A scheme for entangling distant atoms is realized, as proposed in the seminal paper by [C. Cabrillo et al., Phys. Rev. A 59, 1025 (1999)]. The protocol is based on quantum interference and detection of a single photon scattered from two effectively one meter distant laser cooled and trapped atomic ions. The detection of a single photon heralds entanglement of two internal states of the trapped ions with high rate and with a fidelity limited mostly by atomic motion. Control of the entangled state phase is demonstrated by changing the path length of the single-photon interferometer.

In this letter, we report an absorption spectroscopy experiment and the observation of electromagnetically induced transparency from a single trapped atom. We focus a weak and narrowband Gaussian light beam onto an optically cooled 138 Ba + ion using a high numerical aperture lens. Extinction of this beam is observed with measured values of up to 1.3%. We demonstrate electromagnetically induced transparency of the ion by tuning a strong control beam over a two-photon resonance in a three-level Λ-type system. The probe beam extinction is inhibited by more than 75% due to population trapping.PACS numbers: 42.50. Gy, Atom-photon interfaces will be essential building blocks in future quantum networks [1,2]. Here, photons are usually adopted as the messengers due to their robustness in preserving quantum information during propagation, while atoms are used to store the information in stationary nodes. The efficient transfer of quantum information between atoms and photons is then essential and requires controlled photon absorption with a very high probability. The requisite strong coupling can be achieved, for example, using high finesse cavities [3][4][5] or large atomic ensembles [6,7], which are the most studied routes towards such goals.Coupling of radiation to a single atom in free space is generally considered to be weak, however, technological advances, as nowadays available with large aperture lenses [8] and mirrors [9], recently led to reconsider this point of view. Novel experiments demonstrated extinctions of about 10% from single Rubidium atoms [10], single molecules [11,12] and quantum dots [13]. More recently, a light phase shift of one degree was observed by tuning an off-resonant laser to a single Rubidium atom [14], and non-linear switching was demonstrated with a single molecule [15]. These experiments demonstrate first steps towards quantum optical logic gates and quantum memories with single atoms in free space.Long term and controlled storage of quantum information will likely require electromagnetically induced transparency (EIT). This technique is widely used to control the absorption of weak light pulses or single photons in atomic ensembles [7,16] and in high-finesse cavities [17]. Here, a two-photon Raman transition in lambda-type three-level atoms is driven by the weak probe light together with a strong control laser. The control laser leads to splitting of the excited state by the AC Stark effect, which suppresses the absorption of the resonant probe light. Consequently, the change of the control laser intensity can gate the propagating probe field between absorption and transmission. Furthermore, adiabatic switching of the control light can trigger the storage and retrieval of a probe photon onto and from the long-lived atomic ground states [1,18].So far, EIT has been a phenomenon specific to optically thick media consisting of ensembles of many atoms [18], where both the optical fields and the atomic states are modified. However, quantum information processing requires single well-define...

By tightly focusing a laser field onto a single cold ion trapped in front of a far-distant dielectric mirror, we could observe a quantum electrodynamic effect whereby the ion behaves as the optical mirror of a Fabry-Pérot cavity. We show that the amplitude of the laser field is significantly altered due to a modification of the electromagnetic mode structure around the atom in a novel regime in which the laser intensity is already changed by the atom alone. We propose a direct application of this system as a quantum memory for single photons.

Single atoms or atom-like emitters are the purest source of single photons, they are intrinsically incapable of multi-photon emission. To demonstrate this degree of photon number-state purity we have realized a single-photon source using a single ion trapped at the common focus of high numerical aperture lenses. Our trapped-ion source produces single-photon pulses with = ´g 0 1.9 0.2 10 2 3 ( ) ( ) without any background subtraction. After subtracting detector dark counts the residual g 0 2 ( ) is less than 3×10 −4 (95% confidence interval). The multi-photon component of the source light field is low enough that we measure violation of a quantum non-Gaussian state witness, by this characterization the source output is indistinguishable from ideal attenuated single photons. In combination with efforts to enhance collection efficiency from single emitters, our results suggest that single trapped ions are not only ideal stationary qubits for quantum information processing, but promising sources of light for scalable optical quantum networks.

We report measurements of an intensity-field correlation function of the resonance fluorescence of a single trapped 138Ba+ ion. Detection of a photon prepares the atom in its ground state, and we observe its subsequent evolution under interaction with a laser field of well-defined phase. We record the regression of the resonance fluorescence source field. This provides a direct measurement of the field of the radiating dipole of a single atom and exhibits its strong nonclassical behavior. In the setup, an interference measurement is conditioned on the detection of a fluorescence photon.