Microfabricated strontium atomic vapor cells.

We demonstrate strontium (Sr) atomic vapor cells having a total external volume of 0.63 cm3 that can operate above 300 °C for times exceeding 380 h. The cells are fabricated using micromachined silicon frames anodically bonded to glass windows that have a 20-nm thick protective layer of Al2O3 deposited on the interior surfaces. The presence of Sr vapor in the cell is confirmed through laser absorption spectroscopy for the 1S0 → 1P1 transition in Sr at 461 nm. Measurements of sub-Doppler linewidths indicated negligible (<3 MHz) broadening of this transition from residual background gas collisions. This compact and manufacturable, high-temperature atomic vapor cell can enable narrow-line optical frequency references based on strontium and other alkaline earth species.

High-performance, compact optical standard.

We describe a high-performance, compact optical frequency standard based on a microfabricated Rb vapor cell and a low-noise, external cavity diode laser operating on the Rb two-photon transition at 778 nm. The optical standard achieves an instability of 1.8×10-13τ-1/2 for times less than 100 s and a flicker noise floor of 1×10-14 out to 6000 s. At long integration times, the instability is limited by variations in optical probe power and the ac Stark shift. The retrace was measured to 5.7×10-13 after 30 h of dormancy. Such a simple, yet high-performance optical standard could be suitable as an accurate realization of the meter or, if coupled with an optical frequency comb, as a compact atomic clock comparable to a hydrogen maser.

Miniaturized optical frequency reference for next-generation portable optical clocks.

Optical frequency standards, or lasers stabilized to atomic or molecular transitions, are widely used in length metrology and laser ranging, provide a backbone for optical communications and lie at the heart of next-generation optical atomic clocks. Here we demonstrate a compact, low-power optical frequency reference based on the Doppler-free, two-photon transition in rubidium-87 at 778 nm implemented on a micro-optics breadboard. Our optical reference achieves a fractional frequency instability of 2.9×10-12/τ for averaging times τ less than 103 s, has a volume of ≈35 cm3 and operates on ≈450 mW of electrical power. The advanced optical integration presented here demonstrates a key step towards the development of compact optical clocks and the broad dissemination of SI-traceable wavelength references.

A Chip-Scale Optical Frequency Reference for the Telecommunication Band Based on Acetylene.

Lasers precisely stabilized to known transitions between energy levels in simple, well-isolated quantum systems such as atoms and molecules are essential for a plethora of applications in metrology and optical communications. The implementation of such spectroscopic systems in a chip-scale format would allow to reduce cost dramatically and would open up new opportunities in both photonically integrated platforms and free-space applications such as lidar. Here the design, fabrication, and experimental characterization of a molecular cladded waveguide platform based on the integration of serpentine nanoscale photonic waveguides with a miniaturized acetylene chamber is presented. The goal of this platform is to enable cost-effective, miniaturized, and low power optical frequency references in the telecommunications C band. Finally, this platform is used to stabilize a 1.5 µm laser with a precision better than 400 kHz at 34 s. The molecular cladded waveguide platform introduced here could be integrated with components such as on-chip modulators, detectors, and other devices to form a complete on-chip laser stabilization system.

Microresonator Brillouin laser stabilization using a microfabricated rubidium cell.

We frequency stabilize the output of a miniature stimulated Brillouin scattering (SBS) laser to rubidium atoms in a microfabricated cell to realize a laser system with frequency stability at the 10-11 level over seven decades in averaging time. In addition, our system has the advantages of robustness, low cost and the potential for integration that would lead to still further miniaturization. The SBS laser operating at 1560 nm exhibits a spectral linewidth of 820 Hz, but its frequency drifts over a few MHz on the 1 hour timescale. By locking the second harmonic of the SBS laser to the Rb reference, we reduce this drift by a factor of 103 to the level of a few kHz over the course of an hour. For our combined SBS and Rb laser system, we measure a frequency noise of 4 × 104 Hz2/Hz at 10 Hz offset frequency which rapidly rolls off to a level of 0.2 Hz2/Hz at 100 kHz offset. The corresponding Allan deviation is ≤2 × 10-11 for averaging times spanning 10-4 to 103 s. By optically dividing the signal of the laser down to microwave frequencies, we generate an RF signal at 2 GHz with phase noise at the level of -76 dBc/Hz and -140 dBc/Hz at offset frequencies of 10 Hz and 10 kHz, respectively.

Rotational State Microwave Mixing for Laser Cooling of Complex Diatomic Molecules.

We demonstrate the mixing of rotational states in the ground electronic state using microwave radiation to enhance optical cycling in the molecule yttrium (II) monoxide (YO). This mixing technique is used in conjunction with a frequency modulated and chirped continuous wave laser to slow longitudinally a cryogenic buffer-gas beam of YO. We generate a flux of YO below 10 m/s, directly loadable into a three-dimensional magneto-optical trap. This technique opens the door for laser cooling of diatomic molecules with more complex loss channels due to intermediate states.

Cold state-selected molecular collisions and reactions.

