The dephasing of particle plasmons is investigated using light-scattering spectroscopy on individual gold nanoparticles. We find a drastic reduction of the plasmon dephasing rate in nanorods as compared to small nanospheres due to a suppression of interband damping. The rods studied here also show very little radiation damping, due to their small volumes. These findings imply large local-field enhancement factors and relatively high light-scattering efficiencies, making metal nanorods extremely interesting for optical applications. Comparison with theory shows that pure dephasing and interface damping give negligible contributions to the total plasmon dephasing rate.

The dipole blockade between Rydberg atoms has been proposed as a basic tool in quantum information processing with neutral atoms. Here we demonstrate experimentally the Rydberg blockade of two individual atoms separated by 4 µm. Moreover, we show that, in this regime, the single atom excitation is enhanced by a collective two-atom behavior associated with the excitation of an entangled state. This observation is a crucial step towards the deterministic manipulation of entanglement of two or more atoms using the Rydberg dipole interaction.PACS numbers: 32.80. Rm, 03.67.Lx, 32.80.Pj, 42.50.Ct A large experimental effort is nowadays devoted to the production of entanglement, that is quantum correlations, between individual quantum objects such as atoms, ions, superconducting circuits, spins, or photons. Entangled states are important in many areas of physics such as quantum information and quantum metrology, the study of strongly correlated systems in many-body physics, and more fundamentally in the understanding of quantum physics.There are several ways to engineer entanglement in a quantum system. Here, we focus on a method that relies on a blockade mechanism where the strong interaction between different parts of a system prevents their simultaneous excitation by the same driving pulse. Single excitation is still possible, but it is delocalized over the whole system, and results in the production of an entangled state. This approach to entanglement is deterministic and can be used to realize quantum gates [1] or to entangle mesoscopic ensembles, provided that the blockade is effective over the whole sample [2]. Blockade effects have been observed in systems where interactions are strong such as systems of electrons using the Coulomb force [3] or the Pauli effective interaction [4], as well as with photons and atoms coupled to an optical cavity [5]. Recently, atoms held in the ground state of the wells of an optical lattice have been shown to exhibit interaction blockade, due to s-wave collisions [6].An alternative approach uses the comparatively strong interaction between two atoms excited to Rydberg states, which have very large dipole moments. This strong interaction gives rise to the so-called Rydberg blockade, which has been observed in clouds of cold atoms [7,8,9,10,11,12] as well as in a Bose condensate [13]. A collective behavior associated with the blockade has been reported in an ultra-cold atomic cloud [14]. Recently, an experiment demonstrated the blockade between two atoms 10 µm apart, by showing that when one atom is excited to a Rydberg state, the excitation of the second one is greatly suppressed [15].In the present work, we study two individual atoms, held at a distance of ∼ 4 µm by two optical tweezers. We demonstrate that under this condition, the atoms are in the Rydberg blockade regime since only one atom can be excited. Furthermore, we show that the single atom excitation is enhanced by a collective two-atom behavior, associated with the production of a two-atom entangled state between th...

We report the generation of entanglement between two individual 87Rb atoms in hyperfine ground states |F=1,M=1> and |F=2,M=2> which are held in two optical tweezers separated by 4 microm. Our scheme relies on the Rydberg blockade effect which prevents the simultaneous excitation of the two atoms to a Rydberg state. The entangled state is generated in about 200 ns using pulsed two-photon excitation. We quantify the entanglement by applying global Raman rotations on both atoms. We measure that 61% of the initial pairs of atoms are still present at the end of the entangling sequence. These pairs are in the target entangled state with a fidelity of 0.75.

tors (17). The resulting density matrix has only positive eigenvalues, and hence it represents a physically possible state. Its fidelity with respect to the expected Bell state, |Y-〉 from Eq. 2, is F = 86.0(4)%, with 0.5 < F ≤ 1 proving entan-glement (18). From the density matrix, following (16), we derive a concurrence of C = 0.73(7), with 0 < C ≤ 1 also proving entanglement. Because of technical imperfections, e.g., of polarizers in the detection setups, the observed fidelity/concurrence sets a lower bound for both the atom-photon and photon-photon entangle-ment achieved. The same measurements were done for B = −0.13 G and t S = 2.8 ms for which the atomic superposition state accumulates a p phase shift (compare to Fig. 3). Therefore, a density matrix corresponding to the Bell state jY þ 〉 ≡ 1 ffiffi 2 p ðjþ1; s − 〉 þ j−1; s þ 〉Þ is expected. This is indeed observed (Fig. 4B) with a fidelity of F = 82.9(6)% and a concurrence of C = 0.72(13). The state evolves between the two photon detections as a result of the constant magnetic field. Future experiments could produce a time-independent |Y + 〉 Bell state by applying a pulsed magnetic field to the atom between entanglement generation and state mapping. Moreover, partial driving of the Raman transition in combination with atomic state manipulation should allow production of highly entangled multiphoton states (12). Our technique applied to a quasi-permanently trapped intracavity atom (3, 19) will push the probability of success even further, making the scheme truly deterministic. Two (or more) such systems operated in parallel are perfectly suited for teleportation and entanglement experiments in a quantum network (20-22) or quantum gate operations in a distributed and, hence, scalable quantum computer (23, 24). Inorganic porous materials are being developed for use as molecular sieves, ion exchangers, and catalysts, but most are oxides. We show that various sulfide and selenide clusters, when bound to metal ions, yield gels having porous frameworks. These gels are transformed to aerogels after supercritical drying with carbon dioxide. The aerogels have high internal surface area (up to 327 square meters per gram) and broad pore size distribution, depending on the precursors used. The pores of these sulfide and selenide materials preferentially absorb heavy metals. These materials have narrow energy gaps (between 0.2 and 2.0 electron volts) and low densities, and they may be useful in optoelectronics, as photocatalysts, or in the removal of heavy metals from water. I norganic porous materials are at the foundation of broad applications such as molecular sieves, ion exchangers, and catalysts (1, 2). Zeolites and aluminosilicate mesoporous materials constitute the vast majority of this class. Aerogels are another kind of porous inorganic amorphous polymer in which nanosized blocks are interconnected to yield high internal surface area, very low densities, and large open pores (3, 4). Although the sol-gel chemistry of oxide-based materials (e.g., SiO 2 , Al 2 O 3 ,...

