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arXiv:1405.6776 · Physical Review A

Microtoroidal cavity QED with fiber overcoupling and strong atom-field coupling: A single-atom quantum switch for coherent light fields

A. S. Parkins, Takao Aoki

Published Nov 11, 2014• 20 citations
arXivHTMLPDF

Abstract

We propose a scheme for single-atom, quantum control of the direction of propagation of a coherent field incident, via a tapered fiber, upon a microtoroidal whispering-gallery-mode (WGM) resonator. The scheme involves overcoupling of the fiber taper to the resonator and strong coupling of an atom to the evanescent field of the WGM, i.e., an atom-field coupling that exceeds the total WGM linewidth. In contrast to previous, related schemes that operate in the bad-cavity regime, the proposed scheme can operate effectively with much stronger incident fields, while also preserving their coherent nature. It can also serve to prepare an entangled state of the atom and coherent optical pulses propagating in opposite directions along the fiber. We evaluate the fidelity of preparation of such a state taking into account absorption and atomic spontaneous emission and demonstrate that high fidelities should be possible with realistic parameters.

DOI
10.1103/physreva.90.053822

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We propose a scheme for single-atom, quantum control of the direction of propagation of a coherent field incident, via a tapered fiber, upon a microtoroidal whispering-gallery-mode (WGM) resonator. The scheme involves overcoupling of the fiber taper to the resonator and strong coupling of an atom to the evanescent field of the WGM, i.e., an atom-field coupling that exceeds the total WGM linewidth. In contrast to previous, related schemes that operate in the bad-cavity regime, the proposed scheme can operate effectively with much stronger incident fields, while also preserving their coherent nature. It can also serve to prepare an entangled state of the atom and coherent optical pulses propagating in opposite directions along the fiber. We evaluate the fidelity of preparation of such a state taking into account absorption and atomic spontaneous emission and demonstrate that high fidelities should be possible with realistic parameters.
We investigate a two-way cascaded cavity QED system consisting of microtoroidal resonators coupled through an optical fiber. Each microtoroidal cavity supports two counterpropagating whispering-gallery modes coupled to single atoms through their evanescent fields. We focus on a pair of atom-microtoroid systems and compute the spectrum of spontaneous emission into the fiber with one atom initially excited. Explicit results are presented for strong-coupling and bad-cavity regimes, where the latter allows the effective atom-atom interaction to be controlled through the atom-cavity coupling and detuning: the atoms exhibit either collective spontaneous emission with no dipole-dipole interaction or a (coherent) dipole-dipole interaction and independent (single-atom) emission. This capacity for switching the character of the interaction is a feature of bidirectional coupling and connects our two-way cascaded system to work on one-dimensional waveguides. Building upon our bad-cavity results, we generalize to many atom-microtoroid systems coupled through an optical fiber.
We demonstrate highly efficient switching of optical signals between two optical fibers controlled by a single atom. The key element of our experiment is a whispering-gallery-mode bottle microresonator, which is coupled to a single atom and interfaced by two tapered fiber couplers. This system reaches the strong coupling regime of cavity quantum electrodynamics, leading to a vacuum Rabi splitting in the excitation spectrum. We systematically investigate the switching efficiency of our system, i.e., the probability that the fiber-optical switch redirects the light into the desired output. We obtain a large redirection efficiency reaching a raw fidelity of more than 60% without postselection. Moreover, by measuring the second-order correlation functions of the output fields, we show that our switch exhibits a photon-number-dependent routing capability.
The interaction between single quantum emitters and non-transversally polarized photons for which the electric field vector amplitude has a significant component in the direction of propagation is investigated. Even though this situation seems to be at odds with the description of light as a transverse wave, it regularly occurs when interfacing or manipulating emitters with non-paraxial, guided, or evanescent light. Here, this phenomenon for the case of single atoms that strongly interact with whispering-gallery-mode (WGM) microresonators is quantitatively investigated. These resonators confine light by continuous total internal reflection and offer the advantage of very long photon lifetimes in conjunction with near-lossless in- and out-coupling of light via tapered fiber couplers. In the setup, a novel type of high-Q WGM microresonator - a so-called bottle microresonator, which has the additional advantage of being fully tunable and provides a mode geometry that enables near-lossless simultaneous coupling of two independent tapered fiber couplers, is employed.
We propose a dissipative scheme to prepare maximally entangled steady states in cavity QED setup, consisting of two two-level atoms interacting with the two counter-propagating whispering-gallery modes (WGMs) of a microtoroidal resonator. Using spontaneous emission and cavity decay as the dissipative quantum dynamical source, we show that the steady state of this system can be steered into a two-atom single state as well as into a two-mode single state. We probed the compound system with weak field coupled to the system via a tapered fiber waveguide, finding it is possible to determine whether the two atoms or two modes are driven to a maximally entangled state. Through the transmission and reflection measurements, without disturbing the atomic state, when the cavity modes are being driven, or without disturbing the cavity field state, when a single atom being driven, one can get the information about the maximal entanglement. We also investigated for both subsystem, two-atom and two-mode states, the entanglement generation and under what conditions one can transfer entanglement from one subsystem to the other. Our scheme can be selectively used to prepare both maximally entangled atomic state as well as maximally entangled cavity-modes state, providing an efficient method for quantum information processing.
We demonstrate highly efficient switching of optical signals between two optical fibers controlled by a single atom.The key element of our experiment is a whispering-gallery-mode bottle microresonator, which is coupled to a single atom and interfaced by two tapered fiber couplers.This system reaches the strong coupling regime of cavity quantum electrodynamics, leading to a vacuum Rabi splitting in the excitation spectrum.We systematically investigate the switching efficiency of our system, i.e., the probability that the fiber-optical switch redirects the light into the desired output.We obtain a large redirection efficiency reaching a raw fidelity of more than 60% without postselection.Moreover, by measuring the second-order correlation functions of the output fields, we show that our switch exhibits a photon-number-dependent routing capability.