Carrie Segal

Stony Brook University

Abstract Spring 2010

The Quantum Eraser: demonstrating the effect of path knowledge on optical interference

Carrie Segal, Thomas Mittiga, John Noe

The Quantum Eraser demonstration that we recreated illustrates fundamental quantum mechanical effects in a simple table-top optical experiment. Specifically, it shows how making a measurement alters the state of a quantum mechanical system, and how subsequently "erasing" the path knowledge gained by the measurement can restore the system to its original state. Because information about two possible optical paths is involved this type of experiment is sometimes called a "Welcher Weg" or "Which-way" experiment. Such experiments have been imagined, performed and debated since the earliest days of quantum mechanics in the 1920's.

One important quantum mechanical principle is that light behaves as both a wave and a particle. Experiments by Thomas Young in the early 1800's demonstrated diffraction and interference of light, wave phenomena. A century later, careful experiments by Robert Millikan on the photoelectric effect confirmed Einstein's suggestion in a famous 1905 paper that light consists of photons, discreet quantities of energy. When the two experiments are taken together light gains a dual aspect. It is a wave distributed in space and it is also a particle, traveling with a specific momentum along a single path. (The converse is also true, material particles display wave-like properties.)

Our experiment utilizes an interferometer similar to the well-known Mach-Zehnder type. Vertically-polarized light from a HeNe laser is split into two equal intensity beams which follow separate paths and are then recombined. A half-wave plate placed in one arm of the interferometer allows the polarization in that beam path to be shifted to horizontal orientation. The resulting orthogonal linear polarizations in the two interferometer arms then provide in principle a way to tell which path the light has followed. This in-principle path knowledge is sufficient to destroy the interference pattern that would otherwise be observed on a screen after the interferometer, and only a uniform spot of light is seen. In effect, when the fringes are seen the light is behaving like a wave and when they disappear the light is behaving like a particle that must chose between the two possible paths.

Remarkably, placing a linear polarizer oriented midway between horizontal and vertical after the interferometer "erases" the path knowledge and restores the same high contrast fringe pattern that is observed without path knowledge. At oblique angles other than + 45° the fringes are only partially restored and the pattern loses contrast. We are currently making measurements to verify the expected relationship D^2 +V^2 = 1 between path knowledge and fringe visibility. We are also working on formulating a satisfactory explanation for our chance observation that the fringe pattern inverts (dark fringes become bright ones and vice versa) when the "eraser" is moved between +45 and -45 degrees.

The effects we see can be explained without invoking quantum mechanics, using just the classical features of the polarization of light. In the future we hope to develop more advanced experiments of this type based on entangled photons, which do not have a classical counterpart.

Stony Brook Laser Teaching Center June-2011