arXiv subjects

J. W. Pollock

Publications and source records attributed to J. W. Pollock.

9 recordsLinked to original sources

arXiv2025

Investigating the hyperfine systematic error and relative phase in low spin-polarization alkali FID magnetometers

Alkali-metal optically-pumped magnetometers are prone to inaccuracies arising from the overlap of the average F = I + 1/2 and F = I - 1/2 ground-state Zeeman resonances. We employ density-matrix simulations and experiments to investigate how this hyperfine systematic error varies with spin polarization in a $^{87}$Rb free-induction-decay (FID) magnetometer. At low spin polarizations, ($P \leq 0.5$), this effect causes single-frequency magnetic-field extraction techniques to exhibit inaccuracies up to approximately 3.5 nT. Density-matrix simulations reveal that this bias can be traced to the relative amplitude and phase between the F = I $\pm$ 1/2 hyperfine ground-state manifolds in the FID spin precession signal. We show that this systematic error can be mitigated using either a double-frequency fitting model that accounts for the relative amplitude and phase or synchronous-pulse pumping, that minimizes the F = 1 contribution to the FID signal. Theoretical simulations predict accuracies within 0.5 nT for both techniques across a wide range of spin polarizations, suggesting a sevenfold enhancement over single-frequency extraction methods. Our experiments validate this, showcasing a variation in the extracted field below 1 nT, a 3.5-fold improvement compared to single-frequency extraction methods. Furthermore, the mitigation techniques demonstrate agreement of the extracted magnetic field within 1.5 nT.

physics.atom-ph↗

arXiv2020

AC Stark Shifts of Dark Resonances Probed with Ramsey Spectroscopy

The off-resonant AC Stark shift for coherent population trapping (CPT) resonances probed with Ramsey spectroscopy is investigated experimentally and theoretically. Measurements with laser-cooled $^{87}$Rb atoms show excellent quantitative agreement with a simple theory. The shift depends on the relative intensity of the two CPT light fields, but depends only weakly on the total intensity. Since the origin of the shift is through couplings of the interrogation light to off-resonant excited state hyperfine levels, the size and sign of the shift depend on the specific interrogation scheme. However, the simple theory shows that for several commonly used interrogation schemes the off-resonant shift goes to zero at specific values of this relative intensity.

physics.atom-ph↗

arXiv2020

Advanced light-shift compensation protocol in a continuous-wave microcell atomic clock

Light-shifts are known to be an important limitation to the mid- and long-term fractional frequency stability of different types of atomic clocks. In this article, we demonstrate the experimental implementation of an advanced anti-light shift interrogation protocol onto a continuous-wave (CW) microcell atomic clock based on coherent population trapping (CPT). The method, inspired by the Auto-Balanced Ramsey (ABR) spectroscopy technique demonstrated in pulsed atomic clocks, consists in the extraction of atomic-based information from two successive light-shifted clock frequencies obtained at two different laser power values. Two error signals, computed from the linear combination of signals acquired along a symmetric sequence, are managed in a dual-loop configuration to generate a clock frequency free from light-shift. Using this method, the sensitivity of the clock frequency to both laser power and microwave power variations can be reduced by more than an order of magnitude compared to normal operation. In the present experiment, the consideration of the non-linear light-shift dependence allowed to enhance light-shift mitigation. The implemented technique allows a clear improvement of the clock Allan deviation for time scales higher than 1000 s. This method could be applied in various kinds of atomic clocks such as CPT-based atomic clocks, double-resonance Rb clocks, or cell-stabilized lasers.

physics.atom-ph↗

arXiv2020

Universal methods for suppressing the light shift in atomic clocks using power modulation

We show that the light shift in atomic clocks can be suppressed using time variation of the interrogation field intensity. By measuring the clock output at two intensity levels, error signals can be generated that simultaneously stabilize a local oscillator to an atomic transition and correct for the shift of this transition caused by the interrogating optical field. These methods are suitable for optical clocks using one- and two-photon transitions, as well as for microwave clocks based on coherent population trapping or direct interrogation. The proposed methods can be widely used both for high-precision scientific instruments and for a wide range of commercial clocks, including chip-scale atomic clocks.

physics.atom-ph↗

arXiv2019

Optical pumping, decay rates and light shifts of cold-atom dark states

Coherent dark states in atoms, created by simultaneous interaction of two coherent light fields with a 3-level system, are of prime importance in quantum state manipulation. They are used extensively in quantum sensing and quantum information applications to build atomic clocks, magnetometers, atomic interferometers and more. Here we study the formation and decay of coherent dark-states in an ensemble of laser-cooled free-falling atoms. We measure the optical-pumping rate into the dark-state in the $σ^+-σ^-$ polarization configuration. We find that the pumping rate is linear with the optical field intensity, but about an order-of-magnitude slower than the rate predicted by the commonly used, but simplistic, three-level analytic formula. Using a numerical model we demonstrate that this discrepancy is due to the multi-level Zeeman manifold. Taking into account the slower pumping rate we explain quantitatively the relation between the light-shift and the duration of pumping into dark-state in Ramsey spectroscopy. We also measure the decay of the dark-state coherence and find that in our apparatus it is dominated by the mechanical motion of the atoms out of the probing region, while the atomic decoherence is negligible.

