Karela Fry

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Engineering quantum mechanics

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A major step forward in technological use of quantum information was reported in an article in Science by Lee, Sprague, Walmsley, and friends. A good description of their work is given in a news report in Nature:

A pair of diamond crystals has been linked by quantum entanglement. This means that a vibration in the crystals could not be meaningfully assigned to one or other of them: both crystals were simultaneously vibrating and not vibrating.

Quantum entanglement — interdependence of quantum states between particles not in physical contact — has been well established between quantum particles such as atoms at ultra-cold temperatures. But like most quantum effects, it doesn’t tend to survive either at room temperature or in objects large enough to see with the naked eye.

Entanglement occurs when two quantum particles 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.

Until a measurement is made of 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. The act of measuring ‘collapses’ this superposition into just one of the possible states.

But if the particles are entangled, then this measurement also determines the state of the other particle — even if they have become separated by a vast distance. The effect of the measurement is transmitted instantaneously to the other particle, through what Albert Einstein sceptically called ‘spooky action at a distance’.

Weird as it is, quantum entanglement is real — and could be useful. In a technique called quantum cryptography, entangled photons of light have been used to transmit information in such a way that any interception is detectable.

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.

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

Walmsley is … 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.”

How is the entanglement actually created? Start with a laser and create two photons (bundles of light) which are in the spooky state. Use one to excite a phonon (bundle of a sound wave) in one crystal, the other in another crystal. The entanglement of the photons is then transferred to the phonons. Since the phonons involve coherent motions of quintillions of atoms together, so they are hard to disrupt.

An editorial in Science puts this achievement in perspective:

The results confirm that quantum phenomena may persist in ambient conditions in a laboratory-scale system and point toward a possible platform for ultrafast quantum information processing at room temperature, based on optical phonons.

A hundred year old theory may be about to spawn technology for your desktop.

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