  The phase estimation algorithm, which is at the heart of a variety of quantum
algorithms, including Shor's factoring algorithm, allows a quantum computer to
accurately determine an eigenvalue of an unitary operator. Quantum
nondemolition measurements are a quantum mechanical procedure, used to overcome
the standard quantum limit when measuring an observable. We show that the phase
estimation algorithm, in both the discrete and continuous variable setting, can
be viewed as a quantum nondemolition measurement.
