Quantum computers have failed. So now for the science

Bouncing oil droplets reveal slippery truth behind the magical promises

Classical musings

Most physicists nowadays are sceptical that quantum mechanics can arise from an underlying classical system: see for example the Wired article on bouncing droplets. The reason for this, and the argument for quantum computing, hinges on the "Bell tests". Let me explain.

In 1935, Einstein had argued that quantum mechanics could not be the whole story, because if two electrons were generated by the same atomic decay, then quantum mechanics tells us that their states are "entangled", which means that they can be described by a single wave function and thus will remain correlated until one of them interacts with another particle.

So if you measure the momentum of one of them, you could not then measure the position of the other without violating the Heisenberg uncertainty principle, which states that you can't measure two conjugate attributes (like position and momentum) accurately at the same time. But how would particle A know that an observer had measured one particular attribute of particle B? This seems to require that information travels faster than light, contrary to special relativity. Einstein was dead against "Spukhafte Fernwirkung", or "spooky action at a distance"; his 1935 paper was the most cited in physics for many decades.

In 1965, the Northern Irish physicist John Bell proved that the correlation between two classical particles such as billiard balls would be strictly less than the correlation that quantum mechanics predicts between entangled electrons, thus opening the prospect of a decisive experiment. But doing it with electrons was still too hard and in 1969 the Americans John Clauser, Mike Horne, Abner Shimony, and Dick Holt produced a version for photons, called the CHSH inequality.

They assumed that the polarisation of a photon is "carried by and localised within" it, and proved that if this were so then the probability of observing a correlation between the polarisation of two co-generated photons would be strictly less than is predicted by quantum mechanics.

Experiments were then done, first by Stuart Freedman and John Clauser in 1972, then by Alain Aspect and many others, and showed that the correlation between photons is indeed greater than classical physics appears to allow.