Publications
120 results found
Cotter J, Cameron R, 2019, Multi-path interferometry using single photons, Journal of Physics Communications
In most experiments which search for violations of Born’s rule using lightthe diffractive elements are formed from material slits, or waveguides, which are treated classically. In this article we propose an alternative approach where the internal energy levels of particles are used instead of slits to test the superposition principle, thus removing the effect of the finite width of the slits. By quantising both the light field and the diffraction elements we propose a new way to probe Born’s rule for light in a fully quantum mechanical way.
Boissier S, Schofield R, Major K, et al., 2018, Efficient excitation of dye molecules for single photon generation, Journal of Physics Communications, Vol: 2, ISSN: 2399-6528
A reliable photon source is required for many aspects of quantum technology. Organic molecules are attractive for this application because they can have high quantum yield and can be photostable, even at room temperature. To generate a photon with high probability, a laser must excite the molecule efficiently. We develop a simple model for that efficiency and discuss how to optimise it. We demonstrate the validity of our model through experiments on a single dibenzoterrylene (DBT) molecule in an anthracene crystal. We show that the excitation probability cannot exceed 75% at room temperature, but can increase to over 99% if the sample is cooled to liquid nitrogen temperature. The possibility of high photon generation efficiency with only modest cooling is a significant step towards a reliable photon source that is simple and practical.
Williams HJ, Caldwell L, Fitch NJ, et al., 2018, Magnetic trapping and coherent control of laser-cooled molecules, Physical Review Letters, Vol: 120, ISSN: 0031-9007
We demonstrate coherent microwave control of the rotational, hyperfine and Zeeman states of ultracold CaF molecules, and the magnetic trapping of these molecules in a single, selectable quantum state. We trap about 5 X 10³ molecules for almost 2s at a temperature of 70(8) μK and a density of 1.2 X 10⁵ cm⁻³. We measure the state-specific loss rate due to collisions with background helium.
Lim J, Almond J, Trigatzis M, et al., 2018, Laser cooled YbF molecules for measuring the electron's electric dipole moment, Physical Review Letters, Vol: 120, ISSN: 0031-9007
We demonstrate one-dimensional sub-Doppler laser cooling of a beam of YbF molecules to 100 μK. This is a key step towards a measurement of the electron's electric dipole moment using ultracold molecules. We compare the effectiveness of magnetically-assisted and polarization-gradient sub-Doppler cooling mechanisms. We model the experiment and fi nd good agreement with our data.
Jarvis KN, Devlin JA, Wall TE, et al., 2018, Blue-detuned magneto-optical trap, Physical Review Letters, Vol: 120, ISSN: 0031-9007
We present the properties and advantages of a new magneto-optical trap (MOT) where blue- detuned light drives ‘type-II’ transitions that have dark ground states. Using ⁸⁷Rb, we reach a radiation-pressure-limited density exceeding 10¹¹ cm⁻³ and a temperature below 30 μK. The phase-space density is higher than in normal atomic MOTs, and a million times higher than comparable red-detuned type-II MOTs, making the blue-detuned MOT particularly attractive for molecular MOTs which rely on type-II transitions. The loss of atoms from the trap is dominated by ultracold collisions between Rb atoms. For typical trapping conditions, we measure a loss rate of 1.8(4) × 10⁻¹⁰ cm³ s⁻¹.
Truppe S, Williams HJ, Hambach M, et al., 2017, Molecules cooled below the Doppler limit, Nature Physics, Vol: 13, Pages: 1173-1176, ISSN: 1745-2473
The ability to cool atoms below the Doppler limit -- the minimum temperaturereachable by Doppler cooling -- has been essential to most experiments withquantum degenerate gases, optical lattices and atomic fountains, among manyother applications. A broad set of new applications await ultracold molecules,and the extension of laser cooling to molecules has begun. A molecularmagneto-optical trap has been demonstrated, where molecules approached theDoppler limit. However, the sub-Doppler temperatures required for mostapplications have not yet been reached. Here we cool molecules to 50 uK, wellbelow the Doppler limit, using a three-dimensional optical molasses. Theseultracold molecules could be loaded into optical tweezers to trap arbitraryarrays for quantum simulation, launched into a molecular fountain for testingfundamental physics, and used to study ultracold collisions and ultracoldchemistry.
