Cloud Of Cold Atoms Research Articles

Spatial light modulators for the manipulation of individual atoms

We propose a versatile arrangement for the trapping and manipulation of single atoms in optical tweezers formed by the direct image of a spatial light modulator (SLM). The scheme incorporates a high numerical aperture microscope to map the intensity distribution of a SLM onto a cloud of cold atoms. The regions of high intensity act as optical dipole-force traps. With a SLM fast enough to modify the trapping potential in real time, this technique is well suited for the controlled addressing and manipulation of arbitrarily selected atoms.

Engineering two-mode squeezed states of cold atomic clouds with a superconducting stripline resonator

A scheme is proposed for engineering two-mode squeezed states of two separated cold atomic clouds positioned near the surface of a superconducting stripline resonator. Based on the coherent magnetic coupling between the atomic spins and a stripline resonator mode, the desired two-mode squeezed state can be produced via precisely control on the dynamics of the system. This scheme may be used to realize scalable on-chip quantum networks with cold atoms coupling to stripline resonators.

Realization of an 85Rb Atomic Fountain

An atomic fountain with 85Rb cold atoms is reported. A series of time-of-flight signals is obtained, and the measured temperature of the cold atomic cloud is about 2.4 μK. It will have potential new applications in the precise measurement of fundamental constants and the proof of the Einstein's equivalence principle.

Dynamics of a cold atom cloud in an anharmonic trap

A combined gravitomagnetic trap was used to measure how the anharmonicity of a trapping potential affects the dynamics of a cold atomic cloud. A displacement of the effective potential minimum as a function of the ensemble temperature was observed. The effect is a direct consequence of the thermal nature of the atomic sample. The results of both a theory approach to the dynamical evolution of the atomic cloud and a numerical simulation are in very good agreement with the experimental observations. The effect can be exploited to devise a novel measurement technique for the temperature of a trapped atomic cloud, as well as to separate thermal and condensed phases in Bose-Einstein condensation experiments.

Two-photon free-induction decay with electromagnetically induced transparency

We report the observation of coherent two-photon free-induction decay (FID) in a three-level Lambda system excited by a low-intensity square-modulated laser pulse. Using electromagnetically induced transparency in an Rb85 cold atomic cloud, for what we believe to be the first time, we observe FID signals with coherence time exceeding the relevant atomic excited-state natural lifetime. Because of the on-resonance enhancement, the two-photon FID signal can be observed at the falling edge without using heterodyne means.

The pump-probe approach to coherent backscattering of intense laser light from cold atoms

We present a new approach to near-resonant coherent backscattering of light from cold two-level atoms. In the dilute regime, where the distance between atoms is much larger than the laser wavelength, this approach is able to account for multi-photon scattering processes between many atoms through solutions of single-atom optical Bloch equations. We elaborate the method for double scattering from two atoms, and discuss the way of its extension for dilute, cold atomic clouds.

Nonadiabatic transport of cold atoms in a magnetic quadrupole potential

We experimentally demonstrated the transport of cold atoms in a magnetic quadrupole trap in the nonadiabatic regime on the order of the trap period. The trap potential velocity was controlled by varying the currents of the two pairs of anti-Helmholtz coils. We efficiently transported cold atoms with two velocity regimes. For example, with a potential velocity of 18 (28 cm/s), the final velocity of a cold atom cloud is 0.6±2.7 (58.6±1.5 cm/s). The trap motion with proper velocity could expel energetic atoms from the trap, resulting in a decline in temperature of high residual velocity atoms.

Controlled generation of field squeezing with cold atomic clouds coupled to a superconducting transmission line resonator

We propose an efficient method for controlled generation of field squeezing with cold atomic clouds trapped close to a superconducting transmission line resonator. It is shown that, based on the coherent strong magnetic coupling between the collective atomic spins and microwave fields in the transmission line resonator, two-mode or single-mode field squeezed states can be generated through coherent control on the dynamics of the system. The degree of squeezing and preparing time can be directly controlled through tuning the external classical fields. This protocol may offer a promising platform for implementing scalable on-chip quantum information processing with continuous variables.

Manipulation of slow light with orbital angular momentum in cold atomic gases

We study the propagation of a weak pulse of slow light in a cloud of cold atoms controlled by two additional laser beams of larger intensity in a tripod configuration of the light-matter coupling. We consider a case where one of the control beams has an optical vortex and thus has a zero intensity at the center. The presence of the second control beams restores adiabaticity in the propagation of the probe beam. This makes it possible to exchange the optical vortex between the control and probe fields during the storage. We analyze conditions for the vortex of the control beam to be transferred efficiently to the restored probe beam.

Non-iterative imaging of inhomogeneous cold atom clouds using phase retrieval from a single diffraction measurement

We demonstrate a new imaging technique for cold atom clouds based on phase retrieval from a single diffraction measurement. Most single-shot diffractive imaging methods for cold atoms assume a monomorphic object to extract the column density. The method described here allows quantitative imaging of an inhomogeneous cloud, enabling recovery of either the atomic density or the refractive index, provided the other is known. Using ideas borrowed from density functional theory, we calculate the approximate paraxial diffracted intensity derivative from the measured diffracted intensity distribution and use it to solve the Transport of Intensity Equation (TIE) for the phase of the wave at the detector plane. Back-propagation to the object plane yields the object exit surface wave and then provides a quantitative measurement of either the atomic column density or refractive index. Images of homogeneous clouds showed good quantitative agreement with conventional techniques. An inhomogeneous cloud was created using a cascade electromagnetically induced transparency scheme and images of both phase and amplitude parts of refractive index across the cloud were separately retrieved, showing good agreement with theoretical results.