Over the past decade, and particularly the past five years, a quiet revolution has been building at the border between atomic physics and experimental quantum chemistry. The rapid development of techniques for producing cold and even ultracold molecules without a perturbing rare-gas cluster shell is now enabling the study of chemical reactions and scattering at the quantum scattering limit with only a few partial waves contributing to the incident channel. Moreover, the ability to perform these experiments with nonthermal distributions comprising one or a few specific states enables the observation and even full control of state-to-state collision rates in this computation-friendly regime: This is perhaps the most elementary study possible of scattering and reaction dynamics.

2D Magneto-optical trapping of diatomic molecules.

We demonstrate one- and two-dimensional transverse laser cooling and magneto-optical trapping of the polar molecule yttrium (II) oxide (YO). In a 1D magneto-optical trap (MOT), we characterize the magneto-optical trapping force and decrease the transverse temperature by an order of magnitude, from 25 to 2 mK, limited by interaction time. In a 2D MOT, we enhance the intensity of the YO beam and reduce the transverse temperature in both transverse directions. The approach demonstrated here can be applied to many molecular species and can also be extended to 3D.

Evaporative cooling of the dipolar hydroxyl radical.

Atomic physics was revolutionized by the development of forced evaporative cooling, which led directly to the observation of Bose-Einstein condensation, quantum-degenerate Fermi gases and ultracold optical lattice simulations of condensed-matter phenomena. More recently, substantial progress has been made in the production of cold molecular gases. Their permanent electric dipole moment is expected to generate systems with varied and controllable phases, dynamics and chemistry. However, although advances have been made in both direct cooling and cold-association techniques, evaporative cooling has not been achieved so far. This is due to unfavourable ratios of elastic to inelastic scattering and impractically slow thermalization rates in the available trapped species. Here we report the observation of microwave-forced evaporative cooling of neutral hydroxyl (OH(•)) molecules loaded from a Stark-decelerated beam into an extremely high-gradient magnetic quadrupole trap. We demonstrate cooling by at least one order of magnitude in temperature, and a corresponding increase in phase-space density by three orders of magnitude, limited only by the low-temperature sensitivity of our spectroscopic thermometry technique. With evaporative cooling and a sufficiently large initial population, much colder temperatures are possible; even a quantum-degenerate gas of this dipolar radical (or anything else it can sympathetically cool) may be within reach.

Cold heteromolecular dipolar collisions.

Cold molecules promise to reveal a rich set of novel collision dynamics in the low-energy regime. By combining for the first time the techniques of Stark deceleration, magnetic trapping, and cryogenic buffer gas cooling, we present the first experimental observation of cold collisions between two different species of state-selected neutral polar molecules. This has enabled an absolute measurement of the total trap loss cross sections between OH and ND(3) at a mean collision energy of 3.6 cm(-1) (5 K). Due to the dipolar interaction, the total cross section increases upon application of an external polarizing electric field. Cross sections computed from ab initio potential energy surfaces are in agreement with the measured value at zero external electric field. The theory presented here represents the first such analysis of collisions between a (2)Π radical and a closed-shell polyatomic molecule.

Cold N+NH collisions in a magnetic trap.

We present an experimental and theoretical study of atom-molecule collisions in a mixture of cold, trapped N atoms and NH molecules at a temperature of ∼600 mK. We measure a small N+NH trap loss rate coefficient of k(loss)(N+NH)=9(5)(3)×10(-13) cm(3) s(-1). Accurate quantum scattering calculations based on ab initio interaction potentials are in agreement with experiment and indicate the magnetic dipole interaction to be the dominant loss mechanism. Our theory further indicates the ratio of N+NH elastic-to-inelastic collisions remains large (>100) into the mK regime.

Suppression of angular momentum transfer in cold collisions of transition metal atoms in ground States with nonzero orbital angular momentum.

The Zeeman relaxation rate in cold collisions of Ti(3d(2)4s(2) 3F2) with He is measured. We find that collisional transfer of angular momentum is dramatically suppressed due to the presence of the filled 4s(2) shell. The degree of electronic interaction anisotropy, which is responsible for Zeeman relaxation, is estimated to be about 200 times smaller in the Ti-He complex than in He complexes with typical non-S-state atoms.

Magnetic trapping of rare-earth atoms at millikelvin temperatures.

The ability to create quantum degenerate gases has led to the realization of Bose-Einstein condensation of molecules, atom-atom entanglement and the accurate measurement of the Casimir force in atom-surface interactions. With a few exceptions, the achievement of quantum degeneracy relies on evaporative cooling of magnetically trapped atoms to ultracold temperatures. Magnetic traps confine atoms whose electronic magnetic moments are aligned anti-parallel to the magnetic field. This alignment must be preserved during the collisional thermalization of the atomic cloud. Quantum degeneracy has been reached in spherically symmetric, S-state atoms (atoms with zero internal orbital angular momentum). However, collisional relaxation of the atomic magnetic moments of non-S-state atoms (non-spherical atoms with non-zero internal orbital angular momentum) is thought to proceed rapidly. Here we demonstrate magnetic trapping of non-S-state rare-earth atoms, observing a suppression of the interaction anisotropy in collisions. The atoms behave effectively like S-state atoms because their unpaired electrons are shielded by two outer filled electronic shells that are spherically symmetric. Our results are promising for the creation of quantum degenerate gases with non-S-state atoms, and may facilitate the search for time variation of fundamental constants and the development of a quantum computer with highly magnetic atoms.