Biphoton de Broglie Wavelength Measured

In 1995, Joseph Jacobson and colleagues at Stanford University in California suggested that the de Broglie wavelength of an ensemble of N photons of wavelength λ is λ/N, and they described a setup to verify this proposition by causing a wave train to interfere with itself. Now physicists at Osaka University in Toyonaka, Japan, have accomplished this task with biphotons. Their findings, reported in the Nov. 18 issue of Physical Review Letters, may have implications for such applications as quantum lithography.

The Osaka researchers produced entangled pairs of photons by spontaneous parametric down conversion of 860-nm radiation from a Ti:sapphire laser in a KNbO₃ crystal and injected them into a Mach-Zehnder interferometer composed of two beamsplitters. Biphotons were created by Hong-Ou-Mandel interference when two photons simultaneously entered the input ports of one of the beamsplitters.

By blocking one of the input ports, the scientists were able to compare the patterns of interference fringes created by a biphoton and by a single photon. The former displayed a fringe period of approximately 430 nm and the latter approximately 860 nm, in accordance with the predictions of the Stanford group.