Particles of light that effectively exist in 37 dimensions at once have been used to test an extreme version of a quantum paradox.
鈥淭his experiment shows that quantum physics is more nonclassical than many of us thought. It could be [that] 100 years after its discovery, we are still only seeing the tip of the iceberg,鈥� says at the Technical University of Denmark.
He and his colleagues focused on the Greenberger-Horne-Zeilinger (GHZ) paradox, which shows quantum particles can stay connected across large distances for over 30 years. In the simplest version of the paradox, three particles are connected through quantum entanglement, a special link that allows observers to learn something about one particle by interacting with the other two.
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If researchers assume the particles can influence each other only when they are in close proximity 鈥� in other words, when so-called spooky action at a distance is forbidden 鈥� then equations and experiments get snarled in mathematical impossibilities. In fact, the paradox can be expressed through a calculation that results in 1 and -1 being equal, which cannot be true. In the 1990s, physicists realised that the only way to avoid such impossibilities was to accept that particles can participate in quantum spookiness.
Liu and his colleagues wanted to construct the most extreme version of this paradox yet. Specifically, they wanted to find states of photons, or particles of light, whose behaviour in a GHZ experiment would be the most different from that of purely classical particles.
Their calculations revealed that photons had to be in quantum states as intricate as if they existed in 37 dimensions. Just as your position right now must be determined in reference to three spatial and one temporal dimension of our world, each photon鈥檚 state had to use 37 such references.
The researchers then tested this idea by translating a multidimensional version of the GHZ paradox into a series of pulses of very coherent light 鈥� light that is extremely even in its colour and wavelength 鈥� which they could then manipulate.
鈥淭he state encoded by the light and the measurement on it is governed by the same math underneath the quantum physics. Our experiment can thus produce some of the most nonclassical effects in the quantum world,鈥� says Liu. This type of 鈥渜uantum simulation鈥� is extremely technically challenging and requires very stable and precisely calibrated devices, he says.
鈥淭his is a result 鈥榝or eternity鈥�, in the sense that it can be relevant in [a] hundred years,鈥� says at the University of Siegen in Germany. He says that, beyond probing the limits of quantumness, the new work could also have implications for how quantum states of light and atoms are used for information processing, like in quantum computing.
Liu says this is what he wants to study next as well: how to make computations faster by encoding information into quantum states similar to those his team has already studied.
Science Advances
