  For any translation-invariant quantum lattice system with a symmetry group G,
we propose a practical and universal construction of order parameters which
identify quantum phase transitions with symmetry-breaking order. They are
defined in terms of the fidelity between a ground state and its
symmetry-transformed counterpart, and are computed through tensor network
representations of the ground-state wavefunction. To illustrate our scheme, we
consider three quantum systems on an infinite lattice in one spatial dimension,
namely, the quantum Ising model in a transverse magnetic field, the quantum
spin-1/2 XYX model in an external magnetic field, and the quantum spin-1 XXZ
model with single-ion anisotropy. All these models have symmetry group Z_2 and
exhibit broken-symmetry phases. We also discuss the role of the order
parameters in identifying factorized states.
