Laser-cooled Rubidium Research Articles

Subpicosecond Ultracold Electron Source.

We present the first observation of subpicosecond electron bunches from an ultracold electron source. This source is based on near-threshold, two-step, femtosecond photoionization of laser-cooled rubidium gas in a grating magneto-optical trap. Bunch lengths as short as 735±7 fs (rms) have been measured in the self-compression point of the source by means of ponderomotive scattering of the electrons by a 25fs, 800nm laser pulse. The observed temporal structure of the electron bunch depends on the central wavelength of the ionization laser pulse, in agreement with detailed simulations of the atomic photoionization process. This shows that the bunch length limit imposed by the atomic photoionization process has been reached.

Collective atom-cavity coupling and nonlinear dynamics with atoms with multilevel ground states

Typically, physicists treat cold atoms in a cavity as simple two-level atoms. Here, the authors go beyond this approximation to show that adding more levels causes light-matter coupling to be time dependent and results in nonlinear behavior. They experimentally confirm these effects using laser-cooled rubidium.

Space charge effect in low-density ultracold ion bunches

We have implemented an approach to investigate the space charge effect (SCE) in the ultracold ion bunch produced through the near-threshold photoionization of laser-cooled rubidium atoms trapped in a magneto-optical trap. The non-linear broadening of spatial profile of the ultracold ion bunch induced by SCE within the initial density range of 3.7 × 106–4.5 × 107/cm3 was explored using a time-of-flight spectrometer coupled with an imaging detector. A charged particle tracing simulation accounting for all pairwise ion–ion Coulomb interactions and an analytical model calculation, which predicts the dependence of the ion bunch density on time evolution and initial density, reproduced the experimental results successfully, indicating that the study could capture the evolution dynamics of ion bunch dominated by SCE. The aim of this work is to extend the investigation on SCE to extreme low-density regions of the order of 106 /cm3 and is expected to be useful in optimizing the performance of ultracold ion/electron sources.

Ellipticity-dependent sequential over-barrier ionization of cold rubidium

We perform high-resolution measurements of momentum distribution on Rb$^{n+}$ recoil ions up to charge state $n=4$, where laser-cooled rubidium atoms are ionized by femtosecond elliptically polarized lasers with the pulse duration of 35 fs and the intensity of 3.3$\times$10$^{15}$ W/cm$^2$ in the over-barrier ionization (OBI) regime. The momentum distributions of the recoil ions are found to exhibit multi-band structures as the ellipticity varies from the linear to circular polarizations. The origin of these band structures can be explained quantitatively by the classical OBI model and dedicated classical trajectory Monte Carlo simulations with Heisenberg potential. Specifically, with back analysis of the classical trajectories, we reveal the ionization time and the OBI geometry of the sequentially released electrons, disentangling the mechanisms behind the tilted angle of the band structures. These results indicate that the classical treatment can describe the strong-field multiple ionization processes of alkali atoms.

Long-lived circular Rydberg states of laser-cooled rubidium atoms in a cryostat

The exquisite properties of Rydberg levels make them particularly appealing for emerging quantum technologies. The lifetime of low-angular-momentum laser-accessible levels is however limited to a few $100\,\mu\mathrm{s}$ by optical transitions and microwave blackbody radiation (BBR) induced transfers at room temperature. A considerable improvement would be obtained with the few $10\,\mathrm{ms}$ lifetime of circular Rydberg levels in a cryogenic environment reducing the BBR temperature. We demonstrate the preparation of long-lived circular Rydberg levels of laser-cooled Rubidium atoms in a cryostat. We observe a $3.7\,\mathrm{ms}$ lifetime for the circular level of principal quantum number $n=52$. By monitoring the transfers between adjacent circular levels, we estimate in situ the microwave BBR temperature to be $(11\pm 2)\,\mathrm{K}$. The measured atomic coherence time ($270\,\mu\mathrm{s}$) is limited here only by technical magnetic field fluctuations. This work opens interesting perspectives for quantum simulation and sensing with cold circular Rydberg atoms.

