When does a quantum measurement end?

Surprisingly, quantum physicists cannot agree. Some say the measurement ends when you register a result on a piece of classical equipment such as a photomultiplier. Others says the measurement ends when the information in the quantum system has irreversibly leaked into the environment. There are still more who believe in the manyworlds interpretation of quantum mechanics and say a quantum measurement never ends but exists ad infinitum in several parallel universes.

This may sound like an ineffectual academic scrap but it actually has hugely important consequences for the quantum property of entanglement.

Entanglement is the state in which two physically separated particles share the same quantum existence, so that a measurement on one instantaneously affects the other. Yep, that’s instantaneously. It’s what Einstein described as “spooky action at distance”.

For some years, physicists have been measuring this “spooky action at a distance” in tests known as Bell experiments.

These tests depend crucially on the measurement ending quickly. Because if it were to drag on, the particles might be able to communicate at light speed by some currently unknown mechanism.

But because nobody has actually determined when a measurement ends, all the experiments to date are potentially open to this loophole.

Perhaps there is no spooky action at a distance after all, just long quantum measurements during which the particles communicate at the speed of light in some quite ordinary way.

Now Nicolas Gisin and colleagues at the University of Geneva have closed this loophole using the ideas of the Oxford theorist Roger Penrose. A few years ago, he suggested that the end of a quantum measurement is realted to the gravitational energy of the mass distribution of the resulting quantum superposition. In other words, the measurement ends when a massive object receives a decent kick.

So Gisin and buddies set up a Bell experiment which involved sending entangled photons in each direction from the midpoint of an 18 km fibre. At the ends of the fibre were piezoelectric actuators attached to small but massive mirrors. When the photons hit, they triggered the actuators causing the mirrors to move and deflect a beam of light.

The experiment was carefully set up so that the mirrors were heavy enough to please Penrose and far enough apart that no light speed signal could travel between them in the time it took for a pair of entangled photons to “kick” them.

The result? Gisin’s team confirmed that “spooky action at a distance” still governs the behaviour of the entangled photons.

If you believe Penrose, this is the first experiment to ever prove “spooky action at a distance”. Impressive, huh?

More interesting, is the idea of gravity and quantum mechanics coming under the microscope in the same experiment for the first time.

It won’t be the last. There are plenty of other mysteries about gravity that quantum mechanics can probe. It’s about time physicists bit the bullet and started testing them.

Ref: arxiv.org/abs/0803.2425: Space-like Separation in a Bell Test assuming Gravitationally Induced Collapses