arXiv subjects

M. R. Tarbutt

Publications and source records attributed to M. R. Tarbutt.

At least 19 recordsLinked to original sources

arXiv2026

Spin interferometry in a beam of ultracold molecules

We describe a spin interferometer using ultracold YbF molecules and develop the complete set of techniques needed to measure the electron's electric dipole moment, $d_e$, with this apparatus. The molecules are cooled in an optical molasses and prepared in a single internal quantum state. A Raman transition prepares a spin superposition which evolves in parallel magnetic and electric fields before a second Raman transition maps the phase onto the populations of two hyperfine states. These populations are read out using detectors that have spatial and temporal resolution and approach unit efficiency. We characterize the efficiencies and fidelities of all these steps and evaluate the sensitivity of this approach to measuring $d_e$.

physics.atom-ph↗

arXiv2026

Trapping and cooling mechanisms in blue-detuned magneto-optical traps of molecules

In red-detuned magneto-optical traps (MOTs) of molecules, sub-Doppler heating competes with Doppler cooling, resulting in high temperature and low density. A solution is offered by the blue-detuned MOT where sub-Doppler cooling dominates and the cloud is compressed. Several blue-detuned molecular MOTs have been implemented. A recent implementation relies on a pair of orthogonally polarized components whose frequency separation is smaller than the transition linewidth. We identify the trapping force in these MOTs. At a certain magnetic field, there is a state that is dark to the laser propagating in one direction, but not to the counter-propagating one. This Zeeman-induced dark state (ZIDS) sets up an imbalance in the photon scattering rate, leading to a restoring force. We also study the role of the moving lattices generated by the closely-spaced frequency components of the light. We show that there is a velocity-dependent force that drives the molecules towards the speeds of these moving lattices, and that over a relevant range of magnetic fields this combines with the ZIDS force to transport molecules towards the centre of the MOT. Here, gray molasses cooling, assisted by non-adiabatic transitions driven by the time-varying polarization of the light field, cools the molecules towards zero velocity. We study these mechanisms for model systems with simple level structures, then extend them to molecules with ground state hyperfine structure.

physics.atom-ph↗

arXiv2025

Slowing YbF molecules using radiation pressure

We report radiation pressure slowing of YbF molecules to low velocity. In YbF, laser slowing is hindered by leaks out of the optical cycle attributed to low-lying metastable electronic states arising from inner-shell excitation. We bring this population back into the optical cycle once it has decayed to the electronic ground state using microwaves to couple the relevant rotational levels. We measure the scattering rate and closure of the optical cycle as repumps are added, and study the destabilzation of dark states by a magnetic field and by polarization modulation, finding that both are helpful for maximizing the scattering rate. Starting from a beam with a mean speed of 80 m/s, and using frequency broadened slowing light, we reduce the mean speed of the beam and produce a substantial flux in the low velocity tail of the distribution. Slowing increases the fraction of molecules below 40 m/s from 0.4(1)% to 7.0(2)%, and the fraction below 30 m/s from zero to 3.2(1)%. The establishment of a nearly-closed optical cycle and the production of molecules at low velocity are important steps towards trapping YbF molecules for future measurements of the electron's electric dipole moment.

physics.atom-ph↗

arXiv2025

AION-10: Technical Design Report for a 10m Atom Interferometer in Oxford

This Technical Design Report presents AION-10, a 10-meter atom interferometer to be located at Oxford University using ultracold strontium atoms to make precision measurements of fundamental physics. AION-10 serves as both a prototype for future larger-scale experiments and a versatile scientific instrument capable of conducting its own diverse physics programme. The design features a 10-meter vertical tower housing two atom interferometer sources in an ultra-high vacuum environment. Key engineering challenges include achieving nanometer-level vibrational stability and precise magnetic field control. Solutions include active vibration isolation, specialized magnetic shielding, and a modular assembly approach using professional lifting equipment. Detailed analysis confirms the design meets all performance requirements, with critical optical components remaining within our specifications 97% of the time under realistic operating conditions. Vacuum and vibration measurements in the host building validate that the instrument will achieve the precision needed for quantum sensing applications. This work establishes the technical foundation for scaling atom interferometry to longer baselines while creating a cutting-edge facility for precision measurements that could advance our understanding of fundamental physics.

