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Towards a weak measurement of transverse momentum in a matter-wave interferometer

Joel Morley

Published Nov 28, 2020• 1 citation
OpenAlex

Abstract

This thesis describes the construction of an atomic matter-wave interferometer, combined with a spin-state interferometer, used to weakly measure the average transverse momentum of the atoms. A velocity-tuneable cold atomic beam was constructed and characterised. By applying radiation pressure to a magneto-optical trap, the beam's velocity was selected between 1-52 ms-1. The beam was used to produce interference fringes in a matter-wave interferometer which consisted of a mutli-slit Si3Ni4 grating and a planar atom detector placed below the grating. The interference pattern was used to measure the average beam velocity and the Van der Waals coefficient between the atoms and the grating. This was the first instance of such a measurement. Below the grating a longitudinal Stern-Gerlach interferometer was constructed. The phase shift of the atom's spin, due to the Zeeman effect from the interferometer's magnetic field, was measured. The phase shift provided a measurement of the interferometer's magnetic field in the μT range. An experiment combining the two interferometers to weakly measure the atom's transverse momentum is described and modelled.

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European Quantum Electronics Conference (EQEC)•Aug 24, 2005•0 citations
This thesis describes the construction of an atomic matter-wave interferometer, combined with a spin-state interferometer, used to weakly measure the average transverse momentum of the atoms. A velocity-tuneable cold atomic beam was constructed and characterised. By applying radiation pressure to a magneto-optical trap, the beam's velocity was selected between 1-52 ms-1. The beam was used to produce interference fringes in a matter-wave interferometer which consisted of a mutli-slit Si3Ni4 grating and a planar atom detector placed below the grating. The interference pattern was used to measure the average beam velocity and the Van der Waals coefficient between the atoms and the grating. This was the first instance of such a measurement. Below the grating a longitudinal Stern-Gerlach interferometer was constructed. The phase shift of the atom's spin, due to the Zeeman effect from the interferometer's magnetic field, was measured. The phase shift provided a measurement of the interferometer's magnetic field in the μT range. An experiment combining the two interferometers to weakly measure the atom's transverse momentum is described and modelled.
We describe the creation and characterisation of a velocity tunable, spin-polarized beam of slow metastable argon atoms. We show that the beam velocity can be determined with a precision below 1% using matter-wave interferometry. The profile of the interference pattern was also used to determine the velocity spread of the beam, as well as the Van der Waals (VdW) co-efficient for the interaction between the metastable atoms and the multi-slit silicon nitride grating. The VdW co-efficient was determined to be C_{3} = 1.84 ± 0.17 a.u., in good agreement with values derived from spectroscopic data. Finally, the spin polarization of the beam produced during acceleration of the beam was also measured, demonstrating a spatially uniform spin polarization of 96% in the m = +2 state.
Although interferometry's earliest and most familiar use is with photons, the discovery of matter-wave (deBroglie-wave) interference for electrons demanded the development of quantum mechanics. Since then, matter-wave interferometry has been performed with neutrons, Cooper electron-pairs, and most recently, with whole atoms and diatomic molecules. This talk describes our recent high-flux atom interferometry experiments using the generalized Talbot-vonLau effect. Our interferometer consists of a sequence of three planar vacuum-slit diffraction gratings, microfabricated from silicon nitride membranes. DeBroglie-wave interference fringes are sensed by measuring the transmission of potassium atoms on a hot- wire as a function of grating relative position. Different spatial Fourier components (up to sixth) in the diffraction pattern are resonant in the interferometer at different atomic velocities (i.e., at different wavelengths). When a laser cooled slow atomic beam is incident, various different diffraction patterns are observed as a function of atomic velocity. In an alternative `Heisenberg Microscope' configuration an incident thermal beam produces a velocity average over different fringe Fourier components. AC modulated weak laser light passing through the interferometer interacts selectively with atoms at a specific velocity. The associated fringe pattern is then ac modulated and revealed by its selective destruction.
Abstract We describe the creation and characterisation of a velocity tunable, spin-polarized beam of slow metastable argon atoms. We show that the beam velocity can be determined with a precision below 1% using matter-wave interferometry. The profile of the interference pattern was also used to determine the velocity spread of the beam, as well as the Van der Waals (VdW) co-efficient for the interaction between the metastable atoms and the multi-slit silicon nitride grating. The VdW co-efficient was determined to be C 3 = 1.84 ± 0.17 a.u., in good agreement with values derived from spectroscopic data. Finally, the spin polarization of the beam produced during acceleration of the beam was also measured, demonstrating a spatially uniform spin polarization of 96% in the m = +2 state.
An atom interferometer using two Stern-Gerlach magnets as polariser and analyser is described. The interferometer was first operated with a thermal beam of ground state potassium atoms. In that case the beam splitters are two radio-frequency zones within a transverse homogeneous magnetic field building a coherent superposition of Zeeman states. Ramsey fringes are obtained by scanning the RF-frequency through the resonance profile. Pulsing the RF power allows to get the time-of-flight distribution of the beam. Atomic interference fringes (``Stern-Gerlach fringes'') are observed when the magnitude of an inhomogeneous magnetic field located in between the beam splitters is scanned. When the inhomogeneous field is also pulsed, the scalar Bohm-Aharonov effect is observed. The use of metastable helium atoms (He*23S) in place of potassium atoms is proposed. In addition to the usual advantages provided by metastable atoms of high internal energy, this atom offers the specific advantage to behave as an ideal spin-one particle. This allows us to greatly simplify the polarisation scheme of the interferometer. Possible applications involving transversally inhomogeneous magnetic fields are given and limitations due to the finite source size and angular aperture are discussed.
Atom interferometers have been successfully applied for precision measurements and the realization of basic gedanken experiments [1]. They consist of three or more atom optical elements to split, redirect, and recombine the motional state of the atoms. These elements are implemented either by freestanding microstructures or by light fields. For high precision applications, such as gyroscopes [2–5], a large throughput of the initial atomic flux is advantageous [6]. In order to efficiently use all atoms in an uncollimated thermal atomic beam, the ideal atom interferometer should produce equally spaced fringes for a wide range of transverse and longitudinal velocities. So called “white light” interferometers solve the problem for different longitudinal velocities by the recombination of only two velocity independent transverse momentum states at the output of the interferometer. Examples for this are the three-grating interferometer [7], operating with a transversely collimated beam, and the Raman interferometer [8]. Operation using a transversely uncollimated atomic beam can also be realized by the Raman interferometer and, for interferometers that do not require light, by the Talbot-Lau interferometer [9]. The latter, in contrast to Ref. [7], operates with three gratings in the Fresnel regime of matter wave diffraction. Since, in this case, many different momentum components build up the interference pattern, the typical length scale of the setup, the Talbot length, depends on the atomic velocity. The Talbot-Lau interferometer is, therefore, chromatic. In this work, we study a charged wire as the central element in an atom interferometer that requires only microstructures, operates in the Fresnel regime, and accepts, therefore, an uncollimated beam of atoms. At the same time, it has essentially white light character over the range of the atomic velocities in our thermal atomic beam. This makes future extensions of our approach based on microstructures potentially as powerful as, for example, the Raman interferometer gyroscope [5]. The attractive interaction between the induced dipole and the charged wire redirects the atomic trajectories to form a simple interferometer with two spatially separated paths. It can, therefore, also be used to measure the recently proposed topological phase shifts for moving electric dipoles [10–13].