physics.atom-ph↗

arXiv2019

Reduction of light shifts in Ramsey spectroscopy with a combined error signal

Light-induced frequency shifts can be a key limiting contribution to the mid and long-term frequency instability in atomic clocks. In this letter, we demonstrate the experimental implementation of the combined error signal interrogation protocol to a cold-atom clock based on coherent population trapping (CPT) and Ramsey spectroscopy. The method uses a single error signal that results from the normalized combination of two error signals extracted from two Ramsey sequences of different dark periods. The single combined error signal is used to stabilize the atomic clock frequency. Compared to the standard Ramsey-CPT interrogation, this method reduces the clock frequency sensitivity to light-shift variations by more than one order of magnitude. This method can be applied in various kinds of Ramsey-based atomic clocks, sensors and instruments.

physics.atom-ph↗

arXiv2018

Ramsey Spectroscopy with Displaced Frequency Jumps

Sophisticated Ramsey-based interrogation protocols using composite laser pulse sequences have been recently proposed to provide next-generation high-precision atomic clocks with a near perfect elimination of frequency shifts induced during the atom-probing field interaction. We propose here a simple alternative approach to the auto-balanced Ramsey (ABR) interrogation protocol and demonstrate its application to a cold-atom microwave clock based on coherent population trapping. The main originality of the method, based on two consecutive Ramsey sequences with different dark periods, is to sample the central Ramsey fringes with frequency-jumps finely-adjusted by an additional frequency-displacement concomitant parameter, scaling as the inverse of the dark-period. The advantage of this displaced frequency-jumps Ramsey (DFJR) method is that the local oscillator frequency is used as a single physical variable to control both servo loops of the sequence, simplifying its implementation and avoiding noise associated with controlling the LO phase. Compared to the usual Ramsey-CPT technique, the DFJR scheme reduces the sensitivity of the clock frequency to variations of the CPT sideband ratio and to the one-photon laser detuning by more than an order of magnitude. This simple method could be applied in a wide variety of Ramsey-spectroscopy based applications including frequency metrology with CPT-based and optical atomic clocks, mass spectrometry, and precision spectroscopy.

physics.atom-ph↗

arXiv2018

Combined error signal in Ramsey spectroscopy of clock transitions

We have developed a universal method to form the reference signal for the stabilization of arbitrary atomic clocks based on Ramsey spectroscopy. Our approach uses an interrogation scheme of the atomic system with two different Ramsey periods and a specially constructed combined error signal (CES) computed by subtracting two error signals with the appropriate calibration factor. CES spectroscopy allows for perfect elimination of probe-induced light shifts and does not suffer from the effects of relaxation, time-dependent pulse fluctuations and phase-jump modulation errors and other imperfections of the interrogation procedure. The method is simpler than recently developed auto-balanced Ramsey spectroscopy techniques [Ch. Sanner, et al., Phys. Rev. Lett. 120, 053602 (2018); V. I. Yudin, et al., Phys. Rev. Appl. 9, 054034 (2018)], because it uses a single error signal that feeds back on the clock frequency. CES universal technique can be applied to many applications of precision spectroscopy.

physics.atom-ph↗

arXiv2018

Generalized auto-balanced Ramsey spectroscopy of clock transitions

When performing precision measurements, the quantity being measured is often perturbed by the measurement process itself. This includes precision frequency measurements for atomic clock applications carried out with Ramsey spectroscopy. With the aim of eliminating probe-induced perturbations, a method of generalized auto-balanced Ramsey spectroscopy (GABRS) is presented and rigorously substantiated. Here, the usual local oscillator frequency control loop is augmented with a second control loop derived from secondary Ramsey sequences interspersed with the primary sequences and with a different Ramsey period. This second loop feeds back to a secondary clock variable and ultimately compensates for the perturbation of the clock frequency caused by the measurements in the first loop. We show that such a two-loop scheme can lead to perfect compensation of measurement-induced light shifts and does not suffer from the effects of relaxation, time-dependent pulse fluctuations and phase-jump modulation errors that are typical of other hyper-Ramsey schemes. Several variants of GABRS are explored based on different secondary variables including added relative phase shifts between Ramsey pulses, external frequency-step compensation, and variable second-pulse duration. We demonstrate that a universal anti-symmetric error signal, and hence perfect compensation at finite modulation amplitude, is generated only if an additional frequency-step applied during both Ramsey pulses is used as the concomitant variable parameter. This universal technique can be applied to the fields of atomic clocks, high-resolution molecular spectroscopy, magnetically induced and two-photon probing schemes, Ramsey-type mass spectrometry, and to the field of precision measurements. Some variants of GABRS can also be applied for rf atomic clocks using CPT-based Ramsey spectroscopy of the two-photon dark resonance.

physics.atom-ph↗