Laser-cooled Atoms Research Articles

Progress toward an EDM measurement in 225Ra

Permanent electric dipole moments (EDMs) in atoms or molecules are a signature of time-reversal and parity violation and represent an important window onto physics beyond the Standard Model. We are developing a next generation EDM search based on laser-cooled and trapped 225Ra atoms. Due to octupole deformation of the nucleus, 225Ra is predicted to be two to three orders of magnitude more sensitive to T-violating interactions than 199Hg, which currently sets the most stringent limits in the nuclear sector. We will discuss progress toward realizing a first EDM measurement for 225Ra.

Stacked Optical Precursors from Amplitude and Phase Modulations

We report the generation of stacked optical precursors from a laser beam whose amplitude or phase is modulated by sequenced on-off step waveforms. Making use of the constructive interference between the precursors produced from different steps, as well as the main field, we generate optical transient pulses having peak powers of eight times the input power with electromagnetically induced transparency in laser-cooled atoms.

Optical Interface Created by Laser-Cooled Atoms Trapped in the Evanescent Field Surrounding an Optical Nanofiber

Trapping and optically interfacing laser-cooled neutral atoms are essential requirements for their use in advanced quantum technologies. Here we simultaneously realize both of these tasks with cesium atoms interacting with a multicolor evanescent field surrounding an optical nanofiber. The atoms are localized in a one-dimensional optical lattice about 200 nm above the nanofiber surface and can be efficiently interrogated with a resonant light field sent through the nanofiber. Our technique opens the route towards the direct integration of laser-cooled atomic ensembles within fiber networks, an important prerequisite for large scale quantum communication schemes. Moreover, it is ideally suited to the realization of hybrid quantum systems that combine atoms with, e.g., solid state quantum devices.

Recombination during expansion of ultracold plasma

Signals of ultracold plasma are observed by two-photon ionization of laser-cooled caesium atoms in a magneto-optical trap. Recombination of ions and electrons into Rydberg atoms during the expansion of ultracold plasma is investigated by using state-selective field ionization spectroscopy. The dependences of recombination on initial electron temperature (1–70 K) and initial ion density (∼1010 cm−3) are investigated. The measured dependence on initial ion density is N1.547+0.004 at a delay time of 5 μs. The recombination rate rapidly declines as initial electron temperature increases when delay time is increased. The distributions of Rydberg atoms on different values of principal quantum number n, i.e. n = 30–60, at an initial electron temperature of 3.3 K are also investigated. The main experimental results are approximately explained by the three-body recombination theory.

Molecular heat pump for rotational states

In this work, we investigate the theory for three different unidirectional population-transfer schemes in trapped multilevel systems which can be utilized to cool molecular ions. The approach we use exploits the laser-induced coupling between the internal and motional degrees of freedom so that the internal state of a molecule can be mapped onto the motion of that molecule in an external trapping potential. By sympathetically cooling the translational motion back into its ground state, the mapping process can be employed as part of a cooling scheme for molecular rotational levels. This step is achieved through a common mode involving a laser-cooled atom trapped alongside the molecule. For the coherent mapping, we will focus on adiabatic passage techniques which may be expected to provide robust and efficient population transfers. By applying far-detuned chirped adiabatic rapid passage pulses, we are able to achieve an efficiency of better than $98%$ for realistic parameters and including spontaneous emission. Even though our main focus is on cooling molecular states, the analysis of the different adiabatic methods has general features which can be applied to atomic systems.

Nagaoka ferromagnetism in large-spin fermionic and bosonic systems

We study the magnetic properties of itinerant quantum magnetic particles, described by a generalized Hubbard model with large spin $(S>1/2)$, which may be realized in optical lattices of laser-cooled atom systems. In fermion systems (half-integer spins), an extended form of Nagaoka ferromagnetism may be realized. However, as novel aspects of the large-spin cases, we found that the condition on the lattice connectivity is more stringent than in the case of $S=1/2$ particles and that the system shows a peculiar degenerate structure of the ground state in which the ferromagnetic state is included. In contrast, it turns out that the ground state of itinerant bosonic systems (integer spins) has a degenerate structure similar to that of fermion system with $S>1/2$ regardless of the shape, connectivity, or filling of the lattice, and that the state with the maximum total spin is always one of the ground states. Because the system consists of $2S+1$ types of particles and we study a $\text{SU}(2S+1)$ invariant model, the degeneracy of the ground state is given by the multiplets of the fully symmetric Young tableau of $\text{SU}(2S+1)$ if the state with maximum total spin belongs to the ground state.

