Abstract. Laser trapping and interfacing of laser-cooled atoms in an optical fiber network is an important capability for quantum information science. Following the pioneering work of Balykin et al. and Vetsch et al., we propose a robust method of trapping single Cesium atoms with a two-color state-insensitive evanescent wave around a dielectric nanofiber. Specifically, we show that vector light shifts (i.e., effective inhomogeneous Zeeman broadening of the ground states) induced by the inherent ellipticity of the forward-propagating evanescent wave can be effectively canceled by a backward-propagating evanescent wave. Furthermore, by operating the trapping lasers at the magic wavelengths, we remove the differential scalar light shift between ground and excited states, thereby allowing for resonant driving of the optical D 2 transition. This scheme provides a promising approach to trap and probe neutral atoms with long trap and coherence lifetimes with realistic experimental parameters. † These authors contributed equally to this work.

Single photons from a coherent input are efficiently redirected to a separate output by way of a fiber-coupled microtoroidal cavity interacting with individual Cesium atoms. By operating in an overcoupled regime for the input-output to a tapered fiber, our system functions as a quantum router with high efficiency for photon sorting. Single photons are reflected and excess photons transmitted, as confirmed by observations of photon antibunching (bunching) for the reflected (transmitted) light. Our photon router is robust against large variations of atomic position and input power, with the observed photon antibunching persisting for intracavity photon number 0.03 n 0.7. PACS numbers:Cavity quantum electrodynamics (cQED) offers systems in which the coherent interaction between matter and light can dominate irreversible channels of dissipation [1,2,3,4]. Diverse systems based upon radiative interactions in cQED are thereby promising candidates for the physical implementation of quantum networks, where, for example, atoms in optical cavities (quantum nodes) are linked by photons in optical fiber (quantum channels) [4]. Although many important capabilities for quantum nodes have been demonstrated within the setting of cQED with single atoms in Fabry-Perot cavities [5,6,7,8,9,10], an outstanding challenge is high efficiency transport of quantum fields into and out of optical cavities [4], as is required to link large numbers of quantum nodes.In this regard, the coupling rate κ of photons to and from the quantum channel should dominate the rates for any other input-output mechanisms. One way to achieve this is to operate the nodes in an overcoupled regime [11], where external coupling dominates internal system losses. In the microwave domain, "circuit QED" systems routinely operate in the overcoupled regime [3] for high external efficiency.In this Letter, we realize a cQED system in the optical domain with efficient input-output coupling while still maintaining high internal efficiency for coupling to a single atom. We use a microtoroidal cavity interacting with single Cesium atoms [12,13,14], with coupling to and from the cavity implemented with a tapered optical fiber in an overcoupled regime [11]. As a proof of principle, we demonstrate an efficient and robust photon router for which single photons are extracted from an incident coherent state and redirected to a separate output with efficiency ξ 0.6.To model photon transport for the atom-cavity system, we consider the interaction of one atom with the evanescent fields of a microtoroidal cavity, as shown in Fig. 1(a), with g tw the rate of coherent atom-cavity coupling [15]. Near the atomic resonance at frequency ω A , T (τ = 0)) and reflected (g (2) R (τ = 0)) fields. (e) Schematic of our experiment. A Cesium MOT is formed in a separate chamber, and atoms are transfered to the main chamber via an optical conveyor belt. Atoms are then dropped onto a toroid, which is coupled to a tapered fiber as in (a). A probe beam ain is injected into the taper, and the transmitte...

