Focusing on precision measurements to probe fundamental physics
One of the most precise tools devised by humankind is the optical atomic clock, one of which is under development here at UWA. Such clocks can test numerous aspects of physics, e.g. the constancy of fundamental constants, the isotropy of space and general relativistic effects. More recently, they have been used to search for Dark Matter transients and Dark Matter force mediators.
Our work involves a computational component as well as experimental. Recent computational work included atomic structure calculations for Yb (Phys. Rev. A, 104, 022806, 2021), which is relevant to the search for phenomena lying beyond the Standard Model. Atomic clocks can always be improved upon, so we have continual development on the experimental side, which involves laser frequency control, atom manipulation systems, optical setups, circuit design, optoelectronic control systems and coding. The lab commenced in 2013.
Announcement
Congratulations to Jesse Schelfhout on receiving WA’s 2022 Rhodes Scholarship. Jesse completed his Physics Honours and Master degrees in the Atomic Clock Lab.
Performance limits of a synchronous frequency-to-voltage converter for laser cooling
The aim is to increase the versatility of a clock-synchronised frequency-to-voltage converter. This has commercial potential in laser stabilisation applications (and perhaps beyond).
A femto-second laser and optimising pulse widths for frequency combs
Femto-second lasers have applications ranging from tooth enamel ablation, to atomic clock read-outs, to high speed communications. Here we are building a femto-second laser to enhance the properties of an existing frequency comb used for clock read-out and laser stabilisation.
Optical lattice trapping of ytterbium
The aim is to trap Yb in a focused laser beam at the magic wavelength. This will increase the the accuracy of our Yb clock some thousand-fold.
An optical trap simulator (in Matlab)
Our optical trap simulator has been heavily tested for optical lattice trapping in 1D, but further testing/optimisation is required for the single-beam optical trapping and 2D trapping. This is relevant to most ultracold atom experiments.
Atomic structure calculations for multi electron atoms
The aim is to compute multi-configuration Dirac-Hartree-Fock calculations for atomic levels relevant to atomic clocks. The calculations yield important atomic and nuclear parameters, such as hyperfine structure constants and isotope shift parameters. Computational software is freely available for these calculations.
Part of the ytterbium clock at UWA. Photo credit: Sean Middleton.
Phys. Rev. A, 104, 022806 (2021)
J. Opt. Soc. Am. B, 38, 36 (2021)
Phys. Rev. A, 100, 042505 (2019)
Appl. Opt., 58, 3128 (2019)
Recent publications
D. M. Jones, F. van Kann and J. J. McFerran, “Intercombination line frequencies in 171Yb validated with the clock transition”, Appl. Opt., 62, 3932 (2023).
We have carried absolute frequency measurements of the (6𝑠2)1𝑆0−(6𝑠6𝑝)3𝑃1 transition in 171Yb (intercombination line), where the spin-1/2 isotope yields two hyperfine lines. The measurements rely on sub-Doppler spectroscopy to yield a discriminator to which a 556 nm laser is locked. The frequency reference for the optical frequency measurements is a high-quality quartz oscillator steered to the GNSS time scale that is bridged with a frequency comb. The reference is validated to ~3×10−12 by spectroscopy on the 1𝑆0−3𝑃0 (clock) line in laser cooled and trapped 171Yb atoms. From the hyperfine separation between the 𝐹 = 1/2 and 𝐹 = 3/2 levels of 3𝑃1, we determine the hyperfine constant to be 𝐴(3𝑃1) = 3957833(28)kHz.
J. S. Schelfhout and J. J. McFerran, “Multiconfiguration Dirac-Hartree-Fock calculations for Hg and Cd with estimates for unknown clock-transition frequencies”, Phys. Rev. A, 105, 022805 (2022).
By use of the grasp2018 package we perform multiconfiguration Dirac-Hartree-Fock (MCDHF) calculations with configuration interaction (CI) for the 1S0 and 3P0,1 levels in neutral cadmium and mercury. By supplying the resultant atomic state functions to the ris4 program, we evaluate the mass- and field-shift parameters for the 1S0−3P0 (clock) and1S0−3P1 (intercombination) lines. We make revised estimates of the nuclear charge parameters λA,A′ and differences in mean-square charge radii δ(r2)A,A′ for both elements and point out a discrepancy with tabulated data for Cd. In constructing a King plot with the Hg lines we examine the second-order hyperfine interaction for the 3P0,1 levels. Isotope shifts for the clock transition have been estimated, from which we predict the unknown clock line frequencies in the bosonic Hg isotopes and all the naturally occurring isotopes of Cd
J. S. Schelfhout and J. J. McFerran, “Isotope shifts for 1S0 - 3P0,1 Yb lines from multiconfiguration Dirac-Hartree-Fock calculations”, Phys. Rev. A, 104, 022806 (2021).
Relativistic multiconfiguration Dirac-Hartree-Fock calculations with configuration interaction are carried out for the 1S0 and 3Po0,1 states in neutral ytterbium by use of the available grasp2018 package. From the resultant atomic state functions and the ris4 program, we evaluate the mass- and field-shift parameters for the 1S0−3Po0 (clock) and 1S0−3Po1 (intercombination) lines. We present improved estimates of the nuclear charge parameters λA,A′ and differences in mean-square charge radii δ⟨r2⟩A,A′ and examine the second-order hyperfine interaction for the 3Po0,1 states. Isotope shifts for the clock transition have been estimated by three largely independent means from which we predict the unknown clock-line frequencies in bosonic Yb isotopes. Knowledge of these line frequencies has implications for King-plot nonlinearity tests and the search for beyond-standard-model signatures.