Polarization Gradient Cooling by Zeeman-Effect-Assisted Saturated Absorption

A novel and simple method to realize polarization gradient cooling (PGC) is reported. The stabilizing, shifting and rapid tuning of the frequency of the external cavity diode laser is realized by using the Zeeman-effect-assisted Doppler-free saturated absorption technique. Based on this convenient technique, 87Rb cold atoms are captured from room-temperature background vapor in the magneto-optical trap (MOT). Meanwhile, the steady-state number, the density and the lifetime of atoms in the MOT are measured. Subsequently, a frequency-fast-varying circuit is designed to realize PGC, which is demonstrated effectively and reliably in experiments. The temperature of the cold atom cloud is measured by two different methods, which coincide with each other.

The role of collisions and strong coupling in ultracold plasmas

Ultracold plasmas are formed by photo-exciting clouds of cold atoms and molecules near the ionization threshold. They explore a new region of plasma physics and display effects of strong coupling, which is characterized by a ratio of Coulomb energy to kinetic energy that is greater than unity. Collisions of many types play a role in the creation, equilibration, and expansion of these systems.

Cold atomic clouds and Bose-Einstein condensates passing through a Gaussian beam

We have experimentally studied and numerically simulated the behavior of cold atomic clouds and Bose-Einstein condensates passing through a far red-detuned Gaussian beam. Several exotic phenomena such as the focusing and advancement of atomic clouds have been exhibited. The cooling effect and the effective interaction length of the dipole potential to the atoms have also been discussed.

Realization of Green MOT for Ytterbium Atoms

We report the experimental realization of a magneto-optical trap (MOT) of 174Yb atoms operating on the 1So−3 P1 intercombination transition at 555.8 nm. The green MOT is loaded by a Zeeman-slowed atomic beam. In order to increase the capture velocity of the MOT, we use the trapping laser beams consisting of five discrete frequency components obtained by modulating the laser light through an electro-optic modulator. The trapped atomic number of the 174Yb isotope is about 6.2 × 105, and the temperature of the cold atomic cloud is estimated to be about 100 μK. The success of the green MOT is an important step towards the goal of an ytterbium optical clock.

Threshold of a random laser based on Raman gain in cold atoms

We address the problem of achieving a random laser with a cloud of cold atoms, in which gain and scattering are provided by the same atoms. In this system, the elastic scattering cross-section is related to the complex atomic polarizability. As a consequence, the random laser threshold is expressed as a function of this polarizability, which can be fully determined by spectroscopic measurements. We apply this idea to experimentally evaluate the threshold of a random laser based on Raman gain between non-degenerate Zeeman states and find a critical optical thickness on the order of 200, which is within reach of state-of-the-art cold-atom experiments.

Optimized temperature measurement with time-of-flight method

Driven by resonance probe light background Rubidium gas can emit fluorescence, which interacts with the falling atomic cloud in the temperature measurement with the time-of-flight (TOF) method and influence the experimental results. The dependencies of the acceleration and the temperature of the atomic cloud on the detuning of the probe beams were studied. We propose that the deviation of the acceleration from the gravity acceleration can be taken as a criterion of the accuracy of the temperature measurement using TOF method. Moreover, using the principle of the radiation force on the cold atomic cloud, one may measure the considerably weak intensity of the fluorescence.

Properties of the Photonic Hall Effect in Cold Atomic Clouds

On the basis of exact numerical simulations and analytical calculations, we describe qualitatively and quantitatively the interference processes at the origin of the photonic Hall effect for resonant Rayleigh (point-dipole) scatterers in a magnetic field. For resonant incoming light, the induced giant magneto-optical effects result, even for magnetic field strength as low as a few mT, in relative Hall currents in the percent range. This suggests that the observation of the photonic Hall effect in cold atomic vapors is within experimental reach.

Threshold of a Random Laser with Cold Atoms

We address the problem of achieving an optical random laser with a cloud of cold atoms, in which gain and scattering are provided by the same atoms. The lasing threshold can be defined using the on-resonance optical thickness b0 as a single critical parameter. We predict the threshold quantitatively, as well as power and frequency of the emitted light, using two different light transport models and the atomic polarizability of a strongly pumped two-level atom. We find a critical b0 on the order of 300, which is within reach of state-of-the-art cold-atom experiments. Interestingly, we find that random lasing can already occur in a regime of relatively low scattering.

The Laser Glow of an Atom Cloud

Calculations reveal that a dense cloud of cold atoms pumped with lasers can glow with its own laser light. Such ‘random lasers’ have previously been made only from solid and liquid materials.

Tapered optical fibers as tools for probing magneto-optical trap characteristics

We present a novel technique for measuring the characteristics of a magneto-optical trap (MOT) for cold atoms by monitoring the spontaneous emission from trapped atoms coupled into the guided mode of a tapered optical nanofiber. We show that the nanofiber is highly sensitive to very small numbers of atoms close to its surface. The size and shape of the MOT, determined by translating the cold atom cloud across the tapered fiber, is in excellent agreement with measurements obtained using the conventional method of fluorescence imaging using a charge coupled device camera. The coupling of atomic fluorescence into the tapered fiber also allows us to monitor the loading and lifetime of the trap. The results are compared to those achieved by focusing the MOT fluorescence onto a photodiode and it was seen that the tapered fiber gives slightly longer loading and lifetime measurements due to the sensitivity of the fiber, even when very few atoms are present.