  By quantum calibration we name an experimental procedure apt to completely
characterize an unknown measurement apparatus by comparing it with other
calibrated apparatuses. Here we show how to achieve the calibration of an
arbitrary measuring apparatus, by using it in conjunction with a
``tomographer'' in a correlation setup with an input bipartite system. The
method is robust to imperfections of the tomographer, and works for practically
any input state of the bipartite system.
