  The well-known algorithm for quantum phase estimation requires that the
considered unitary is available as a conditional transformation depending on
the quantum state of an ancilla register. We present an algorithm converting an
unknown n-qubit pair-interaction Hamiltonian into a conditional one such that
standard phase estimation can be applied to measure the energy. Our essential
assumption is that the considered system can be brought into interaction with a
quantum computer. For large n the algorithm could still be applicable for
estimating the density of energy states and might therefore be useful for
finding energy gaps in solid states.

Quantum Physics