Wall-coated Cell Research Articles

Note: Pulsed optically pumped atomic clock based on a paraffin-coated cell

We report on the implementation of a pulsed optically pumped atomic clock based on a paraffin-coated cell. The relaxation times are measured, with the longitudinal relaxation time, T1 = 9.7 ± 0.4 ms, and the transversal relaxation time, T2 = 0.40 ± 0.03 ms. We demonstrated that the measured frequency stability of the clock is 3.9 × 10-13 τ-1/2 (1 s ≤ τ ≤ 100 s) and reaches a value of 3.1 × 10-14 for τ = 1000 s, where τ is the averaging time. This is an unprecedented result for a paraffin-coated vapor cell clock, and it makes significant contributions toward improving the performance of the wall-coated vapor cell atomic clock.

The influence of signals correlation on the long-term stability of a tandem of quantum magnetometers with laser pumping

The results of studies of the long-term frequency stability as a function of the correlation coefficient for a tandem of two quantum magnetometers with laser pumping of 87Rb in wall-coated vapour cell are represented. Measurement scheme includes a low-frequency self-generating magnetometer and a quantum microwave discriminator working at magnetic dipole transitions of radio-optical end state resonance. The difference of synchronously detected signals is processed to determine the Allan variance as a function of averaging time and correlation coefficient of signals. These parameters are essentially dependent both on the pumping light intensity and polarization and the intensity of radio fields that are produced in the working cell.

Temperature and number evolution of cold cesium atoms inside a wall-coated glass cell**Project supported by the National Natural Science Foundation of China (Grant No. 11304177).

We report an experimental study on the temperature and number evolution of cold cesium atoms diffusively cooled inside a wall-coated glass cell by measuring the absorption profile of the 62S1/2 (F = 4) → 62P3/2 (F′ = 5) transition line with a weak probe laser in the evolution process. We found that the temperature of the cold atoms first gradually decreases from 16 mK to 9 mK, and then rapidly increases. The number of cold atoms first declines slowly from 2.1 × 109 to 3.7 × 108 and then falls drastically. A theoretical model for the number evolution is built and includes the instantaneous temperature of the cold atoms and a fraction p, which represents the part of cold cesium atoms elastically reflected by the coated cell wall. The theory is overall in good agreement with the experimental result, and a nonzero value is obtained for the fraction p, which indicates that the cold cesium atoms are not all heated to the ambient temperature by a single collision with the coated cell wall. These results can provide helpful insight for precision measurements based on diffuse laser cooling.

Microfabricated alkali vapor cell with anti-relaxation wall coating

We present a microfabricated alkali vapor cell equipped with an anti-relaxation wall coating. The anti-relaxation coating used is octadecyltrichlorosilane and the cell was sealed by thin-film indium-bonding at a low temperature of 140 °C. The cell body is made of silicon and Pyrex and features a double-chamber design. Depolarizing properties due to liquid Rb droplets are avoided by confining the Rb droplets to one chamber only. Optical and microwave spectroscopy performed on this wall-coated cell are used to evaluate the cell's relaxation properties and a potential gas contamination. Double-resonance signals obtained from the cell show an intrinsic linewidth that is significantly lower than the linewidth that would be expected in case the cell had no wall coating but only contained a buffer-gas contamination on the level measured by optical spectroscopy. Combined with further experimental evidence this proves the presence of a working anti-relaxation wall coating in the cell. Such cells are of interest for applications in miniature atomic clocks, magnetometers, and other quantum sensors.

Ac Stark shift in double resonance and coherent population trapping in a wall-coated cell for compact Rb atomic clocks

We present a comparative study of the light-shifts (ac Stark shift) in a Rb vapour cell using two possible schemes for Rb atomic clocks: double resonance (DR) and coherent population trapping (CPT). For both schemes, the same wall-coated cell in a compact atomic resonator was used. The light-shift resulting from a monochromatic (DR) or a non-monochromatic (CPT) optical excitation was measured as a function of the laser intensity and the laser frequency and compared with existing theoretical results.

Light effects in the atomic-motion-induced Ramsey narrowing of dark resonances in wall-coated cells

We report on light shift and broadening in the atomic-motion-induced Ramsey narrowing of dark resonances prepared in alkali-metal vapors contained in wall-coated cells without buffer gas. The atomic-motion-induced Ramsey narrowing is due to the free motion of the polarized atomic spins in and out of the optical interaction region before spin relaxation. As a consequence of this effect, we observe a narrowing of the dark resonance linewidth as well as a reduction of the ground states' light shift when the volume of the interaction region decreases at constant optical intensity. The results can be intuitively interpreted as a dilution of the intensity effect similar to a pulsed interrogation due to the atomic motion. Finally the influence of this effect on the performance of compact atomic clocks is discussed.