Single-photon-level light storage in cold atoms using the Autler-Townes splitting protocol

Broadband spin-photon interfaces for long-lived storage of photonic quantum states are key elements for quantum information technologies. Yet, reliable operation of such memories in the quantum regime is challenging due to photonic noise arising from technical and/or fundamental limitations in the storage-and-recall processes controlled by strong electromagnetic fields. Here, we experimentally implement a single-photon-level spin-wave memory in a laser-cooled Rubidium gas, based on the recently proposed Autler-Townes splitting (ATS) protocol. We demonstrate storage of 20-ns-long laser pulses, each containing an average of 0.1 photons, for 200 ns with an efficiency of $12.5\%$ and signal-to-noise ratio above 30. Notably, the robustness of ATS spin-wave memory against motional dephasing allows for an all-spatial filtering of the control-field noise, yielding an ultra-low unconditional noise probability of $3.3\times10^{-4}$, without the complexity of spectral filtering. These results highlight that broadband ATS memory in ultracold atoms is a preeminent option for storing quantum light.

Efficient quantum memory for single-photon polarization qubits

A quantum memory, for storing and retrieving flying photonic quantum states, is a key interface for realizing long-distance quantum communication and large-scale quantum computation. While many experimental schemes of high storage-retrieval efficiency have been performed with weak coherent light pulses, all quantum memories for true single photons achieved so far have efficiencies far below 50%, a threshold value for practical applications. Here, we report the demonstration of a quantum memory for single-photon polarization qubits with an efficiency of >85% and a fidelity of >99 %, basing on balanced two-channel electromagnetically induced transparency in laser-cooled rubidium atoms. For the single-channel quantum memory, the optimized efficiency for storing and retrieving single-photon temporal waveforms can be as high as 90.6 %. Our result pushes the photonic quantum memory closer to its practical applications in quantum information processing.

Twists in nonlinear magneto-optic rotation with cold atoms.

We observe a narrow secondary dispersive feature nested within conventional nonlinear magneto-optical rotation (NMOR) signals obtained with a laser-cooled rubidium vapor. A similar feature has been previously named a "twist" by Budker et. al., in the context of warm vapor optical magnetometry [Phys. Rev. A. 81, 5788-5791 (1998)], and was ascribed to simultaneous optical pumping through multiple nearby hyperfine levels. In this work the twist is observed in a cold atom vapor, where the hyperfine levels are individually addressable, and thus is due to a different mechanism. We experimentally and numerically characterize this twist in terms of magnetic field strength, polarization, and optical intensity and find good agreement between our data and numerical models. We find that the twist width is proportional to the magnetic field in the transverse direction, and therefore two independent directions of the magnetic field can be measured simultaneously. This technique is useful as a simple and rapid in-situ method for nulling background magnetic fields.

Prospects for Precise Measurements with Echo Atom Interferometry

Echo atom interferometers have emerged as interesting alternatives to Raman interferometers for the realization of precise measurements of the gravitational acceleration g and the determination of the atomic fine structure through measurements of the atomic recoil frequency ω q . Here we review the development of different configurations of echo interferometers that are best suited to achieve these goals. We describe experiments that utilize near-resonant excitation of laser-cooled rubidium atoms by a sequence of standing wave pulses to measure ω q with a statistical uncertainty of 37 parts per billion (ppb) on a time scale of ∼50 ms and g with a statistical precision of 75 ppb. Related coherent transient techniques that have achieved the most statistically precise measurements of atomic g-factor ratios are also outlined. We discuss the reduction of prominent systematic effects in these experiments using off-resonant excitation by low-cost, high-power lasers.

Collisional state-changing of OH− rotations by interaction with Rb atoms in cold traps

We employ an accurate, ab initio potential energy surface (PES) which describes the electronic interaction energy between the molecular anion OH− (1Σ+) and the neutral rubidium atom Rb (2S), to evaluate the elastic and inelastic cross sections over a range of energies representative of the conditions of low-T experiments in MOT traps, when combined with laser-cooled rubidium gas. The system is considered to be in its vibrational ground state, while the first four rotational levels are taken to be involved in the cooling and heating collisional processes that are computed here.The corresponding cooling and heating rates up to about 35K are obtained from the calculations and compared with the recent results in a similar experiments, where He was the partner atom of the current anion.

