A Bi-Photon de Broglie Wavelength

A bi-photon de Broglie wavelength has been directly measured in an interference experiment for the first time. In the early days of quantum mechanics, Louis de Broglie argued that if waves could act like particles (photoelectric effect) then why couldn't particles act like waves?

They could, as was borne out in numerous experiments (the double-slit experiment for electrons was voted the "most beautiful" experiment in a recent poll—see Physics World, Sept 2002).

In fact, intact atoms in motion and even molecules can be thought of as "de Broglie waves." Molecules as large as buckyballs (carbon-60) have been sent through an interferometer, creating a characteristic interference pattern (see Update 579).

The measured wavelength for a composite object like C-60 will in part depend on the internal bonds of the molecule. What then if the corporate object is a pair of entangled photons?

One of the more fascinating predictions made regarding quantum entanglement (Jacobson et al., Physical Review Letters, 12 Jun 1995) was the suggestion that the de Broglie wavelength for an ensemble consisting of N entangled photons (each with a wavelength of L) would be L/N.

This proposition has been verified now by physicists at Osaka University (Keiichi Edamatsu, 81-6-6850-6507, eda@mp.es.osaka-u.ac.jp) for the case of two entangled photons. The daughter photons were created by the process of parametric down-conversion, in which an incident photon entering a special crystal will split into two correlated photons. These photons are then sent through an interferometer (see figure).

The resultant interference pattern shows that the photons behave as if they acted as a single entity with a wavelength half that for either photon alone, a feature which might improve the sharpness of future quantum lithography (the narrowness of lines on a circuit board being no better than the wavelength of light used in the fabrication process).

But since the parent photon already had this shorter wavelength, what will have been gained by splitting the photon in half? The advantage will come when, at some point in the future it will be possible to generate entangled photons from non-entangled photons of the same wavelength, a process called hyper-parametric scattering. (Edamatsu et al., Physical Review Letters, 18 November 2002.)