Proposal of a frequency-synthesis chain between the microwave and optical frequencies of the Ca intercombination line at 657 nm using diode lasers

We propose a frequency synthesis chain which can directly connect a microwave atomic clock with a visible laser. We design this chain for the frequency measurement of a visible laser locked on the intercombination transition of Ca at 657 nm. The proposed chain is based on both an optical difference frequency divider and an optical frequency comb generator, and it is designed to use nine visible and near-infrared diode lasers. We discuss the technical requirements to realize the frequency measurement accuracy level of 10^–14.     

Two-color photoionization of calcium using SHG and LED light

We present a photoionization method to load single ⁴⁰Ca ions in a linear Paul trap from an atomic beam. Neutral Ca I atoms are resonantly excited from the ground state to the intermediate 4s4p ¹P₁-level using coherent 423 nm radiation produced by single-pass second harmonic generation in a periodically poled KTiOPO₄ crystal pumped with an 120 mW extended-cavity diode laser. Ionization is then attained with a high-power light emitting diode imaged to the trap center, using an appropriately designed optical system composed of standard achromatic doublet lenses. The setup simplifies previous implementations at similar efficiency, and it hardly requires any maintenance at all.     

Improved uncertainty budget for optical frequency measurements with microkelvin neutral atoms: Results for a high-stability 40Ca optical frequency standard

Using a Ca optical frequency standard at 657 nm, we demonstrate a method that reduces uncertainties in absolute frequency measurements of optical transitions using freely expanding neutral atoms. Working with atoms that have been laser cooled to 10 μK, we have developed and employed a new technique that combines launching of cold atom clouds with atom interferometry to measure and optimise spectroscopy beam parameters. When applied to a frequency standard with laser beams of high spatial quality, this approach can potentially reduce residual Doppler effect uncertainties to well below one part in 10¹⁶. With Doppler uncertainties greatly suppressed, we investigate other potential shifts at the 1-Hz level with a multiplexed measurement system that takes advantage of the low instability of the calcium frequency standard (4×10^-15 at 1 s). The resultant fractional frequency uncertainty for the standard is 6.6×10^-15, the lowest uncertainty reported to date for a neutral atom optical standard. PACS 06.30.Ft; 32.30.-r; 39.20.+q     

Frequency measurement of I2 lines in the NIR using Ca and CH4 optical frequency standards

2 lines have been carried out using difference-frequency mixing of the emission of CH₄ and Ca frequency standards at 88 THz (or 3.4 μm) and 456 THz (or 657 nm), respectively. A power of ≈10 pW for the mixing product at 815 nm was obtained using a critically phase-matched KTP crystal and input powers of the order of mW. The relative uncertainty of the measurement was a few times 10^-11, limited by the frequency instability of the laser source locked to I₂. Received: 3 December 1997/Revised version: 16 February 1998     

Subkilohertz enhanced-power diode-laser spectrometer in the visible

We report on a high-performance diode-laser spectrometer operating near 657 nm with narrow linewidth (<0.6kHz) , enhanced power (as much as 40 mW), and low drift (<10Hz/s) . The spectrometer comprised an extended-cavity diode-laser frequency stabilized to a high-finesse optical resonator and a broad-area antireflection coated laser diode as an amplifier with a single-lobe emission pattern of good spatial purity. The spectrometer was used to record time-domain optical Ramsey spectra of laser-cooled Ca atoms with a resolution of 0.6 kHz.     

Direct comparison of two cold-atom-based optical frequency standards by using a femtosecond-laser comb

With a fiber-broadened, femtosecond-laser frequency comb, the 76-THz interval between two laser-cooled optical frequency standards was measured with a statistical uncertainty of 2x10(-13) in 5 s , to our knowledge the best short-term instability thus far reported for an optical frequency measurement. One standard is based on the calcium intercombination line at 657 nm, and the other, on the mercury ion electric-quadrupole transition at 282 nm. By linking this measurement to the known Ca frequency, we report a new frequency value for the Hg(+) clock transition with an improvement in accuracy of ~10(5) compared with its best previous measurement.     

Studying the possibility of deep laser cooling of <Superscript>24</Superscript>Mg atoms in an optical lattice: Two-level quantum model

The possible deep laser cooling of ²⁴Mg atoms in a deep optical lattice in the presence of an additional pumping field resonant to the narrow 3s3s¹S₀ → 3s3p³P₁ (λ = 457 nm) optical transition is studied. Two quantum models of the laser cooling of atoms in the optical trap are compared. One is based on the direct numerical solution to the kinetic quantum equation for an atomic density matrix; it considers both optical pumping and quantum recoil effects during interaction between the atoms and field photons. The second, simplified model is based on decomposing the states of the atoms over the levels of vibration in the optical trap and analyzing the evolution of these states. The comparison allows derivation of optical field parameters (pumping field intensity and detuning) that ensure cooling of the atoms to minimal energies. The conditions for fast laser cooling in an optical trap are found.     

