  The measurement of quantum states is one of the most important problems in
quantum mechanics. We introduce a quantum state tomography technique in which
the state of a qubit is reconstructed, while the qubit remains undetected. The
key ingredients are: (i) employing an additional qubit, (ii) aligning the
undetected qubit with a known reference state by using path identity, and (iii)
measuring the additional qubit to reconstruct the undetected qubit state. We
theoretically establish and experimentally demonstrate the method with photonic
polarization states. The principle underlying our method could also be applied
to quantum entities other than photons.
