Observing and manipulating quantum information: some experiments with photons and atoms. - Aephraim Steinberg

Throughout the 20th century, the question of quantum measurement has confused and intrigued physicists. At the dawn of the 21st, these issues have taken on new practical importance due to the birth of an interdisciplinary science of "quantum information." The realization that quantum mechanics allows communications more secure than one could ever have classically, and computation exponentially more efficient than any classical known algorithms, has incited a huge amount of research into this new area, which has in turn provided an exciting new perspective on quantum mechanics. Motivated in part by these considerations, my lab has been carrying out a variety of experiments on controlling simple quantum systems and comparing different techniques for "measuring" their wave functions, density matrices, or phase-space distributions. I will describe some of the current issues in measurement and characterisation of quantum systems, and show the results of some of our recent experiments. In particular, I will discuss how many seeming paradoxes in quantum mechanics appear to be resolved if one considers the outcomes of so-called "weak measurements"... but only at the expense of accepting some truly surreal results. We also discuss our most recent efforts on generating and characterizing multi-photon entangled states for phase super-resolution, and on optimizing pulse sequences to preserve the coherence of atoms trapped in optical lattices.