## Title: Dispersive Quantum Interface with Atoms and Nanophotonic Waveguides

: June 13, 2018. 10:00am.*Time*Strong coupling between atoms and light is critical for quantum information processing and precise sensing. A nanophotonic waveguide is a promising platform for realizing an atom-light interface that reaches the strong coupling regime. We study the dispersive response theory of the nanowaveguide system as the means to create an entangling atom-light interface, with applications to quantum non-demolition (QND) measurement and spin squeezing. We propose an enhanced QND measurement and spin squeezing protocol based on the Faraday effect, which is modeled using first-principles stochastic master equations. We simulate the metrological squeezing effect for a nanofiber and a square waveguide. Counterintuitively, by placing the atoms at an azimuthal position where the guided probe mode has the lowest intensity, we increase the cooperativity. This arises because the measurement strength depends on the interference between the probe and scattered light into an orthogonal mode, while the decoherence rate depends on the local intensity of the probe. We find 6.3 dB and 13 dB of peak squeezing for the nanofiber and the square waveguide, respectively, with 2500 atoms.*Abstract:*I will also briefly discuss a preliminary work on the optimal control theory of atoms preloaded in an optical lattice near a nanophotonic waveguide. The controllability of the system relies on the enhanced inhomogeneous interactions due to multiple scattering of photons among atoms and the global control of internal atomic states of the atoms and the lattice geometry using a microwave control field and the guided modes of the waveguide near the atoms. I will discuss our protocol and demonstrate numerical evidences that one may be able to design universal optimal control waveforms to generate arbitrary collective states and unitary evolution operators in a product quantum space of atoms’ internal states and lattice states with a finite size. An immediate application of our protocol is to demonstrate Boson sampling and hence the quantum supremacy using atoms or atom-like particles on an optical lattice.

*References:*

- [1]. Phys. Rev. A 97, 033829 (2018).
- [2]. Phys. Rev. A 93, 023817 (2016).
- [3]. Phys. Rev. A 57, 1972 (1998).
- [4]. Phys. Rev. A 95, 033818 (2017).
- [5]. arXiv:1805.01858 [quant-ph].