Implementing the new kelvin by molecular precision spectroscopy

International audienceNext autumn, the redefinition of the international System of Units (SI) will be accepted by the General Conference on Weights and Measures. This major reform will establish a new definition of units in terms of a set of 7 defining constants. The temperature unit, the kelvin, the definition of which is currently based on the triple point of water (TPW), will for example be redefined by fixing the value of the Boltzmann constant kB. For the implementation of the new kelvin, various primary thermometry methods are currently being developed such as Doppler-broadening thermometry (DBT), acoustic gas thermometry, Johnson noise thermometry, dielectric constant gas thermometry, etc... We have previously proposed and developped the DBT method and have used it to demonstrate an accurate determination of kBa,b. Once kB fixed in the new SI, DBT will become a primary spectroscopic method for thermodynamic temperature measurements and thus for the implementation of the new kelvin. DBT is based on the precise measurement of the Doppler broadening of absorption line of a gaz phase atomic or molecular species, an ammonia ro-vibrational transition in the mid-infrared range (∼ 10μm) in our case, combined with some highly accurate modeling of the line profile. We are currently developing DBT in thetemperature range 300-430K, in order to demonstrate its potential and study its limitations beyond the temperature of the TPW. We will present our progress towards temprature measurements with uncertainties at the 25ppm level. The existing set-up (previously used for the determination of kB) has been upgraded. We have placed the spectroscopic cell in a variable thermostat, the temperature stability and gradient of which has been characterized. We have also improved our mid-infrared spectrometer to investigate, in particular the influence of the line-mixing on the temperature measurement accuracy

Probing molecules in gas cells of subwavelength thickness with high frequency resolution

Abstract Miniaturizing and integrating atomic vapor cells is widely investigated for the purposes of fundamental measurements and technological applications such as quantum sensing. Extending such platforms to the realm of molecular physics is a fascinating prospect that paves the way for compact frequency metrology as well as for exploring light-matter interactions with complex quantum objects. Here, we perform molecular rovibrational spectroscopy in a thin-cell of micrometric thickness, comparable to excitation wavelengths. We operate the cell in two distinct regions of the electromagnetic spectrum, probing ν1 + ν3 resonances of acetylene at 1.530 µm, within the telecommunications wavelength range, as well as the ν3 and ν2 resonances of SF6 and NH3 respectively, in the mid-infrared fingerprint region around 10.55 µm. Thin-cell confinement allows linear sub-Doppler transmission spectroscopy due to the coherent Dicke narrowing effect, here demonstrated for molecular rovibrations. Our experiment can find applications extending to the fields of compact molecular frequency references, atmospheric physics or fundamental precision measurements

Ultra-sensitive heterodyne detection at room temperature in the atmospheric windows

International audienceWe report room temperature heterodyne detection of a quantum cascade laser beaten with a local oscillator on a unipolar quantum photodetector in two different atmospheric windows, at 4.8 µm and 9 µm. A noise equivalent power of few pW is measured by employing an active stabilization technique in which the local oscillator and the signal are locked in phase. The measured heterodyne noise equivalent power is six orders of magnitude lower than that obtained with direct detection