Imperial College London

DrThomasWall

Faculty of Natural SciencesDepartment of Physics

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Blackett LaboratorySouth Kensington Campus

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Summary

 

Publications

Publication Type
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22 results found

Wright S, Wall T, Tarbutt M, 2019, Microwave trap for atoms and molecules, Physical Review Research, Vol: 1, ISSN: 2643-1564

We demonstrate a trap that confines polarizable particles around the antinode of a standing-wave microwave field. The trap relies only on the polarizability of the particles far from any resonances, so can trap a wide variety of atoms and molecules in a wide range of internal states, including the ground state. The trap has a volume of about 10 cm³, and a depth approaching 1K for many polarmolecules. We measure the trap properties using ⁷Li atoms, showing that when the input microwave power is 610W, the atoms remain trapped with a 1/e lifetime of 1.76(12) s, oscillating with an axial frequency of 28.55(5) Hz and a radial frequency of 8.81(8) Hz. The trap could be loaded with slowmolecules from a range of available sources, and is particularly well suited to sympathetic cooling and evaporative cooling of molecules.

Journal article

Cournol A, Manceau M, Pierens M, Lecordier L, Tran DBA, Santagata R, Argence B, Goncharov A, Lopez O, Abgrall M, Le Coq Y, Le Targat R, Martinez HA, Lee WK, Xu D, Pottie P-E, Hendricks RJ, Wall TE, Bieniewska JM, Sauer BE, Tarbutt MR, Amy-Klein A, Tokunaga SK, Darquie Bet al., 2019, A new experiment to test parity symmetry in cold chiral molecules using vibrational spectroscopy, QUANTUM ELECTRONICS, Vol: 49, Pages: 288-292, ISSN: 1063-7818

Journal article

Jarvis KN, Devlin JA, Wall TE, Sauer BE, Tarbutt MRet 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.

Journal article

Truppe S, Williams HJ, Fitch NJ, Hambach M, Wall TE, Hinds EA, Sauer BE, Tarbutt MRet 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.

Journal article

Cheng C, van der Poel APP, Jansen P, Quintero-Perez M, Wall TE, Ubachs W, Bethlem HLet al., 2016, Molecular Fountain, PHYSICAL REVIEW LETTERS, Vol: 117, ISSN: 0031-9007

Journal article

Wall TE, 2016, Preparation of cold molecules for high-precision measurements, Journal of Physics B - Atomic Molecular and Optical Physics, ISSN: 0953-4075

Journal article

Dufour G, Cassidy DB, Crivelli P, Debu P, Lambrecht A, Nesvizhevsky VV, Reynaud S, Voronin AY, Wall TEet al., 2015, Prospects for Studies of the Free Fall and Gravitational Quantum States of Antimatter, Advances in High Energy Physics, Vol: 2015, ISSN: 1687-7357

Different experiments are ongoing to measure the effect of gravity on cold neutral antimatter atoms such as positronium, muonium, and antihydrogen. Among those, the project GBAR at CERN aims to measure precisely the gravitational fall of ultracold antihydrogen atoms. In the ultracold regime, the interaction of antihydrogen atoms with a surface is governed by the phenomenon of quantum reflection which results in bouncing of antihydrogen atoms on matter surfaces. This allows the application of a filtering scheme to increase the precision of the free fall measurement. In the ultimate limit of smallest vertical velocities, antihydrogen atoms are settled in gravitational quantum states in close analogy to ultracold neutrons (UCNs). Positronium is another neutral system involving antimatter for which free fall under gravity is currently being investigated at UCL. Building on the experimental techniques under development for the free fall measurement, gravitational quantum states could also be observed in positronium. In this contribution, we report on the status of the ongoing experiments and discuss the prospects of observing gravitational quantum states of antimatter and their implications.

Journal article

Cooper BS, Alonso AM, Deller A, Wall TE, Cassidy DBet al., 2015, A trap-based pulsed positron beam optimised for positronium laser spectroscopy, Review of Scientific Instruments, Vol: 86, ISSN: 0034-6748

We describe a pulsed positron beam that is optimised for positronium (Ps) laser-spectroscopy experiments. The system is based on a two-stage Surko-type buffer gas trap that produces 4 ns wide pulses containing up to 5 × 105 positrons at a rate of 0.5-10 Hz. By implanting positrons from the trap into a suitable target material, a dilute positronium gas with an initial density of the order of 107 cm−3 is created in vacuum. This is then probed with pulsed (ns) laser systems, where various Ps-laser interactions have been observed via changes in Ps annihilation rates using a fast gamma ray detector. We demonstrate the capabilities of the apparatus and detection methodology via the observation of Rydberg positronium atoms with principal quantum numbers ranging from 11 to 22 and the Stark broadening of the n = 2 → 11 transition in electric fields.

