Measuring measurement

Measurement connects the world of quantum phenomena to the world of classical events. It plays both a passive role, observing quantum systems, and an active one, preparing quantum states and controlling them. Surprisingly - in the light of the central status of measurement in quantum mechanics - there is no general recipe for designing a detector that measures a given observable. Compounding this, the characterization of existing detectors is typically based on partial calibrations or elaborate models. Thus, experimental specification (i.e. tomography) of a detector is of fundamental and practical importance. Here, we present the realization of quantum detector tomography: we identify the optimal positive-operator-valued measure describing the detector, with no ancillary assumptions. This result completes the triad, state, process, and detector tomography, required to fully specify an experiment. We characterize an avalanche photodiode and a photon number resolving detector capable of detecting up to eight photons. This creates a new set of tools for accurately detecting and preparing non-classical light.Comment: 6 pages, 4 figures,see video abstract at http://www.quantiki.org/video_abstracts/0807244

Optomechanical device actuation through the optical gradient force

Optical forces are widely used to manipulate microparticles such as living cells, DNA and bacteria. The forces used in these 'optical tweezers' originate from the strongly varying electromagnetic field in the focus of a high-power laser beam. This field gradient polarizes the particle, causing the positively and negatively charged sides of the dipole to experience slightly different forces. It was recently realized that the strong field gradient in the near-field of guided wave structures can also be exploited for actuating optomechanical devices, and initial theoretical work in this area was followed rapidly by several experimental demonstrations. This Review summarizes the rapid development in this field. First, the origin of the optical gradient force is discussed in detail. Several experimental demonstrations and approaches for enhancing the strength of the effect are then discussed. Finally, some of the possible applications of the effect are reviewed