Atom-interferometric studies of light scattering

We have used an echo-type atom interferometer that manipulates laser-cooled atoms in a single ground state to investigate the effect of light scattering from pulsed and continuous-wave light. The interferometer uses two off-resonant standing-wave pulses applied at times t=0 and t=T to diffract and recombine momentum states separated by 2({Dirac_h}/2{pi})k at t=2T. Matter wave interference is associated with the formation of a density grating with period {lambda}/2 in the vicinity of this echo time. The grating contrast is measured by recording the intensity of coherently backscattered light. The interferometer is perturbed by an additional pulse applied at t=2T-{delta}T or by continuous-wave background light. If the additional pulse is a standing wave, the momentum states interfering at t=2T are displaced and the grating contrast can be completely recovered due to constructive interference. In this case, the contrast shows a periodic modulation at the atomic recoil frequency as a function of {delta}T. In a recent work, it was shown that the atomic recoil frequency can be measured easily and precisely when using coherence functions to model the signal shape. This paper provides an alternative description of the signal shape through an analytical calculation of echo formation in the presence of an additionalmore » standing-wave pulse. Using this treatment, it is possible to model the effects of spontaneous emission and spatial profile of the laser beam on the signal shape. Additionally, the theory predicts scaling laws as a function of the pulse area and the number of additional standing-wave pulses. These scaling laws are investigated experimentally and can be exploited to improve precision measurements of the atomic recoil frequency. We also show that coherence functions can be used to make a direct measurement of the populations of momentum states associated with the ground state under conditions where the Doppler-broadened velocity distribution of the sample is much larger than the recoil velocity. These measurements are consistent with Monte Carlo wave-function simulations. If the additional pulse is a traveling wave, we find that the grating contrast measured as a function of {delta}T can be modeled by a quasiperiodic coherence function as in previous experiments that utilized atomic beams. In this work, we investigate the dependence of the photon scattering rate on the intensity and detuning of the traveling wave. We also study the effects of perturbing the interferometer with continuous-wave light and find that the dependence of the photon scattering rate on the intensity and detuning of the perturbing field is consistent with expectations.« less

Exchanging the ionic partner in a linear Paul trap: the MgH+(X1Σ+) ion with neutral Rb(2S)

In the present computational study we discuss the behavior of the MgH+(X1Σ+) molecular ion interacting with Rb(2S) and argue that it constitutes an amenable system for ultracold chemistry studies in a Coulomb crystal environment. In particular, we find from our calculations that there is a number of different ways in which this system can chemically evolve and we present results from a diabatic representation of the potential energy surfaces (PESs) of the electronic states which are relevant for the realistic description of an experimental setup in which a cloud of ultracold laser-cooled Rb atoms is superimposed onto a Coulomb crystal arrangement of MgH+ molecular ions. Our findings suggest the most likely pathways for either the formation of a new Coulomb crystal of Rb ions or the combination of cold Mg+, MgH+ and Rb+ mixtures within the trap under consideration.

Stationary Light Pulses in Cold Atomic Media and without Bragg Gratings

We study the creation of stationary light pulses (SLPs), i.e., light pulses without motion, based on the effect of electromagnetically induced transparency with two counterpropagating coupling fields in cold atoms. We show that the Raman excitations created by counterpropagating probe and coupling fields prohibit the formation of SLPs in media of cold and stationary atoms such as laser-cooled atom clouds, Bose condensates or color-center crystals. A method is experimentally demonstrated to suppress these Raman excitations and SLPs are realized in laser-cooled atoms. Furthermore, we report the first experimental observation of a bichromatic SLP at wavelengths for which no Bragg grating can be established. Our work advances the understanding of SLPs and opens a new avenue to SLP studies for few-photon nonlinear interactions.

