Entanglement Studies Suggest Interferometry Application

Anne L. Fischer

Two groups of scientists have independently demonstrated means of entangling photons that may enable applications in gravity-wave detection and optical interferometry. Using different approaches to entanglement, the groups have proved that the phenomenon may be used to beat the diffraction limit and, thereby, to create interference patterns typically associated with much shorter wavelengths.

Researchers at Universität Wein and Österreichische Akademie der Wissenschaften, both in Vienna, Austria, used two entangled blue photons to generate four entangled infrared photons for their interferometer by type-II spontaneous parametric down-conversion in a BBO crystal. Andrea Aglibut of Universität Wein's Institut für Experimentalphysik said that dealing with four entangled photons becomes more difficult when the relative phase between the four-photon states must be actively controlled.

Research teams in Austria and Canada have independently demonstrated means of beating the diffraction limit in an interferometer by using entangled photons. Courtesy of Morgan W. Mitchell.

To demonstrate interference between their entangled states, they used a motor to control not only the movement of one of the mirrors in the interferometer, but also the stability of the interferometer. The challenge, Aglibut said, was that the paths had to be stable within a few nanometers over the course of the measurements, which could take several hours.

Using their setup, the researchers obtained interference fringes with a periodicity of half of the blue photon wavelength, or one-quarter of the infrared wavelength. Reducing the effective de Broglie wavelength of the four-photon state, they believe, opens new possibilities, especially in the area of quantum metrology.

In a separate experiment, scientists at the University of Toronto combined three photons of different polarizations into a single spatial mode, creating a "NooN" quantum state, in which all three photons have the same polarization, and providing subdiffraction-limit resolution. In this case, the challenge was to get a sufficiently bright source of photons, said research team member Morgan W. Mitchell.

They wanted the photons to be the same color, to have the same spatial mode and to reach the detector at the same time. The first two came from a spontaneous down-conversion process in BBO, but the third came from a weak pulse of less than a photon on average, which was split from the same mode-locked Ti:sapphire laser used to produce the other photons.

To get the photons to look and act the same, the scientists employed narrowband-wavelength filters and spatial filters with single-mode fibers.

Such a process for entangling three, four or more quantum states may facilitate such measurements as x-ray diffraction in crystallography and optical interferometry in gravitational-wave studies. But while the entanglement of three photons is significant in itself, Mitchell believes it will be some time before the research finds any technical applications.

The next step is to extend the approach to the entanglement of larger numbers of photons. Another avenue of research is the creation of different sorts of entangled photons. "We used polarization entanglement, but position- or momentum-entangled would be interesting," he said.