Accumulation of chromium metastable atoms into an optical trap

We report the fast accumulation of a large number of metastable ⁵²Cr atoms in a mixed trap, formed by the superposition of a strongly confining optical trap and a quadrupolar magnetic trap. The steady state is reached after about 400 ms, providing a cloud of more than one million metastable atoms at a temperature of about 100 μK, with a peak density of 10¹⁸ atoms m^-3. We have optimized the loading procedure, and measured the light shift of the ⁵D₄ state by analyzing how the trapped atoms respond to a parametric excitation. We compare this result to a theoretical evaluation based on the available spectroscopic data for chromium atoms.     

Resonant interaction of femtosecond radiation with a cloud of cold <Superscript>87</Superscript>Rb atoms

The resonant interaction of ⁸⁷Rb atoms in a magneto-optical trap with femtosecond laser radiation in the spectral range 760–820 nm has been investigated experimentally. It has been demonstrated that femtosecond laser radiation with a spectral width of 10 nm interacts with an atomic ensemble as a set of spectrally narrow modes and as an ionizing laser field simultaneously. The dynamics of trap loading in the presence of ionization by femtosecond radiation has been studied, and the 5D _5/2 level population produced by an additional weak laser field has been measured.     

Pulsed polarization gradient cooling in an optical dipole trap with a Laguerre-Gaussian laser beam

We utilized a blue-detuned Laguerre-Gaussian (doughnut) laser beam to trap cold rubidium atoms by optical dipole force. ”Pulsed” polarization gradient cooling was applied to the trapped atoms to suppress the trap loss due to heating caused by random photon scattering of the trapping light. In this trap about 10⁸ atoms were initially captured and the trap lifetime was 1.5 s, which was consistent with losses due to background gas collisions. This trap can readily be applied to atom guiding, compression, and evaporative cooling. Received: 10 July 1997 / Received in final form: 5 January 1998 / Accepted: 16 January 1998     

High-resolution spectroscopy with laser-cooled and trapped calcium atoms

We report on high-resolution spectroscopy with two different samples of calcium atoms, in a laser-cooled and deflected beam and in a magneto-optical trap. The atomic beam was excited by spatially separated laser fields. For spectroscopy with stored atoms in a magneto-optical trap we used a multiple-pulse excitation scheme. The resolution as low as 2.5 kHz was limited by residual frequency fluctuations of our dye-laser spectrometer. The results should allow to establish a frequency standard with a relative uncertainty below 10^–14.     

High-resolution diode-laser spectroscopy of calcium

Saturated-absorption signals on the calcium 657 nm transition are observed by direct absorption using diode lasers and a high flux atomic-beam cell. Line-widths as narrow as 65 kHz are observed with a high signal-to-noise ratio. Prospects for using this system as a compact wavelength/frequency reference are considered.     

Feasibility of narrow-line cooling in optical dipole traps

We have investigated the influence of narrow-line laser cooling on the loading of Ca atoms into optical dipole traps. To describe the narrow-line cooling of alkaline-earth atoms in combination with optical dipole trapping, we have developed a model that takes into account the light shifts of the cooling transition in three dimensions. The model is compared with two experimental realizations of optical dipole traps for calcium at the wavelengths 514 nm and 10.6 μm.     

Prospects for precision measurements of atomic helium using direct frequency comb spectroscopy

We analyze several possibilities for precisely measuring electronic transitions in atomic helium by the direct use of phase-stabilized femtosecond frequency combs. Because the comb is self-calibrating and can be shifted into the ultraviolet spectral region via harmonic generation, it offers the prospect of greatly improved accuracy for UV and far-UV transitions. To take advantage of this accuracy an ultracold helium sample is needed. For measurements of the triplet spectrum a magneto-optical trap (MOT) can be used to cool and trap metastable 2³S state atoms. We analyze schemes for measuring the two-photon 2³S →4³S interval, and for resonant two-photon excitation to high Rydberg states, 2³S →3³P →n³S, D. We also analyze experiments on the singlet-state spectrum. To accomplish this we propose schemes for producing and trapping ultracold helium in the 1¹S or 2¹S state via intercombination transitions. A particularly intriguing scenario is the possibility of measuring the 1¹S →2¹S transition with extremely high accuracy by use of two-photon excitation in a magic wavelength trap that operates identically for both states. We predict a “triple magic wavelength” at 412 nm that could facilitate numerous experiments on trapped helium atoms, because here the polarizabilities of the 1¹S, 2¹S and 2³S states are all similar, small, and positive.     

