Over the past decade, strong interactions of light and matter at the single-photon level have enabled a wide set of scientific advances in quantum optics and quantum information science. This work has been performed principally within the setting of cavity quantum electrodynamics with diverse physical systems, including single atoms in Fabry-Perot resonators, quantum dots coupled to micropillars and photonic bandgap cavities and Cooper pairs interacting with superconducting resonators. Experiments with single, localized atoms have been at the forefront of these advances with the use of optical resonators in high-finesse Fabry-Perot configurations. As a result of the extreme technical challenges involved in further improving the multilayer dielectric mirror coatings of these resonators and in scaling to large numbers of devices, there has been increased interest in the development of alternative microcavity systems. Here we show strong coupling between individual caesium atoms and the fields of a high-quality toroidal microresonator. From observations of transit events for single atoms falling through the resonator's evanescent field, we determine the coherent coupling rate for interactions near the surface of the resonator. We develop a theoretical model to quantify our observations, demonstrating that strong coupling is achieved, with the rate of coherent coupling exceeding the dissipative rates of the atom and the cavity. Our work opens the way for investigations of optical processes with single atoms and photons in lithographically fabricated microresonators. Applications include the implementation of quantum networks, scalable quantum logic with photons, and quantum information processing on atom chips.
Beyond traditional nonlinear optics with large numbers of atoms and photons, qualitatively new phenomena arise in a quantum regime of strong interactions between single atoms and photons. By using a microscopic optical resonator, we achieved such interactions and demonstrated a robust, efficient mechanism for the regulated transport of photons one by one. With critical coupling of the input light, a single atom within the resonator dynamically controls the cavity output conditioned on the photon number at the input, thereby functioning as a photon turnstile. We verified the transformation from a Poissonian to a sub-Poissonian photon stream by photon counting measurements of the input and output fields. The results have applications in quantum information science, including for controlled interactions of single light quanta and for scalable quantum processing on atom chips.
The prospect of quantum networks, in which quantum information is carried by single photons in photonic circuits, has long been the driving force behind the effort to achieve all-optical routing of single photons. We realized a single-photon-activated switch capable of routing a photon from any of its two inputs to any of its two outputs. Our device is based on a single atom coupled to a fiber-coupled, chip-based microresonator. A single reflected control photon toggles the switch from high reflection (R ~ 65%) to high transmission (T ~ 90%), with an average of ~1.5 control photons per switching event (~3, including linear losses). No additional control fields are required. The control and target photons are both in-fiber and practically identical, making this scheme compatible with scalable architectures for quantum information processing.
[aps,prl,twocolumn,showpacs,groupaddress]We experimentally demonstrate two-photon absorption (TPA) with broadband down-converted light (squeezed vacuum). Although incoherent and exhibiting the statistics of a thermal noise, broadband downconverted light can induce TPA with the same sharp temporal behavior as femtosecond pulses, while exhibiting the high spectral resolution of the narrowband pump laser. Using pulse-shaping methods, we coherently control TPA in Rubidium, demonstrating spectral and temporal resolutions that are 3-5 orders of magnitude below the actual bandwidth and temporal duration of the light itself. Such properties can be exploited in various applications such as spread-spectrum optical communications, tomography and nonlinear microscopy.
Maximizing nonlinear light-matter interactions is a primary motive for compressing laser pulses to achieve ultrashort transform limited pulses. Here we show how, by appropriately shaping the pulses, resonant multiphoton transitions can be enhanced significantly beyond the level achieved by maximizing the pulse's peak intensity. We demonstrate the counterintuitive nature of this effect with an experiment in a resonant two-photon absorption, in which, by selectively removing certain spectral bands, the peak intensity of the pulse is reduced by a factor of 40, yet the absorption rate is doubled. Furthermore, by suitably designing the spectral phase of the pulse, we increase the absorption rate by a factor of 7.
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...
We experimentally demonstrate shaping of the two-photon wave function of entangled-photon pairs, utilizing coherent pulse-shaping techniques. By performing spectral-phase manipulations we tailor the second-order correlation function of the photons exactly like a coherent ultrashort pulse. To observe the shaping we perform sum-frequency generation with an ultrahigh flux of entangled photons. At the appropriate conditions, sum-frequency generation performs as a coincidence detector with an ultrashort response time (approximately 100 fs), enabling a direct observation of the two-photon wave function. This property also enables us to demonstrate background-free, high-visibility two-photon interference oscillations.
We experimentally demonstrate sum-frequency generation with entangled photon pairs, generating as many as 40,000 photons per second, visible even to the naked eye. The nonclassical nature of the interaction is exhibited by a linear intensity dependence of the nonlinear process. The key element in our scheme is the generation of an ultrahigh flux of entangled photons while maintaining their nonclassical properties. This is made possible by generating the down-converted photons as broadband as possible, orders of magnitude wider than the pump. This approach can be applied to other nonlinear interactions, and may become useful for various quantum-measurement tasks.
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