OAM entangles four photons, not just two

Physicists at the Leiden Institute of Physics have entangled four photons in their orbital angular momentum (OAM) for the first time.

Leiden physicists sent short ultraviolet laser pulses of 2 ps through a crystal. This leads to the creation of four photons that are entangled in their orbital angular momentum (OAM)--depicted here as red blue spirals. The rainbow-colored circles illustrate the phase (color) and intensity (brightness) of the photon's cross section. (Image credit: Leiden Institute of Physics)
Leiden physicists sent short ultraviolet laser pulses of 2 ps through a crystal. This leads to the creation of four photons that are entangled in their orbital angular momentum (OAM)--depicted here as red blue spirals. The rainbow-colored circles illustrate the phase (color) and intensity (brightness) of the photon's cross section. (Image credit: Leiden Institute of Physics)

IMAGE:Leiden physicists sent short ultraviolet laser pulses of 2 ps through a crystal. This leads to the creation of four photons that are entangled in their orbital angular momentum (OAM)--depicted here as red blue spirals. The rainbow-colored circles illustrate the phase (color) and intensity (brightness) of the photon’s cross section. (Image credit: Leiden Institute of Physics)

An article in Physical Review Letters from physicists at the Leiden Institute of Physics (Leiden, Netherlands) explains how, for the first time, four photons have been entangled in their orbital angular momentum (OAM). Leiden physicists sent a laser through a crystal, thereby creating four photons with coupled 'rotation'. So far this has only been done for two photons. The researchers say the discovery makes uncrackable secret communication of complex information possible between multiple parties.

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Entanglement holds a great promise, with applications in perfectly secret communication and quantum computing. If two photons are created simultaneously, they are each other's counterpart, so that their rotation is always reversed with respect to the other. If we measure left rotation for one photon, then the other will always rotate to the right after measurement with a similar filter. This is called entanglement. Before the measurement, each photon's rotation is undetermined.

This rotation is a property of photons that scientists discovered in 1992 in Leiden; physicists call this OAM. And this property has more than two values. It covers an infinitely large alphabet of information. So with this you can transfer much more information per photon than with a property like polarization, which contains only two possible values.

In 2001, scientists managed to entangle two photons in OAM for the first time. Now, Leiden physicist Wolfgang Löffler and his colleagues are the first ones to entangle four photons in this way.

During their successful experiment, the researchers sent short ultraviolet laser pulses of 2 ps through a crystal. Occasionally this leads to the creation of four entangled photons. This is extremely rare, but by generating 80 million pulses per second they managed to detect on average two so-called photon quadruplets each second. To confirm these were indeed entangled in OAM, the team used a spatial phase modulator that converts this ‘rotation’ back to light traveling as a plane wave. They registered this ‘normal’ light with single-photon detectors.

SOURCE: Leiden Institute of Physics; http://www.physics.leidenuniv.nl/index.php?id=11573&news=824&type=LION&ln=EN

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