Journal article

Andersen SL, Cassidy DB, Chevallier J, Cooper BS, Deller A, Wall TE, Uggerhøj UIet al., 2015, Positronium emission and cooling in reflection and transmission from thin meso-structured silica films, Journal of Physics B: Atomic, Molecular and Optical Physics, Vol: 48, ISSN: 0953-4075

Measurements of the positronium (Ps) energy and formation fraction in reflection and transmission from a thin meso-structured silica target have been conducted using single-shot positron annihilation lifetime spectroscopy and Doppler spectroscopy. The silica sample is made using glancing angle deposition of vaporized SiO2 on a suspended thin carbon foil. Optical access through the silica sample facilitates measurement of the longitudinal Ps energy, and the Ps energy in the reflection geometry is found to decrease with positron energy as expected, with a minimum achievable Ps energy of 0.203(12) and 0.26(3) eV for the transverse and longitudinal directions, respectively. In the transmission geometry cooling of Ps becomes evident at the minimum positron impact energy required for the positrons to penetrate the carbon foil and enter the meso-structured silica. The minimum energies for this geometry are 0.210(12) and 0.287(14) eV in the transverse and longitudinal directions, respectively, and the minimum achievable Ps energy is found to be limited by the thickness of the structured silica target, since the same energy was found in both geometries.

Journal article

Wall TE, Alonso AM, Cooper BS, Deller A, Hogan SD, Cassidy DBet al., 2015, Selective Production of Rydberg-Stark States of Positronium, Physical Review Letters, Vol: 114, ISSN: 1079-7114

Rydberg positronium (Ps) atoms have been prepared in selected Stark states via two-step (1s→2p→nd/ns) optical excitation. Two methods have been used to achieve Stark-state selection: a field ionization filter that transmits the outermost states with positive Stark shifts, and state-selected photoexcitation in a strong electric field. The former is demonstrated for n=17 and 18 while the latter is performed for n=11 in a homogeneous electric field of 1.9  kV/cm. The observed spectral intensities and their dependence on the polarization of the laser radiation are in agreement with calculations that include the perturbations of the intermediate n=2 manifold. Our results pave the way for the generation of Rydberg Ps atoms with large electric dipole moments that are required for the realization of schemes to control their motion using inhomogeneous electric fields, an essential feature of some proposed Ps free-fall measurements requiring focused beams of long-lived atoms.

Journal article

Deller A, Cooper BS, Wall TE, Cassidy DBet al., 2015, Positronium emission from mesoporous silica studied by laser-enhanced time-of-flight spectroscopy, New Journal of Physics, Vol: 17, ISSN: 1367-2630

The use of mesoporous silica films for the production and study of positronium (Ps) atoms has become increasingly important in recent years, providing a robust source of free Ps in vacuum that may be used for a wide variety of experiments, including precision spectroscopy and the production of antihydrogen. The ability of mesoporous materials to cool and confine Ps has also been utilized to conduct measurements of Ps–Ps scattering and Ps2 molecule formation, and this approach offers the possibility of making a sufficiently dense and cold Ps ensemble to realize a Ps Bose–Einstein condensate. As a result there is great interest in studying the dynamics of Ps atoms inside such mesoporous structures, and how their morphology affects Ps cooling, diffusion and emission into vacuum. It is now well established that Ps atoms are initially created in the bulk of such materials and are subsequently ejected into the internal voids with energies of the order of 1 eV, whereupon they rapidly cool via hundreds of thousands of wall collisions. This process can lead to thermalisation to the ambient sample temperature, but will be arrested when the Ps deBroglie wavelength approaches the size of the confining mesopores. At this point diffusion through the pore network can only proceed via tunneling, at a much slower rate. An important question then becomes, how long does it take for the Ps atoms to cool and escape into vacuum? In a direct measurement of this process, conducted using laser-enhanced positronium time-of-flight spectroscopy, we show that cooling to the quantum confinement regime in a film with approximately 5 nm diameter pores is nearly complete within 5 ns, and that emission into vacuum takes ~10 ns when the incident positron beam energy is 5 keV. The observed dependence of the Ps emission time on the positron implantation energy supports the idea that quantum confined Ps does not sample all of the available pore volume, but rather is limited to a subset of th

Journal article

Wall TE, Cassidy DB, Hogan SD, 2014, Single-color two-photon spectroscopy of Rydberg states in electric fields, Physical Review A, Vol: 90, ISSN: 1050-2947

Rydberg states of atomic helium with principal quantum numbers ranging from n=20 to n=100 have been prepared by non-resonance-enhanced single-color two-photon excitation from the metastable 23S1 state. Photoexcitation was carried out using linearly and circularly polarized pulsed laser radiation. In the case of excitation with circularly polarized radiation, Rydberg states with azimuthal quantum number |mℓ|=2 were prepared in zero electric field and in homogeneous electric fields oriented parallel to the propagation axis of the laser radiation. In sufficiently strong electric fields, individual Rydberg–Stark states were resolved spectroscopically, highlighting the suitability of non-resonance-enhanced multiphoton excitation schemes for the preparation of long-lived high-|mℓ| hydrogenic Rydberg states for deceleration and trapping experiments. Applications of similar schemes for Doppler-free excitation of positronium atoms to Rydberg states are also discussed.

