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Chinese Physics B

A combined system for generating a uniform magnetic field and its application in the investigation of Efimov physics

Rui Yao, Zhendong Sun, Shuyu Zhou, Ying Wang, Yuzhu Wang

Published Jan 1, 2018• 7 citations
Publisher

Abstract

We propose a scheme to produce a uniform magnetic field with a system comprising a pair of coils and an atom chip. After optimizing the parameters of the chip wires, we improve the homogeneity of the magnetic field by two orders of magnitude. We exhibit that this method can be applied in the investigation of Efimov physics in 87 Rb– 40 K mixture.

DOI
10.1088/1674-1056/27/1/016703

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We propose a scheme to produce a uniform magnetic field with a system comprising a pair of coils and an atom chip. After optimizing the parameters of the chip wires, we improve the homogeneity of the magnetic field by two orders of magnitude. We exhibit that this method can be applied in the investigation of Efimov physics in 87 Rb– 40 K mixture.
In this paper, we have developed a combined magnetic field system to explore an alternative technical route for the generation of Bose–Einstein condensates containing numerous atoms on an atom chip. The system is characterized by the fact that the quadrupole magnetic field required by the magneto-optic trap is generated by U-shaped current-carrying wires combined with bias magnetic fields, whereas the quadrupole magnetic field adopted for the magnetic trap is generated by anti-Helmholtz coils. By fine-tuning the bias magnetic fields, the collection of atoms in the mirror MOT and the loading of the quadrupole magnetic trap can be optimized. The initial number of [Formula: see text]Rb atoms in the magnetic trap is approximately [Formula: see text], and the lifetime of the atoms is over 30 s. This scheme can facilitate the chip-surface design and is suitable for the precise manipulation of Bose–Einstein condensates near the chip surface.
In order to take quantum mechanical systems out of the lab and produce practical devices, a method of miniaturizing and integrating apparatus components is needed. In the context of ultracold atomic gas experiments, the ideal method of achieving this miniaturization is the implementation of atom chip technology. By depositing micro-scale current carrying wires onto planar substrates, it is possible to create a magnetic trap capable of confining a cloud of neutral atoms in sufficient proximity to the chip surface for solid state devices to interact with the cloud. This thesis presents our lab’s first steps towards creating an atom chip capable of confining a cloud of rubidium-87. The chip prototypes were developed with the intent to incorporate them into the hybrid system apparatus within our lab. The chips are intended to serve as a platform for facilitating the interaction between the atomic gas ensemble and mesoscopic solid-state devices, allowing for the hybridization of the two systems. In this work I describe the design of the trapping wire configuration and the fabrication of the first two generations of chip prototypes. Additionally, I provide numerical simulations of the produced magnetic field, from which estimates of the trap frequency and depth can be calculated. Finally, I introduce some basic characterization tests of the absolute current limitations and resistive behavior of the chip wires, and the dimensional uniformity of the fabricated wire profiles.
In this thesis I discuss work that has been carried out on the dynamics of a Bose Einstein condensate of Rb 87 produced near an atom chip. A Bose Einstein Condensate (BEC) is a quantum state of matter where a single quantum state becomes occupied by a macroscopic number of identical Bosons. In our case this is achieved by cooling a system of trapped identical rubidium 87 atoms to its ground state. To reach temperatures of condensation we initially laser cool atoms from room temperature, before loading them into a magnetic trap. The magnetic trap is produced through a combination of uniform magnetic fields from coils outside our vacuum chamber and currents running through wires on an atom chip. The atom chip is a microfabricated device, produced by a coating a silicon chip with a thin layer of gold and etching wires into it. Together, these fields create a magnetic field minimum 120μm from the surface of the chip which can be used to confine low field seeking hyperfine states of the atom in an elongated harmonic trap. Once the atoms are confined in the magnetic trap we used force evaporative cooling out to reach the phase space densities required for Bose Einstein condensation. The BEC is used to investigate the relative dynamics between the fraction of the atoms in the condensate to those not in the condensate. Our atom chip provided a suitable environment to investigate this due to fragmentation of the magnetic potential close to the chip. Small imperfections in the wires on our atom chip mean that the trapping potential isn’t smooth. Small regions of higher trapping frequency - or fragments - are formed. Due to the small size of these fragments it is possible to find a position where a condensate can form in the fragment, and see a potential of high frequency, whereas a non-condensed atom will see a lower frequency potential. We exploit this to set the condensed fraction moving relative to the non condensed part and investigate the subsequent damping of their motion relative to each other.
We propose two kinds of wire configurations fabricated on an atom chip surface for creating two-dimensional (2D) adiabatic rf guide with an inhomogeneous rf magnetic field and a homogenous dc magnetic field. The guiding state can be selected by changing the detuning between the frequency of rf magnetic field and the resonance frequency of two Zeeman sublevels. We also discuss the optimization of loading efficiency and the trap depth and how to decide proper construction when designing an rf atom chip.
Several techniques for manipulating neutral atoms (more precisely, ultracold clouds of neutral atoms) in chip-based magnetic traps and atomic waveguides have been demonstrated. Such traps and waveguides are promising components of future quantum sensors that would offer sensitivities much greater than those of conventional sensors. Potential applications include gyroscopy and basic research in physical phenomena that involve gravitational and/or electromagnetic fields. The developed techniques make it possible to control atoms with greater versatility and dexterity than were previously possible and, hence, can be expected to contribute to the value of chip-based magnetic traps and atomic waveguides. The basic principle of these techniques is to control gradient magnetic fields with suitable timing so as to alter a trap to exert position-, velocity-, and/or time-dependent forces on atoms in the trap to obtain desired effects. The trap magnetic fields are generated by controlled electric currents flowing in both macroscopic off-chip electromagnet coils and microscopic wires on the surface of the chip. The methods are best explained in terms of examples. Rather than simply allowing atoms to expand freely into an atomic waveguide, one can give them a controllable push by switching on an externally generated or a chip-based gradient magnetic field. This push can increase the speed of the atoms, typically from about 5 to about 20 cm/s. Applying a non-linear magnetic-field gradient exerts different forces on atoms in different positions a phenomenon that one can exploit by introducing a delay between releasing atoms into the waveguide and turning on the magnetic field.