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Using graphene multilayers to transform the performance and functionality of atom chips for use in quantum sensors

Rosemary Crawford

Published Jul 17, 2019• 0 citations
OpenAlex

Abstract

Current atom chips, conventionally made from metal wires, suffer from anumber of different problems, which we have shown can be overcome byusing graphene conductors, specifically; •Metal wires have a large Johnson noise due to the high carrier density.Graphene has a much lower charge carrier density(≈8 orders of magnitude lower), and thus, Johnson noise that is four orders of magnitude lower than metal. This leads to a corresponding major increase in the lifetime of the atom cloud above the wire. We have shown that an atom cloud trapped 1μm above a graphene wire has its lifetime increased by around 4 orders of magnitude compared to metal, i.e from 0.1 s to>10 minutes. This extends the Johnson noise limited lifetime so much that it becomes negligible as it is far beyond the limit imposed by the background gas collisions. •Metal wires exert a large Casimir-Polder attraction on atoms trapped near them, thereby limiting the minimum trapping distance to 10-100μm. Using a transfer matrix method in conjunction with the Lifshitz approach, we have demonstrated that the Casimir-Polder attraction between a graphene layer and an atom is approximately 50% that of the attraction between an atom and a thin gold layer. This enables atoms to be trapped up to 2 orders of magnitude closer to a graphene atom chip and so achieve sub-micron trapping distances. •Metal wires are spatially imperfect on 200 nm scales [1], which can lead to fragmented atom clouds. We have that shown that current lithography techniques can produce conducting paths of an arbitrary shape in graphene with roughness on only the≈10 nm scale. This leads to smoother traps and therefore smoother clouds even at sub-micron trapping distances. •Finally, we note that graphene will also allow for a greater degree of integration between all the different components of the cold atom system, trapping wires, Ultra High Vacuum (UHV) environments and optics,thus aiding the miniaturisation of the experimental set-up.

