Session T3: Cavities and Laser Cooling

Superradiant Supercooling

We will describe recent progress learning to exploit collective effects of many laser-cooled atoms in a medium finesse optical cavity. We will first describe experimental work attempting to observe a newly predicted collective cooling mechanism in which rubidium atoms are cooled as they undergo lasing in a deep bad-cavity or superradiant regime. It is predicted that the cooling rate is collectively enhanced by the number of atoms, and the ultimate temperature limits can be well below the limit for standard cavity cooling. We will describe a novel trapping geometry for allowing the atoms to undergo guided falling in the cavity mode, and if time permits, we will discuss the generation of 10 dB of spatially-homogeneous spin-squeezing for future work on entanglement-enhanced atom interferometry.   

Laser cooled anions as a sympathetic coolant

Several ongoing experiments at CERN aim at testing the CPT theorem and the weak equivalence principle using antimatter, among them the AEgIS experiment. For the latter, antiprotons inside a Penning trap interacting with Rydberg positronium form antihydrogen, which will then be used for precision measurements. The achievable sensitivity of these measurements is determined by the antihydrogen temperature which, for this production scheme, is determined by the temperature of the antiprotons. We are investigating the use of laser-cooled anionic molecules to sympathetically cool antiprotons confined in the same trapping potential. A test setup to produce cold ground state C2- molecules is currently being commissioned. This setup will be presented, together with a theoretical study on the feasibility of several laser cooling schemes, including one using the AC-Stark shift. Laser cooling of anions --- which has so far never been achieved --- would also enable the sympathetic cooling of any other negatively charged species, opening new opportunities in a variety of research areas.   

Cooperatively coupled motion with superradiant and subradiant atoms

We investigate the coupled motion of cooperative atoms subjected to the Doppler dissipative force. The dipole-dipole interaction introduces mutual decay channel and splits the super-radiant and sub-radiant states. The Doppler force is thus modified due to the collective emission and coupled recoil. Such a cooperative effect is more evident when the inter-atom separation is less than or comparable to a wavelength. In an optical molasses, we find that, along the axis of two atoms, there presents an effective potential with mechanically stable and unstable regions alternatively as their separation increases. Taking the cooperative Lamb shift into account, we map out the stability diagram and investigate the blockade effect.   

Build-up cavity enhanced photoionization of ultracold atoms as a source for focused ion beams

Focused ion beams (FIBs) are indispensable tools in the semiconductor industry and materials science as they offer the possibility for in situ sample manipulation at the nanometer length scale. The FIB probe size is limited by aberrations of the electrostatic lens system and the transverse reduced brightness and energy spread of the ion beam. Here we present measurements of these beam parameters for a new FIB source that is based on the photoionization of a magneto-optically compressed beam of $^{85}$Rb atoms. Recent measurements have shown that the transverse reduced brightness of the atomic beam is six times higher than that of the industry standard liquid metal ion source. Furthermore, comparison of current measurements with numerical calculations of the two-step photoionization shows that 75\% of the atoms can be ionized when using a build-up cavity to enhance the intensity in the ionization laser beam. The maximum current measured to date is 600 pA. A retarding field analyser is used to measure the energy spread of the beam which is mostly determined by the range over which the atoms are ionized inside an electric field that is needed to prevent disorder-induced heating. The experimental results will be presented together with their implications for the FIB probe size.   

Emergence of Coherence from Incoherence in Cavity-Coupled Arrays of Three-level Atoms

We investigate the emergence of many-body synchronization in macroscopic arrays of V-type three-level atoms. The two optical transitions are separately coupled to two cavity modes in the bad cavity regime, meaning that for these modes the cavity decay rate is larger than all other relevant system frequencies. While synchronization and superradiance have been demonstrated in two-level arrays coupled to one bad cavity mode, the three-level case, possessing more degrees of freedom, is anticipated to exhibit richer physics. Using the cumulant expansion approach, we find both transitions can individually synchronize when the ground state is incoherently pumped to the two excited states. Of particular interest is the fact that the two-point correlation function between the excited states becomes nonzero and oscillates in time, indicating an emergent coherence between these two levels even in the absence of any external coherent drive. The oscillations are robust and only decay at the collective decay rate (the smallest frequency scale in the problem). We derive analytical expressions for the oscillation frequency and the associated linewidth. We further examine the phase diagrams to determine the parameter regime where the emergent coherence exists.   

