State Characterization

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State characterization is the process by which a series of quantum mechanical measurements on an identical ensemble allow the reconstruction of that ensemble's quantum state. As every quantum information protocol or quantum computational algorithm requires the ability to verify which state is being used in the experiment, state characterization is of paramount importance.

As a specific example of state characterization, we have implemented an automated and extremely precise two-qubit measurement system. By using waveplates to implement unitary transformations on the polarization light and polarizers to implement projectors, we are able to construct a series of separable two-qubit measurements (pairs of arbitrary one-qubit measurements). By looking at a given state from a number of different directions (projecting into a number of different bases), we are able to implement a quantum tomography, and learn how that quantum state looks from every direction.