  In this work we study the interplay between the crystal structure and
magnetism of the pyroarsenate \alpha-Cu_2As_2O_7 by means of magnetization,
heat capacity, electron spin resonance and nuclear magnetic resonance
measurements as well as density functional theory (DFT) calculations and
quantum Monte Carlo (QMC) simulations. The data reveal that the magnetic Cu-O
chains in the crystal structure represent a realization of a quasi-one
dimensional (1D) coupled alternating spin-1/2 Heisenberg chain model with
relevant pathways through non-magnetic AsO_4 tetrahedra. Owing to residual 3D
interactions antiferromagnetic long range ordering at T_N\simeq10K takes place.
Application of external magnetic field B along the magnetically easy axis
induces the transition to a spin-flop phase at B_{SF}~1.7T (2K). The
experimental data suggest that substantial quantum spin fluctuations take place
at low magnetic fields in the ordered state. DFT calculations confirm the
quasi-one-dimensional nature of the spin lattice, with the leading coupling J_1
within the structural dimers. QMC fits to the magnetic susceptibility evaluate
J_1=164K, the weaker intrachain coupling J'_1/J_1 = 0.55, and the effective
interchain coupling J_{ic1}/J_1 = 0.20.
