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Controlling the dimensionality of the Stern-Gerlach effect for neutral atom manipulation

Kevin Melin

Published May 1, 2019• 0 citations
Publisher

Abstract

The Stern-Gerlach experiment performed in 1922 is a seminal work of modern physics. It was the experiment that first showed the quantized nature of the spin of an atom. This dissertation returns to this early experiment and describes the design, construction, and characterization of a variation that produces a one-dimensional Stern-Gerlach effect, or 1D kick, on a beam of neutral lithium atoms. Our technique relies on the combination of a magnetic field gradient and a strong bias field to generate a one-dimensional force on lithium entrained in a supersonic helium beam. By adjusting the bias field via variable, pulsed currents, we demonstrate the transition from a purely dispersive kick to a one-dimensional kick where off-axis heating of the beam has been eliminated. Pure one-dimensional forces enable access to new atomic and molecular cooling methods that can replace or supplement existing techniques to produce larger samples of cold atoms currently limited by evaporative cooling. Simulations suggest that the proposed magneto-optical cooling utilizing one-dimensional kicks will increase the total number of atoms in cold atom traps by orders of magnitude thus opening applications such as high flux atom lasers and more sensitive atom interferometry.

DOI
10.26153/tsw/2914

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The Stern-Gerlach experiment performed in 1922 is a seminal work of modern physics. It was the experiment that first showed the quantized nature of the spin of an atom. This dissertation returns to this early experiment and describes the design, construction, and characterization of a variation that produces a one-dimensional Stern-Gerlach effect, or 1D kick, on a beam of neutral lithium atoms. Our technique relies on the combination of a magnetic field gradient and a strong bias field to generate a one-dimensional force on lithium entrained in a supersonic helium beam. By adjusting the bias field via variable, pulsed currents, we demonstrate the transition from a purely dispersive kick to a one-dimensional kick where off-axis heating of the beam has been eliminated. Pure one-dimensional forces enable access to new atomic and molecular cooling methods that can replace or supplement existing techniques to produce larger samples of cold atoms currently limited by evaporative cooling. Simulations suggest that the proposed magneto-optical cooling utilizing one-dimensional kicks will increase the total number of atoms in cold atom traps by orders of magnitude thus opening applications such as high flux atom lasers and more sensitive atom interferometry.
We report a different type of optical Stern-Gerlach effect, where a magnetic-field gradient is replaced with a light-intensity gradient and a paramagnetic atom is deflected according to its magnetic quantum number. The laser light is detuned between the D1D1D1 and D2D2D2 frequencies, with the size of the detuning from the D2D2D2 resonance being twice that from the D1D1D1 resonance, and it is circularly polarized to produce an ac Stark shift that takes the form of a pure Zeeman shift. Slow rubidium atoms are extracted from a magneto-optical trap and then spin polarized. The atoms traversing the laser-intensity gradient on one side of the Gaussian beam profile show deflections that depend on the atomic spin state and the laser polarization. When the laser-beam axis is aligned with the slit that defines the atomic beam, we observe focusing and defocusing of the atomic beam.
This dissertation describes a general approach of generating a high flux of cold atoms that can be confined in a magneto-optical trap. As an alternative to the state-of-the-art laser cooling method, this approach widens our ability to cool and control atoms without relying on a specific atomic transition and availability of laser. In this dissertation I will discuss the design, construction of the experiment and characterization of a pulsed cold atom source in detail. This work is based on a new paradigm: entrainment of atoms in the carrier gas of a supersonic beam, followed by the magnetic deceleration and trapping. Our methodology is based on the supersonic beam created by the expansion of a dense carrier gas from a pulsed release of gas through a small aperture. Cold noble gas emerging from this pulsed high pressure supersonic nozzle acts as a carrier, into which atoms of interest are then entrained. In our studies with the lithium atoms, up to 10¹¹ atoms per pulse could be entrained into a supersonic beam of helium at a translation temperature of below 100 mK. The supersonic valve is typically operated at a rate of below 1 Hz in the experiment. A much larger flux can be achieved at a higher repetition rate. These atoms are moving at a speed of 500 ms⁻¹ and need to be decelerated almost to a complete stop, in order to be trapped and cooled to quantum degeneracy. A 2.5-meter long moving-trap magnetic decelerator with 480 coil pairs was built and characterized. Atoms moving at a speed around 500 ms⁻¹ were trapped and decelerated to various final velocities ranging from 400 ms⁻¹ to 50 ms⁻¹, at a resulting temperature of 30 mK and a flux of 10⁸ ~ 10⁹ atoms per pulse. This whole process takes place within only 10 ms, at a repetition rate of 100 Hz 10¹⁰ ~ 10¹¹ atoms can be delivered per second. This approach is very general compared to the laser cooling, since most atoms in the periodic table have a magnetic moment in their ground state or can be optically pumped to a long-lived para-magnetic metastable state. In this dissertation, I show the working principle of each component of our experiment and characterize the atom beam at multiple stages. I describe in detail the construction and optimization of our magnetic decelerator, and demonstrate the control and monitoring system with the experiment results. Also implemented in this work is a flexible laser system that is composed of a reference diode laser and two tapered amplifiers to control and probe the internal states of atoms, as well as provide the trapping force. I also explore the optical molasses and chirped cooling techniques which help load the atoms into a magneto-optical trap. The successful demonstration of this method of creating a cold atom source leads us to believe that the magnetically decelerated supersonic atoms will play an important role in the area of cold atom physics
We demonstrate high-fidelity manipulation of the quantized motion of a single 87 Rb atom in an optical tweezer via microwave couplings induced by Stern–Gerlach splitting. The Stern–Gerlach splitting is mediated by polarization gradient of a strongly focused tweezer beam that functions as fictitious magnetic field gradient. The spatial splitting removes the orthogonality of the atomic spatial wavefunctions, thus enables the microwave couplings between the motional states. We obtain coherent Rabi oscillations for up to third-order sideband transitions, in which a high fidelity of larger than 0.99 is obtained for the spin-flip transition on the first order sideband after subtraction of the state preparation and detection error. The Stern–Gerlach splitting is measured at a precision of better than 0.05 nm. This work paves the way for quantum engineering of motional states of single atoms, and may have wide applications in few body physics and ultracold chemistry.
The Stern-Gerlach (SG) effect, discovered almost a century ago, has become a paradigm of quantum mechanics. Surprisingly there is little evidence that the original scheme with freely propagating atoms exposed to gradients from macroscopic magnets is a fully coherent quantum process. Specifically, no high-visibility spatial interference pattern has been observed with such a scheme, and furthermore no full-loop SG interferometer has been realized with the scheme as envisioned decades ago. On the contrary, numerous theoretical studies explained why it is a near impossible endeavor. Here we demonstrate for the first time both a high-visibility spatial SG interference pattern and a full-loop SG interferometer, based on an accurate magnetic field, originating from an atom chip, that ensures coherent operation within strict constraints described by previous theoretical analyses. This also allows us to observe the gradual emergence of time-irreversibility as the splitting is increased. Finally, achieving this high level of control over magnetic gradients may facilitate technological applications such as large-momentum-transfer beam splitting for metrology with atom interferometry, ultra-sensitive probing of electron transport down to shot-noise and squeezed currents, as well as nuclear magnetic resonance and compact accelerators.