Light is often described as a fully transverse-polarized wave, i.e., with an electric field vector that is orthogonal to the direction of propagation. However, light confined in dielectric structures such as optical waveguides or whispering-gallery-mode microresonators can have a strong longitudinal polarization component. Here, using single 85 Rb atoms strongly coupled to a whispering-gallerymode microresonator, we experimentally and theoretically demonstrate that the presence of this longitudinal polarization fundamentally alters the interaction between light and matter.PACS numbers: 42.50. Pq,42.50.Ct,42.60.Da,42.25.Ja The interaction between light and matter underlies basically every optical process and application. For essentially plane waves in isotropic media, it has been quantitatively investigated in a number of groundbreaking experiments at the level of single atoms and single photons in high-finesse cavities [1][2][3][4][5][6][7][8]. In order to further enhance the light-matter coupling strength, an increasing number of recent experiments rely on waveguide structures [9][10][11], or high NA objectives [12][13][14]. However, in these situations, the physics changes drastically from the plane wave case because the polarization of the light fields is in general no longer transversal but exhibits a longitudinal component in the direction of propagation. This tags the propagation direction of the light by its polarization state and fundamentally renders full destructive interference of two counter-propagating waves impossible. One would thus expect this effect to have striking consequences for the physics of light-matter interaction.Here, we quantitatively investigate this phenomenon in a model system consisting of single atoms that strongly interact with a whispering-gallery-mode (WGM) microresonator [15]. These resonators confine light by continuous total internal reflection and offer the advantage of very long photon lifetimes in conjunction with nearlossless in-and out-coupling of light via tapered fibre couplers [16]. As recently demonstrated in a series of pioneering experiments with toroidal WGM microresonators [17][18][19][20][21][22], single atoms as well as solid state quantum emitters can be strongly coupled to WGMs. Beyond their importance in strong light-matter coupling, WGM microresonators are highly versatile photonic devices which have found applications in a large variety of disciplines. They have enabled, e.g., on-chip detection of single nanoparticels [23] and single viruses [24], the generation of optical frequency combs [25] as well as squeezed and correlated twin-beams and single and pair photons [26,27]. Moreover, WGM microresonators provide a successful experimental platform in the thriving field of cavity optomechanics [28,29].However, thus far, the non-transversal polarization of WGMs has not been taken into account in the description of the quantum mechanical interaction of light and mat-FIG. 1. a) Schematic view of our bottle microresonator interfaced with a tapered fibre cou...

The realization of nanophotonic optical isolators with high optical isolation even at ultralow light levels and low optical losses is an open problem. Here, we employ the link between the local polarization of strongly confined light and its direction of propagation to realize low-loss nonreciprocal transmission through a silica nanofiber at the single-photon level. The direction of the resulting optical isolator is controlled by the spin state of cold atoms. We perform our experiment in two qualitatively different regimes, i.e., with an ensemble of cold atoms where each atom is weakly coupled to the waveguide and with a single atom strongly coupled to the waveguide mode. In both cases, we observe simultaneously high isolation and high forward transmission. The isolator concept constitutes a nanoscale quantum optical analog of microwave ferrite resonance isolators, can be implemented with all kinds of optical waveguides and emitters, and might enable novel integrated optical devices for fiber-based classical and quantum networks.

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 bottlemicroresonator, 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 (CQED), 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 CQED 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 post-selection. Moreover, by measuring the second order correlation functions of the output fields, we show that our switch exhibits a photon number-dependent routing capability.PACS numbers: 42.50. Pq,42.50.Ct,42.60.Da, Fiber-optical switches are devices that enable optical signals to be rerouted to different fiber output ports and play a vital role in optical communication networks. Scaling such a device into the quantum domain, where a single quantum system controls the flow of light, would enable the implementation of quantum communication and information protocols with atoms and photons as well as the preparation of non-classical light, useful for interferometric schemes in quantum metrology [1,2].The physical realization of such a quantum switch requires an enhanced light-matter interaction that can, e.g., be reached by coupling an atom to an optical microresonator. However, this requires to reach the singleatom strong coupling regime, where the so-called critical atom number N 0 = 2κγ/g 2 [3], has to be much smaller than one. Here, κ and γ are the decay constants of the cavity field and the atomic dipole and g is the single photon-single atom coupling strength. This regime has been investigated in the optical domain in numerous groundbreaking experiments using high finesse Fabry-Pérot microresonators [4][5][6][7][8][9][10]. While these experiments clearly demonstrate the high potential of CQED systems for future applications, light absorption and scattering in the mirrors limit the efficiency of coupling light into and out of the resonator to typically a few tens of percent.In this context, whispering-gallery-mode (WGM) microresonators combine very high atom-light coupling strengths [11][12][13] and low coupling losses in the same system. WGM resonators are monolithic dielectric structures, such as microspheres [14] and microtori [15], in which the light is guided near the surface by continuous total internal reflection [16]. The light can be coupled in and out by frustrated total internal reflection with near 100% efficiency using tapered fiber couplers [17], thereby largely outperforming all other types of optical resonators.Strong coupling of single atoms and solid state quan- tum emitters to WGM microresonators has recently been demonstrated in a number of experiments [18][19][20][21][22][23][24]. Moreover, using toroi...

