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The quantum world keeps getting bigger. An experiment on crystals made from 10 billion atoms proves that even relatively large objects follow the weird rules of quantum mechanics.

One such rule is entanglement: the properties of two particles can be linked together so that measuring the state of one determines the measured state of the other in what Einstein called “spooky action at a distance”.

A standard test for entanglement is Bell’s inequality, which sets a limit on how often two particles can end up in the same state by chance, with no quantum weirdness involved. If a pair of particles violates Bell’s inequality, it shows that quantum mechanics really is at work.

Most of the time, we test this using single particles, often photons of light. Now, Simon Gröblacher at the Delft University of Technology in the Netherlands and his colleagues have raised the stakes with a Bell test on crystals made of about 10 billion atoms, bringing quantum weirdness one step closer to the human scale.

Cold crystals

The experiment starts with two silicon crystals, each far smaller than a grain of sand, cooled to near absolute zero to dampen extra vibrations. A laser pulse is split in two and directed towards the crystals. Most photons from the laser pass through them, but about one in every 100 bounces off.

This causes those photons to lose some of their energy and the crystals to vibrate, entangling quantum states of the photons and crystals.

The researchers repeated this experiment about 1 billion times, taking different combinations of measurements of the vibrations and photons.

The photon energies and crystal vibrations matched far more often than would be expected if their behaviour wasn’t ruled by quantum mechanics, violating Bell’s inequality and showing that the system was truly quantum.

“It’s astounding that they’ve been able to do this with an object with 10 billion atoms instead of a single atom,” says David Kaiser at the Massachusetts Institute of Technology. “It’s a breathtaking result.”

Quantum writ large

That leap in size is important because we aren’t sure how big an object can get before quantum mechanics no longer applies. We know that anything large enough for us to interact with in everyday life doesn’t need quantum mechanics and that individual particles do – but in between is a grey area.

“The ultimate goal is to really test how we go from quantum to classical physics, whether there is some fundamental reason why big things don’t behave quantum mechanically,” says Gröblacher.

Building larger objects that behave according to quantum mechanics may also have practical uses. It could help with technologies such as quantum computers, sensors and communication.

“As we get closer and closer to a world where many of us depend on devices that exploit quantum entanglement, they’ve shown that entanglement can be robust even in devices that can actually be manufactured,” says Kaiser.

This article appeared in print under the headline “Crystal test beats quantum record”