Entangled diamonds blur quantum-classical divide

Entangled diamonds blur quantum-classical divide

THIS WEEK Lisa Grossman PATRICK LLEWELYN-DAVIES/SCIENCE PHOTO LIBRARY ‘Big’ diamonds do quantum dance absolute zero was the only way to keep its at...

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Lisa Grossman


‘Big’ diamonds do quantum dance absolute zero was the only way to keep its atoms from jostling each other and destroying entanglement’s delicate links, or coherence. “We said, let’s not bother doing that,” says Ian Walmsley, head of the lab where the diamonds were entangled. “It turns out all you need to do is look on a very short timescale, before all that mugging around has a chance to destroy the coherence.” The team used a beam-splitter to zap two diamonds with a laser

QUANTUM physics is the realm of the tiny, right? Increasingly, that’s not the case. Now two diamonds as wide as earring studs have been made to share the spooky quantum state of entanglement. Entanglement links the fates of two or more particles – even when they are far apart. For example, electrons have been entangled so that changing the quantum spin of one immediately affects the spins of its entangled partners. “Ever larger physical Macroscopic objects, on the systems behave according other hand, are supposed to to the strange predictions mind their own business – of quantum mechanics” flipping one coin shouldn’t force another flipped coin to come up heads, for example. But something that emitted pulses of light lasting akin to that happened with two 100 femtoseconds (10-13 seconds). Every so often, according to 3-millimetre-wide diamonds in classical physics, one of the laser’s a lab at the University of Oxford. pulses should set the atoms in one Physicists there led by Ka Chung diamond vibrating. That saps Lee and Michael Sprague were some energy from the photon, able to put the diamonds into which then moves on with less a shared vibrational state at energy to a detector. Each room temperature. That temperature is impressive diamond would be left either vibrating or not vibrating. because it was thought that But if the diamonds behaved as cooling an object down to quantum objects, they would be fractions of a degree above

–Entanglement’s new best friend–

placed in a superposition – both of them vibrating and not vibrating at the same time. To show that the diamonds were entangled, the researchers quickly hit them with a second laser pulse. That pulse picked up the energy left behind by the first, and reached the detector as an extra-energetic photon. If the system were classical, the second photon should pick up extra energy only half the time –

Super-sizing quantum effects The boundary between the quantum world and the “classical” everyday one has been weakening for years. Photons of light behave not only like particles but also like waves, producing interference patterns when sent through gratings. In 1999, Anton Zeilinger at the University of Vienna in Austria and colleagues demonstrated that buckyballs – molecules of 60 carbon atoms – do the same thing, acting like waves when they pass through gratings. And in 2003 the same team performed the feat with

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tetraphenylporphyrin, a large molecule related to chlorophyll, which set the record for the heaviest object to show wave-particle duality. Quantum effects have also nudged into the realm of objects visible to the naked eye (see main story). In 2010, a group led by Andrew Cleland at the University of California, Santa Barbara, made a 0.06-millimetre-long supercooled metal strip simultaneously vibrate and not vibrate, putting it into a quantum superposition of states. Physicists have also suggested

ways to show quantum effects in living things, beginning with a scheme to put a virus in a superposition of states. Is there any limit to how large an object can be and still show quantum effects? The more atoms an object has, the more likely those atoms are to interact with each other and their environment, destroying fragile quantum effects. “That is a big challenge,” Zeilinger says. “But there is nothing in quantum mechanics which says there is a limit.”

only if it happened to hit the diamond where the energy was first deposited. But in 200 trillion trials, the team found that the second photon picked up extra energy every time. That means the energy was not limited to one diamond or the other – both shared the same vibrational state (Science, DOI: 10.1126/ science.1211914). “We think it is the first time that a room-temperature, solidstate system has been put in this entangled state,” Walmsley says. “This is an interesting avenue for thinking about how quantum mechanics can emerge into the classical world” (see “Super-sizing quantum effects”, left). Erika Andersson of HeriotWatt University in Edinburgh, UK, agrees. “We want to push and see how far quantum mechanics goes,” she says. “The reported work is a major step in trying to push quantum mechanics to its limits, showing that larger and larger physical systems can behave according to the strange predictions of quantum mechanics.” n