Diamond vibrations neither here nor there

Here’s the pre-edited version of my latest news story for Nature online.
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Two objects big enough for the eye to see have been placed in a weirdly connected quantum state.

A pair of diamond crystals has been spookily linked by quantum entanglement by researchers working in Oxford, Canada and Singapore.

This means that vibrations detected in the crystals could not be meaningfully assigned to one or other of them: both crystals were simultaneously vibrating and not vibrating.

Quantum entanglement is well established between quantum particles such as atoms at ultra-cold temperatures. But like most quantum effects, it doesn’t usually tend to survive either at room temperature or in objects large enough to see with the naked eye.

The team, led by Ian Walmsley of Oxford University, found a way to overcome both those limitations – demonstrating that the weird consequences of quantum theory don’t just apply at very small scales.

The result is “clever and convincing” according to Andrew Cleland, a specialist in the quantum behaviour of nanometre-scale objects at the University of California at Santa Barbara.

Entanglement was first mooted by Albert Einstein and two of his coworkers in 1935, ironically as an illustration of why quantum theory could not tell the whole story about the microscopic world.
Einstein considered two quantum particles that interact with each other so that their quantum states become interdependent. If the first particle is in state A, say, then the other must be in state B, and vice versa. The particles are then said to be entangled.

Until a measurement is made on one of the particles, its state is undetermined: it can be regarded as being in both states A and B simultaneously, known as a superposition. But a measurement ‘collapses’ this superposition into just one state or the other.

The trouble is, Einstein said, that if the particles are entangled then this measurement determines which state the other particle is in too – even if they have become separated by a vast distance. The effect of the measurement is transmitted instantaneously to the other particle, via what Einstein called ‘spooky action at a distance’. That can’t be right, he argued.

But it is, as countless experiments have since shown. Quantum entanglement is not only real but could be useful. Entangled photons of light have been used to transmit information in a way that cannot be intercepted and read without that being detectable – a technique called quantum cryptography.

And entangled quantum states of atoms or light can be used in quantum computing, where the superposition states allow much more information to be encoded in them than in conventional two-state bits.

But superpositions and entanglement are usually seen as delicate states, easily disrupted by random atomic jostling in a warm environment. This scrambling also tends to happen very quickly if the quantum states contain many interacting particles – in other words, for larger objects.

Walmsley and colleagues got round this by entangling synchronized atomic vibrations called phonons in diamond. Phonons – wavelike motions of many atoms, rather like sound waves in air – occur in all solids. But in diamond, the stiffness of the atomic lattice means that the phonons have very high frequencies and energy, and are therefore not usually active even at room temperature.

The researchers used a laser pulse to stimulate phonon vibrations in two crystals 3 mm across and 15 cm apart. They say that each phonon involves the coherent vibration of about 10**16 atoms, corresponding to a region of the crystal about 0.05 mm wide and 0.25 mm long – large enough to see with the naked eye.

There are three crucial conditions for getting entangled phonons in the two diamonds. First, a phonon must be excited with just one photon from the laser’s stream of photons. Second, this photon must be sent through a ‘beam splitter’ which directs it into one crystal or the other. If the path isn’t detected, then the photon can be considered to go both ways at once: to be in a superposition of trajectories. The resulting phonon is then in an entangled superposition too.

“If we can’t tell from which diamond the photon came, then we can’t determine in which diamond the phonon resides”, Walmsley explains. “Hence the phonon is ‘shared’ between the two diamonds.”

The third condition is that the photon must not only excite a phonon – also, part of its energy must be converted into a lower-energy photon, called a Stokes photon, that signals the presence of the phonon.

“When we detect the Stokes photon we know we have created a phonon, but we can’t know even in principle in which diamond it now resides”, says Walmsley. “This is the entangled state, for which neither the statement ‘this diamond is vibrating’ nor ‘this diamond is not vibrating’ is true.”

To verify that it’s been made, the researchers fire a second laser pulse into the two crystals to ‘read out’ the phonon, from which it draws extra energy. All the necessary conditions are satisfied only very rarely during the experiment. “They have to perform an astronomical number of attempts to get a very finite number of desired outcome”, says Cleland.

He doubts that there will be any immediate applications, partly because the entanglement is so short-lived. “I am not sure where this particular work will go from here”, he says. “I can’t think of a particular use for entanglement that lasts for only a few picoseconds [10**-12 s].”

But Walmsley is more optimistic. “Diamond could form the basis of a powerful technology for practical quantum information processing”, he says. “The optical properties of diamond make it ideal for producing tiny optical circuits on chips.”

1. K. C. Lee et al., Science 334, 1253-1256 (2011).