Strong Coupling of Two Light Fields mediated by a Single Atom

Light fields consist of photons that carry neither mass nor charge and therefore do not interact in vacuum. Even in nonlinear optical media, typical interaction strengths are negligible at the level of individual quanta. In novel quantum systems, strong interactions between individual photons of a single light field have been demonstrated based on e.g. photon or Rydberg blockade. This led to the realization of single-photon switches, transistors, and phase shifters. Here, we demonstrate how two optical fields coupled to different longitudinal modes of a cavity can be brought to interaction using a single atom. While each field by itself achieves full transmission, already a single photon in one mode suppresses the transmission of the other mode. In analogy to the cavity quantum electrodynamics situation, we refer to this as strongly-coupled light fields. The novel quantum system exhibits single-photon switching and strong correlations between different light fields.   

Combining sideband cooling schemes for fluorescence imaging of fermions in an optical lattice

Quantum gas microscopes offer a unique tool with which to study strongly interacting cold atom systems. We report on the combination of Raman sideband cooling and electromagnetically-induced transparency (EIT) cooling of potassium-40 for this purpose. EIT cooling is performed in the plane perpendicular to the imaging axis via the D$_1$ transition, and provides the fluorescence necessary for imaging. Other laser beams detuned by 25\,GHz from the D$_2$ line drive Raman transitions to lower energy states, enabling an additional cooling mechanism. The scheme is implemented in a 1200\,Er lattice, with a bias magnetic field, and beam polarization keeps the atoms confined to stretched states. Collection of scattered photons through a 0.8\,NA microscope objective results in detection of 600 photons per atom in a two second exposure, which is sufficient to resolve individual atoms with a PSF of FWHM 600\,nm. The combination of these two cooling schemes resulted in a five-fold improvement in photon collection rate relative to either individual scheme for our system, while still allowing for 94\,\% of the atoms to remain pinned between two successive exposures. Fluorescence imaging of samples will allow for characterization of the dynamics of interacting fermions in periodic potentials.   

Success probability of atom-molecule sympathetic cooling: A statistical approach

Sympathetic cooling with ultracold atoms is a promising route toward creating colder and denser ensembles of polar molecules at temperatures below 1 mK. Rigorous quantum scattering calculations can be carried out to identify atom-molecule collision systems with suitable collisional properties for sympathetic cooling experiments. The accuracy of such calculations is limited by the uncertainties of the underlying ab initio interaction potentials. To overcome these limitations, we introduce a statistical approach based on cumulative probability distributions for the ratio of elastic-to-inelastic collision cross sections, from which the success probability of atom-molecule sympathetic cooling can be estimated. Our analysis shows that, for a range of experimentally relevant collision systems, the cumulative probabilities are not sensitive to the number of rotational states in the basis set, potentially leading to a dramatic reduction of the computational cost of simulating cold molecular collisions in external fields.   

Sympathetic laser cooling of antiprotons with molecular anions.

Antimatter experiments conducted at the Antiproton Decelerator (AD) at CERN address the fundamental questions why primordial antimatter is not observed in the present Universe. The weak equivalent principle (WEP) can be tested measuring the gravitational acceleration of antihydrogen atoms in the Earth's gravitational field that are horizontally emitted from a Penning trap. The antihydrogen atoms can be produced via resonant charge exchange of Rydberg positronium and antiprotons at temperatures potentially determined by the recoil limit of the constituents To prepare an ensemble of cold antihydrogen with a narrow velocity spread we plan to extend the existing electron cooling mechanism of antiprotons by laser-cooling techniques of negative C$_{\mathrm{2}}^{\mathrm{-}}$ molecules in a Penning trap in order to sympathetically cool antiprotons to the mK regime. The generation of cold antihydrogen atoms can ultimately also be used for precision spectroscopy experiments of electromagnetic interaction as a test of CPT symmetry. In this presentation the status of the experiment at CERN and a computational study of sympathetic cooling of antiprotons using photodetachment cooling, Doppler and AC Stark Sisyphus cooling of C$_{\mathrm{2}}^{\mathrm{-}}$ will be presented.   

Toward laser cooling and trapping lanthanum ions

Trapped atomic ions are a leading candidate for applications in quantum information. For scalability and applications in quantum communication, it would be advantageous to interface ions with telecom light. We present progress toward laser cooling doubly-ionized lanthanum, which should require only infrared, telecom-compatible light. Since the hyperfine structure of this ion has not been measured, we are using optogalavanic spectroscopy in a hollow cathode lamp to measure the hyperfine spectrum of transitions in lanthanum. Using laser ablation to directly produce ions from a solid target, we laser cool and trap barium ions, and explore extending this technique to lanthanum ions.