A new caesium fountain frequency standard (NPL-CsF1) at the National Physical Laboratory is described. Procedures for evaluation of the systematic frequency shifts are presented. The NPL-CsF1 has a short-term stability σ y (τ ) = 1.4 × 10 −13 τ −1/2 , predominantly due to the local oscillator phase noise. The accuracy of 1 part in 10 15 is limited by the uncertainty of the frequency shift due to collisions between cold atoms.
An accuracy evaluation of the caesium fountain NPL-CsF2 as a primary frequency standard is reported. The device operates with a simple one-stage magneto-optical trap as the source of cold atoms. Both the uncertainty in and magnitude of the cold collision frequency shift are reduced by taking advantage of the dependence of the cross section on the effective collision energy in an expanding atomic cloud. The combined type B uncertainty (typically 4 × 10−16) is dominated by an estimate of the frequency shift due to the distributed cavity phase. When operated at single density, the short-term fractional frequency instability of NPL-CsF2 is 1.7 × 10−13 at 1 s and limited by the noise of the room temperature quartz-based local oscillator. During a typical frequency measurement campaign, the fountain is operated in an alternating mode at high and low density in order to measure and correct for a residual collision shift. This increases the effective fractional frequency instability to 5.4 × 10−13 at 1 s; consequently the averaging time required for the type A uncertainty level to match that of the type B is 20 days.
We have observed that the collisional frequency shift in primary caesium fountain clocks varies with the clock state population composition and, in particular, is zero for a given fraction of the |F = 4, m F = 0〉 atoms, depending on the initial cloud parameters. We present a theoretical model explaining our observations. The possibility of the collisional shift cancellation implies an improvement in the performance of caesium fountain standards and a simplification in their operation. Our results also have implications for test operation of fountains at multiple π/2 pulse areas. The primary caesium clocks have increased their accuracy and stability by an order of magnitude over the last decade [1]. This staggering progress has mainly resulted from the implementations of laser cooling and the fountain configuration. Slow atoms allow for long interrogation times and observation of narrow resonances (≤ 1 Hz) leading to improvements in the short-term stability. At the same time many systematic effects, which limited the performance of thermal beam clocks, have been significantly reduced in the fountains resulting in improvements of their accuracy. The use of ultracold atoms gave rise, however, to a frequency shift due to collisions, an effect generally neglected in thermal beam clocks. For the several operational fountain frequency standards, the collisions constitute one of the major systematic effects limiting the standards' performance. Generally, the fountain standards are corrected for the collisional frequency shift by extrapolating the measured frequency to zero atom density, assuming a linear dependence between the shift and the density. However, the atomic density is not measured directly in the fountains. The changes in the density are derived from changes in the number N at of detected atoms. A potential deviation from linearity between N at and the atomic density (and hence the shift) gives rise to an uncertainty of the correction. One way to minimize this uncertainty is to operate the fountain at a very low density [2]. Unfortunately, with a low atom number the detection signal-to-noise ratio (and short-term stability of the standard) is reduced. Another way relies on the implementation of a technique based on adiabatic passage to link unambiguously changes in density with changes in N at [3]. In this case, the accuracy of the collisional shift measurement is ultimately limited by a residual amount of atoms in |F = 3, m F ≠ 0〉 colliding with atoms in the clock states during the ballistic flight [4]. Ways of cancelling the collisional shift have been studied earlier. The cancellation would be possible if the cross-sections of the frequency changing collisions of the two clock states |3,0〉 and |4,0〉 were of opposite sign. Then, if a certain composition of the clock states were excited the two contributions to the clock shift would cancel. In the 1990s the relevant cross-sections for various Cs isotopes were calculated and it was shown that the
A radio-frequency tunable atomic magnetometer with a sensitivity of about 1 fT/Hz1/2 in a range of 10–500 kHz is demonstrated. The magnetometer is operated in the orientation configuration in which atoms are pumped to the stretched atomic state by a scheme based on indirect optical pumping using only one unmodulated, low-power laser. The magnetometer operates with cesium atoms, which have sufficient vapor pressure near room temperature to enable high magnetometric sensitivities. The technique enables a compact and robust module to be constructed that could become an in-the-field device.
We demonstrate imaging of ferromagnetic carbon steel samples and we detect the thinning of their profile with a sensitivity of 0.1 mm using a Cs radio-frequency atomic magnetometer. Images are obtained at room temperature, in magnetically unscreened environments. By using a dedicated arrangement of the setup and active compensation of background fields, the magnetic disturbance created by the samples' magnetization is compensated. Proof-of-concept demonstrations of non-destructive structural evaluation in the presence of concealing conductive barriers are also provided. Relevant impact for steelwork inspection and health and usage monitoring without disruption of operation is envisaged, with direct benefit for industry, from welding in construction, to pipelines inspection and corrosion under insulation in the energy sector.
Imaging of structural defects in a material can be realized with a radio-frequency atomic magnetometer by monitoring the material's response to a radio-frequency excitation field. We demonstrate two measurement configurations that enable the increase of the amplitude and phase contrast in images that represent a structural defect in electrically conductive and magnetically permeable samples. Both concepts involve the elimination of the excitation field component, orthogonal to the sample surface, from the atomic magnetometer signal. The first method relies on the implementation of a set of coils that directly compensates the excitation field component in the magnetometer signal. The second takes advantage of the fact that the radio-frequency magnetometer is not sensitive to the magnetic field oscillating along one of its axes. Results from simple modelling confirm the experimental observation and are discussed in detail. Published under license by AIP Publishing. https://doi.Published under license by AIP Publishing.FIG. 3. The modelled change in the signal phase (dotted black line) and the amplitude (solid red line) of the magnetic resonance signal over a recess recorded by a magnetometer for various amplitudes of the primary field components. The vertical axis of the image array represents changes in the vertical component, while the horizontal axis represents changes in the horizontal component of the primary field. Amplitude is expressed in units of b y,max .Journal of Applied Physics ARTICLE scitation.org/journal/jap
Imaging of structural defects in a material can be realized with a radio-frequency atomic magnetometer by monitoring the material's response to a radio-frequency excitation field. We demonstrate two measurement configurations that enable the increase of the amplitude and phase contrast in images that represent a structural defect in highly electrically conductive and high magnetic permeability samples. Both concepts involve the elimination of the excitation field component, orthogonal to the sample surface, from the atomic magnetometer signal. The first method relies on the implementation of a set of coils that directly compensates the excitation field component in the magnetometer signal. The second takes advantage of the fact that the radio-frequency magnetometer is not sensitive to the magnetic field oscillating along one of its axes. Results from simple modelling confirm the experimental observation and are discussed in detail.
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