Observation of Ramsey fringes using stimulated Raman transitions in a laser-cooled continuous rubidium atomic beam

We performed the Ramsey experiment of optical separated oscillatory fields based on a cold continuous \(^{87}\hbox {Rb}\) atomic beam. The beam had a free evolution time of 3.4 ms over a short interaction-zone separation of 42 mm, producing a central Raman–Ramsey fringe with a linewidth of 146 Hz (full width at half maximum). This Raman–Ramsey interferometer based on an atomic beam offers a promising solution for developing a highly stable, compact atomic clock.

Nanostructured optical nanofibres for atom trapping

We propose an optical dipole trap for cold, neutral atoms based on the electric field produced from the evanescent fields in a hollow, rectangular slot cut through an optical nanofibre. In particular, we discuss the trap performance in relation to laser-cooled rubidium atoms and show that a far off-resonance, blue-detuned field combined with the attractive surface-atom interaction potential from the dielectric material forms a stable trapping configuration. With the addition of a red-detuned field, we demonstrate how three dimensional confinement of the atoms at a distance of 140–200 nm from the fibre surface within the slot can be accomplished. This scheme facilitates optical coupling between the atoms and the nanofibre that could be exploited for quantum communication schemes using ensembles of laser-cooled atoms.

Measuring the atomic recoil frequency using a modified grating-echo atom interferometer

We describe progress toward a precise measurement of the recoil energy of an atom measured using a perturbative grating-echo atom interferometer (AI) that involves three standing-wave (sw) pulses. With this technique, a perturbing sw pulse is used to shift the phase of excited momentum states---producing a modulation in the contrast of the interference pattern. The signal exhibits narrow fringes that revive periodically at twice the two-photon recoil frequency, $2\omega_q$, as a function of the onset time of the pulse. Experiments are performed using samples of laser-cooled rubidium atoms with temperatures $\lesssim 5$ $\mu$K in a non-magnetic apparatus. We demonstrate a measurement of $\omega_q$ with a statistical uncertainty of 37 parts per $10^9$ (ppb) on a time scale of $\sim 45$ ms in 14 hours. Further statistical improvements are anticipated by extending this time scale and narrowing the signal fringe width. However, the total systematic uncertainty is estimated to be $\sim 6$ parts per $10^6$ (ppm). We describe methods of reducing these systematic errors.

Analogs of Basic Electronic Circuit Elements in a Free-Space Atom Chip

Using a thermal sample of laser-cooled rubidium atoms, we have constructed a neutral-atom circuit analogous to an electronic capacitor discharged through a resistor. The atoms are confined using what we call a free-space atom chip, an optical dipole trap created using a generalized phase-contrast imaging technique. We have also calculated theoretical values for the capacitance and resistance, which agree with our experiments, as well as theoretical value for an atomic analog of electrical inductance. We show that atomic capacitance is analogous to the quantum capacitance, the atomic resistance is analogous to the ballistic, or Sharvin resistance, and the atomic inductance is analogous to kinetic inductance.

A new mini-magnetic lens proposal for coverging the clod atoms

A mini-magnetic lens for laser-cooled rubidium atoms is experimentally demonstrated in this paper. The key component of the mini-magnetic lens is a mini-coil with a radius of 2 mm. When the cold atomic clouds are transported in the vicinity of the coil along its axial, they are compressed in the longitudinal direction due to the interaction with the non-uniform magnetic field of the coil. Given a current carrying time of about 10 ms, the atomic clouds tend to be gradually compressed in the axial direction with the increase of the coil current. When the coil current is greater than 0.9 A, the atomic clouds begin to expand. At a threshold value of 0.9 A, the focus length of the mini-magnetic lens is determined to be about 1.3 mm. Compared with the case that no current passes through the mini-coil, the dimension of the focused atom clouds is one order of magnitude smaller. Moreover, the focus length can be controlled by both the coil current and its carrying time. Numerical simulations are also given which are in agreement with the experimental results.

