W. Chalupczak

Evaluation of the primary frequency standard NPL-CsF1

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 σy(τ) = 1.4 × 10−13τ−1/2, predominantly due to the local oscillator phase noise. The accuracy of 1 part in 1015 is limited by the uncertainty of the frequency shift due to collisions between cold atoms.

First accuracy evaluation of the NPL-CsF2 primary frequency standard

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.

Room temperature femtotesla radio-frequency atomic magnetometer

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.

Optical-radio-frequency resonances free from power broadening

We have demonstrated a new mode of operation of an optical-radio-frequency double-resonance measurement, which allows high-resolution rf spectroscopy with negligible power broadening. The method is based on saturating, resonant excitation, and nonresonant detection of an atomic alignment of alkali-metal atoms by magneto-optical means. Its application to precision measurements, in particular to atomic magnetometry, is discussed.

Collisions in a ballistically expanding cloud of cold atoms in an atomic fountain

We have employed an intuitive model and 3D Monte Carlo simulations to study the effect of ballistic expansion of the atomic cloud on the collision energies and the resulting frequency shift in a fountain clock. In particular, we show that the energies relevant for collisions contributing to the clock shift correspond to temperatures which may be significantly smaller than the temperature of the cloud at launch. Both the assumptions and predictions of our model are related to realistic parameters of operating caesium fountain clocks.

Initial evaluation of the NPL caesium fountain frequency standard

An initial performance evaluation of the caesium fountain primary frequency standard at the NPL is reported. Improved magnetic shielding has resulted in the inhomogeneity of the C-field being below 1 nT and has enabled the observation of full-contrast Ramsey fringes. A direct evaluation of the fountains stability in comparison to a hydrogen maser is shown to agree with the short-term stability inferred from the signal-to-noise ratio. The stability is consistent with the limit set by the local oscillator. The measurement of the frequency bias due to cold collisions is described, together with the associated uncertainty.

Multiple π/2 pulse area operation of caesium fountains and the collisional frequency shift

Typically, primary fountain frequency standards are tested at elevated microwave powers in order to check for potential frequency shifts due to, e.g., microwave leakage or cavity phase gradients. Usually such tests are performed at nπ/2 microwave pulse area (n an odd integer), where the microwave power is set to these values typically by maximizing the contrast of the central Ramsey fringe. We have experimentally demonstrated that in this case for different n a varying clock state composition after the first Ramsey interaction can be obtained, if the average microwave power seen by the expanding atom cloud is different during the first and the second transition through the Ramsey cavity. In the presence of a clock state composition dependent collisional shift, this effect gives rise to different collisional shifts for operation at different nπ/2 pulse area.

Reply to the comment on 'Evaluation of the primary frequency standard NPL-CsF1'

In response to the comment by Jefferts and Levi on page L11 of this issue, we review the uncertainty budget quoted in our earlier paper on the evaluation of the caesium primary frequency standard NPL-CsF1. As a result of this re-evaluation, performed in the light of new analyses and experimental data on the microwave leakage shift, we expand the fractional frequency uncertainty for this standard to 2.0 × 10−15 (1σ).

High-precision measurement of the 87 Rb ground-state hyperfine splitting in an atomic fountain

The results on accurate measurement of the absolute frequency of the 87Rb hyperfine ground-state transition in the NPL Rb fountain frequency standard will be presented. Preliminary frequency measurements of the Rb transition against the new NPL Cs fountain (after the subtraction of the quadratic Zeeman, BBR and gravitational Red shifts) agrees within 5×10−15 with the previous measurements at the BNM-SYRTE dual fountain [1,2] performed in 1999 and 2002. The accuracy of the measurements is expected to be further improved after complete characterisation of all the systematic shifts of the Rb hyperfine clock transition currently in progress.

Manipulation of the collisional frequency shift in caesium fountain clocks

The frequency shift due to atomic collisions is a major, and in some cases the dominant, limitation to the accuracy of caesium fountain primary frequency standards. A correction for this shift is usually obtained by measuring the frequency of the standard as a function of atomic density and performing an extrapolation to zero density. In general this means that additional measurement time is needed to reach a given statistical resolution. Recently, we have observed that, for a certain range of fountain parameters (i.e. the initial size of the atom cloud and its temperature at launch), the collisional frequency shift varies significantly when the population of the clock states (set by the first Ramsey interaction) is varied. In particular, the collisional shift can be zero for a certain value of the population ratio. This demonstration of collisional shift cancellation offers the intriguing prospect of operating the fountain at the zero-shift point, avoiding the need for extrapolation. In this contribution we provide further experimental validation of the theoretical model describing the collisional shift variation. We also discuss requirements for and benefits of the operation at the zero shift point. In addition, we point out the possible consequences of collisional shift variation for operation of a fountain standard at elevated microwave power, a mode of operation frequently used for the evaluation of other systematic frequency shifts.