February 8, 2001
Presented at the 3rd Workshop on Mysteries, Puzzles and Paradoxes in Quantum Mechanics,
Gargnano, Italy, September 17 -23 , 2000.
We show that it is possible to associate a de Broglie wavelength to a composite system even
when the constituent particles are separated spatially. The nonlocal de Broglie wavelength (A/2)
of a two-photon system separated spatially is measured with an appropriate detection system.
The two-photon system is prepared in an entangled state in space-momentum variables. - Pacs:
Key words: Quantum
References  M. Kacprowicz, R. Demkowicz-Dobrzanski, W. Wasilewski, K. Banaszek, and I. A. Walmsley, “Experimental quantum-enhanced estimation of a lossy phase shift”, Nature Phot. 4 , 357 (2010).  J. Abadie, et al. (the LIGO Scientific Collaboration), A gravitational wave observatory operating beyond the quantum shot-noise limit , Nat. Phys. 7 , 962 (2011).  R. Demkowicz-Dobrzanski, K. Banaszek, and R. Schnabel, “Fundamental quantuminterferometry bound for the squeezed-light-enhanced gravitational wave detector GEO 600”, Phys. Rev. A 88 , 041802(R
in the QIOPA is discussed in [8 , 9].
 S. Popescu, in: Introduction to Quantum Computation
and Information, World Scientific, New York 1998.
perspective in the fundamental endeavour on quantum
We acknowledge enlightening discussions and col
laboration with S. Branca, V. Mussi, F. Bovino, and
M. Lucamarini. We thank M.U.R.S.T. and the FET
European Network IST-2000-29681 (ATESIT) on
“Quantum Information” for funding.
 G. Di Giuseppe, F. De Martini, D. Boschi, and
S. Branca, in: QuantumInterferometry, F. De Martini