  We propose a protocol that achieves fast adiabatic transfer between two
orthogonal states of a qubit by coupling with an ancilla. The qubit undergoes
Landau-Zener dynamics, whereas the coupling realizes a time-dependent
Hamiltonian, which is diagonal in the spin's instantaneous Landau-Zener
eigenstates. The ancilla (or meter), in turn, couples to a thermal bath, such
that the overall dynamics is incoherent. We analyse the protocol's fidelity as
a function of the strength of the coupling and of the relaxation rate of the
meter. When the meter's decay rate is the largest frequency scale of the
dynamics, the spin dynamics is encompassed by a master equation describing
dephasing of the spin in the instantaneous eigenbasis. In this regime the
fidelity of adiabatic transfer improves as the bath temperature is increased.
Surprisingly, the adiabatic transfer is significantly more efficient in the
opposite regime, where the time scale of the ancilla dynamics is comparable to
the characteristic spin time scale. Here, for low temperatures the coupling
with the ancilla tends to suppress diabatic transitions via effective cooling.
The protocol can be efficiently implemented by means of a pulsed, stroboscopic
coupling with the ancilla and is robust against moderate fluctuations of the
experimental parameters.