Williams H, Truppe S, Hambach M, et al., 2017, Characteristics of a magneto-optical trap of molecules, New Journal of Physics, Vol: 19, ISSN: 1367-2630
We present the properties of a magneto-optical trap (MOT) of CaFmolecules. We study the process of loading the MOT from a decelerated bu er-gas-cooled beam, and how best to slow this molecular beam in order to capture the most molecules. We determine how the number of molecules, the photon scattering rate, the oscillation frequency, damping constant, temperature, cloud size and lifetime depend on the key parameters of the MOT, especially the intensity and detuning of the main cooling laser. We compare our results to analytical and numerical models, to the properties of standard atomic MOTs, and to MOTs of SrF molecules. We load up to 2 x 10⁴ molecules, and measure a maximum scattering rate of 2.5 x 10⁶ s⁻¹ per molecule, a maximum oscillation frequency of 100 Hz, a maximum damping constant of 500 s⁻¹, and a minimum MOT rms radius of 1.5 mm. A minimum temperature of 730 μK is obtained by ramping down the laser intensity to low values. The lifetime, typically about 100 ms, is consistent with a leak out of the cooling cycle with a branching ratio of about 6 x 10⁻⁶. The MOT has a capture velocity of about 11 m/s.
Truppe S, Hambach M, Skoff S, et al., 2017, A buffer gas beam source for short, intense and slow molecular pulses, Journal of Modern Optics, Vol: 65, Pages: 246-254, ISSN: 0950-0340
Experiments with cold molecules usually begin with a molecular source. We describe the construction and characteristics of a cryogenic buff er gas source of CaF molecules. The source emits pulses with a typical duration of 240 μs, a mean speed of about 150 m/s, and a flux of 5x 10¹⁰ molecules per steradian per pulse in a single rotational state.
Sauer BE, Devlin JA, Rabey IM, 2017, A big measurement of a small moment, New Journal of Physics, Vol: 19, ISSN: 1367-2630
A beam of ThO molecules has been used to make the most precise measurement of the electron's electric dipole moment (EDM) to date. In their recent paper, the ACME collaboration set out in detail their experimental and data analysis techniques. In a tour-de-force, they explain the many ways in which their apparatus can produce a signal which mimics the EDM and show how these systematic effects are measured and controlled.
LIM J, Almond J, Tarbutt MR, et al., 2017, The [557]-X²∑⁺ and [561]- X²∑⁺ bands of ytterbium fluoride,¹⁷⁴YbF, Journal of Molecular Spectroscopy, Vol: 338, Pages: 81-90, ISSN: 1096-083X
The [557] - X2Σ+(v = 0, 1, 2, 3) and the [561] - X2Σ+ (v = 2) visible electronic transitions of YbF have been recorded at a near-natural linewidth spectral resolution using both supersonically cooled and buffer-gas cooled beams. The 174YbF isotopologue spectral features were analyzed to produce a new set of spectroscopic parameters for the X2Σ+(v = 2), X2Σ+(v = 3), [557], and [561] states. This study will facilitate ongoing work to produce ultracold YbF molecules by laser cooling in order to improve measurements of the electron’s electric dipole moment.
Tokunaga SK, Hendricks RJ, Tarbutt MR, et al., 2017, High-resolution mid-infrared spectroscopy of buffer-gas-cooled methyltrioxorhenium molecules, New Journal of Physics, Vol: 19, ISSN: 1367-2630
We demonstrate cryogenic buffer-gas cooling of gas-phase methyltrioxorhenium (MTO). This molecule is closely related to chiral organometallic molecules where the parity-violating energy differences between enantiomers is measurable. The molecules are produced with a rotational temperature of approximately 6 K by laser ablation of an MTO pellet inside a cryogenic helium buffer gas cell. Facilitated by the low temperature, we demonstrate absorption spectroscopy of the 10.2 μm antisymmetric Re=O stretching mode of MTO with a resolution of 8 MHz and a frequency accuracy of 30 MHz. We partially resolve the hyperfine structure and measure the nuclear quadrupole coupling of the excited vibrational state. Our ability to produce dense samples of complex molecules of this type at low temperatures represents a key step towards a precision measurement of parity violation in a chiral species.
