The aim of this thesis is to investigate the interaction of neutral atoms with conducting and superconducting surfaces. Experimental advances in the magnetic confinement of ultracold atoms have shown that they can act as a powerful tool in a wide range of phenomena such as electric and magnetic field imaging and matter wave interferometry. Coherent manipulation of atoms and ever smaller magnetic traps are essential elements in the implementation of integrated quantum devices for fundamental research, quantum information processing and precision measurements. This thesis considers main influences on atoms placed within three different environments which are useful in achieving miniaturization and efficient control in atomic magnetic traps: carbon nanotubes, dielectric surfaces and superconducting thin films. The possibility of holding atoms near the outside of a carbon nanotubes will be addressed. In order to give a qualitative analysis of the atom-nanotube interaction, thermally induced spin-flips and the Casimir-Polder potential have been considered. The comparison between these two effects is presented in this thesis. It indicates that the Casimir-Polder force is the dominant loss mechanism and an estimation of the minimum trapping distance is given based on its effect. Secondly, a first-principles derivation of spatial atomic-sublevel decoherence near dielectric and metallic surfaces will be presented. The rate obtained for the decay of spatial coherence has dual implications, on one hand, it can be considered as a measure of the coherence length of the fluctuations of the electromagnetic field arising from a given substrate. On the other hand, it turns out to be relevant for quantum information encoding in double well potentials. Finally, the known spin-flip transition rate will be linked to the flux noise spectrum in superconducting thin films showing the feasibility of using cold atomic clouds in the investigation of vortex dynamics.
We describe measurements of the trap lifetime of an ultracold atom cloud near niobium and copper surfaces at cryogenic temperatures. Large lifetimes of 35-160 seconds at 20-30 micrometers from superconducting niobium exceed predictions for a high conductivity normal metal with thermally fluctuating currents. This demonstrates clearly for the first time the absence of Johnson noise in the vicinity of the superconductor. Nonetheless we observed a drop in lifetime within 30 micrometers of the niobium wire, a result of the reduced trap depth due to the Meissner effect and corresponding surface evaporation. By comparison, near copper, a further reduction of the lifetime in this range indicates the presence of Johnson noise.
We compute the evanescent-wave Johnson noise (EWJN) in the vacuum half-space above a superconductor, and the resulting relaxation time (
T1) of spin and charge qubits placed at nanometer distances from the surface. The electromagnetic response is described by a single microscopic transverse current-response kernel
Q(q,ω) for a BCS superconductor. This is computed for varying densities of both non-magnetic impurities and magnetic impurities, for arbitrary frequency and temperature and for wave vectors
q≪kF (the Fermi wavevector). When combined with the fluctuation-dissipation theorem and the nonlocal surface impedances of the half-space, this yields the magnetic and electric field noise at any distance
z≫kF−1 from the surface, from which we obtain
T1. Just below
Tc the magnetic noise is enhanced relative to the normal state by the coherence (Hebel-Slichter-type) peak of the dissipative conductivity and drops exponentially at lower temperatures; the electric noise shows no coherence peak. The theory predicts that there is a zero-temperature noise floor induced by magnetic impurities. In the gapless regime produced by pair breaking, the finite subgap density of states
ν(0) yields a temperature-independent noise spectral density and a relaxation rate bounded by
T1−1(T)≤[ν(0)/νF]2T1,N−1(T) for
T≪Tc, with equality in the extreme nonlocal regime. Here
ν(0) and
νF are the superconducting and normal-state densities of states at the Fermi energy, and
T1,N(T) is the relaxation time the same electrode would produce in its normal state at the same temperature.
Atomic spin-based comagnetometers are powerful tools for precision sensing and tests of fundamental physics. Compared with the widely used gas-cell comagnetometer systems, cold-atom systems offer access to much shorter distance scales and allow implementation of {optical} quantum control techniques. However, in order to realize long spin coherence times with cold atoms, it is necessary to employ diamagnetic atoms and overcome decoherence induced by light shifts. Here we demonstrate a cold-atom comagnetometer based on the nuclear spins of
171Yb (spin-1/2) and
173Yb (spin-5/2), jointly trapped in an optical lattice. Vector light shifts are suppressed by enforcing linear polarization of the lattice, while tensor shifts in
173Yb are suppressed via the use of a Schrödinger cat state. This enables simultaneous Ramsey interferometry on both isotopes with a spin coherence time of 60 s. We achieve a magnetic noise suppression factor exceeding
3×104, and determine the ratio of nuclear magnetic moments to 4 ppm precision. Our results establish a new cold-atom platform for spin-based sensing and open pathways toward quantum-enhanced searches for physics beyond the Standard Model.
We investigate theoretically the magnetic spin-flip transitions of neutral atoms trapped near a superconducting slab. Our calculations are based on a quantum-theoretical treatment of electromagnetic radiation near dielectric and metallic bodies. Specific results are given for rubidium atoms near a niobium superconductor. At the low frequencies typical of atomic transitions, we find that BCS theory greatly overestimates coherence effects, which are much less pronounced when quasiparticle lifetime effects are included through Eliashberg theory. At
4.2K, the typical atomic spin lifetime is found to be larger than
1000s, even for atom-superconductor distances of one $1\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{m}$. This constitutes a large enhancement in comparison with normal metals.
A single-spin qubit placed near the surface of a conductor acquires an additional contribution to its 1/T 1 relaxation rate due to magnetic noise created by electric current fluctuations in the material.We analyze this technique as a wireless probe of superconductivity in atomically thin two-dimensional materials.At temperatures T T c , the dominant contribution to the qubit relaxation rate is due to transverse electric current fluctuations arising from quasiparticle excitations.We demonstrate that this method enables detection of metal-to-superconductor transitions, as well as investigation of the symmetry of the superconducting gap function, through the noise scaling with temperature.We show that scaling of the noise with sample-probe distance provides a window into the nonlocal quasistatic conductivity of superconductors, both clean and disordered.At low temperatures the quasiparticle fluctuations get suppressed, yet the noise can be substantial due to resonant contributions from collective longitudinal modes, such as plasmons in monolayers and Josephson plasmons in bilayers.Potential experimental implications are discussed.