Research Challenges Our Understanding of Quantum Theory

A team of researchers at the University of East Anglia (UEA), UK, has discovered a new mechanism involved in the generation of paired photons.

In this illustration, one photon (purple) carries a million times the energy of another (yellow). Image credit: NASA / Sonoma State University / Aurore Simonnet.

The team, led by UEA Professor David Andrews, has shown that when photons are created in pairs, they can emerge from different, rather than the same, location.

The findings could have significant implications for quantum physics, the theoretical basis of modern physics.

Until now, the general assumption was that such photon pairs necessarily originate from single points in space.

Quantum entanglement — when particles are linked so closely that what affects one directly affects the other — is widely used in labs in numerous processes from quantum cryptography to quantum teleportation.

Prof. Andrews and his colleagues were studying a process called spontaneous parametric down-conversion (SPDC), in which photon beams are passed through a crystal to generate entangled pairs of photons.

“At a fundamental level, this process entails the conversion of pump photons into conascent, phase-matched pairs, executed by material interactions that entail the second-order nonlinear optical susceptibility: a third-order electric dipole response,” they explained.

“Each generated pair of photons has a combined energy and momentum equal to that of the corresponding annihilated photon, and they also exhibit correlated polarization.”

“When the emergent photons equally share the energy of the input, the process is known as degenerate down-conversion (DDC).”

“Until now, it has been assumed that such paired photons come from the same location,” Prof. Andrews said.

“Now, the identification of a new delocalized mechanism shows that each photon pair can be emitted from spatially separated points, introducing a new positional uncertainty of a fundamental quantum origin.”

The entanglement of the quantum states in each pair has important applications in quantum computing — theoretical computation systems that could potentially process big data problems at incredible speeds — as well as other areas of quantum physics.

The team’s results are also significant because they place limits on spatial resolution.

“Everything has a certain quantum ‘fuzziness’ to it, and photons are not the hard little bullets of light that are popularly imagined,” Prof. Andrews said.

The research was published online in the journal Physical Review Letters.

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Kayn A. Forbes et al. Nonlocalized generation of correlated photon pairs in degenerate down-conversion. Physical Review Letters, in press;

This article is based on text provided by the University of East Anglia.