A KITTEN playing with string is adorable, but when Schr枚dinger鈥檚 cat becomes entangled, things get weird. That鈥檚 the upshot of two experiments that add a new tier of quantum oddity to a famous thought experiment.
The feat has entranced quantum geeks. 鈥淭here aren鈥檛 so many experimentally accessible demonstrations of quantum weirdness,鈥� says Seth Lloyd of the Massachusetts Institute of Technology, who was not involved in the work. 鈥淭his is one of the coolest.鈥�
Erwin Schr枚dinger imagined a cat in a box that is both dead and alive until we open the box, an act of measurement that forces it to be in one state or the other. A similar situation occurs in the 鈥渄elayed choice鈥� test involving a device that splits and then recombines beams of light, producing an interference pattern. The pattern is visible if the light is sent one photon at a time, because single photons can behave like waves too. If the recombining capability is disabled, however, then there is no interference and the light emerges as particles. Bizarrely, if the recombination is turned off only after the beam has passed through the splitter, we still see no interference, even though the photon should have 鈥渄ecided鈥� to be wave-like when the beam was split.
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In one of the two new experiments, Alberto Peruzzo at the University of Bristol, UK, and colleagues added a twist to the delayed choice test by entangling the state of the recombiner with that of a photon outside the device. The recombiner is both operational and disabled simultaneously, and only becomes one or the other when the second photon is measured.
In this set-up, the recombiner supposedly determines whether light inside the device behaves as a wave or a particle. But Peruzzo鈥檚 team found that the photon within could procrastinate for even longer 鈥� only becoming wave or particle after it has passed through the recombiner. S茅bastien Tanzilli at the University of Nice Sophia-Antipolis in France and colleagues have shown the same thing using a different set-up (Science, and ).
It鈥檚 as if, long after the cat has supposedly been killed, or not, one can choose to determine if it is dead or alive, or determine if it is dead and alive, says Lloyd.
聯Long after the cat is meant to be dead, or not, one can choose to determine if it is dead, alive, or indeed both聰