Modern research in optical physics has achieved quantum control of strong interactions between a single atom and one photon within the setting of cavity quantum electrodynamics (cQED) 1 . However, to move beyond current proof-of-principle experiments involving one or two conventional optical cavities to more complex scalable systems that employ N 1 microscopic resonators 2 requires the localization of individual atoms on distance scales 100nm from a resonator's surface. In this regime an atom can be strongly coupled to a single intracavity photon 3 while at the same time experiencing significant radiative interactions with the dielectric boundaries of the resonator 4 . Here, we report an initial step into this new regime of cQED by way of real-time detection and high-bandwidth feedback to select and monitor single Cesium atoms localized ∼ 100 nm from the surface of a micro-toroidal optical resonator. We employ strong radiative interactions of atom and cavity field to probe atomic motion through the evanescent field of the resonator. Direct temporal and spectral measurements reveal both the significant role of Casimir-Polder attraction 5 and the manifestly quantum nature of the atom-cavity dynamics. Our work sets the stage for trapping atoms near micro-and nano-scopic optical resonators for applications in quantum information science, including the creation of scalable quantum networks composed of many atom-cavity systems that coherently interact via coherent exchanges of single photons 2 .The proximity of dielectric boundaries fundamentally alters atomic radiative processes as compared to quantum electrodynamics in free space. For example, freespace Lamb shifts and Einstein-A coefficients (i.e., level positions and decay rates) are modified for atom-surface distances comparable to the relevant transition wavelengths, as considered in the pioneering analyses of Casimir and Polder 5 and of Purcell 6 in the late 1940s. Seminal experiments in the 1970s investigated radiative decay for organic dye molecules near a metal mirror 7 and were followed in the 1980s by landmark observations of the inhibition of spontaneous emission for a trapped electron 8 and an atom in a waveguide 9 . The ensuing years have witnessed the development of cavity quantum electrodynamics (cQED) in this perturbative regime of boundary-modified linewidths and level shifts 4,10,11 , with applications ranging from measurements of fundamental constants 12 to the development of novel semiconductor devices 13 .With increased interaction strength, a non-perturbative regime of cQED becomes possible and is characterized not by irreversible decay but rather by the cyclic, reversible exchange of excitation between atom and photon 14 . The experimental quest for strong atomphoton coupling had its initial success in 1985 in the microwave regime with the realization of the micromaser 15 , with strong coupling in the optical domain achieved some years later 16 . By now the coherent control of atomic radiative dynamics has become possible on a photon-byphoton ...

Laser trapping and interfacing of laser-cooled atoms in an optical fiber network is an important tool for quantum information science. Following the pioneering work of Balykin et al (2004 Phys. Rev. A 70 011401) and Vetsch et al (2010 Phys. Rev. Lett. 104 203603), we propose a robust method for trapping single cesium atoms with a two-color state-insensitive evanescent wave around a dielectric nanofiber. Specifically, we show that vector light shifts (i.e. effective inhomogeneous Zeeman broadening of the ground states) induced by the inherent ellipticity of the forward-propagating evanescent wave can be effectively canceled by a backward-propagating evanescent wave. Furthermore, by operating the trapping lasers at the magic wavelengths, we remove the differential scalar light shift between ground and excited states, thereby allowing for resonant driving of the optical D 2 transition. This scheme provides a promising approach to trap and probe neutral atoms with long trap and coherence lifetimes with realistic experimental parameters.

In realistic continuous variable quantum key distribution protocols, an eavesdropper may exploit the additional Gaussian noise generated during transmission to mask her presence. We present a theoretical framework for a post-selection based protocol which explicitly takes into account excess Gaussian noise. We derive a quantitative expression of the secret key rates based on the Levitin and Holevo bounds. We experimentally demonstrate that the post-selection based scheme is still secure against both individual and collective Gaussian attacks in the presence of this excess noise.PACS numbers: 03.67. Dd, 42.50.Dv, 89.70.+c Continuous variable quantum key distribution (CV-QKD) [1] was introduced as an alternative to the original discrete variable single photon schemes [2]. CV-QKD promises to offer higher secret key rates, better detection efficiencies and higher bandwidths than its single photon counterpart and is easily adapted to current communication systems. Currently the two main protocols in CV-QKD are post-selection (PS) [3] and reverse reconciliation (RR) [4]. These protocols are based on the random Gaussian modulation of coherent states using either homodyne [4] or heterodyne [5] detection and both have been experimentally demonstrated [6,7,8,9,10]. At present PS-based CV-QKD has practical advantages in terms of key distillation and has been demonstrated experimentally for up to 90% channel loss [7].Reverse reconciliation CV-QKD, due to its inherent nature, easily incorporates excess noise into the protocols, and security proof have been demonstrated in the case of individual Gaussian attacks [4,5], non-Gaussian attacks [11], collective attacks [12,13] (with their Gaussian optimality [14]) and coherent states using homodyne detection [15]. For PS CV-QKD, the addition of excess noise into the analysis is quite difficult. The original protocol [3] only considered pure or vacuum states in its scheme and so far all post-selection protocols since have concentrated on the unrealistic case of zero excess noise [7,16,17]. Recently however, excess noise using a hybrid protocol, consisting of both post-selection and either direct or reverse reconciliation, was considered for the case of collective attacks [18].In this paper, we present a protocol for calculating the effect of excess Gaussian noise (EGN) on post-selection where two way classical communication is permitted, and show its security when considering either individual or collective attacks. We apply our analysis to an experimental demonstration and conclude that good key rates can be obtained under the realistic condition of channel with loss and excess Gaussian noise.We extend the original PS CV-QKD protocol [3] as follows. The sender, Alice draws two random numbers S x A and S p A from Gaussian distributions of variances V x A and V p A re-AM