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An extension of the classical orthogonal functions invariant to the quantum domain is presented. This invariant is expressed in terms of the Hamiltonian. Unitary transformations which involve the auxiliary function of this quantum invariant are used to solve the time-dependent Schrödinger equation for a harmonic oscillator with time-dependent parameter. The solution thus obtained is in agreement with the results derived using other methods which invoke the Lewis invariant in their procedures.

We study the time evolution for the quantum harmonic oscillator subjected to a sudden change of frequency. It is based on an analytic solution to the time dependent Ermakov equation for an approximated step function, and therefore, ours is a continuous treatment that differs from former studies that have limiting procedures for times when the step occurs.

We show that by using the quantum orthogonal functions invariant, we found a solution to coupled time-dependent harmonic oscillators where all the time-dependent frequencies are arbitrary. This system may be found in many applications such as nonlinear and quantum physics, biophysics, molecular chemistry, and cosmology. We solve the time-dependent coupled harmonic oscillators by transforming the Hamiltonian of the interaction using a set of unitary operators. In passing, we show that N time-dependent and coupled oscillators have a generalized orthogonal functions invariant from which we can write a Ermakov–Lewis invariant.

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The scator product in 1 + n dimensions for n > 1, is associative if all possible product pairs have a non vanishing additive scalar component. The product is in general, not associative in the additive representation whenever the additive scalar component of a product pair is zero. A particular case of this statement is non associativity due to zero products of non zero factors. These features of scator algebra could be used to model the quantum wave function evolution and collapse in a unified description.

The correspondence between classical and quantum invariants is established. The Ermakov Lewis quantum invariant of the time dependent harmonic oscillator is translated from the coordinate and momentum operators into amplitude and phase operators. In doing so, Turski's phase operator as well as Susskind-Glogower operators are generalized to the time dependent harmonic oscillator case. A quantum derivation of the Manley-Rowe relations is shown as an example.