title: Accurate lineshape spectroscopy and the Boltzmann constant.
abstract: Spectroscopy has an illustrious history delivering serendipitous discoveries and providing a stringent testbed for new physical predictions, including applications from trace materials detection, to understanding the atmospheres of stars and planets, and even constraining cosmological models. Reaching fundamental-noise limits permits optimal extraction of spectroscopic information from an absorption measurement. Here, we demonstrate a quantum-limited spectrometer that delivers high-precision measurements of the absorption lineshape. These measurements yield a very accurate measurement of the excited-state (6P1/2) hyperfine splitting in Cs, and reveals a breakdown in the well-known Voigt spectral profile. We develop a theoretical model that accounts for this breakdown, explaining the observations to within the shot-noise limit. Our model enables us to infer the thermal velocity dispersion of the Cs vapour with an uncertainty of 35 p.p.m. within an hour. This allows us to determine a value for Boltzmann's constant with a precision of 6 p.p.m., and an uncertainty of 71 p.p.m.
authors.affiliation: Institute for Photonics and Advanced Sensing (IPAS) and School of Chemistry and Physics, The University of Adelaide, Adelaide, South Australia 5005, Australia.|School of Physics, The University of Western Australia, Perth, Western Australia 6009, Australia.
authors.forename: G-W
authors.initials: GW
authors.lastname: Truong
authors.affiliation: Institute for Photonics and Advanced Sensing (IPAS) and School of Chemistry and Physics, The University of Adelaide, Adelaide, South Australia 5005, Australia.|School of Physics, The University of Western Australia, Perth, Western Australia 6009, Australia.
authors.forename: J D
authors.initials: JD
authors.lastname: Anstie
authors.affiliation: Centre for Energy, School of Mechanical and Chemical Engineering, The University of Western Australia, Crawley, Western Australia 6009, Australia.
authors.forename: E F
authors.initials: EF
authors.lastname: May
authors.affiliation: ARC Centre for Engineered Quantum Systems, University of Queensland, Brisbane 4072, Australia.
authors.forename: T M
authors.initials: TM
authors.lastname: Stace
authors.affiliation: Institute for Photonics and Advanced Sensing (IPAS) and School of Chemistry and Physics, The University of Adelaide, Adelaide, South Australia 5005, Australia.|School of Physics, The University of Western Australia, Perth, Western Australia 6009, Australia.
authors.forename: A N
authors.initials: AN
authors.lastname: Luiten