Quantum Measurement

Professor Josh Grossman and his students also pursue research on quantum measurement. In particular, we study the practical implementation of positive operator-valued measurements (POVMs) on the polarization of photons.

POVMs represent a much more general class of quantum measurements than the von Neumann measurement operators traditionally treated in quantum mechanics courses. POVMs can have non-orthogonal outcomes, and they need not fully collapse the wavefunction of the measured system. The development of quantum cryptography protocols has motivated recent interest in POVMs, particularly for photon polarization. Polarization state POVMs have been performed previously, but the implementations have always been ad hoc, developed for one specific POVM.

We are beginning by experimentally demonstrating two systematic methods of implementing any single-photon polarization POVM, both of which I proposed while working with Dr. Bill Wootters at Williams College. Later stages of the project will involve both theoretical and experimental investigation of implementation of POVMs on multi-photon polarization states. We will investigate two-photon polarization POVMs, including performing measurements on entangled polarization states generated by Type I parametric down-conversion. Beyond applications in quantum communications, the question of whether a POVM can be performed with local vs. non-local operations relates to the fundamental problem of determining the degree of entanglement of a mixed state.