  The measurement science in realizing and disseminating the unit for pressure
in the International System of Units (SI), the pascal (Pa), has been the
subject of much interest at the National Institute of Standards and Technology
(NIST). Modern optical-based techniques for pascal metrology have been
investigated, including multi-photon ionization and cavity ringdown
spectroscopy. Work is ongoing to recast the pascal in terms of quantum
properties and fundamental constants and in so doing, make vacuum metrology
consistent with the global trend toward quantum-based metrology. NIST has
ongoing projects that interrogate the index of refraction of a gas using an
optical cavity for low vacuum, and count background particles in high vacuum to
extreme high vacuum using trapped laser-cooled atoms.