J. S. Schelfhout, L. D. Toms-Hardman and J. J. McFerran, “Fourier transform detection of weak optical transitions in atoms undergoing cyclic routines”, Appl. Phys. Lett, 118, 014002 (2021). Editor's Pick
We demonstrate a means of detecting weak optical transitions in cold atoms that undergo cyclic (preparation, probing, and detection) routines with improved sensitivity. The gain in sensitivity is made by probing atoms on alternate cycles of a repeated experimental sequence, leading to regular modulation of the ground state atom population when at the resonance frequency. The atomic transition is identified by conducting a fast Fourier transform via an algorithm or instrument. We find an enhancement of detection sensitivity compared to more conventional scanning methods of ∼20 for the same sampling time, and can detect contemporary clock lines with fewer than 103 atoms in a magneto-optical trap. We apply the method to the (6𝑠2) 1𝑆0−(6𝑠6𝑝) 3𝑃0 clock transition in 171Yb and 173Yb. In addition, the ac-Stark shift of this line in 171Yb is measured to be 0.19(3) kHz W−1 m2 at 556 nm.
R. S. Watson and J. J. McFerran, “Simulation of optical lattice trap loading from a cold atomic ensemble”, J. Opt. Soc. Am. B, 38, 36 (2021).
We model the efficiency of loading atoms of various species into a one-dimensional optical lattice from a cold ensemble, taking into account the initial cloud temperature and size, the lattice laser properties affecting the trapping potential, and the atomic parameters. Stochastic sampling and dynamical evolution are used to simulate the transfer, leading to estimates of transfer efficiency for varying trap depth and profile. Tracing the motion of the atoms also enables the evaluation of the equilibrium temperature and site occupancy in the lattice. The simulation compares favorably against a number of experimental results and is used to compute an optimum lattice-waist-to-cloud-radius ratio for a given optical power.
P. E. Atkinson, J. S. Schelfhout and J. J. McFerran, “Hyperfine constants and line separations for the 1S0 - 3P1 intercombination line in neutral ytterbium with sub-Doppler resolution”, Phys. Rev. A, 100, 042505 (2019).
Optical frequency measurements of the intercombination line (6s2)1S0−(6s6p)3P1 in the isotopes of ytterbium are carried out with the use of sub-Doppler fluorescence spectroscopy on an atomic beam. A dispersive signal is generated to which a master laser is locked, while frequency counting of an auxiliary beat signal is performed via a frequency comb referenced to a hydrogen maser. The relative separations between the lines are used to evaluate the 3P1-level magnetic dipole and electric quadrupole constants for the fermionic isotopes. The center of gravity for the 3P1 levels in 171Yb and 173Yb are also evaluated, where we find significant disagreement with previously reported values. These hyperfine constants provide a valuable testbed for atomic many-body computations in ytterbium.
F. C. Reynolds and J. J. McFerran, “Optical frequency stabilization with a synchronous frequency-to-voltage converter”, Appl. Opt., 58, 3128 (2019). Editor's Pick
Between a frequency comb mode and a continuous-wave (cw) laser, we demonstrate that a frequency-to-voltage converter can be used to transfer frequency instability in the 10−14 range for integration times 𝜏 between 0.25 and 2100 s. The technique is relevant when the optical beat between laser signals is weak and a high level of frequency stability is required both in the short term and long term, as in the case of laser cooling with very narrow transitions. The impressive stability transfer arises through the use of a synchronous voltage-to-frequency converter that relies on an external CMOS oscillator. Aided by an atomic reference to the frequency comb, the method grants long-term stability to the cw laser, superior to that achieved with most optical cavities.
A. Guttridge, S. A. Hopkins, M. D. Frye, J. J. McFerran, J. M. Hutson, and S. L. Cornish. “Production of ultracold Cs*Yb molecules by photoassociation”, Phys. Rev. A, 97, 063414 (2018).
We report the production of ultracold heteronuclear Cs*Yb molecules through one-photon photoassociation applied to an ultracold atomic mixture of Cs and Yb confined in an optical dipole trap. We use trap-loss spectroscopy to detect molecular states below the Cs(2P1/2)+Yb(1S0) asymptote. For 133Cs174Yb, we observe 13 rovibrational states with binding energies up to ~500GHz. For each rovibrational state we observe two resonances associated with the Cs hyperfine structure and show that the hyperfine splitting in the diatomic molecule decreases for more deeply bound states. In addition, we produce ultracold fermionic 133Cs173Yb and bosonic 133Cs172Yb and 133Cs170Yb molecules. From mass scaling, we determine the number of vibrational levels supported by the 2(1/2) excited-state potential to be 154 or 155.
J. J. McFerran, “Laser stabilization with a frequency-to-voltage chip for narrow-line laser cooling”, Opt. Lett., 43, 1475 (2018).
We use integrated circuit-based frequency-to-voltage conversion of a frequency comb beat signal as the means for laser frequency stabilization that is suitable for narrow-line laser cooling. The method is compared to an atomic beam lock where the laser frequency instability for the new scheme shows an improvement of 2 orders of magnitude at sub-1 s and grants a lock-capture range that is approximately 30 times greater. We employ the locking method on a 1111.6 nm laser that is frequency doubled and used in a dual-wavelength magneto-optical trap for Yb171 atoms, producing atomic cloud temperatures of ~20 μK.