Truppe S, Williams HJ, Fitch NJ, et al., 2017, An intense, cold, velocity-controlled molecular beam by frequency-chirped laser slowing, NEW JOURNAL OF PHYSICS, Vol: 19, ISSN: 1367-2630
Using frequency-chirped radiation pressure slowing, we precisely control the velocity of a pulsed CaF molecular beam down to a few m s–1, compressing its velocity spread by a factor of 10 while retaining high intensity: at a velocity of 15 m s–1 the flux, measured 1.3 m from the source, is 7 × 105 molecules per cm2 per shot in a single rovibrational state. The beam is suitable for loading a magneto-optical trap or, when combined with transverse laser cooling, improving the precision of spectroscopic measurements that test fundamental physics. We compare the frequency-chirped slowing method with the more commonly used frequency-broadened slowing method.
Asselin P, Berger Y, Huet TR, et al., 2017, Characterising molecules for fundamental physics: an accurate spectroscopic model of methyltrioxorhenium derived from new infrared and millimetre-wave measurements, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 19, Pages: 4576-4587, ISSN: 1463-9076
Precise spectroscopic analysis of polyatomic molecules enables many striking advances in physical chemistry and fundamental physics. We use several new high-resolution spectroscopic devices to improve our understanding of the rotational and rovibrational structure of methyltrioxorhenium (MTO), the achiral parent of a family of large oxorhenium compounds that are ideal candidate species for a planned measurement of parity violation in chiral molecules. Using millimetre-wave and infrared spectroscopy in a pulsed supersonic jet, a cryogenic buffer gas cell, and room temperature absorption cells, we probe the ground state and the Re[double bond, length as m-dash]O antisymmetric and symmetric stretching excited states of both CH3187ReO3 and CH3185ReO3 isotopologues in the gas phase with unprecedented precision. By extending the rotational spectra to the 150–300 GHz range, we characterize the ground state rotational and hyperfine structure up to J = 43 and K = 41, resulting in refinements to the rotational, quartic and hyperfine parameters, and the determination of sextic parameters and a centrifugal distortion correction to the quadrupolar hyperfine constant. We obtain rovibrational data for temperatures between 6 and 300 K in the 970–1015 cm−1 range, at resolutions down to 8 MHz and accuracies of 30 MHz. We use these data to determine more precise excited-state rotational, Coriolis and quartic parameters, as well as the ground-state centrifugal distortion parameter DK of the 187Re isotopologue. We also account for hyperfine structure in the rovibrational transitions and hence determine the upper state rhenium atom quadrupole coupling constant eQq′.
Lien Y-H, Barontini G, Scheucher M, et al., 2016, Observing coherence effects in an overdamped quantum system, Nature Communications, Vol: 7, ISSN: 2041-1723
It is usually considered that the spectrum of an optical cavity coupled to an atomic medium does not exhibit a normal-mode splitting unless the system satisfies the strong coupling condition, meaning the Rabi frequency of the coherent coupling exceeds the decay rates of atom and cavity excitations. Here we show that this need not be the case, but depends on the way in which the coupled system is probed. Measurements of the reflection of a probe laser from the input mirror of an overdamped cavity reveal an avoided crossing in the spectrum that is not observed when driving the atoms directly and measuring the Purcell-enhanced cavity emission. We understand these observations by noting a formal correspondence with electromagnetically induced transparency of a three-level atom in free space, where our cavity acts as the absorbing medium and the coupled atoms play the role of the control field.
