Following the realization of Bose-Einstein condensates in atomic gases, an experimental challenge is the production of molecular gases in the quantum regime. A promising approach is to create the ...molecular gas directly from an ultracold atomic gas; for example, bosonic atoms in a Bose-Einstein condensate have been coupled to electronic ground-state molecules through photoassociation or a magnetic field Feshbach resonance. The availability of atomic Fermi gases offers the prospect of coupling fermionic atoms to bosonic molecules, thus altering the quantum statistics of the system. Such a coupling would be closely related to the pairing mechanism in a fermionic superfluid, predicted to occur near a Feshbach resonance. Here we report the creation and quantitative characterization of ultracold 40K2 molecules. Starting with a quantum degenerate Fermi gas of atoms at a temperature of less than 150 nK, we scan the system over a Feshbach resonance to create adiabatically more than 250,000 trapped molecules; these can be converted back to atoms by reversing the scan. The small binding energy of the molecules is controlled by detuning the magnetic field away from the Feshbach resonance, and can be varied over a wide range. We directly detect these weakly bound molecules through their radio-frequency photodissociation spectra; these probe the molecular wavefunction, and yield binding energies that are consistent with theory.
The realization of superfluidity in a dilute gas of fermionic atoms, analogous to superconductivity in metals, represents a long-standing goal of ultracold gas research. In such a fermionic ...superfluid, it should be possible to adjust the interaction strength and tune the system continuously between two limits: a Bardeen-Cooper-Schrieffer (BCS)-type superfluid (involving correlated atom pairs in momentum space) and a Bose-Einstein condensate (BEC), in which spatially local pairs of atoms are bound together. This crossover between BCS-type superfluidity and the BEC limit has long been of theoretical interest, motivated in part by the discovery of high-temperature superconductors. In atomic Fermi gas experiments superfluidity has not yet been demonstrated; however, long-lived molecules consisting of locally paired fermions have been reversibly created. Here we report the direct observation of a molecular Bose-Einstein condensate created solely by adjusting the interaction strength in an ultracold Fermi gas of atoms. This state of matter represents one extreme of the predicted BCS-BEC continuum.
Micromechanical resonator performance is fundamentally limited by the coupling to a thermal environment. The magnitude of this thermodynamical effect is typically considered in accordance with a ...physical temperature, assumed to be uniform across the resonator's physical span. However, in some circumstances, e.g., quantum optomechanics or interferometric gravitational wave detection, the temperature of the resonator may not be uniform, resulting in the resonator being thermally linked to a spatially varying thermal bath. In this case, the link of a mode of interest to its thermal environment is less straightforward to understand. Here, we engineer a distributed bath on a germane optomechanical platform—a phononic crystal—and utilize both highly localized and extended resonator modes to probe the spatially varying bath in entirely different bath regimes. As a result, we observe striking differences in the modes' Brownian motion magnitude. From these measurements we are able to reconstruct the local temperature map across our resonator and measure nanoscale effects on thermal conductivity and radiative cooling. Our work explains some thermal phenomena encountered in optomechanical experiments, e.g., mode-dependent heating due to light absorption. Moreover, our work generalizes the typical figure of merit quantifying the coupling of a resonator mode to its thermal environment from the mechanical dissipation to the overlap between the local dissipation and the local temperature throughout the resonator. This added understanding identifies design principles that can be applied to the performance of micromechanical resonators.
Micro-mechanical resonator performance is fundamentally limited by the coupling to a thermal environment. The magnitude of this thermodynamical effect is typically considered in accordance with a ...physical temperature, assumed to be uniform across the resonator's physical span. However, in some circumstances, e.g. quantum optomechanics or interferometric gravitational wave detection, the temperature of the resonator may not be uniform, resulting in the resonator being thermally linked to a spatially varying thermal bath. In this case, the link of a mode of interest to its thermal environment is less straightforward to understand. Here, we engineer a distributed bath on a germane optomechanical platform -- a phononic crystal -- and utilize both highly localized and extended resonator modes to probe the spatially varying bath in entirely different bath regimes. As a result, we observe striking differences in the modes' Brownian motion magnitude. From these measurements we are able to reconstruct the local temperature map across our resonator and measure nanoscale effects on thermal conductivity and radiative cooling. Our work explains some thermal phenomena encountered in optomechanical experiments, e.g. mode-dependent heating due to light absorption. Moreover, our work generalizes the typical figure of merit quantifying the coupling of a resonator mode to its thermal environment from the mechanical dissipation to the overlap between the local dissipation and the local temperature throughout the resonator. This added understanding identifies design principles that can be applied to performance of micro-mechanical resonators.
We demonstrate how to measure in situ for heading errors of optically pumped magnetometers (OPMs) in the challenging parameter regime of compact vapor cells with imperfect optical pumping and high ...buffer gas pressure. For this, we utilize microwave-driven Ramsey and Rabi frequency spectroscopy (FS) to independently characterize scalar heading errors in free induction decay (FID) signals. Both of these approaches suppress 5-nT inaccuracies in geomagnetic fields caused by nonlinear Zeeman (NLZ) shifts in FID measurements to below 0.6 nT. For Ramsey FS, we implement short periods of microwave interrogation within a \(\pi/2-t_R-3\pi/2\) Ramsey interferometry sequence, effectively circumventing systematic errors from off-resonant driving. Conversely, Rabi FS leverages an atom-microwave Hamiltonian for accurate modeling of Rabi oscillation frequencies, achieving a measurement precision down to 80 pT\(/ \sqrt{\text{Hz}}\) that is limited primarily by technical microwave noise. We show that the fundamental sensitivity of Rabi FS is 30 pT/\(\sqrt{\text{Hz}}\) with our vapor cell parameters through a Cramér-Rao lower bound (CRLB) analysis. This work paves the way for future investigations into the accuracy of hyperfine structure (HFS) magnetometry and contributes to the broader applicability of OPMs in fields ranging from navigation and geophysics to space exploration and unexploded ordinance detection, where heading error mitigation is essential.
PDF