Journal article

Quintero-Pérez M, Wall TE, Hoekstra S, Bethlem HLet al., 2014, Preparation of an ultra-cold sample of ammonia molecules for precision measurements, Journal of Molecular Spectroscopy, Vol: 300, Pages: 112-115, ISSN: 0022-2852

Journal article

Zhelyazkova V, Cournol A, Wall TE, Matsushima A, Hudson JJ, Hinds EA, Tarbutt MR, Sauer BEet al., 2014, Laser cooling and slowing of CaF molecules, Physical Review A, Vol: 89, ISSN: 1094-1622

We demonstrate slowing and longitudinal cooling of a supersonic beam of CaF molecules using counterpropagating laser light resonant with a closed rotational and almost-closed vibrational transition. A group of molecules are decelerated by about 20 m/s by applying light of a fixed frequency for 1.8 ms. Their velocity spread is reduced, corresponding to a final temperature of about 300 mK. The velocity is further reduced by chirping the frequency of the light to keep it in resonance as the molecules slow down.

Journal article

Jansen P, Quintero-Pérez M, Wall TE, van den Berg JE, Hoekstra S, Bethlem HLet al., 2013, Deceleration and trapping of ammonia molecules in a traveling-wave decelerator, Physical Review A, Vol: 88, ISSN: 1050-2947

We have recently demonstrated static trapping of ammonia isotopologues in a decelerator that consists of a series of ring-shaped electrodes to which oscillating high voltages are applied [Quintero-Pérez et al., Phys. Rev. Lett. 110, 133003 (2013)]. In this paper we provide further details about this traveling-wave decelerator and present new experimental data that illustrate the control over molecules that it offers. We analyze the performance of our setup under different deceleration conditions and demonstrate phase-space manipulation of the trapped molecular sample.

Journal article

Quintero-Pérez M, Jansen P, Wall TE, van den Berg JE, Hoekstra S, Bethlem HLet al., 2013, Static Trapping of Polar Molecules in a Traveling Wave Decelerator, Physical Review Letters, Vol: 110, ISSN: 0031-9007

We present experiments on decelerating and trapping ammonia molecules using a combination of a Stark decelerator and a traveling wave decelerator. In the traveling wave decelerator, a moving potential is created by a series of ring-shaped electrodes to which oscillating high voltages (HV) are applied. By lowering the frequency of the applied voltages, the molecules confined in the moving trap are decelerated and brought to a standstill. As the molecules are confined in a true 3D well, this kind of deceleration has practically no losses, resulting in a great improvement on the usual Stark deceleration techniques. The necessary voltages are generated by amplifying the output of an arbitrary wave generator using fast HV amplifiers, giving us great control over the trapped molecules. We illustrate this by experiments in which we adiabatically cool trapped NH3 and ND3 molecules and resonantly excite their motion.

Journal article

Wall TE, Kanem JF, Dyne JM, Hudson JJ, Sauer BE, Hinds EA, Tarbutt MRet al., 2011, Stark deceleration of CaF molecules in strong- and weak-field seeking states, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 13, Pages: 18991-18999, ISSN: 1463-9076

Journal article

Wall TE, Tokunaga SK, Hinds EA, Tarbutt MRet al., 2010, Nonadiabatic transitions in a Stark decelerator, PHYSICAL REVIEW A, Vol: 81, ISSN: 1050-2947

Journal article

Wall TE, Armitage S, Hudson JJ, Sauer BE, Dyne JM, Hinds EA, Tarbutt MRet al., 2009, Transport of polar molecules by an alternating-gradient guide, PHYSICAL REVIEW A, Vol: 80, ISSN: 1050-2947

Journal article

Wall TE, Kanem JF, Hudson JJ, Sauer BE, Cho D, Boshier MG, Hinds EA, Tarbutt MRet al., 2008, Lifetime of the A(v(')=0) state and Franck-Condon factor of the A-X(0-0) transition of CaF measured by the saturation of laser-induced fluorescence, PHYSICAL REVIEW A, Vol: 78, ISSN: 1050-2947

Journal article

Stucki D, Ribordy G, Stefanov A, Zbinden H, Rarity JG, Wall Tet al., 2001, Photon counting for quantum key distribution with peltier cooled InGaAs/InP APDs, Journal of Modern Optics, Vol: 48, Pages: 1967-1981, ISSN: 0950-0340

Journal article

Rarity JG, Wall TE, Ridley KD, Owens PCM, Tapster PRet al., 2000, Single-photon counting for the 1300–1600-nm range by use of Peltier-cooled and passively quenched InGaAs avalanche photodiodes, Applied Optics, Vol: 39, Pages: 6746-6746, ISSN: 0003-6935

Journal article

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