  Numerical simulations of numerous quantum systems suffer from the notorious
sign problem. Meron-cluster algorithms lead to an efficient solution of sign
problems for both fermionic and bosonic models. Here we apply the meron concept
to quantum spin systems in an arbitrary external magnetic field, in which case
standard cluster algorithms fail. As an example, we simulate antiferromagnetic
quantum spin ladders in a uniform external magnetic field that competes with
the spin-spin interaction. The numerical results are in agreement with analytic
predictions for the magnetization as a function of the external field.