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Current atom chips, conventionally made from metal wires, suffer from anumber of different problems, which we have shown can be overcome byusing graphene conductors, specifically; •Metal wires have a large Johnson noise due to the high carrier density.Graphene has a much lower charge carrier density(≈8 orders of magnitude lower), and thus, Johnson noise that is four orders of magnitude lower than metal. This leads to a corresponding major increase in the lifetime of the atom cloud above the wire. We have shown that an atom cloud trapped 1μm above a graphene wire has its lifetime increased by around 4 orders of magnitude compared to metal, i.e from 0.1 s to>10 minutes. This extends the Johnson noise limited lifetime so much that it becomes negligible as it is far beyond the limit imposed by the background gas collisions. •Metal wires exert a large Casimir-Polder attraction on atoms trapped near them, thereby limiting the minimum trapping distance to 10-100μm. Using a transfer matrix method in conjunction with the Lifshitz approach, we have demonstrated that the Casimir-Polder attraction between a graphene layer and an atom is approximately 50% that of the attraction between an atom and a thin gold layer. This enables atoms to be trapped up to 2 orders of magnitude closer to a graphene atom chip and so achieve sub-micron trapping distances. •Metal wires are spatially imperfect on 200 nm scales [1], which can lead to fragmented atom clouds. We have that shown that current lithography techniques can produce conducting paths of an arbitrary shape in graphene with roughness on only the≈10 nm scale. This leads to smoother traps and therefore smoother clouds even at sub-micron trapping distances. •Finally, we note that graphene will also allow for a greater degree of integration between all the different components of the cold atom system, trapping wires, Ultra High Vacuum (UHV) environments and optics,thus aiding the miniaturisation of the experimental set-up.
Ultracold atomic gases can be utilised as extremely sensitive probes of their surrounding environment. In particular, samples of ultracold atoms confined using chip-based microtraps are an ideal tool for mapping electric and magnetic field landscapes. Over the course of this thesis, a new experiment capable of performing surface microscopy using magnetically trapped clouds of cold rubidium-87 atoms has been built. The focus of the work is on the design and construction of the experimental system, which must incorporate many different aspects for manipulating thermal atomic gases, with a view to positioning them at sub-micron distances from special surfaces. This reduced atom-surface separation is necessary for implementing a high resolution, high sensitivity magnetic field sensor with cold atoms. Although current microfabrication techniques easily enable trapping at distances on the order of micrometres, several distance-dependent surface effects - such as the Casimir-Polder force, Johnson-Nyquist noise, and stray potentials - eventually impede magnetic trapping at the sub-micron level. These surface effects can greatly modify the confining potentials, which reduces the trap depth and consequently leads to an additional loss rate of atoms from the trap. We have explored the possibility of using special surfaces such as nano-membranes of silicon nitride and graphene, which have reduced atom- surface interactions, to enable trapping distances at the sub-micron level. A multilayer printed circuit board chip has been designed to form an initial magnetic trap and then transport the cloud of atoms to a desired location over the samples. This chip, along with various samples, is mounted on a custom-made electrical feedthrough designed to make connections to all conductor that are inside the vacuum chamber. The initial cloud of cold atoms can then be prepared in the central region of the chip and delivered to the location of the samples on either side. The experimental system is able to routinely capture over 10^8 rubidium-87 atoms at a temperature of 80 micro-Kelvin in a magneto-optical trap using a novel scheme of five laser beams. A method is demonstrated for enhancing the atom number in the magneto-optical traps by a factor of two by using laser beams with two slightly different frequencies. Atoms from the magneto-optical trap are then transferred to a purely magnetic trap formed by the wires on the printed circuit board chip. Time-dependent currents in the chip wires then create a dynamic potential, which is shown to successfully transport the atomic sample over a distance of 12 millimetre with minimal atom loss. This thesis describes the development of the apparatus in detail, along with careful characterisation of the cold cloud at various stages of the experimental sequence. Initial results on the long distance atom transport are presented. Finally, the experimental results of the two frequency magneto-optical trap for atom number improvement are discussed.
We present an analysis of magnetic traps for ultracold atoms based on current-carrying wires with sub-micron dimensions. We analyze the physical limitations of these conducting wires as well as how such miniaturized magnetic traps are affected by the nearby surface due to tunneling to the surface, surface thermal noise, electron scattering within the wire and the Casimir–Polder force. We show that wires with cross sections as small as a few tens of nanometers should enable robust operating conditions for coherent atom optics (e.g. tunneling barriers for interferometry). In particular, trap sizes of the order of the de Broglie wavelength become accessible, based solely on static magnetic fields, thereby bringing the atomchip a step closer to fulfilling its promise of a compact device for complex and accurate quantum optics with ultracold atoms.
We show that current in a two-dimensional electron gas (2DEG) can trap ultracold atoms $<$1 $\ensuremath{\mu}$m away with orders of magnitude less spatial noise than a metal trapping wire. This enables the creation of hybrid systems, which integrate ultracold atoms with quantum electronic devices to give extreme sensitivity and control: For example, activating a single quantized conductance channel in the 2DEG can split a Bose-Einstein condensate (BEC) for atom interferometry. In turn, the BEC offers unique structural and functional imaging of quantum devices and transport in heterostructures and graphene.
Atom chips are robust and extremely powerful toolboxes for quantum optical experiments, since they make it possible to create exceedingly precise magnetic traps for neutral atoms with minimal field modulations. Accurate manipulation of trapped atoms is feasible with magnetic and electric fields created on the atom chip. Therefore atom chips with high quality surfaces and extremely well defined wires were build (roughness < 20nm). Furthermore new generations of atom chips were developed, like the multi-layer atom chip with contact-free wire crossings, the sub-micron structured atom chip and the semiconducting atom chip. Extensive characterisation measurements of atom chip wires demonstrated that the wires stand current densities of up to 10^7A/cm^2 for seconds and voltage differences of more than 500V over 10µm. From the different materials tested doped silicon with a thin silicon dioxide layer is the best qualified substrate for atom chip fabrication. The atom chips fabricated during this thesis have been used in many successful experiments, yielding numerous results. Moreover this thesis established the basics for many further experiments in atom physics and quantum optics and delivers complete instructions for the fabrication of all developed atom chips as well as an introduction to the experiments.
The entry into the new millennium was marked in the quantum community by the first experimental realizations of the Bose-Einstein condensation of atoms in a vapor, achieved in different research groups. Twenty-five years later, such systems are 'routinely' produced and are the core of many experiments. At the frontier between condensed matter their y, quantum optics, and atomic physics, ultra-cold atoms are subjects of high scientific interest.Ultra-cold gas can be trapped in optical lattices, giving a realistic way of experimentally realizing a new range of lattice Hamiltonians, that are used to simulate condensed matter systems. In these simulations, ultra-cold atoms are used to mimic the behavior of electrons in a solid-state material and could be used to simulate a wide range of complex condensed matter phenomena, such as the fractional quantum Hall effect, topological insulators, and high-temperature superconductivity.In order to study such systems, diffraction-limited quantum gas microscopes have emerged as a way to manipulate and detect atoms trapped in lattice potentials. Using far-field interference, experiments have mainly been restricted to a lattice spacing of 500 nm. This limits the speed of dynamics of all physical processes.Higher energy regimes could be accessed by reducing the lattice spacing. For this purpose, one of the long-term goals of the team is to trap atoms in the near-field of a surface and to make use of the Casimir-Polder forces to shape sub-wavelength potentials. Thanks to nano-fabrication capabilities, the spacing can be reduced to a few tens of nanometers. However, this requires to adapt the imaging techniques to observe cold-atoms with sub-wavelength spatial resolutions.In this PhD thesis, I report first on the advances toward implementing an atom chip in our experiment, which will serve as an integrated platform to manipulate the quantum gas. I present simulations and study an experimental solution allowing the cloud to be transported to the near-field. The method is based on the interference of a laser radiation in the near field of a rotating surface. It has been fully simulated, and the limitations have been numerically and experimentally assessed. I conclude this part by presenting the first model of chip that has been fabricated.In a second part, I report on the development of a sub-wavelength imaging method of cold Rubidium 87 atoms which is based on excited state dressing. I detail the concept of this nano-metric spatial resolution method and apply it to super-resolve a single longitudinal wave function trapped in a tight lattice that exhibits a width below 30 nm.