physics.atom-ph↗

arXiv2025

A prototype differential atom interferometer for fundamental physics

Gravitational waves and ultralight dark matter are among the most compelling frontiers in fundamental physics, motivating proposals for Very Long-Baseline Atom Interferometers (VLBAIs) such as AION, MAGIS, AICE and AEDGE that aim to detect frequencies at which ground-based and space-borne laser interferometers lose sensitivity. VLBAIs look for signals by comparing the quantum phase evolution of widely separated atomic ensembles interrogated by a common laser. However, their performance depends critically on suppressing noise sources, particularly laser phase noise. Experimental validation of such noise rejection remains an important challenge. Here we demonstrate a prototype differential atom interferometer based on the single-photon clock transition of fermionic 87Sr, realising for the first time a gradiometer configuration with a species intrinsically suited to kilometre-scale and space-baseline operation. The instrument operates at the Standard Quantum Limit with no excess noise beyond atom shot noise, and the differential configuration maintains quantum-limited sensitivity in the presence of several radians of artificially injected laser phase noise per shot, emulating the conditions expected in a VLBAI. We further demonstrate recovery of coherent oscillatory signals across a broad frequency range under fully phase-randomised conditions, a capability that is inaccessible to a single interferometer operating in the same regime. These results provide an experimental validation of the noise-immune measurement principle underlying VLBAIs and mark an important step towards next-generation quantum sensors for gravitational-wave detection and searches for ultralight dark matter.

hep-ex↗

arXiv2025

Low-noise environment for probing fundamental symmetries

We present the design and characterization of a low-noise environment for measuring the electron's electric dipole moment (EDM) with a beam of molecules. To minimize magnetic Johnson noise from metals, the design features ceramic electric field plates housed in a glass vacuum chamber. To suppress external magnetic noise the apparatus is enclosed within a cylindrical four-layer mu-metal shield with a shielding factor exceeding $10^6$ in one radial direction and $10^5$ in the other. Finite element modelling shows that the difference between these shielding factors is due to imperfect joints between sections of mu-metal. Using atomic magnetometers to monitor the magnetic field inside the shield, we measure noise below 40 fT/$\sqrt{{\rm Hz}}$ at 1 Hz and above, rising to 500 fT/$\sqrt{{\rm Hz}}$ at 0.1 Hz. Analytical and numerical studies show that residual magnetic Johnson noise contributes approximately 13 fT/$\sqrt{{\rm Hz}}$. The background magnetic field averaged along the beamline is maintained below 3 pT, with typical gradients of a few nT/m. An electric field of 20 kV/cm is applied without discharges and with leakage currents below 1 nA. Each magnetometer measures the magnetic field correlated with the direction of the applied electric field with a precision of 0.11 fT in 104 hours of data. These results demonstrate that the apparatus is suitable for measuring the electron EDM with precision at the $10^{-31}$ e cm level. The design principles and characterization techniques presented here are broadly applicable to precision measurements probing fundamental symmetries in molecules, atoms, and neutrons.

physics.atom-ph↗

arXiv2024

Wavelength modulation laser spectroscopy of N$_2$O at 17 $\mu$m

Using a mid-infrared quantum cascade laser and wavelength modulation absorption spectroscopy, we measure the frequencies of ro-vibrational transitions of N$_2$O in the 17 $\mu$m region with uncertainties below 5 MHz. These lines, corresponding to the bending mode of the molecule, can be used for calibration of spectrometers in this spectral region. We present a model for the lineshapes of absorption features in wavelength modulation spectroscopy that takes into account Doppler broadening, collisional broadening, saturation of the absorption, and lineshape distortion due to frequency and intensity modulation. Combining our data with previous measurements, we provide a set of spectroscopic parameters for several vibrational states of N$_2$O. The lines measured here fall in the same spectral region as a mid-infrared frequency reference that we are currently developing using trapped, ultracold molecules. With such a frequency reference, the spectroscopic methods demonstrated here have the potential to improve frequency calibration in this part of the spectrum.

physics.atom-ph↗

arXiv2024

Slow molecular beams from a cryogenic buffer gas source

We study the properties of a cryogenic buffer gas source that uses a low temperature two-stage buffer gas cell to produce very slow beams of ytterbium monofluoride molecules. The molecules are produced by laser ablation inside the cell and extracted into a beam by a flow of cold helium. We measure the flux and velocity distribution of the beam as a function of ablation energy, helium flow rate, cell temperature, and the size of the gap between the first and second stages of the cell. We also compare the velocity distributions from one-stage and two-stage cells. The one-stage cell emits a beam with a speed of about 82 m s$^{-1}$ and a translational temperature of 0.63 K. The slowest beams are obtained using the two-stage cell at the lowest achievable cell temperature of 1.8 K. This beam has a peak velocity of 56 m s$^{-1}$ and a flux of $9 \times 10^9$ ground state molecules per steradian per pulse, with a substantial fraction at speeds below 40 m s$^{-1}$. These slow molecules can be decelerated further by radiation pressure slowing and then captured in a magneto-optical trap.

physics.atom-ph↗

arXiv2024

Understanding inner-shell excitations in molecules through spectroscopy of the 4f hole states of YbF

