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Optical and Quantum Sensing and Precision Metrology II

Potential roughness suppression in a RF AC Zeeman atom chip trap

William Miyahira, S. Aubin

Published Mar 13, 2024• 0 citations
Publisher

Abstract

Trapping and manipulating ultracold atoms through the use of atom chips offers the ability to engineer complex electromagnetic field geometries in a compact form factor. One of the primary limitations hindering their use is trap potential roughness which arises as a result of defects in the atom chip wires when atoms are brought close to the chip. This roughness can play a limiting role in atom interferometry and strong 1D confinement experiments. We present an initial experimental demonstration of atom chip potential roughness suppression using radio-frequency (RF) AC Zeeman (ACZ) potentials on an atom chip. Using ∼20 MHz RF magnetic near-fields from the chip to target intra-manifold transitions in the 87Rb ground state, we trap atoms in a spindependent ACZ potential. We compare the axial trapping potential for atoms in comparable 2-wire DC and AC Zeeman traps and show evidence of roughness suppression in the AC trap.

DOI
10.1117/12.3012372

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Physical Review A•Dec 23, 2024•1 citation
Trapping and manipulating ultracold atoms through the use of atom chips offers the ability to engineer complex electromagnetic field geometries in a compact form factor. One of the primary limitations hindering their use is trap potential roughness which arises as a result of defects in the atom chip wires when atoms are brought close to the chip. This roughness can play a limiting role in atom interferometry and strong 1D confinement experiments. We present an initial experimental demonstration of atom chip potential roughness suppression using radio-frequency (RF) AC Zeeman (ACZ) potentials on an atom chip. Using ∼20 MHz RF magnetic near-fields from the chip to target intra-manifold transitions in the 87Rb ground state, we trap atoms in a spindependent ACZ potential. We compare the axial trapping potential for atoms in comparable 2-wire DC and AC Zeeman traps and show evidence of roughness suppression in the AC trap.
We present several toy models for predicting the character and strength of the roughness of ac Zeeman trapping potentials generated by imperfections in wire traces on an atom chip. An ac Zeeman trapping potential is generated by targeting a microwave or rf magnetic field at hyperfine or Zeeman ground-state transitions, respectively, while a dc Zeeman trap uses a static magnetic field to manipulate an atom via its magnetic moment. We find that an ac Zeeman trap suppresses potential roughness by several orders of magnitude with respect to a comparable dc Zeeman trap. This suppression stems from the inherent differences between an atom's response to dc and ac (namely rf and microwave) magnetic fields, in concert with how the ac skin effect smooths out a current distribution disturbed by a conductivity variation. For chip wires fabricated by evaporation, we find that an ac Zeeman trap for 87Rb^{87}\mathrm{Rb}87Rb atoms, located $100\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$ from the chip and operating on a 6.8 GHz hyperfine transition, is expected to suppress potential roughness by a factor of $4\ifmmode\times\else\texttimes\fi{}{10}^{5}$ compared with a similar dc Zeeman trap. For a comparable ac Zeeman trap operating on a 10 MHz Zeeman transition, the suppression is $2\ifmmode\times\else\texttimes\fi{}{10}^{4}$. Conversely, the ac skin effect also slightly exacerbates potential roughness, in ac Zeeman traps, from defects in the sidewall of a wire trace.
Spin-specific trapping and mechanical control of ultracold atoms is difficult with current techniques, but offers the possibility of exploring new physics systems, such as spin-dependent trapped atom interferometers, as well as quantum gates, 1D many-body spin gases, and novel cooling schemes. Microwave near-field potentials based on the AC Zeeman effect provide a mechanism for such spin-specific control of atoms: in principle, independent potentials can be targeted to different spin states simultaneously. A broadband microwave atom chip can be used to generate both AC Zeeman trapping potentials and a microwave lattice for long distance trap positioning. We present recent experimental results on the successful trapping of ultracold rubidium atoms in an AC Zeeman atom chip trap using RF intra-hyperfine manifold transitions. Notably, these traps suppress the potential roughness encountered in traditional micro-magnetic chip traps.
We present a toolbox of microstrip building blocks for microwave atom chips geared towards trapped atom interferometry. Transverse trapping potentials based on the AC Zeeman (ACZ) effect can be formed from the combined microwave magnetic near fields of a pair or a triplet of parallel microstrip transmission lines. Axial confinement can be provided by a microwave lattice (standing wave) along the microstrip traces. Microwave fields provide additional parameters for dynamically adjusting ACZ potentials: detuning of the applied frequency to select atomic transitions and local polarization controlled by the relative phase in multiple microwave currents. Multiple ACZ traps and potentials, operating at different frequencies, can be targeted to different spin states simultaneously, thus enabling spin-specific manipulation of atoms and spin-dependent trapped atom interferometry.
In this dissertation we study the development of microwave and RF near-field potentials for use with atom chip trapped atomic gases. These potentials are inherently spin-dependent, able to target individual spin states simultaneously. In contrast with traditional atom chip potentials, these RF traps can be operated at arbitrary bias magnetic field strengths and thus be combined with magnetic Feshbach resonances. Furthermore, these potentials can strongly suppress the potential roughness that plagues traditional atom chip potentials. We present a dual chamber atom chip apparatus for generating ultracold 87Rb and 39K atomic gases. The apparatus produces quasi-pure Bose-Einstein condensates of 104 87Rb atoms in an atom chip trap that features a dimple and good optical access. We have also demonstrated production of ultracold 39K and subsequent loading into the chip trap. We describe the details of the dual chamber vacuum system, the cooling lasers, the magnetic trap, the multi coil magnetic transport system, and the atom chip. The apparatus is well suited for studies of atom-surface forces, quantum pumping and transport experiments, atom interferometry, novel chip-based traps, and studies of one-dimensional many-body systems.
We measure the ac Zeeman force acting on ultracold 87Rb^{87}\mathrm{Rb}87Rb atoms due to a radio frequency (rf) field driving intramanifold Zeeman transitions near 8 MHz. We observe about 2.5 times the force of gravity for $\ensuremath{\approx}14$ mW of rf power acting on atoms $\ensuremath{\approx}100\phantom{\rule{4pt}{0ex}}\textmu{}\mathrm{m}$ from the atom chip. Our measurements are consistent with theoretical predictions and demonstrate the spin-state dependence of the force, as well as its resonant and bipolar behavior. We demonstrate the ac Stern-Gerlach effect by spatially separating a spin mixture, using initial detuning to determine which spins are ac Zeeman high- and low-field seekers. With an eye towards using this force for atom trapping, we observe the time dependence of dressed state spin mixing as a function of detuning and power.