Quantum process tomography and the search for decoherence-free subspaces

We describe experiments with photon pairs to evaluate, correct for, and avoid sources of error in optical quantum information processing. It is well known that a simple beam-splitter can non-deterministicially prepare or select entangled polarization states. We use quantum process tomography (QPT) to fully characterize this effect, including loss and decoherence. The QPT results identify errors and indicate how well they can be corrected. To evade decoherence in a noisy quantum channel, we identify decoherence-free subspaces using experimental channel characterization, without need for a priori knowledge of the decoherence mechanism or simplifying assumptions. Working with pairs of polarization-encoded photonic qubits, we use tomographic and adaptive techniques to identify 2- and 3-state decoherence-free subspaces for encoding decoherence-free qubits and qutrits within the noisy channel.