What putting big things in quantum states can tell us about reality

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AN APPLE never appears to be in many places at one. That statement hardly seems surprising – until you start burrowing into the depths of quantum weirdness, and realise there’s no fundamental reason why that shouldn’t be so.

The theory of decoherence implies that the reason quantumness vanishes is because the more particles there are in an object, the harder it is to sustain quantum properties like a superposition of locations as it interacts with its environment (see “Why aren’t big things quantum?”). Yet in theory, if those interactions can be restricted by isolating the quantum system, there should be no limit on the size for which an object can keep displaying such quantum behaviour.

Can that really be true? With the right set-up, could we quantumly entangle a pair of Braeburns so that it becomes impossible to say which of them is ripe until we bite one? In recent years, Anton Zeilinger and Markus Arndt at the University of Vienna, Austria, and their colleagues, among others, have been doing their best to find out by attempting to get objects of ever-increasing size to remain quantum – and so perhaps find out where they stop being so.

In the 1990s, the cutting edge in their experiments was beams of large molecules a whole nanometre across, plenty big enough to see in an electron microscope. Arndt and his colleagues subsequently went larger, reporting interference for carbon-based molecules each containing 430 atoms. These were 6 nanometres across, the size of small proteins. They have now reached the scale of 2000-atom molecules, which, says Arndt, “still behave perfectly quantum-mechanically”. Other researchers are preparing …