The photonic de Broglie wavelength of entangled photon pairs

The photonic de Broglie wavelength of entangled photon pairs has been directly measured. In the early days of quantum mechanics, Louis de Broglie argued, and it was soon demonstrated, that if waves could act like particles (as in the photoelectric effect), then particles could also act like waves. By now, the wave nature of molecules as large as buckyballs (carbon-60) has been demonstrated. For composite objects, the de Broglie wavelength depends fundamentally on the object’s internal structure. For an ensemble of photons taken collectively, the de Broglie wavelength is λ/N, the wavelength of an individual photon divided by the number of photons. This was verified in 1999 for a two-photon wavepacket in a double-slit experiment. Physicists at Osaka University in Japan have now demonstrated that the relation still holds for spatially separated, entangled photons. Through parametric down-conversion, they transformed a photon of wavelength λ into an entangled pair of photons (a biphoton) of wavelength 2λ. Those photons were then sent along different paths through an interferometer. When the single-photon interference for either member of the pair was measured, it showed a wavelength of 2λ. However, the measurement that preserved the entanglement yielded λ. The physicists also showed that the coherence length of the biphoton was much longer than for the 2λ single-photon case. They say that the concept remains valid for more than two entangled photons. Eventually, it may be possible to generate entangled photons from nonentangled photons of the same wavelength, a process called hyperparametric scattering. (K. Edamatsu , R. Shimizu , T. Itoh , Phys. Rev. Lett. 89, 213601, 2002 http://dx.doi.org/10.1103/PhysRevLett.89.213601 .)