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Breakthrough in uncrackable quantum encryption

An important discovery makes it possible to communicate complex information among multiple people without the message being cracked. The communication can also relate to complex information.

Coupled rotation

For the first time scientists have managed to entangle four photons in their orbital angular momentum. Leiden physicist Wolfgang Löffler and his colleagues  have sent a laser through a crystal and created four photons with a coupled 'twist'. This had previously only been possible for two photons. Publication on 1 February in  Physical Review Letters. 

Very promising

Our current computers communicate with light signals (photons) via glass fibres. A light signal is a kind of morse code: on-off, on-off, corresponding with the value 0 or 1 of one bit.  Entanglement is one of the great promises of quantum mechanics, with such applications as uncrackable secure communication and quantum computing. If two photons are created simultaneously, they are one another’s mirror image, so they will also have opposite twist: the other one will be right-twisted or vice versa. However, the twist is undetermined before the measurement. This is a key feature of quantum entanglement. 

Milestone

This 'twist' of photons is a characteristic that was discovered in Leiden in 1992; the specialist term for it is orbital angular momentum. Compared to the polarisation of light, which has two possible values, orbital angular momentumcan encode much more information as it can take an intinite number of different values. In 2001 it became possible to entangle two photons in their orbital angular momentun. Leiden physicist Wolfgang Löffler and his colleagues now report in Physical Review Letters that they are the first researchers to successfully entangle four photons using this method. This offers many more possibilities, including transmitting an undecipherable message to multiple recipients.  

Experiment

During their successful experiment they focused very short laser pulses of two picoseconds into a crystal, where a process probabilistically generates four entangled photons. This is a very rare occurrence, but by generating 80 million pulses a second, the researchers were able to detect on average two so-called photon quadruplets every  second. To prove four photon orbital angular momentum entanglement, the research team used a spatial phase modulator that converts the twistback into plane-wave light; this ‘normal’ light iscan beregistered using single-photon detectors.

Header image: Leiden physicists transmitted short ultraviolet laser pulses of two picoseconds through a crystal. Four photons were spontaneously created that were entangled in their orbital angular momentum, shown here by the red-blue spirals. The rainbow-coloured circles represent the phase (colour) and intensity (clarify) of a cross-section of the photon.