Molecules containing a lanthanide atom have sets of electronic states arising from excitation of an inner-shell electron. These states have received little attention, but are thought to play an important role in laser cooling of such molecules and may be a useful resource for testing fundamental physics. We study a series of inner-shell excited states in YbF using resonance-enhanced multi-photon ionisation spectroscopy. We investigate the excited states of lowest energy, 8474, 9013 and 9090 cm$^{-1}$ above the ground state, all corresponding to the configuration 4f$^{13}$6s$^{2}$ ${}^{2}F_{7/2}$ of the Yb$^+$ ion. They are metastable, since they have no electric dipole allowed transitions to the ground state. We also characterize a state at 31050 cm$^{-1}$ that is easily excited from both the ground and metastable states, which makes it especially useful for this spectroscopic study. Finally, we study a state at 48729 cm$^{-1}$, which is above the ionization limit and features strong auto-ionizing resonances that prove useful for efficient detection of the molecules and for identifying the rotational quantum number of each line in the spectrum. We resolve the rotational structures of all these states and find that they can all be described by a very simple model based on Hund's case (c). Our study provides information necessary for laser slowing and magneto-optical trapping of YbF, which is an important species for testing fundamental physics. We also consider whether the low-lying inner-shell states may themselves be useful as probes of the electron's electric dipole moment or of varying fundamental constants, since they are long-lived states in a laser-coolable molecule featuring closely-spaced levels of opposite parity.

physics.atom-ph↗

arXiv2023

Systematic errors arising from polarization imperfections in measurements of the electron's electric dipole moment

The electron's electric dipole moment (eEDM) can be determined by polarizing the spin of an atom or a molecule and then measuring the spin precession frequency in an applied electric field. Radiation is used to polarize the spin and then analyze the precession angle, and the measurement is often sensitive to the polarization of this radiation. We show how systematic errors can arise when both the polarization of the radiation and the magnitude of the electric field are imperfectly controlled. We derive approximate analytical expressions for these errors, confirm their accuracy numerically, and show how they can be corrected empirically. We consider spin manipulation using single-photon pulses, Raman pulses, and Stimulated Raman Adiabatic Passage (STIRAP), and show that STIRAP provides better immunity to these systematic errors. An experimental study of these errors partly supports our findings but also reveals another potential error that is not captured by this analysis.

physics.atom-ph↗

arXiv2023

Centralised Design and Production of the Ultra-High Vacuum and Laser-Stabilisation Systems for the AION Ultra-Cold Strontium Laboratories

This paper outlines the centralised design and production of the Ultra-High-Vacuum sidearm and Laser-Stabilisation systems for the AION Ultra-Cold Strontium Laboratories. Commissioning data on the residual gas and steady-state pressures in the sidearm chambers, on magnetic field quality, on laser stabilisation, and on the loading rate for the 3D Magneto-Optical Trap are presented. Streamlining the design and production of the sidearm and laser stabilisation systems enabled the AION Collaboration to build and equip in parallel five state-of-the-art Ultra-Cold Strontium Laboratories within 24 months by leveraging key expertise in the collaboration. This approach could serve as a model for the development and construction of other cold atom experiments, such as atomic clock experiments and neutral atom quantum computing systems, by establishing dedicated design and production units at national laboratories.

physics.atom-ph↗

arXiv2022

Measuring the nuclear magnetic quadrupole moment in heavy polar molecules

Theories that extend the Standard Model of particle physics often introduce new interactions that violate charge-parity (CP) symmetry. CP-violating effects within an atomic nucleus can be probed by measuring its nuclear magnetic quadrupole moment (MQM). The sensitivity of such a measurement is enhanced when using a heavy polar molecule containing a nucleus with quadrupole deformation. We determine how the energy levels of a molecule are shifted by the MQM and how those shifts can be measured. The measurement scheme requires molecules in a superposition of magnetic sub-levels that differ by many units of angular momentum. We develop a generic scheme for preparing these states. Finally, we consider the sensitivity that can be reached, showing that this method can reduce the current uncertainties on several CP-violating parameters.

physics.atom-ph↗

arXiv2022

An ultracold molecular beam for testing fundamental physics

We use two-dimensional transverse laser cooling to produce an ultracold beam of YbF molecules. Through experiments and numerical simulations, we study how the cooling is influenced by the polarization configuration, laser intensity, laser detuning and applied magnetic field. The ultracold part of the beam contains more than $2 \times 10^5$ molecules per shot and has a temperature below 200 $μ$K, and the cooling yields a 300-fold increase in the brightness of the beam. The method can improve the precision of experiments that use molecules to test fundamental physics. In particular, the beam is suitable for measuring the electron electric dipole moment with a statistical precision better than $10^{-30}$ e cm.

