  A long-lived qubit is usually well-isolated from all other systems and the
environments, and so is not easy to couple with measurement apparatus. It is
sometimes difficult to implement reliable projective measurements on such a
qubit. One potential solution is spin amplification with many ancillary qubits.
Here, we propose a spin amplification technique, where the ancillary qubits
state change depending on the state of the target qubit. The technique works
even in the presence of inhomogeneous broadening. We show that fast and
accurate amplification is possible even if the coupling and frequency of the
ancillary qubits is inhomogeneous. Since our scheme is robust against realistic
imperfections, this could provide a new mechanism for reading out a single spin
that could not have been measured using the previous approaches.
