Hartley and Ray have constructed and studied coherent states for the time- dependent oscillator. Here we show how to construct coherent states for more general time-dependent systems. We also show that these states are equivalent to tl;e well-known squeezed states.
Nieto and Simmons have defined and studied coherent states for arbitrary potentials V(q). We show how to extend their results to certain time-dependent potentials V(q,t). For each V(q) there is a V(q,t), for which we can construct time-dependent coherent states.
Physical consequences of the theorem of equivalence among harmonic oscillators of different frequencies, including a free particle as a special case, are explored. A new pictorial way of understanding time-dependent coherent states and squeezed states are presented and contrasted with the standard view. Three-dimensional spherical analogues of squeezed coherent states are constructed. It is also shown as a corollary of the theorem that a free wave packet which coincides initially with any one of the eigenfunctions of a harmonic oscillator maintains its shape, that is, the probability density of position does not change apart from a time-dependent scaling of spatial coordinates. Most of the results are derived without use of any sophisticated techniques but with the knowledge of free wave packets alone.