Note: A 3D-printed alkali metal dispenser

Advances in Portable Atom Interferometry-Based Gravity Sensing

Gravity sensing is a valuable technique used for several applications, including fundamental physics, civil engineering, metrology, geology, and resource exploration. While classical gravimeters have proven useful, they face limitations, such as mechanical wear on the test masses, resulting in drift, and limited measurement speeds, hindering their use for long-term monitoring, as well as the need to average out microseismic vibrations, limiting their speed of data acquisition. Emerging sensors based on atom interferometry for gravity measurements could offer promising solutions to these limitations, and are currently advancing towards portable devices for real-world applications. This article provides a brief state-of-the-art review of portable atom interferometry-based quantum sensors and provides a perspective on routes towards improved sensors.

Precise quantum measurement of vacuum with cold atoms

We describe the cold-atom vacuum standards (CAVS) under development at the National Institute of Standards and Technology (NIST). The CAVS measures pressure in the ultra-high and extreme-high vacuum regimes by measuring the loss rate of sub-millikelvin sensor atoms from a magnetic trap. Ab initio quantum scattering calculations of cross sections and rate coefficients relate the density of background gas molecules or atoms to the loss rate of ultra-cold sensor atoms. The resulting measurement of pressure through the ideal gas law is traceable to the second and the kelvin, making it a primary realization of the pascal. At NIST, two versions of the CAVS have been constructed: a laboratory standard used to achieve the lowest possible uncertainties and pressures, and a portable version that is a potential replacement for the Bayard-Alpert ionization gauge. Both types of CAVSs are connected to a combined extreme-high vacuum flowmeter and dynamic expansion system to enable sensing of a known pressure of gas. In the near future, we anticipate being able to compare the laboratory scale CAVS, the portable CAVS, and the flowmeter/dynamic expansion system to validate the operation of the CAVS as both a standard and vacuum gauge.

Λ-enhanced gray molasses in a tetrahedral laser beam geometry

We report the observation of sub-Doppler cooling of lithium using an irregular-tetrahedral laser beam arrangement, which is produced by a nanofabricated diffraction grating. We are able to capture 11(2)% of the lithium atoms from a grating magneto-optical trap into Λ-enhanced D₁ gray molasses. The molasses cools the captured atoms to a radial temperature of 60(9) μK and an axial temperature of 23(3) μK. In contrast to results from conventional counterpropagating beam configurations, we do not observe cooling when our optical fields are detuned from Raman resonance. An optical Bloch equation simulation of the cooling dynamics agrees with our data. Our results show that grating magneto-optical traps can serve as a robust source of cold atoms for tweezer-array and atom-chip experiments, even when the atomic species is not amenable to sub-Doppler cooling in bright optical molasses.

Confinement of an alkaline-earth element in a grating magneto-optical trap

We demonstrate a compact magneto-optical trap (MOT) of alkaline-earth atoms using a nanofabricated diffraction grating chip. A single input laser beam, resonant with the broad ¹S₀ → ¹P₁ transition of strontium, forms the MOT in combination with three diffracted beams from the grating chip and a magnetic field produced by permanent magnets. A differential pumping tube limits the effect of the heated, effusive source on the background pressure in the trapping region. The system has a total volume of around 2.4 l. With our setup, we have trapped up to 5 × 10^{6 88}Sr atoms at a temperature of ∼6 mK, and with a trap lifetime of ∼1 s. Our results will aid the effort to miniaturize quantum technologies based on alkaline-earth atoms.

Single-beam Zeeman slower and magneto-optical trap using a nanofabricated grating

We demonstrate a compact (0.25 L) system for laser cooling and trapping atoms from a heated dispenser source. Our system uses a nanofabricated diffraction grating to generate a magnetooptical trap (MOT) using a single input laser beam. An aperture in the grating allows atoms from the dispenser to be loaded from behind the chip, increasing the interaction distance of atoms with the cooling light. To take full advantage of this increased distance, we extend the magnetic field gradient of the MOT to create a Zeeman slower. The MOT traps approximately 10^{6 7}Li atoms emitted from an effusive source with loading rates greater than 10⁶ s^-1. Our design is portable to a variety of atomic and molecular species and could be a principal component of miniaturized cold-atom-based technologies.

Single-beam Zeeman slower and magneto-optical trap using a nanofabricated grating

We demonstrate a compact (0.25 L) system for laser cooling and trapping atoms from a heated dispenser source. Our system uses a nanofabricated diffraction grating to generate a magnetooptical trap (MOT) using a single input laser beam. An aperture in the grating allows atoms from the dispenser to be loaded from behind the chip, increasing the interaction distance of atoms with the cooling light. To take full advantage of this increased distance, we extend the magnetic field gradient of the MOT to create a Zeeman slower. The MOT traps approximately 10^{6 7}Li atoms emitted from an effusive source with loading rates greater than 10⁶ s^-1. Our design is portable to a variety of atomic and molecular species and could be a principal component of miniaturized cold-atom-based technologies.

Light-induced atomic desorption of lithium

We demonstrate loading of a Li magneto-optical trap using light-induced atomic desorption. The magnetooptical trap confines up to approximately 4 × 104 7Li atoms with loading rates up to approximately 4 × 103 atoms per second. We study the Li desorption rate as a function of the desorption wavelength and power. The extracted wavelength threshold for desorption of Li from fused silica is approximately 470 nm. In addition to desorption of lithium, we observe light-induced desorption of background gas molecules. The vacuum pressure increase due to the desorbed background molecules is ≲ 50 % and the vacuum pressure decreases back to its base value with characteristic timescales on the order of seconds when we extinguish the desorption light. By examining both the loading and decay curves of the magneto-optical trap, we are able to disentangle the trap decay rates due to background gases and desorbed lithium. Our results show that light-induced atomic desorption can be a viable Li vapor source for compact devices and sensors.

Quantum-based vacuum metrology at NIST

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 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.

Challenges to miniaturizing cold atom technology for deployable vacuum metrology

Cold atoms are excellent metrological tools; they currently realize SI time and, soon, SI pressure in the ultra-high (UHV) and extreme high vacuum (XHV) regimes. The development of primary, vacuum metrology based on cold atoms currently falls under the purview of national metrology institutes. Under the emerging paradigm of the "quantum-SI", these technologies become deployable (relatively easy-to-use sensors that integrate with other vacuum chambers), providing a primary realization of the pascal in the UHV and XHV for the end-user. Here, we discuss the challenges that this goal presents. We investigate, for two different modes of operation, the expected corrections to the ideal cold-atom vacuum gauge and estimate the associated uncertainties. Finally, we discuss the appropriate choice of sensor atom, the light Li atom rather than the heavier Rb.