  In solid materials, the parameters relevant to quantum effects, such as the
spin quantum number, are basically determined and fixed at the chemical
synthesis, which makes it challenging to control the amount of quantum
correlations. We propose and demonstrate a method for active control of the
classical-quantum crossover in magnetic insulators by applying external
pressure. As a concrete example, we perform high-field, high-pressure
measurements on CsCuCl$_3$, which has the structure of weakly-coupled spin
chains. The magnetization process experiences a continuous evolution from the
semi-classical realm to the highly-quantum regime with increasing pressure.
Based on the idea of "squashing" the spin chains onto a plane, we characterize
the change in the quantum correlations by the change in the value of the local
spin quantum number of an effective two-dimensional model. This opens a way to
access the tunable classical-quantum crossover of two-dimensional spin systems
by using alternative systems of coupled-chain compounds.