physics.atom-ph↗

arXiv2022

Quantum computation in a hybrid array of molecules and Rydberg atoms

We show that an array of polar molecules interacting with Rydberg atoms is a promising hybrid system for scalable quantum computation. Quantum information is stored in long-lived hyperfine or rotational states of molecules which interact indirectly through resonant dipole-dipole interactions with Rydberg atoms. A two-qubit gate based on this interaction has a duration of 1 $μ$s and an achievable fidelity of 99.9%. The gate has little sensitivity to the motional states of the particles -- the molecules can be in thermal states, the atoms do not need to be trapped during Rydberg excitation, the gate does not heat the molecules, and heating of the atoms has a negligible effect. Within a large, static array, the gate can be applied to arbitrary pairs of molecules separated by tens of micrometres, making the scheme highly scalable. The molecule-atom interaction can also be used for rapid qubit initialization and efficient, non-destructive qubit readout, without driving any molecular transitions. Single qubit gates are driven using microwave pulses alone, exploiting the strong electric dipole transitions between rotational states. Thus, all operations required for large scale quantum computation can be done without moving the molecules or exciting them out of their ground electronic states.

quant-ph↗

arXiv2022

Measuring the stability of fundamental constants with a network of clocks

The detection of variations of fundamental constants of the Standard Model would provide us with compelling evidence of new physics, and could lift the veil on the nature of dark matter and dark energy. In this work, we discuss how a network of atomic and molecular clocks can be used to look for such variations with unprecedented sensitivity over a wide range of time scales. This is precisely the goal of the recently launched QSNET project: A network of clocks for measuring the stability of fundamental constants. QSNET will include state-of-the-art atomic clocks, but will also develop next-generation molecular and highly charged ion clocks with enhanced sensitivity to variations of fundamental constants. We describe the technological and scientific aims of QSNET and evaluate its expected performance. We show that in the range of parameters probed by QSNET, either we will discover new physics, or we will impose new constraints on violations of fundamental symmetries and a range of theories beyond the Standard Model, including dark matter and dark energy models.

hep-ph↗

arXiv2021

QSNET, a network of clocks for measuring the stability of fundamental constants

The QSNET consortium is building a UK network of next-generation atomic and molecular clocks that will achieve unprecedented sensitivity in testing variations of the fine structure constant, $α$, and the electron-to-proton mass ratio, $μ$. This in turn will provide more stringent constraints on a wide range of fundamental and phenomenological theories beyond the Standard Model and on dark matter models.

hep-ph↗

arXiv2021

Collisions in a dual-species magneto-optical trap of molecules and atoms

We study inelastic collisions between CaF molecules and $^{87}$Rb atoms in a dual-species magneto-optical trap. The presence of atoms increases the loss rate of molecules from the trap. By measuring the loss rates and density distributions, we determine a collisional loss rate coefficient $k_{2} = (1.43 \pm 0.29) \times 10^{-10}$ cm$^{3}$/s at a temperature of 2.4 mK. We show that this is not substantially changed by light-induced collisions or by varying the populations of excited-state atoms and molecules. The observed loss rate is close to the universal rate expected in the presence of fast loss at short range, and can be explained by rotation-changing collisions in the ground electronic state.

physics.atom-ph↗

arXiv2021

Laser cooling and magneto-optical trapping of molecules analyzed using optical Bloch equations and the Fokker-Planck-Kramers equation

We study theoretically the behavior of laser-cooled calcium monofluoride (CaF) molecules in an optical molasses and magneto-optical trap (MOT), and compare our results to recent experiments. We use multi-level optical Bloch equations to estimate the force and the diffusion constant, followed by a Fokker-Planck-Kramers equation to calculate the time-evolution of the velocity distribution. The calculations are done in three-dimensions, and we include all the relevant energy levels of the molecule and all the relevant frequency components of the light. Similar to simpler model systems, the velocity-dependent force curve exhibits Doppler and polarization-gradient forces of opposite signs. We show that the temperature of the MOT is governed mainly by the balance of these two forces. Our calculated MOT temperatures and photon scattering rates are in broad agreement with those measured experimentally over a wide range of parameters. In a blue-detuned molasses, the temperature is determined by the balance of polarization gradient cooling, and heating due to momentum diffusion, with no significant contribution from Doppler heating. In the molasses, we calculate a damping rate similar to the measured one, and steady-state temperatures that have the same dependence on laser intensity and applied magnetic field as measured experimentally, but are consistently a few times smaller than measured. We attribute the higher temperatures in the experiments to fluctuations of the dipole force which are not captured by our model. We show that the photon scattering rate is strongly influenced by the presence of dark states in the system, but that the scattering rate does not go to zero even for stationary molecules because of the transient nature of the dark states.

physics.atom-ph↗