstein and Niels Bohr publicly debated the philosophy of the physically possible for decades during the first half of the 20th century. Both giants did pioneering work in Quantum Theory (QT) and other greats like Erwin Schrödinger, Werner Heisenberg, Wolfgang Pauli, Paul Dirac, also produced pathbreaking QT papers at that time.

QT works. It offers many predictions about particle behaviour. Those can be experimentally confirmed with great precision. But unlike classical physics, QT is probabilistic in nature. This leads to paradoxes such as illustrated by Schrödinger's famous thought experiment concerning the cat, which is both half-alive and half-dead.

A central QT paradox was described by Schrödinger as "Entanglement". Two particles (or a group of paired particles) may be strongly correlated and these can be described by the same wave-function. When one such entangled particle is affected in terms of spin, polarisation, momentum etc, the paired particle's characteristics instantly change. Entanglement occurs even if particles are separated by significant distances.

Einstein (and others) felt there must be underlying principles, so-called 'hidden variables', which QT had not grasped. Einstein once wrote, "God does not play dice with the universe", implying that the universe could not be purely probabilistic. Bohr disagreed with an equally famous retort, "Don't tell God what to do". (Neither believed in a conventional God, making the exchange interesting in other ways).

In another exchange , Einstein wrote a letter to Max Born describing Entanglement as "spooky action at a distance". Einstein also co-authored a paper with Boris Podolski and Nathan Rosen, where the trio of Einstein Podolski Rosen (EPR) Paradox pointed out how Entanglement could violate Relativity Theory.

Relativity assumes that nothing, not even information, can travel faster than light and Relativity's predictions have been verified experimentally countless times. Yet, if Entanglement works at a distance, information at the least is transmitted instantly to alter the state of entangled particles.

The EPR paradox led the trio to suggest that QT was incomplete and 'hidden variables', or loopholes from classical physics, allowed distant Entanglement.

In the 1960s, John Bell worked out a way to measure Entanglement at "non-local" distances. The states of two separated particles can be measured. If the correlation is higher than the levels calculated in Bell's Inequality, Entanglement occurs, and it cannot be explained by 'hidden variables'.

Multiple experiments have broken the bounds of Bell's Inequality confirming Entanglement occurs at a distance. But every one of those experiments contained potential loopholes. The biggest loopholes are 'communication', and 'detection'.

One communication loophole is simply that affecting a particle could affect its pair if the two are close together. But performing such an experiment with two entangled particles at great distances from each other is hard. Another is that the detectors used to measure the state of two entangled particles may communicate. Also up to 80 per cent of particles used in Entanglement experiments cannot be detected, leaving researchers to guess results by looking at the few they can detect, causing a detection loophole.

A recent paper (refer to http://arxiv.org/abs/1508.05949) from the Delft Institute of technology in Holland, along with researchers at the Institute of Photonic Sciences, Barcelona, and industrial diamond-maker, Element Six, Oxford, describes a single experiment that eliminates these two loopholes.

The researchers took two unentangled electrons in diamond crystals held 1.3 km apart. Each electron was individually entangled with a photon. Those two photons were taken to a third location (hundreds of metres from the other two labs) and then entangled with each other. This caused the partner electrons (originally unentangled) to also become entangled!

The communication loophole is eliminated since there is no communication between the original pair. The detection loophole was shut down by repeating the experiment 245 times. The results break Bell's Inequality.

The experiment has been hailed by many other physicists but the paper is still under review. Many other labs are looking to reproduce this result. The trick is really the clever and original design of the 'entanglement swap'. If this is reproduced, it has huge implications for cryptography and quantum computing, quite apart from energising science-fictional speculation about instantaneous communication.

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First Published: Thu, September 17 2015. 21:48 IST
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