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AN APPLE never appears to be in many places at one. That statement hardly seems surprising 鈥� until you start burrowing into the depths of quantum weirdness, and realise there鈥檚 no fundamental reason why that shouldn鈥檛 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. 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 鈥� 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 , which, says Arndt, 鈥渟till behave perfectly quantum-mechanically鈥�. Other researchers are preparing to put nanoparticles with millions of atoms into quantum superpositions.

At this point, the obstacles to Big Quantum seem to be merely technical. Oriol Romero-Isart at the University of Innsbruck in Austria has proposed that it should be possible, with sufficient control over decoherence, to put a biological particle like a virus or a bacterium into a superposition state 鈥� or even to do so with a microscopic creature like a tardigrade. 鈥淚 don鈥檛 think there is any roadblock to doing these experiments with microorganisms, provided they can withstand a high vacuum,鈥� he says.

As we place larger and larger things in quantum states, however, there is a chance we could discover something new about the process by which quantum becomes classical. Some researchers suspect there might be more to it than decoherence alone. Notably, Roger Penrose at the University of Oxford reckons that gravity, which is a negligible force for atoms but ever more significant as objects get larger, could trigger a switch to classical behaviour, perhaps via an as-yet unobserved physical process that collapses the quantum wave function. If so, efforts to put even nanoparticles into superpositions should fail.

In any case, Romero-Isart says we shouldn鈥檛 take it for granted that quantum mechanics will still hold at large scales. 鈥淭here are extremely exciting questions about the interplay of quantum mechanics and gravity that could perhaps be addressed in the future,鈥� he says.