Dicke narrowing in strong fields: The atom-as-antenna analogy.

It is well known that when a radiator (or absorber) suffers velocity-changing collisions that do not perturb its internal state, the oscillator's successive Doppler shifts are motionally averaged. This process (Dicke narrowing) gives rise to a homogeneously broadened sub-Doppler resonance, which is often astride a broad, apparently Doppler-width, pedestal. Though it has long been held that this pedestal is an inhomogeneous remnant of Doppler broadening, no evidence has ever been obtained to substantiate this expectation. In the present study we investigate the behavior of Dicke-narrowed line shapes in strong fields in an evacuated wall-coated cell. Our experimental results indicate that the hypothesis of an inhomogeneous Doppler remnant pedestal is invalid. Rather, the pedestal is homogeneously broadened and is best understood in terms of the oscillator's response to the electromagnetic field's power spectral density as observed in its rest frame. This conclusion suggests an intimate relationship between the process of Dicke narrowing and the behavior of quantum systems in the presence of stochastic fields. By envisioning the quantum system as a narrow-band antenna that only absorbs a small portion of the field's energy, we develop a simple intuitive model to account for the interaction of the atom with the nonmonochromatic field; the appeal of this simple theory is that it removes the need for extensive computation. Agreement between this ``atom-as-antenna'' analogy and experiment is very good.

Rb O-O hyperfine transition in evacuated wall-coated cell at melting temperature

Using an evacuated wall-coated cell, a high-resolution measurement was made of the behavior of the Rb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">87</sup> O-O hyperfine transition frequency near the melting temperature of the coating. Several features attributed to phase changes in the coating were evident. Notable were a phase transition from a super-cooled state and three regions of zero or small temperature coefficient of hyperfine frequency. The hyperfine resonance persists above the melting point of the coating. Data were analyzed in terms of an adsorption model for the Rb-wall interaction. The 10 Hz FWHM intrinsic hyperfine line width provided evidence for a high-quality wall coating.

Relaxation of optically pumped Cs in wall-coated cells.

Low spin-relaxation rates of optically pumped Cs atoms are obtained in paraffin-wall-coated cells. The data are analyzed to determine the electron randomization rate for cesium-wall collisions. Pumping of cesium atoms by the paraffin wall coating is also observed.

Analysis of Dicke narrowing in wall-coated and buffer-gas-filled atomic storage cells

Spectral lineshape narrowing of atomic vapour emission in a bufferless, wall-coated absorption cell has been analysed using the same formalism developed to describe sub-Doppler linewidths, Dicke narrowing, in a buffered cell. Within this formalism lineshapes are generated using Monte Carlo techniques. Dramatic linewidth reductions in the bufferless cell, compared with the buffered cell, occur because of the high probability that Doppler shifts before and after a particular wall collision will cancel each other. The formalism described allows more physical insight than the classic one-dimensional box model previously invoked to explain spectra narrowing in bufferless, wall-coated cells. Additionally, this formalism reveals a correlation between post-collision velocities and intercollision times in the buffered cell resulting in modifications of the standard Dicke analysis of these cells.

A New Heart for Rb Frequency Standards?: The Evacuated, Wall-Coated Sealed Cell

Rb87 hyperfine resonances observed in an evacuated, wall-coated cell, sealed since 1968, exhibit linewidths of ~10 Hz for the narrow Lorentzian feature. The high-resonance Q of 0.65 × 109 shows good promise for the potential use of such cells in Rb frequency standards.

Narrow 87Rb hyperfine-structure resonances in an evacuated wall-coated cell

Examination of 87Rb δF = 1, δ‖mF‖ = 1 hyperfine resonances in a 200-cm3 evacuated tetracontane wall-coated Pyrex cell which has been sealed since 1968 shows remarkable Dicke-narrowed sub-Doppler Lorentzian line shapes with less than 11 Hz full width. This linewidth is 70 times narrower than that in buffer gas filled cells used in commercial Rb atomic frequency standards. The phase shift per wall collision is 0.058 radians/collision as derived from an observed wall-induced hyperfine frequency shift of −52 Hz at a wall temperature of 26 °C.

