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’Loopy’ Photons Test Hidden-Variable Predictions

Credit: Alan Migdall, JQI

Credit: Alan Migdall, JQI

JQI researchers have devised a new method for creating pairs of entangled photons to test key postulates of quantum mechanics.

JQI Fellow Alan Migdall and colleagues at NIST send a pulse of light [1] into both ends of a twisted loop [2] of optical fiber. Light traveling in each direction in the fiber occasionally undergoes a process known as “four-wave mixing,” in which two photons of the same color transform into two new photons: one redder and the other bluer than the originals. The researchers then monitor these transformed photons.

The setup makes it impossible to determine which path-clockwise or counterclockwise—the newly created photon pairs traveled, even though the fiber twist means that pairs emitted from one end are vertically polarized while pairs emitted from the other end are horizontally polarized. Since these paths are indistinguishable, quantum physics says they are actually in both states horizontal and vertical polarization at the same time. But a photon must assume a single,definite polarization state when it’s measured. If the researchers measure one photon to have horizontal polarization, that immediately forces the other photon to be horizontal, since they traveled the same path.

Entangled photons contradict Einstein’s preferred worldview of “local realism.” He thought that beneath the “spooky action at a distance” behavior exhibited by quantum objects, they possessed still-to-bediscovered properties known as “hidden variables” that would reveal them to be more like classical objects.

Decades of tests have shown that entangled particles do not obey local realism. But could Einstein still be partially right? Non-local hidden variables (NLHV) theories would allow for the possibility of hidden variables but would concede nonlocality. Several variations of such theories exist, each with slightly different scenarios of how the particles would behave.

The NIST researchers used their entangled photon pairs to check different NLHV theories. Each makes predictions about the polarization measurements of the photons. In this way, they were able to rule out some NLHV theories, some of which said that some polarizations were predetermined. Their results agree with other groups that are testing entangled photons.

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