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.
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.
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.
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.
Ultracold atom experiments use a gas of neutral atoms with temperatures less than 100 µK above absolute zero and offer unmatched experimental control of quantum states and coherence, which has allowed ultracold atom-based measurements to be some of the most precise to date. While ultracold atom experiments can control almost all atomic degrees of freedom, spin-dependent trapping and spatial manipulation has remained difficult if not inaccessible. We are developing a method of spin-dependent trapping and spatial manipulation for ultracold neutral atoms using the AC Zeeman force produced by a microwave magnetic near-field gradient generated by an atom chip. We measure the AC Zeeman force on ultracold rubidium atoms by observing its effect on the motion of atoms in free-fall and on those confined in a trap. We have studied the force as a function of microwave frequency detuning from a hyperfine transition at 6.8 GHz at several magnetic field strengths and have observed its characteristic bipolar and resonant features predicted by two-level dressed atom theory. We find that the force is several times the strength of gravity in our setup, and that it can be targeted to a specific hyperfine transition while leaving other hyperfine states and transitions relatively unaffected. We find that our measurements are reasonably consistent with parameter-free theoretical predictions.
The spectacular progress in the field of ultracold quantum gases is intimately connected with the availability of sophisticated techniques for quantum-level control of internal states, motional states, and collisional interactions. Atom chips provide such control in compact, robust, and scalable setups, which makes them attractive for both applications and fundamental studies. In this thesis I report on experiments that use a new method for coherent manipulation of ultracold atoms. The method is based on microwave near-fields, provided by a waveguide structure that is fully integrated on an atom chip. We generate microwave near-field potentials that combine the versatility of optical traps with the robustness and tailorability of static magnetic microtraps. These potentials depend on the internal atomic state, and we use them for state-selective splitting of Rb Bose-Einstein condensates. We show for the first time combined coherent manipulation of internal and motional states on an atom chip, realizing a trapped-atom interferometer with internal state labeling of the interferometer paths. Moreover, we use microwave near-field potentials for the preparation of spin-squeezed states for quantum-enhanced metrology through controlling state-dependent collisional interactions. In addition, very promising proposals exist for the implementation of a quantum phase gate using these potentials. Measuring microwave fields is important for engineering of microwave devices as well as in science, e.g. to characterize the field homogeneity in the interaction regions of an atomic clock. We develop a novel technique that uses clouds of uncondensed ultracold atoms as sensitive, tunable and non-invasive probes for microwave field imaging with micrometer spatial resolution. The microwave magnetic field components drive Rabi oscillations on atomic hyperfine transitions whose frequency can be tuned with a static magnetic field. Readout is accomplished using state-selective absorption imaging. Quantitative data extraction is simple and it is possible to reconstruct the amplitudes and phases of the different microwave magnetic field components. While we demonstrate 2D imaging, an extension to 3D imaging is straightforward. We use the method to determine the microwave near-field distribution around the on-chip waveguide and reconstruct the corresponding current distribution. For our experimental parameters, the method provides a microwave magnetic field sensitivity of 0.2 mG, which can even be improved further with variants discussed. The experiments presented in this thesis open the path for the realization of portable quantum-enhanced interferometer devices, the implementation of a quantum phase gate as well as for a new generation of microwave field sensors.
Atom chip based interferometry holds signicant promise for embedded precise measurements of time, accelerations, rotations, and magnetic fields. It can enable compact sensors for various carriers like planes, boats, and drones, making it advantageous for navigation applications. When measuring acceleration with a cold atom interferometer, it is necessary to separate the two states during the interferometry sequence. Microwave dressed potentials generated from CPWs within a magnetic trap are one approach to achieve such on-chip manipulation. Two specic challenges arise in this process. First, the microwave field emitted by a waveguide introduces an AC Zeeman shifts, leading to a bias phase in the interferometer output. Precise determination of this bias is necessary for accurate parameter readout. Second, positioning the atom trap accurately between the two waveguides is crucial to obtain the necessary symmetry during separation; otherwise, the contrast of the interferometer is signicantly reduced. To address these challenges, we present a method for measuring AC Zeeman shifts using Ramsey interferometry.
Microwave near-fields are a key ingredient for quantum information processing with atom chips. Our goal is to realize a quantum gate with the following features: the qubit is encoded in the hyperfine states |1rang equiv |F=1, mF=-1rang and |2rang equiv |F=2, mF=+1rang of87Rb, which are both magnetically trappable and allow for very long coherence lifetimes. Microwave near-fields guided on the atom chip are used to drive single-qubit rotations and provide state-selectivity to the magnetic trapping potential. The quantum phase gate is implemented by state-selective collisions of two qubit atoms in this potential. Besides applications in quantum information processing, microwave near-fieIds on atom chips can also be used for atom interferometry and chip-based atomic clocks.