Observation and optimization of DAVLL spectra on the S0-P1 transition of neutral mercury atom

The dichroic atomic vapor laser locking (DAVLL) spectra on the ¹S₀-³P₁ transition of neutral mercury atoms are reported for the first time. Two classes of DAVLL line shapes corresponding to single resonant transition and the combinations of several transitions are experimentally observed and compared with numerical simulation. The dependences of peak-to-peak amplitude and the slope near the zero-crossing point on the axial magnetic field and cell temperature are also investigated. A simple model was introduced to briefly estimate the optimal magnetic field with the largest slope. The optimal operating parameters in a 1 mm cell are highlighted to use DAVLL to lock the frequency of the 253.7 nm UV trap laser of mercury atoms.     

Laser cooling and trapping of ytterbium atoms

The experiments on the laser cooling and trapping of ytterbium atoms are reported, including the two-dimensional transversal cooling, longitudinal velocity Zeeman deceleration, and a magneto-optical trap with a broadband transition at a wavelength of 399 nm. The magnetic field distributions along the axis of a Zeeman slower were measured and in a good agreement with the calculated results. Cold ytterbium atoms were produced with a number of about 10⁷ and a temperature of a few milli-Kelvin. In addition, using a 556-nm laser, the excitations of cold ytterbium atoms at ¹S₀-³P₁ transition were observed. The ytterbium atoms will be further cooled in a 556-nm magneto-optical trap and loaded into a three-dimensional optical lattice to make an ytterbium optical clock.      

Optimizing the production of metastable calcium atoms in a magneto-optical trap

We investigate the production of long-lived metastable (4³ P ₂ ) calcium atoms in a magneto-optical trap (MOT) operating on the 4¹ S ₀ ?4¹ P ₁ transition at 423 nm. For excited 4¹ P ₁ atoms a weak decay channel into the triplet states 4³ P ₂ and 4³ P ₁ exists via the 3¹ D ₂ state. The undesired 4³ P ₁ atoms decay back to the ground state within 0.4 ms and can be fully recaptured if the illuminated trap volume is sufficientlylarge. We obtain a flux of above 10¹⁰ atoms/s into the 4³ P ₂ state. We find that our MOT lifetime of 23 ms is mainly limited by this loss channel, and thus the 4³ P ₂ production is not hampered by inelastic collisions. If we close the loss channel by repumping the 3¹ D ₂ atoms with a 671 nm laser back into the MOT cycling transition, a non-exponential 72 ms trap decay is observed, indicating the presence of inelastic two-body collisions between 4¹ S ₀ and 4¹ P ₁ atoms. Received: 10 July 2001 / Revised version: 22 October 2001 / Published online: 23 November 2001     

Optical Ramsey spectroscopy on laser-trapped and thermal Mg atoms

High-resolution spectroscopic measurements on the 457 nm Mg intercombination line are reported. Studies have been performed both on atoms contained in a magneto-optical trap and on atoms in a thermal beam. We present a detailed analysis of various factors influencing resolution and accuracy for both setups, including direct comparisons. With the trapped cold atoms, a stability of 8.7 × 10^–13 within 20 s has been achieved together with an accuracy of 2 × 10^–15. The direct comparison shows the limited stability of the thermal beam apparatus for integration times larger than 300 s. For shorter times, the thermal beam setup is at present more stable by a factor of three, mainly because of a better signal-to-noise ratio.     

Frequency chain towards the Ca intercombination line based on laser diodes: First step

Stable, narrow linewidth operation of red and 1.3 µm free-running laser diodes with external gratings in non-Littrow geometry is demonstrated. The resonance of the saturated fluorescence of an atomic beam with a contrast of 25% and a linewidth of 400 ± 50 kHz of the Ca intercombination line 4¹ S ₀–4³ P ₁ ( = 657 nm) is shown. A high-power (110 mW) single-mode external cavity laser diode at 1.3 µm is used for second-harmonic generation in a KTP crystal. The beat signal (signal to noise ratio about 25 dB) of 10 nW second-harmonic radiation at 1.3 µm and the radiation of a laser diode in the visible spectrum, as a step to realize a frequency chain, is observed.     

An optical lattice clock with spin-polarized 87Sr atoms

We present a new evaluation of an ⁸⁷Sr optical lattice clock using spin polarized atoms. The frequency of the ¹S₀→³P₀ clock transition is found to be 429 228 004 229 873.6 Hz with a fractional uncertainty of 2.6×10^-15, a value that is comparable to the frequency difference between the various primary standards throughout the world. This measurement is in excellent agreement with a previous one of similar accuracy [Phys. Rev. Lett. 98, 083002 (2007)].