  In quantum mechanics, the process of measurement is a subtle interplay
between extraction of information and disturbance of the state of the quantum
system. A quantum non-demolition (QND) measurement minimizes this disturbance
by using a particular system - detector interaction which preserves the
eigenstates of a suitable operator of the quantum system. This leads to an
ideal projective measurement. We present experiments in which we perform two
consecutive measurements on a quantum two -level system, a superconducting flux
qubit, by probing the hysteretic behaviour of a coupled nonlinear resonator.
The large correlation between the results of the two measurements demonstrates
the QND nature of the readout method. The fact that a QND measurement is
possible for superconducting qubits strengthens the notion that these
fabricated mesoscopic systems are to be regarded as fundamental quantum
objects. Our results are also relevant for quantum information processing,
where projective measurements are used for protocols like state preparation and
error correction.