Typical microresonators exhibit a large frequency spacing between resonances and a limited tunability. This impedes their use in a large class of applications which require a resonance of the microresonator to coincide with a predetermined frequency. Here, we experimentally overcome this limitation with highly prolate-shaped whispering-gallery-mode "bottle microresonators" fabricated from standard optical glass fibers. Our resonators combine an ultrahigh quality factor of 3.6 x 10(8), a small mode volume, and near-lossless fiber coupling, characteristic of whispering-gallery-mode resonators, with a simple and customizable mode structure enabling full tunability.

Typical microresonators exhibit a large frequency spacing between resonances and a limited tunability. This impedes their use in a large class of applications which require a resonance of the microcavity to coincide with a predetermined frequency. Here, we experimentally overcome this limitation with highly prolate-shaped whispering-gallery-mode "bottle microresonators" fabricated from standard optical glass fibers. Our resonators combine an ultra-high quality factor of 360 million, a small mode volume, and near lossless fibre coupling, characteristic of whispering-gallery-mode resonators, with a simple and customizable mode structure enabling full tunability.PACS numbers: 42.60. Da, 42.50.Pq Optical microresonators hold great potential for many fields of research and technology [1]. They are used for filters and switches in optical communications [2][3][4], nonlinear optics [5], bio(chemical) sensing [6], microlasers [7][8][9], as well as for cavity quantum electrodynamics applications such as single photon sources [10-12] and interfaces for quantum communication [13,14]. All these applications rely on the spatial and temporal confinement of light by the microresonator, characterized by its mode volume V and its quality factor Q, respectively [1]. The ratio Q/V thus defines a key figure relating the coupling strength between light and matter in the resonator to the dissipation rates of the coupled system. The highest values of Q/V to date have been reached with whisperinggallery-mode (WGM) microresonators [15]. Standard WGM microresonators, like dielectric microspheres, microdisks, and microtori, typically confine the light in a narrow ring along the equator of the structure by continuous total internal reflection at the resonator surface [16]. While such equatorial WGMs have the advantage of a small mode volume they also exhibit a large frequency spacing between consecutive modes. In conjunction with the limited tuning range due to their monolithic design, tuning of equatorial WGM microresonators to an arbitrary frequency has therefore not been realized to date.For this reason, the WGM "bottle microresonator" has recently received considerable attention [17][18][19][20] because it promises a customizable mode structure while maintaining a favourable Q/V ratio [21,22]. Due to its highly prolate shape, the bottle microresonator gives rise to a class of whispering-gallery-modes (WGMs) with advantageous properties, see Fig. 1(a). The light in these "bottle modes" harmonically oscillates back and forth along the resonator axis between two turning points which are defined by an angular momentum barrier [22]. The resulting axial standing wave structure exhibits a significantly enhanced intensity at the so-called "caustics" of the bottle mode, located at the turning points of the harmonic motion. The bottle microresonator possesses an equidistant spectrum of eigenmodes, labelled by the "azimuthal quantum number" m, which counts the number of wave-FIG. 1: (a) Concept of the bottle microresonator. In addition to the r...

We describe a reproducible method of fabricating adiabatic tapers with 3 -4 m diameter. The method is based on a heat-and-pull rig, whereby a CO 2 laser is continuously scanned across a length of fiber that is being pulled synchronously. Our system relies on a CO 2 mirror mounted on a geared stepper motor in order to scan the laser beam across the taper region. We show that this system offers a reliable alternative to more traditional rigs incorporating galvanometer scanners. We have routinely obtained transmission losses between 0.1 and 0.3 dB indicating the satisfactory production of adiabatic tapers. The operation of the rig is described in detail and an analysis on the produced tapers is provided. The flexibility of the rig is demonstrated by fabricating prolate dielectric microresonators using a microtapering technique. Such a rig is of interest to a range of fields that require tapered fiber fabrication such as microcavity-taper coupling, atom guiding along a tapered fiber, optical fiber sensing, and the fabrication of fused biconical tapered couplers.

Divacancy defects in silicon carbide have long-lived electronic spin states and sharp optical transitions. Because of the various polytypes of SiC, hundreds of unique divacancies exist, many with spin properties comparable to the nitrogen-vacancy center in diamond. If ensembles of such spins can be all-optically manipulated, they make compelling candidate systems for quantum-enhanced memory, communication, and sensing applications. We report here direct all-optical addressing of basal plane-oriented divacancy spins in 4H-SiC. By means of magneto-spectroscopy, we fully identify the spin triplet structure of both the ground and the excited state, and use this for tuning of transition dipole moments between particular spin levels. We also identify a role for relaxation via intersystem crossing. Building on these results, we demonstrate coherent population trapping -a key effect for quantum state transfer between spins and photons- for divacancy sub-ensembles along particular crystal axes. These results, combined with the flexibility of SiC polytypes and device processing, put SiC at the forefront of quantum information science in the solid state.

We review our recent work on tunable, ultra-high quality factor whispering-gallery-mode bottle microresonators and highlight their applications in nonlinear optics and in quantum optics experiments. Our resonators combine ultra-high quality factors of up to Q = 3.6 × 10 8 , a small mode volume, and near-lossless fiber coupling, with a simple and customizable mode structure enabling full tunability. We study, theoretically and experimentally, nonlinear all-optical switching via the Kerr effect when the resonator is operated in an add-drop configuration. This allows us to optically route a single-wavelength cw optical signal between two fiber ports with high efficiency. Finally, we report on progress towards strong coupling of single rubidium atoms to an ultra-high Q mode of an actively stabilized bottle microresonator.