Squeezing equivalence of quantum harmonic oscillators under different frequency modulations

Abstract: The papers by Janszky and Adam [Phys. Rev. A {\bf 46}, 6091 (1992)] and Chen \textit{et al.} [Phys. Rev. Lett. {\bf 104}, 063002 (2010)] are examples of works where one can find the following equivalences: belonging to the following class: quantum harmonic oscillators subjected to different time-dependent frequency modulations, during a certain time interval $\tau$, exhibit exactly the same final null squeezing parameter ($r_f=0$). In the present paper, we discuss a more general case of squeezing equivalence, where the final squeezing parameter can be non-null ($r_f\geq0$). We show that when the interest is in controlling the forms of the frequency modulations, but keeping free the choice of the values of $r_f$ and $\tau$, this in general demands numerical calculations to find these values leading to squeezing equivalences (a particular case of this procedure recovers the equivalence found by Jansky and Adams). On the other hand, when the interest is not in previously controlling the form of these frequencies, but rather $r_f$ and $\tau$ (and also some constraints, such as minimization of energy), one can have analytical solutions for these frequencies leading to squeezing equivalences (particular cases of this procedure are usually applied in problems of shortcuts to adiabaticity, as done by Chen \textit{et al.}). In this way, this more general squeezing equivalence discussed here is connected to recent and important topics in the literature as, for instance, generation of squeezed states and the obtaining of shortcuts to adiabaticity.