Of the quantum entanglement phenomena that Einstein described as "spooky action at a distance," quantum holography may be the spookiest to date.
Researchers at Boston University's Quantum Imaging Laboratory propose to create holographic images of objects concealed in a spherical chamber.
Ideally, a small opening in the chamber wall permits light to enter, but lets no light out. The photons in a beam of light directed through the hole scatter from the enclosed object, and ultimately strike the inner wall of the chamber.
According to the scheme, the inside of the chamber would be designed to detect the time when a photon hits the wall but not where it hits. Classically, there is no way to generate an image of an object with this sort of configuration.
Quantum mechanically, however, it's possible to build a hologram of the hidden object provided that the photons in the illuminating beam are entangled with photons in another beam.
Each photon in an entangled pair has properties (such as momentum or polarization) that are unknown until a measurement is performed on one photon or the other. When a property of one of the photons is measured, corresponding information about its entangled mate is instantly determined.
That may seem spooky enough, but in quantum holography, things get spookier still. Holograms are typically constructed with interfering beams of light, which provides more information about a subject than simple illumination can. The additional information helps build a three dimensional image of a three dimensional object.
In quantum holography, the researchers measure the simultaneous arrivals of an illuminating photon that is sent into the chamber and a companion photon in the other entangled beam. This measurement tells the researchers about the interference of various possible paths that the single photon inside the chamber could travel.
It's the interference of the possible paths that encodes the holographic image of the hidden object, which is very spooky indeed.
For the moment, quantum holography exists only on paper. But the BU researchers assert that there are no technological obstacles to the proposal, and they hope to begin building an experimental system soon.
(Reference: Ayman F. Abouraddy, Bahaa E. A. Saleh, Alexander V. Sergienko, and Malvin C. Teich, Optics Express, 5 November 2001. Text and figures at this URL.)
(Editor's Note: This story is based on an edited version of an item in PHYSICS NEWS UPDATE, the American Institute of Physics Bulletin of Physics News, Number 566, November 21, 2001, by Phillip F. Schewe, Ben Stein and James Riordon.)
[Contact: Bahaa Saleh]