The kinematic cooling of molecules with laser-cooled atoms

We propose a new scheme for the production of milli-Kelvin molecules via kinematic cooling through collisions with atoms in a magneto-optical trap (MOT). We will discuss the kinematic conditions necessary for producing cold molecules, the limits of the final attainable temperatures and the experimental implementation of this technique. Finally, we will look at some specific physical systems and discuss the effectiveness of kinematic cooling inside a MOT.

A three‐dimensional Brownian motor, realised with symmetric optical lattices

AbstractA three‐dimensional Brownian motor is realised using lasercooled caesium atoms trapped in a system of two static, and individually symmetric, optical lattices; a so‐called double optical lattice. Isotropic fluctuations, emanating from light scattering, are rectified, and the diffusion of the ensemble of atoms is biased, with a resulting constant velocity that is controllable both in direction and magnitude. The working principle of the Brownian motor can be seen as a pulsation between two different potentials, both symmetric but around different points. The correlation between interferometric spatial offsets, and imbalance in optical pumping rates, leads to a spatio‐temporal asymmetry sufficient for generating a controlled, directed motion (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optimizing the retrieval efficiency of stored light pulses

We study the retrieval efficiency of stored light pulses based on electromagnetically induced transparency in multiple simultaneously driven Lambda-systems. The light pulses are stored in coherences between different Zeeman states of laser-cooled atoms. When the stray magnetic field from the environment is minimized by compensation coils we observed a smaller retrieved probe pulse amplitude than for a small externally applied magnetic field, i.e., a seemingly shorter coherence time. We identify this effect as a beating of several coherences due to a very small uncompensated dc magnetic stray field. By intentionally applying a small magnetic field larger than this stray field we were able to increase the retrieved probe pulse amplitude up to five-fold to the value determined by the true coherence time of our system.

Method for producing ultracold molecular ions

We propose a method for the production of ultracold molecular ions. This method utilizes sympathetic cooling due to the strong collisions between appropriately chosen molecular ions and laser-cooled neutral atoms to realize ultracold internal ground-state molecular ions. In contrast to other experiments producing cold molecular ions, our proposed method efficiently cools both the internal and external molecular-ion degrees of freedom. The availability of an ultracold, absolute ground-state sample of molecular ions would have broad impact to fields as diverse as quantum chemistry, astrophysics, and fundamental physics and may lead to the development of a robust scalable quantum computer.

Technique for measuring atomic recoil frequency using coherence functions

We have developed a technique for measuring the atomic recoil frequency using a single-state echo-type atom interferometer that manipulates laser-cooled atoms in the ground state. The interferometer relies on momentum-state interference due to two standing-wave pulses that produce density gratings. The interference is modified by applying a third standing-wave pulse during the interferometer pulse sequence. As a result, the grating contrast exhibits periodic revivals at the atomic recoil frequency ${\ensuremath{\omega}}_{r}$ as a function of the time at which the third pulse is applied, allowing ${\ensuremath{\omega}}_{r}$ to be measured easily and precisely. The contrast is accurately described by a coherence function, which is the Fourier transform of the momentum distribution, produced by the third pulse and by the theory of echo formation. If the third pulse is a traveling wave, loss of grating contrast is observed, an effect also described by a coherence function. The decay of the grating contrast as a function of continuous-wave light intensity is used to infer the cross section for photon absorption.

