  While characterization of coherent wavefields is essential to laser, x-ray
and electron imaging, sensors measure the squared magnitude of the field,
rather than the field itself. Holography or phase retrieval must be used to
characterize the field. The need for a reference severely restricts the utility
of holography. Phase retrieval, in contrast, is theoretically consistent with
sensors that directly measure coherent or partially coherent fields with no
prior assumptions. Unfortunately, phase retrieval has not yet been successfully
implemented for large-scale fields. Here we show that both holography and phase
retrieval are capable of quantum-limited coherent signal estimation and we
describe phase retrieval strategies that approach the quantum limit for >1
megapixel fields. These strategies rely on group testing using networks of
interferometers, such as might be constructed using emerging integrated
photonic, plasmonic and/or metamaterial devices. Phase-sensitive sensor planes
using such devices could eliminate the need both for lenses and reference
signals, creating a path to large aperture diffraction limited laser imaging.
