The Universe Really Is Weird: A Landmark Quantum Experiment Has Finally Proved It So

via IFLScience

Only last year the world of physics celebrated the 50th anniversary of Bell’s theorem, a mathematical proof that certain predictions of quantum mechanics are incompatible with local causality. Local causality is a very natural scientific assumption and it holds in all modern scientific theories, except quantum mechanics.

Local causality is underpinned by two assumptions. The first is Albert Einstein’s principle of relativistic causality, that no causal influences travels faster than the speed of light. This is related to the “local” bit of local causality.

The second is a common-sense principle named after the philosopher Hans Reichenbach which says roughly that if you could know all the causes of a potential event, you would know everything that is relevant for predicting whether it will occur or not.

Although quantum mechanics is an immensely successful theory – it has been applied to describe the behaviour of systems from subatomic particles to neutron stars – it is still only a theory.

Thus, because local causality is such a natural hypothesis about the world, there have been decades of experiments looking for, and finding, the very particular predictions of quantum mechanics that John Bell discovered in 1964.

But none of these experiments definitively ruled out a locally causal explanation of the observations. They all had loopholes because they were not done quite in the way the theorem demanded.

No Loopholes

Now, the long wait for a loophole-free Bell test is over. In a paper published today in Nature, a consortium of European physicists has confirmed the predictions required for Bell’s theorem, with an experimental set-up without the imperfections that have marred all previous experiments.

A Bell experiment requires at least two different locations or laboratories (often personified as named fictional individuals such as Alice and Bob) where measurements are made on quantum particles. More specifically, at each location:

    a setting for the measurement is chosen randomly
    the measurement is performed with the chosen setting
    the result is recorded.

The experiment will only work if the particles in the different laboratories are in a so-called entangled state. This is a quantum state of two or more particles which is only defined for the whole system. It is simply not possible, in quantum theory, to disentangle the individual particles by ascribing each of them a state independent of the others.

The two big imperfections, or loopholes, in previous experiments were the separation and efficiency loophole.

To close the first loophole, it is necessary that the laboratories be far enough apart (well separated). The experimental procedures should also be fast enough so that the random choice of measurement in any one laboratory could not affect the outcome recorded in any other laboratory be any influence travelling at the speed of light or slower. This is challenging because light travels very fast.

To close the second, it is necessary that, once a setting is chosen, a result must be reported with high probability in the time allowed. This has been a problem with experiments using photons (quantum particles of light) because often a photon will not be detected at all.

The Experiment

Most previous Bell-experiments have used the simplest set up, with two laboratories, each with one photon and the two photons in an entangled state. Ronald Hanson and colleagues have succeeded in making their experiment loophole-free by using three laboratories, in a line of length 1.3km.

In the laboratories at either ends, Alice and Bob create an entangled state between a photon and an electron, keep their electron (in a diamond lattice) and send their photons to the laboratory in the middle (which I will personify as Juanita). Alice and Bob then each choose a setting and measure their electrons while Juanita performs a joint measurement on the two photons.

Alice and Bob’s measurements can be done efficiently, but Juanita’s, involving photons, is actually very inefficient. But it can be shown that this does not open a loophole, because Juanita does not make any measurement choice but rather always measures the two photons in the same way.

The experiment, performed in the Netherlands, was very technically demanding and only just managed to convincingly rule out local causality. This achievement could, in principle, be applied to enable certain very secure forms of secret key distribution. With continuing improvements in the technology one day this hopefully will become a reality.

For the moment, though, we should savour this result for its scientific significance. It finally proves that either causal influences propagate faster than light, or a common-sense notion about what the word “cause” signifies is wrong.

One thing this experiment has not resolved is which of these options we should choose. Physicists and philosophers remain as divided as ever on that question, and what it means for the nature of reality.

Reality doesn’t exist until you look at it, pioneering quantum physics experiment finds

by Andrew Griffin

Reality doesn’t exist until it’s measured, at least for very small things, new research has found.

By replicating a famous experiment where an object is given a choice for how to behave, physicists found that the object doesn’t actually make its decision until it is seen. The finding proves one of the central parts of quantum theory, a branch of science that has been applied to make much of our modern technology.

Scientists gave an object a chance either to act like a wave or a particle, and looked to find out when the decision was made. While one might expect that it is either one or the other, quantum physics predicts that it will only act one way or the other when it is measured, after it has decided — and that was confirmed by the experiment.

To perform the test, scientists isolated a single helium atom. They then dropped it through a laser beam, and used a random number to decide whether a second set of beams were added that would interfere with the atom’s path.

But the researchers found that number wasn’t calculated until after the atom had passed through the crossroads of lasers and decided which way to go. Though the atom had already decided whether it would behave like a wave or a particle, it wasn’t decided which way it would go until it was measured, the research found.

"The atoms did not travel from A to B. It was only when they were measured at the end of the journey that their wave-like or particle-like behavior was brought into existence," said associate professor Andrew Truscott, the scientist who led the experiment.

The test is known as the delayed-choice thought experiment, and was proposed by John Wheeler. When he came up with it in 1978, it seemed impossible that it could ever be put to the test — but researchers from the Australian National University were able to run a slightly varied version of it in tests.

"It proves that measurement is everything. At the quantum level, reality does not exist if you are not looking at it," Truscott said.

Truscott and his team published their findings in Nature Physics.