Dynamics of a laser-cooled and trapped radiator interacting with the Holstein–Primakoff SU(1,1) coherent state

We study the interaction between a laser-cooled and trapped equidistant three-level atom (ion) and a single-mode cavity field. The dipole moment matrix transition elements between the adjacent atomic energy levels d12 and d23 are assumed to be different. This problem generalizes the model of a pair of indistinguishable two-level atoms. An intensity-dependent coupling is assumed between the three-level atom and the radiation field. In this situation, we suppose that at the initial moment the field is in the Holstein–Primakoff SU(1,1) coherent state and obtain the exact analytical solution for the atom–field state vector. The quantum-statistical and squeezing properties of the radiation field are investigated. The obtained results are compared with those for the single two-level atom model. We observe that the exact periodicity of the squeezing revivals that was observed in the case of the single two-level atom is violated in the model involving the equidistant three-level radiator with different dipole moment matrix transition elements. In other words, the exact periodicity of the physical quantities can be destroyed only when more than two levels of the single-atom model are taken into account. Two limiting cases are considered. In the first case, when d12 → d23, the quantum-statistical and squeezing properties of the single-mode cavity field are similar to those for a pair of indistinguishable two-level atoms. In the second case, when d12 → 0, the exact periodicity of the squeezing revivals takes place. This limiting case is equivalent to the single two-level atom model.

Efficient generation of cold atoms towards a source for atom lithography

We report on the efficient generation of cold rubidium atoms as a potential coherent atom source for atom lithography. We successfully trapped and cooled 2.6 × 108 atoms in 5 s with a conventional magneto-optical trap simply by enlarging the diameter of the laser beam to 20 mm. The size of the laser-cooled atom cloud was measured to be 10 × 7 × 7 mm3. The number of trapped atoms was approximately 10 times as large as that of previous typical results, while the loading time of atoms remained the same.

Creation of an Ultracold Plasma by Photoionizing Laser-Cooled Caesium Atom

The signals of ultracold plasma are observed by two-photon ionization of laser-cooled atom in a caesium magnetooptical trap. A simple model has been introduced to explain the creation of plasma, and the mechanism is further investigated by changing the energy of a pulsed dye laser and the number of initial cooled atoms.

Formation of Ultracold Polar Molecules in the Rovibrational Ground State

Ultracold LiCs molecules in the absolute ground state X1Sigma+, v'' = 0, J'' = 0 are formed via a single photoassociation step starting from laser-cooled atoms. The selective production of v'' = 0, J'' = 2 molecules with a 50-fold higher rate is also demonstrated. The rotational and vibrational state of the ground state molecules is determined in a setup combining depletion spectroscopy with resonant-enhanced multiphoton ionization time-of-flight spectroscopy. Using the determined production rate of up to 5 x 10(3) molecules/s, we describe a simple scheme which can provide large samples of externally and internally cold dipolar molecules.

Low-frequency-noise diode laser for atom interferometry

We investigate the influence of fast frequency fluctuations of an interrogation laser on the obtainable signal-to-noise ratio in Ramsey-Borde atom interferometers. It is shown that the excessive high-frequency noise of diode lasers can degrade the signal-to-noise ratio significantly in an atom interferometer. To overcome these limitations, we have devised a low-noise light source by frequency filtering a diode laser in a high-finesse cavity and further amplifying the transmitted power by injection locking a solitary laser diode. When this laser source was used for atom interferometry with laser-cooled calcium atoms, the signal-to-noise ratio could be improved further by a factor of 3.5 compared with the use of the unfiltered 1 Hz linewidth laser source.

Magneto-Optical-Trap-Based, High Brightness Ion Source for Use as a Nanoscale Probe

We report on the demonstration of a low emittance, high brightness ion source based on magneto-optically trapped neutral atoms. Our source has ion optical properties comparable to or better than those of the commonly used liquid metal ion source. In addition, it has several advantages that offer new possibilities, including high resolution ion microscopy with ion species tailored for specific applications, contamination-free ion milling, and nanoscale implantation of a variety of elements, either in large quantities, or one at a time, deterministically. Using laser-cooled Cr atoms, we create an ion beam with a normalized rms (root-mean-square) emittance of 6.0 x 10 (-7) mm mrad M e V and approximately 0.25 pA of current, yielding a brightness as high as 2.25 A cm (-2) sr (-1) eV (-1). These values of emittance and brightness show that, with suitable ion optics, an ion beam with a useful amount of current can be produced and focused to spot sizes of less than 1 nm.