  The presence of a quantum critical point (QCP) can significantly affect the
thermodynamic properties of a material at finite temperatures T. This is
reflected, e.g., in the entropy landscape S(T, r) in the vicinity of a QCP,
yielding particularly strong variations for varying the tuning parameter r such
as pressure or magnetic field B. Here we report on the determination of the
critical enhancement of $ \delta S / \delta B$ near a B-induced QCP via
absolute measurements of the magnetocaloric effect (MCE), $(\delta T / \delta
B)_S$, and demonstrate that the accumulation of entropy around the QCP can be
used for efficient low-temperature magnetic cooling. Our proof of principle is
based on measurements and theoretical calculations of the MCE and the cooling
performance for a Cu$^{2+}$-containing coordination polymer, which is a very
good realization of a spin-1/2 antiferromagnetic Heisenberg chain - one of the
simplest quantum-critical systems.