Grandi S, Major KD, Polisseni C, et al., 2016, Quantum dynamics of a driven two-level molecule with variable dephasing, Physical Review A, Vol: 94, ISSN: 1094-1622
The longitudinal (1) and transverse (2) decay rates of a two-level quantum system have a profound influenceon its evolution. Atomic systems with 2 = 121 have been studied extensively, but with the rise of solid-statequantum devices it is also important to consider the effect of stronger transverse relaxation due to interactionswith the solid environment. Here we study the quantum dynamics of a single organic dye molecule driven by alaser. We measure the variation of 2 with temperature and determine the activation energy for thermal dephasingof the optical dipole. Then we measure the second-order correlation function g(2)(τ ) of the light emitted by themolecule for various ratios 2/1 and saturation parameters S. We show that the general solution to the opticalBloch equations accurately describes the observed quantum dynamics over a wide range of these parameters,and we discuss the limitations of the various approximate expressions for g(2)(τ ) that appear in the literature.DOI:
Devlin J, Tarbutt M, 2016, Three-dimensional Doppler, polarization-gradient, and magneto-optical forces for atoms and molecules with dark states, New Journal of Physics, Vol: 18, ISSN: 1367-2630
We theoretically investigate the damping and trapping forces in a three-dimensional magneto-optical trap (MOT), by numerically solving the optical Bloch equations. We focus on the case where there are dark states because the atom is driven on a ”type-II" system where the angular momentum of the excited state, F', is less than or equal to that of the ground state, F. For these systems we find that the force in a three-dimensional light field has very different behaviour to its one-dimensional counterpart. This differs from the more commonly used “type-I" systems (F' = F +1) where the 1D and 3D behaviours are similar. Unlike type-I systems where, for red-detuned light, both Doppler and sub-Doppler forces damp the atomic motion towards zero velocity, in type-II systems in 3D, the Doppler force and polarization gradient force have opposite signs. As a result, the atom is driven towards a non-zero equilibrium velocity, v₀, where the two forces cancel. We find that v₀² scales linearly with the intensity of the light and is fairly insensitive to the detuning from resonance. We also discover a new magneto-optical force that alters the normal MOT force at low magnetic fields and whose influence is greatest in the type-II systems. We discuss the implications of these findings for the laser cooling and magneto-optical trapping of molecules where type-II transitions are unavoidable in realising closed optical cycling transitions.
Rabey IM, Devlin JA, Hinds EA, et al., 2016, Low magnetic Johnson noise electric field plates for precision measurement, Review of Scientific Instruments, Vol: 87, ISSN: 1089-7623
We describe a parallel pair of high voltage electric field plates designed and constructed to minimise magnetic Johnson noise. They are formed by laminating glass substrates with commercially available polyimide (Kapton) tape, covered with a thin gold film. Tested in vacuum, the outgassing rate is less than 5 x 10 exp(-5) mbar.l/s. The plates have been operated at electric fields up to 8.3 kV/cm, when the leakage current is at most a few hundred pA. The design is discussed in the context of a molecular spin precession experiment to measure the permanent electric dipole moment of the electron.
Fitch N, Tarbutt M, 2016, Principles and design of a Zeeman-Sisyphus decelerator for molecular beams, Chemphyschem, Vol: 17, Pages: 3609-3623, ISSN: 1439-7641
We explore a technique for decelerating molecules using a static magnetic eld and optical pumping. Molecules travel through a spatially varying magnetic fi eld and are repeatedly pumped into a weak-field seeking state as they move towards each strong field region, and into a strong-fi eld seeking state as they move towards weak fi eld. The method is time-independentand so is suitable for decelerating both pulsed and continuous molecular beams. By using guiding magnets at each weak field region, the beamcan be simultaneously guided and decelerated. By tapering the magnetic field strength in the strong field regions, and exploiting the Doppler shift, the velocity distribution can be compressed during deceleration. We develop the principles of this deceleration technique, provide a realistic design, use numerical simulations to evaluate its performance for a beam of CaF, and compare this performance to other deceleration methods.
Cotter JP, McGilligan JP, Griffin PF, et al., 2016, Design and fabrication of diffractive atom chips for laser cooling and trapping, Applied Physics B, Vol: 122, ISSN: 0946-2171
It has recently been shown that optical reflection gratings fabricated directly into an atom chip provide a simple and effective way to trap and cool substantial clouds of atoms (Nshii et al. in Nat Nanotechnol 8:321–324, 2013; McGilligan et al. in Opt Express 23(7):8948–8959, 2015). In this article, we describe how the gratings are designed and microfabricated and we characterise their optical properties, which determine their effectiveness as a cold atom source. We use simple scalar diffraction theory to understand how the morphology of the gratings determines the power in the diffracted beams.
