TRY to imagine a tiny ball sitting on one fingertip yet also on your shoulder at the same instant. Are you struggling? Most of us can’t conceive of an object being in two places at once – yet physicists have just demonstrated the effect over a distance of half a metre, smashing previous records.
It’s an example of superposition, the idea that an object can exist in two quantum states at the same time. This persists until it is observed, causing a property called its wave function to collapse into one state or the other. The same principle allows Schrödinger’s cat to be both dead and alive inside a box until you open the lid.
We often think of quantum mechanics as applying only to subatomic particles, but there is nothing in the theory that limits its range. That’s why experiments try to probe the transition between the quantum and everyday realms. “We’re all wondering whether there is some regime where superpositions turn into classical states of matter,” says Mark Kasevich of Stanford University in California.
“We’re wondering if there is some regime where superpositions turn into ordinary states of matter”
To find out, Kasevich and his colleagues created a Bose-Einstein condensate (BEC) – a cloud of 10,000 rubidium atoms, all in the same quantum state. They shot this cloud, just a few millimetres across, up a 10-metre-high chamber using lasers, which also gradually push the atoms into two separate states.
By the time the BEC reaches the top of the chamber, its wave function is a 50-50 mixture of those states, representing positions 54 centimetres apart. It stays in this superposition for about a second, then falls back down. At the bottom, the lasers turn the two states back into one, and this reveals that the atoms appear to arrive from two different heights, confirming that the BEC was indeed in a superposition at the top of the chamber (Nature, DOI: 10.1038/nature16155).
Previous experiments demonstrated superposition over much smaller distances and times – the old record was just under a centimetre for a quarter of a second. Kasevich’s work pushes it into human scales. “Our work really is definitive for large separations,” he says. “Nobody else has done that.”
Pulling it off involved cooling the chamber to a fraction above absolute zero, and minutely adjusting the BEC’s trajectory to account for Earth’s rotation during the experiment. “They’ve explored the unknown,” says Klaus Hornberger of the University of Duisburg-Essen, Germany.
But it may be less of a breakthrough than it seems. In 2013, Hornberger helped devise a “weirdness scale” that scores experiments according to how far they show quantum effects extending into the everyday world. Kasevich’s work extends the distance scale but compromises in other ways, so scores about the same as previous attempts, says Hornberger. That means a true Schrödinger’s cat is still far from being realised.