Quantum light has shown that the 鈥渟pookiness鈥� that can inextricably link two distant particles can be equivalent to an equally odd property of just one. This may be useful for developing quantum technologies and help researchers understand why quantum particles can become entangled to begin with.
鈥淭he quantum world operates in ways that are fundamentally different from the familiar, deterministic laws of classical physics,鈥� says at Xiamen University in China. He and his colleagues studied two prime examples of strange quantum behaviour, known as contextuality and nonlocality.
Contextuality means that the result of a measurement of a quantum object depends on which other measurements are being performed on it at the same time. Think of soup 鈥� what a bean in a broth tastes like to you depends on what other ingredients you are tasting alongside it. Nonlocality, on the other hand, means that measuring the properties of one quantum object, like a particle, can immediately reveal something about another one, even when it is very distant. This entanglement is sometimes called 鈥渟pooky action at a distance鈥�, and researchers disagree on how exactly it happens.
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Strikingly, Sheng and his colleagues have now shown that one of these properties can be converted into another.
They used entangled pairs of particles of light, or photons. The researchers put each photon in a quantum state that had intricate rotational properties: had they been part of a beam of light, that beam would have been twisted in the shape of a helix.
The more twisty they made the photons, the more intricate their quantum states got. In fact, they could create photons whose quantum states had up to six dimensions, behaving as if the photon existed in a six-dimensional quantum world. You could say that the soup had many layers of flavour.
Because the photons were entangled, the team knew they would experience nonlocality. For instance, making one of them travel through a lens or hit a detector, which changed its properties, always meant that they immediately knew the corresponding new properties of the other photon without having to measure it. To connect this with contextuality, they used a mathematical recipe for converting between nonlocality and contextuality that was previously developed by at the University of Seville in Spain.
Cabello created a formula that the researchers could plug their measurements into to check whether the conversion worked. Sheng and his colleagues found that it did. Entanglement can also be multidimensional, so Sheng says that being able to put photons into multidimensional states lets them share many correlations with their partner, which was key in making this test unambiguous.
鈥淲hen I started [studying] all this stuff many years ago, these experiments were dreams. And the experiment really matches the quantum prediction. It is an extremely beautiful confirmation that nature is really following quantum mechanics to an incredible degree of precision,鈥� says Cabello.
He says the connection between contextuality and nonlocality has not been studied as much as phenomena like quantum entanglement, but may be the key for understanding how either works at all. What mechanism enables two entangled photons to keep their properties correlated even when they are pulled far apart? This long-standing question about the fundamentals of quantum physics is exactly what experiments like these may shed light on, says Cabello.
The new work may also have practical implications. Being able to convert technologically difficult contextuality experiments with a single quantum particle into more common experiments with pairs of particles makes contextuality a more readily available resource for technologies like ultra-secure quantum communication, efficient quantum cryptography and some types of quantum computing, says Sheng.
Physical Review Letters