Determination ofg-Factor Ratios for FreeRb85andRb87Atoms

The ratio of the nuclear $g$ factor to the electronic $g$ factor in the ground electronic state of free ${\mathrm{Rb}}^{85,87}$ atoms has been determined using optical pumping techniques in a magnetic field of \ensuremath{\le}50 G. Typical linewidths were \ensuremath{\le}15 cps. The linewidth contribution due to magnetic field effects was less than 1 cps. The ratio of electronic $g$ factors for the Rb isotopes was also measured. The results are $\ensuremath{-}\frac{{g}_{I}}{{g}_{J}({\mathrm{Rb}}^{87})}=4.9699147\ifmmode\times\else\texttimes\fi{}(1\ifmmode\pm\else\textpm\fi{}0.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, $\ensuremath{-}\frac{{g}_{I}}{{g}_{J}({\mathrm{Rb}}^{85})}=1.4664908\ifmmode\times\else\texttimes\fi{}(1\ifmmode\pm\else\textpm\fi{}2.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, and $\frac{{g}_{J}({\mathrm{Rb}}^{87})}{{g}_{J}({\mathrm{Rb}}^{85})}=1.0000000041\ifmmode\times\else\texttimes\fi{}(1\ifmmode\pm\else\textpm\fi{}6.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9})$. These results were obtained from evacuated wall-coated cells having the Lorentzian line shape. Cells filled with inert buffer gases exhibited small, uncontrollable, systematic error due to non-Lorentzian line shape. In both types of cells, the Zeeman resonances exhibited frequency shifts proportional to the pumping-light intensity. The ratio $\frac{{g}_{I}({\mathrm{Rb}}^{85})}{{g}_{I}({\mathrm{Rb}}^{87})}=0.2950736\ifmmode\times\else\texttimes\fi{}(1\ifmmode\pm\else\textpm\fi{}2.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6})$ was obtained by combining the results above. The combination with results of other researchers yields the chemical shift of ${\mathrm{Rb}}^{+}$ in aqueous solution relative to the free atom as $\ensuremath{\Delta}\ensuremath{\sigma}(\frac{{{\mathrm{Rb}}^{+}}_{\mathrm{aq}}}{\mathrm{Rb}})=\ensuremath{-}(211.6\ifmmode\pm\else\textpm\fi{}1.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$. Absolute values in units of the Bohr magneton for the shielded nuclear moments are derived using only $g$-factor ratios for free atoms: ${g}_{I}({\mathrm{Rb}}^{87})=\ensuremath{-}0.9951414\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\ifmmode\times\else\texttimes\fi{}(1\ifmmode\pm\else\textpm\fi{}1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6})$ and ${g}_{I}({\mathrm{Rb}}^{85})=\ensuremath{-}0.2936400\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\ifmmode\times\else\texttimes\fi{}(1\ifmmode\pm\else\textpm\fi{}2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6})$.

Experimental Upper Limit for the Permanent Electric Dipole Moment ofRb85by Optical-Pumping Techniques

An optical-pumping experiment designed to detect a permanent electric dipole moment in the ground state of the ${\mathrm{Rb}}^{85}$ atom is described. It has the advantage of averaging out the motional magnetic fields that limit the more recent related atomic-beam results. The applied electric fields were necessarily smaller. Zeeman transitions in free ${\mathrm{Rb}}^{85}$ atoms in the earth's magnetic field (about 210 kc/sec) were observed in evacuated, wall-coated absorption cells under electric fields up to \ifmmode\pm\else\textpm\fi{}2000 V/cm. Modulation of the light beam by precessing atoms was used to operate the samples as self-excited oscillators. The difference in frequency between two ${\mathrm{Rb}}^{85}$ oscillators operating side by side in the same light beam was measured to reduce the effect of magnetic field fluctuations. This frequency difference (about 300 cps) was found to be stable to within \ifmmode\pm\else\textpm\fi{}0.1 cps (e.g., corresponding to magnetic-field variations between samples of 2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}7}$ G for 30-min periods, under favorable conditions), and was found to be independent of the electric field applied to one of them within that limit. It was concluded that the permanent electric dipole moment of the ${\mathrm{Rb}}^{85}$ atomic ground state, if it exists, is less than $e\ifmmode\times\else\texttimes\fi{}({10}^{\ensuremath{-}18} \mathrm{cm})$, where $e$ is the electronic charge. The theory of optically pumped alkali oscillators is developed for arbitrary nuclear spin in the limit of low light intensities (the high-resolution case).