Hopkins SA, Butler K, Guttridge A, et al., 2016, A versatile dual-species Zeeman slower for caesium and ytterbium (vol 87, 043109, 2016), Review of Scientific Instruments, Vol: 87, ISSN: 1089-7623
Hopkins SA, Butler K, Guttridge A, et al., 2016, A versatile dual-species Zeeman slower for caesium and ytterbium, Review of Scientific Instruments, Vol: 87, ISSN: 1089-7623
We describe the design, construction, and operation of a versatile dual-species Zeeman slower for both Cs and Yb, which is easily adaptable for use with other alkali metals and alkaline earths. With the aid of analytic models and numerical simulation of decelerator action, we highlight several real-world problems affecting the performance of a slower and discuss effective solutions. To capture Yb into a magneto-optical trap(MOT), we use the broad ¹S0 to ¹P1 transition at 399 nm for the slower and the narrow ¹S0 to ³P1 intercombination line at 556 nm for the MOT. The Cs MOT and slower both use the D2 line (6²S1/2 to 6²P3/2 at 852 nm. The slower can be switched between loading Yb or Cs in under 0.1 s. We demonstrate that within a few seconds the Zeeman slower loads more than 10⁹ Yb atoms and 10⁸ Cs atoms into their respective MOTs. These are ideal starting numbers for further experiments on ultracold mixtures and molecules.
Polisseni C, Major K, Boissier S, et al., 2016, A stable, single-photon emitter in a thin organic crystal for application to quantum-photonic devices, Optics Express, ISSN: 1094-4087
Kemp SL, Butler KL, Freytag R, et al., 2016, Production and characterization of a dual species magneto-optical trap of cesium and ytterbium, Review of Scientific Instruments, Vol: 87, ISSN: 1089-7623
We describe an apparatus designed to trap and cool a Yb and Cs mixture. The apparatus consists of a dual species effusive oven source, dual species Zeeman slower, magneto-optical traps in a single ultra-high vacuum science chamber, and the associated laser systems. The dual species Zeeman slower is used to load sequentially the two species into their respective traps. Its design is flexible and may be adapted for other experiments with different mixtures of atomic species. The apparatus provides excellent optical access and can apply large magnetic bias fields to the trapped atoms. The apparatus regularly produces 108 Cs atoms at 13.3 μK in an optical molasses, and 109174Y b atoms cooled to 22 μK in a narrowband magneto-optical trap.
Norrgard EB, McCarron DJ, Steinecker MH, et al., 2016, Submillikelvin dipolar molecules in a radio-frequency magneto-optical trap, Physical Review Letters, Vol: 116, ISSN: 1079-7114
We demonstrate a scheme for magneto-optically trapping strontium monofluoride (SrF) molecules at temperatures one order of magnitude lower and phase space densities 3 orders of magnitude higher than obtained previously with laser-cooled molecules. In our trap, optical dark states are destabilized by rapidly and synchronously reversing the trapping laser polarizations and the applied magnetic field gradient. The number of molecules and trap lifetime are also significantly improved from previous work by loading the trap with high laser power and then reducing the power for long-term trapping. With this procedure, temperatures as low as 400 μK are achieved.
Lim J, Frye MD, Hutson JM, et al., 2015, Modeling sympathetic cooling of molecules by ultracold atoms, Physical Review A, Vol: 92, ISSN: 1094-1622
We model sympathetic cooling of ground-state CaF molecules by ultracold Li and Rb atoms. The molecules are moving in a microwave trap, while the atoms are trapped magnetically. We calculate the differential elastic cross sections for CaF-Li and CaF-Rb collisions, using model Lennard-Jones potentials adjusted to give typical values for the s-wave scattering length. Together with trajectory calculations, these differential cross sections are used to simulate the cooling of the molecules, the heating of the atoms, and the loss of atoms from the trap. We show that a hard-sphere collision model based on an energy-dependent momentum transport cross section accurately predicts the molecule cooling rate but underestimates the rates of atom heating and loss. Our simulations suggest that Rb is a more effective coolant than Li for ground-state molecules, and that the cooling dynamics is less sensitive to the exact value of the s-wave scattering length when Rb is used. Using realistic experimental parameters, we find that molecules can be sympathetically cooled to 100μK in about 10 s. By applying evaporative cooling to the atoms, the cooling rate can be increased and the final temperature of the molecules can be reduced to 1 μK within 30 s.
