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Physica

Spectral decomposition of the operator p2–q2

N. G. van Kampen

Published Jan 1, 1958• 8 citations
Publisher
DOI
10.1016/s0031-8914(58)80068-3

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We give the explicit form of the common eigenstate \ensuremath{\Vert}\ensuremath{\xi}〉 of the center-of-mass coordinate ${\mathrm{\ensuremath{\mu}}}_{1}$Q1{\mathit{Q}}_{1}Q1​+${\mathrm{\ensuremath{\mu}}}_{2}$Q2{\mathit{Q}}_{2}Q2​ and the mass-weighted relative momentum ${\mathrm{\ensuremath{\mu}}}_{2}$P1{\mathit{P}}_{1}P1​-${\mathrm{\ensuremath{\mu}}}_{1}$P2{\mathit{P}}_{2}P2​ of two particles, which is more complicated than the common eigenstate of the other pair of commutative operators Q2{\mathit{Q}}_{2}Q2​-Q1{\mathit{Q}}_{1}Q1​ and P1{\mathit{P}}_{1}P1​+P2{\mathit{P}}_{2}P2​. The orthonormal and completeness relation of \ensuremath{\Vert}\ensuremath{\xi}〉 are investigated easily by virtue of the technique of integration within an ordered product of operators. The normally ordered squeezing operator for ${\mathrm{\ensuremath{\mu}}}_{1}$Q1{\mathit{Q}}_{1}Q1​+${\mathrm{\ensuremath{\mu}}}_{2}$Q2{\mathit{Q}}_{2}Q2​ and ${\mathrm{\ensuremath{\mu}}}_{2}$P1{\mathit{P}}_{1}P1​-${\mathrm{\ensuremath{\mu}}}_{1}$P2{\mathit{P}}_{2}P2​ is also derived by using the \ensuremath{\Vert}\ensuremath{\xi}〉 representation.
We give the explicit form of the common eigenvectors of the relative position Q1{\mathit{Q}}_{1}Q1​-Q2{\mathit{Q}}_{2}Q2​ and the total momentum P1{\mathit{P}}_{1}P1​+P2{\mathit{P}}_{2}P2​, of two particles which were considered by Einstein, Podolsky, and Rosen [Phys. Rev. 47, 777 (1935)] in their argument that the quantum-mechanical state vector is not complete. Orthonormality and completeness of such eigenvectors, as well as their use in constructing the unitary operator for simultaneously squeezing Q1{\mathit{Q}}_{1}Q1​-Q2{\mathit{Q}}_{2}Q2​ and P1{\mathit{P}}_{1}P1​+P2{\mathit{P}}_{2}P2​, are derived by using the technique of integration within an ordered product of operators.
Statistical reformulation of quantum mechanics in terms of phase-space distribution functions as given by Moyal using Weyl's correspondence rule between classical functions and operators has been extended to various different correspondence rules. The dynamical bracket in the Weyl correspondence (the ``Moyal'' or the ``sine'' bracket) is shown to be a Lie bracket. It is further shown that if the theory is restricted to Lie brackets of the form [u(p, q),υ(p, q)]={f(∂2∂q1 ∂p2−∂2∂q2 ∂p1)u(p1, q1)υ(p2, q2)},evaluated for p1 = p2 = p; q1 = q2 = q after differentiation, then the only admissible functional form of f is f(x) = β[(sin αx)/α], where α and β are constants. A law of multiplication which is associative and distributive with respect to addition is also introduced in each case. It gives a correct correspondence between operator multiplication and the multiplication of classical functions. The dynamical brackets obtained in each case are also found to be Lie brackets. Conditions on the phase-space distribution functions to describe pure states are also given.