Hybrid quantum systems can be formed that combine the strengths of multiple platforms while avoiding the weaknesses. Here we report on progress toward a hybrid quantum system of neutral atom spins coupled to superconducting qubits. We trap laser-cooled rubidium atoms in the evanescent field of an ultrathin optical fiber, which will be suspended a few microns above a superconducting circuit that resonates at the hyperfine frequency of the Rb atoms, allowing magnetic coupling between the atoms and superconductor. As this will be done in a dilution refrigerator environment, the technical demands on the optical fiber is severe. We have developed and optimized a tapered fiber fabrication system, achieving optical transmission in excess of 99.95% , and fibers that can sustain 400 mW of optical power in a UHV environment. We have also optimized tapered fibers that can support higher order optical modes with high transmission (> 97%), which may be useful for different optical potential geometries. We have developed an in-situ tunable high-Q superconducting microwave resonator that can be tuned to within the resonator linewidth of the 6.8 GHz frequency of the Rb hyperfine transition.
We have demonstrated efficient propagation of the first excited TE01, TM01, and HE21 modes in a nanofiber
with a radius of 400 nm. As we decrease the taper angle from 4 mrad to 1 mrad, the propagation becomes more
adiabatic and the transmission improves from 20% to 85%. We have also demonstrated that the choice of drawn
fiber can have a significant impact on the propagation characteristics.
We explore the entanglement between a single atom and a single, resonant field mode of a driven optical cavity, focusing on the strong driving regime. We show that, in the absence of spontaneous emission, there are special initial conditions that lead to approximately disentangled trajectories, whereas spontaneous emission results in coherent superpositions of such trajectories that may lead to (transient) near-maximally entangled atom-field states. We also discuss the possibility of using a special "asymmetric" field correlation function to track the time evolution of this entanglement.