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 pollsee Physics World, Sept
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
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
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, email@example.com) 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.)