Hyper-entangled Photons


News of another advance in the search for practical applications of entanglement:

“The record for the most amount of information sent by a single photon has been broken by researchers at the University of Illinois. Using the direction of ‘wiggling’ and ‘twisting’ of a pair of hyper-entangled photons, they have beaten a fundamental limit on the channel capacity for dense coding with linear optics.

‘Dense coding is arguably the protocol that launched the field of quantum communication,’ said Paul Kwiat, a John Bardeen Professor of Physics and Electrical and Computer Engineering. ‘Today, however, more than a decade after its initial experimental realization, channel capacity has remained fundamentally limited as conceived for photons using conventional linear elements.’

In classical coding, a single photon will convey only one of two messages, or one bit of information. In dense coding, a single photon can convey one of four messages, or two bits of information.

‘Dense coding is possible because the properties of photons can be linked to one another through a peculiar process called quantum entanglement,’ Kwiat said. ‘This bizarre coupling can link two photons, even if they are located on opposite sides of the galaxy.’

[…]Through the process of spontaneous parametric down conversion in a pair of nonlinear crystals, the researchers first produce pairs of photons simultaneously entangled in polarization, or ‘wiggling’ direction, and in orbital angular momentum, or ‘twisting’ direction. They then encode a message in the polarization state by applying birefringent phase shifts with a pair of liquid crystals.

‘While hyper-entanglement in spin and orbital angular momentum enables the transmission of two bits with a single photon,’ Barreiro said, ‘atmospheric turbulence can cause some of the quantum states to easily decohere, thus limiting their likely communication application to satellite-to-satellite transmissions.'”

In other words, researchers have demonstrated the possibility of encoding much more information in a single “bit” of information than would be possible without entanglement. The upshot is that for some conditions, quantum computers will be even more efficient than first thought.

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