- Position operator
quantum mechanics, the position operator corresponds to the position observable of a particle. Consider, for example, the case of a spinless particle moving on a line. The state space for such a particle is "L"2(R), the Hilbert spaceof complex-valued and square-integrable (with respect to the Lebesgue measure) functions on the real line. The position operator, "Q", is then defined by
Since all continuous functions with
compactsupport lie in "D(Q)", "Q" is densely defined. "Q", being simply multiplication by "x", is a self adjoint operator, thus satisfying the requirement of a quantum mechanical observable. Immediately from the definition we can deduce that the spectrum consists of the entire real line and that "Q" has purely continuous spectrum, therefore no eigenvalues. The three dimensional case is defined analogously. We shall keep the one-dimensional assumption in the following discussion.
As with any quantum mechanical
observable, in order to discuss measurement, we need to calculate the spectral resolution of "Q":
Since "Q" is just multiplication by "x", its spectral resolution is simple. For a Borel subset "B" of the real line, let denote the
indicator functionof "B". We see that the projection-valued measureΩ"Q" is given by
i.e. Ω"Q" is multiplication by the indicator function of "B". Therefore, if the system is prepared in state "ψ", then the probability of the measured position of the particle being in a
Borel set"B" is
where "μ" is the Lebesgue measure. After the measurement, the wave function collapses to , where is the Hilbert space norm on "L"2(R).
Unitary equivalence with momentum operator
For a particle on a line, the
momentum operator"P" is defined by
usually written in
with appropriate domain. "P" and "Q" are unitarily equivalent, with the
unitary operatorbeing given explicitly by the Fourier transform. Thus they have the same spectrum. In physical language, "P" acting on momentum space wave functions is the same as "Q" acting on position space wave functions (under the image of Fourier transform).
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