Sub-microkelvin Temperatures Research Articles

An ultrastable laser system at 689 nm for cooling and trapping of strontium

We present a 689-nm cavity-based laser system for cooling and trapping strontium atoms. The laser is stabilized to a high-finesse cavity by the Pound–Drever–Hall technique, exhibiting a frequency stability in the $$10^{-14}$$ range for averaging times up to 100 s. A cavity drift of 8 kHz per day is mapped out and compensated. At short times, the laser exhibits a linewidth of a few kilohertz. With this laser system, we realize a magneto-optical trap of strontium operated on the narrow inter-combination transition yielding sub-microkelvin temperatures, and demonstrate absorption spectroscopy on the strontium inter-combination line.

Ionic Impurity in a Bose-Einstein Condensate at Submicrokelvin Temperatures.

Rydberg atoms immersed in a Bose-Einstein condensate interact with the quantum gas via electron-atom and ion-atom interaction. To suppress the typically dominant electron-neutral interaction, Rydberg states with a principal quantum number up to n=190 are excited from a dense and tightly trapped micron-sized condensate. This allows us to explore a regime where the Rydberg orbit exceeds the size of the atomic sample by far. In this case, a detailed line shape analysis of the Rydberg excitation spectrum provides clear evidence for ion-atom interaction at temperatures well below a microkelvin. Our results may open up ways to enter the quantum regime of ion-atom scattering for the exploration of charged quantum impurities and associated polaron physics.

Prospects for narrow-line cooling of KRb molecules in the rovibrational ground state

We propose and experimentally investigate a scheme for narrow-line cooling of KRb molecules in the rovibrational ground state. We show that the spin-forbidden $\mathrm{X^1\Sigma^+} \rightarrow \mathrm{b^3\Pi_{0^+}}$ transition of KRb is ideal for realizing narrow-line laser cooling of molecules because it has highly diagonal Franck-Condon factors and narrow linewidth. In order to confirm the prediction, we performed the optical and microwave spectroscopy of ultracold $^{41}$K$^{87}$Rb molecules, and determined the linewidth ($2\pi\times$ 4.9(4) kHz) and Franck-Condon factors for the $\mathrm{X^1\Sigma^+} (v''=0) \rightarrow \mathrm{b^3\Pi_{0^+}} (v'=0)$ transition (0.9474(1)). This result opens the door towards all-optical production of polar molecules at sub-microkelvin temperatures.

Universalities in ultracold reactions of alkali-metal polar molecules

We consider ultracold collisions of ground-state heteronuclear alkali-metal dimers that are susceptible to four-center chemical reactions $2AB\ensuremath{\rightarrow}{A}_{2}+{B}_{2}$ even at submicrokelvin temperatures. These reactions depend strongly on species, temperature, electric field, and confinement in an optical lattice. We calculate ab initio van der Waals coefficients for these interactions and use a quantum formalism to study the scattering properties of such molecules under an external electric field and optical lattice. We also apply a quantum threshold model to explore the dependence of reaction rates on the various parameters. We find that, among the heteronuclear alkali-metal fermionic species, LiNa is the least reactive, whereas LiCs is the most reactive. For the bosonic species, LiK is the most reactive in zero field, but all species considered, LiNa, LiK, LiRb, LiCs, and KRb, share a universal reaction rate once a sufficiently high electric field is applied. For indistinguishable bosons, the inelastic/reactive rate increases as ${d}^{2}$ in the quantum regime, where $d$ is the dipole moment induced by the electric field. This is a weaker power-law dependence than for indistinguishable fermions, for which the rate behaves as ${d}^{6}$.

Sub-Doppler magneto-optical trap for calcium

We explore an efficient method for preparing large samples of ultracold calcium atoms. An optimized conventional (Doppler-limited) magneto-optical trap collects atoms from a Zeeman cooled atomic beam using a strong dipole transition within the singlet system. This transition is not completely closed thus yielding an intense flux of atoms into the metastable triplet state ${}^{3}{P}_{2}.$ A second magneto-optical trap sharing the same magnetic-field gradient is superimposed which captures and further cools the metastables using the narrow-band infrared transition ${}^{3}{\stackrel{\ensuremath{\rightarrow}}{{P}_{2}}}^{3}{D}_{3}.$ In our present experiment we were able to prepare $3\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms at temperatures below 20 microkelvin within 250 ms. Minor technical improvements of our setup promise to yield above ${10}^{10}$ atoms at submicrokelvin temperatures within 1 s.

The behaviour of nuclear spin systems at spin temperatures below 1μK

At present two techniques are used to study nuclear spin systems at submicro-Kelvin temperatures. The first technique uses brute force nuclear polarization and adiabatic nuclear demagnetization. The second one uses dynamic nuclear polarization and adiabatic demagnetization in the rotating frame. A third method is envisaged using microwave optical nuclear polarization. In Leiden the second method is used to study the proton spin system in Ca(OH) 2. A ferromagnetic ordering with a domain structure and a transverse helical ordering were produced and studied.