Laser-cooled atoms inside a hollow-core photonic-crystal fiber

We describe the loading of laser-cooled rubidium atoms into a single-mode hollow-core photonic-crystal fiber. Inside the fiber, the atoms are confined by a far-detuned optical trap and probed by a weak resonant beam. We describe different loading methods and compare their trade-offs in terms of implementation complexity and atom-loading efficiency. The most efficient procedure results in loading of ~30,000 rubidium atoms, which creates a medium with optical depth ~180 inside the fiber. Compared to our earlier study this represents a six-fold increase in maximum achieved optical depth in this system.

磁光阱与磁学阱中测量超冷Rb原子总碰撞截面

The implementation of a technique to measure total collision cross sections using laser-cooled rubidium atoms along with the introduction of room-temperature background gases, confined in both magnetooptic and magnetic traps, is proposed. Atom loss from the trap and total collision cross section can be inferred from the knowledge of the density of background gases. The measured cross sections from the magneto-optic and magnetic traps are represented, compared, and found to be significantly different. The measurements using this technique have a very small error in the range of approximately 2%-7%.

Sensitive gravity-gradiometry with atom interferometry: progress towards an improved determination of the gravitational constant

We present here the current status of our high-sensitivity gravity-gradiometer based on atom interferometry. In our apparatus, two clouds of laser-cooled rubidium atoms are launched in a fountain configuration and simultaneously interrogated by a Raman-pulse interferometry sequence. The system has recently been upgraded and its stability re-evaluated. We also discuss the recent progress of the experiment towards a precise determination of the Newtonian gravitational constant G. The signal-to-noise ratio and the long-term stability of the gravity gradiometer demonstrated interesting perspectives for pushing the G measurement precision below the 100 ppm level.

A compact dual atom interferometer gyroscope based on laser-cooled rubidium

We present a compact and transportable inertial sensor for precision sensing of rotations and accelerations. The sensor consists of a dual atom interferometer operated with laser-cooled 87Rb. Raman processes are employed to coherently manipulate the matter waves. We describe and characterize the experimental apparatus. A method for passing from a compact geometry to an extended interferometer with three independent atom-light interaction zones is proposed and investigated. The extended geometry will enhance the sensitivity by more than two orders of magnitude which is necessary to achieve sensitivities better than 10-8rad/s/ $\sqrt{\rm Hz}$ .

Observation of superfluorescent emissions from laser-cooled atoms

We study superfluorescence (SF) from spherical and cigar-shaped clouds of laser-cooled Rubidium atoms from the $5{D}_{5/2}$ level through the $6{P}_{3/2}$ level to the $5{S}_{1/2}$ ground level. The atomic system is initially excited to the $5{D}_{5/2}$ level from the ground state via two-photon excitation through the intermediate $5{P}_{3/2}$ level. The fluorescence on the $6P\text{\ensuremath{-}}5S$ transition at 420 nm is recorded using time-resolved measurements. The time delays of the observed SF emission peaks typically scale as $\ensuremath{\sim}{N}^{\ensuremath{-}1}$, where $N$ is the atom number, and are much smaller than the time delay expected for uncorrelated cascade fluorescence. Since $N$ is significantly smaller than the threshold number for SF on the 420 nm transition, and larger than the threshold number for the $5D\text{\ensuremath{-}}6P$ transition at $5.2\text{ }\ensuremath{\mu}\text{m}$, our observations suggest that the 420 nm SF emission is triggered by rapid deexcitation of the $5D$ to the $6P$ level via SF at $5.2\text{ }\ensuremath{\mu}\text{m}$. The observed SF time delays for 420 nm emission agree with SF time-delay estimates for the $5.2\text{ }\ensuremath{\mu}\text{m}$ transition. For spherical clouds, the SF is isotropic. For cigar-shaped clouds, the SF is highly anisotropic. Along the long axis of cigar-shaped atom clouds, SF and incoherent cascade fluorescence produce temporally well-resolved peaks in the detected signal. In this case, the SF component of the signal is highly concentrated along a direction in between the directions of the two almost parallel excitation beams. The observed SF intensities scale as $N$, suggesting that the $5D$ level is regeneratively pumped during the SF decay.