  We present an exact quantum Monte Carlo method for spin systems coupled to
dissipative bosonic baths which makes use of nonlocal wormhole updates to
simulate the retarded spin-flip interactions originating from an off-diagonal
spin-boson coupling. The method is closely related to the stochastic series
expansion and extends the scope of the global directed-loop updates to nonlocal
moves through a world-line configuration. We test our method for the
U(1)-symmetric two-bath spin-boson model, where the off-diagonal components of
a spin-$1/2$ particle are coupled to identical independent baths with power-law
spectra, and get a precise estimate of the critical coupling between the
critical and the localized phase. Our method applies to impurity systems and
lattice models in any spatial dimension coupled to bosonic modes with arbitrary
spectral distributions.