Tarbutt MR, Steimle TC, 2015, Modeling magneto-optical trapping of CaF molecules, Physical Review A, Vol: 92, ISSN: 1094-1622
Magneto-optical trapping forces for molecules are far weaker than for alkali atoms because thephoton scattering rate is reduced when there are multiple ground states, and because of opticalpumping into dark states. The force is further reduced when the upper state has a much smallerZeeman splitting than the lower state. We use a rate model to estimate the strength of the trappingand damping forces in a magneto-optical trap (MOT) of CaF molecules, using either the A2Π1/2- X2Σ+ transition or the B2Σ+ - X2Σ+ transition. We identify a new mechanism of magnetoopticaltrapping that arises when, in each beam of the MOT, two laser components with oppositepolarizations and different detunings address the same transition. This mechanism produces a strongtrapping force even when the upper state has little or no Zeeman splitting. It is the main mechanismresponsible for the trapping force when the A2Π1/2 - X2Σ+ transition is used.
Hills DJA, Grutter AM, Hudson JJ, 2015, An algorithm for discovering Lagrangians automatically from data, PEERJ COMPUTER SCIENCE, ISSN: 2376-5992
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Yuen B, Barr I, Cotter J, et al., 2015, Enhanced oscillation lifetime of a Bose–Einstein condensate in the 3D/1D crossover, New Journal of Physics, Vol: 17, ISSN: 1367-2630
We have measured the damped motion of a trapped Bose–Einstein condensate, oscillating withrespect to a thermal cloud. The cigar-shaped trapping potential provides enough transverseconfinement that the dynamics of the system are intermediate between three-dimensional and onedimensional.We find that the scaling of the damping rate with temperature is consistent with Landautheory, but that the damping rate for axial oscillations at a given temperature is consistently smallerthan expected for a three-dimensional gas. We attribute this to the suppressed density of states forlow-energy transverse excitations(essential excitations for axial Landau damping), which results fromthe quantization of the radial motion.
major K, lien Y, polisseni C, et al., 2015, Growth of optical-quality anthracene crystals, doped with dibenzoterrylenefor controlled single photon production, Review of Scientific Instruments, Vol: 86, ISSN: 1089-7623
Dibenzoterrylene (DBT) molecules within a crystalline anthracene matrix show promise as quantum emitters for controlled, single photon production. We present the design and construction of a chamber in which we reproducibly grow doped anthracene crystals of optical quality that are several mm across and a few micrometres thick. We demonstrate control of the DBT concentration over the range 6 { 300 parts per trillion and show that these DBT molecules are stable single-photon emitters. We interpret our data with a simple model that provides some information on the vapour pressure of DBT.
Devlin J, Tarbutt MR, Kokkin DL, et al., 2015, Measurements of the Zeeman effect in the A(2)Pi and B-2 Sigma(+) states of calcium fluoride, Journal of Molecular Spectroscopy, Vol: 317, Pages: 1-9, ISSN: 1096-083X
The magnetic tuning of the low rotational levels in the (v = 0) X2Σ+, (v = 0) A2Π, and (v = 0) B2Σ+ electronic states of calcium monofluoride, CaF, have been experimentally investigated using high resolution optical Zeeman spectroscopy of a cold molecular beam. The observed Zeeman shifts and splittings are successfully modeled using a traditional effective Hamiltonian approach to account for the interaction between the (v = 0) A2Π and (v = 0) B2Σ+ states. The determined magnetic g-factors for the X2Σ+, B2Σ+ and A2Π states are compared to those predicted by perturbation theory.
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