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The interference of two single photons impinging on a beam splitter is measured in a time-resolved manner. Using long photons of different frequencies emitted from an atom-cavity system, a quantum beat with a visibility close to 100% is observed in the correlation between the photodetections at the output ports of the beam splitter. The time dependence of the beat amplitude reflects the coherence properties of the photons. Most remarkably, simultaneous photodetections are never observed, so that a temporal filter allows one to obtain perfect twophoton coalescence even for non-perfect photons. 03.67.Mn, 42.50.Xa, 42.50.Dv, 42.65.Dr The quantum nature of light impressively manifests itself in the fourth-order interference of two identical and mutually coherent single photons that impinge simultaneously on a beam splitter (BS). The photons coalesce and both leave the beam splitter in the same direction. Hong et al. first demonstrated this phenomenon with photon pairs from parametric down conversion [1] and Santori et al. used the same effect to show the indistinguishability of independently generated photons that are successively emitted from a quantum dot embedded in a micro cavity [2]. In all experiments performed so far, the photons were short compared to the time resolution of the employed detectors, so that interference phenomena were only observed as a function of the spatial delay between the interfering photons [3].To investigate the temporal dynamics behind this interference phenomenon, we now use an adiabatically driven strongly coupled atom-cavity system as single-photon emitter [4,5,6,7]. Photons are generated by a unitary process, so that their temporal and spectral properties can be arbitrarily adjusted. In fact, the duration of the photons used in our experiment exceeds the time resolution of the employed singlephoton counters by three orders of magnitude. This allows for the first time an experimental investigation of fourth-order interference phenomena in a time-resolved manner with photons arriving simultaneously at the beam splitter [8]. We find perfect interference even if the frequency difference between the two photons exceeds their bandwidths. This surprising result is very robust against all kinds of fluctuations and opens up new possibilities in all-optical quantum information processing [9].The principal scheme of the experiment is sketched in Fig. 1. We consider an initial situation where two single photons in modes A and B impinge simultaneously on a BS. In front of the BS, we distinguish states |1 A,B and |0 A,B , where either a photon is present or where it has been annihilated by transmission through the BS and subsequent detection by detector C or D. Mode A is an extended spatiotemporal photonic field mode, traveling along an optical fiber, which initially carries a photon. The photon in mode B emerges from a strongly coupled atom-cavity system, which is driven in a way that the photon is deterministically generated by a vacuum-stimulated Raman transition between two long-lived S...

The interference of two independent single-photon pulses impinging on a beam splitter is analysed in a generalised time-resolved manner. Different aspects of the phenomenon are elaborated using different representations of the single-photon wave packets, like the decomposition into single-frequency field modes or spatio-temporal modes matching the photonic wave packets. Both representations lead to equivalent results, and a photon-by-photon analysis reveals that the quantum-mechanical two-photon interference can be interpreted as a classical one-photon interference once a first photon is detected. A novel time-dependent quantum-beat effect is predicted if the interfering photons have different frequencies. The calculation also reveals that full two-photon fringe visibility can be achieved under almost any circumstances by applying a temporal filter to the signal.PACS 42.50.Dv, 42.50.Ct

Photon blockade is a dynamical quantum-nonlinear effect in driven systems with an anharmonic energy ladder. For a single atom strongly coupled to an optical cavity, we show that atom driving gives a decisively larger optical nonlinearity than cavity driving. This enhances single-photon blockade and allows for the implementation of two-photon blockade where the absorption of two photons suppresses the absorption of further photons. As a signature, we report on three-photon antibunching with simultaneous two-photon bunching observed in the light emitted from the cavity. Our experiment constitutes a significant step towards multiphoton quantum-nonlinear optics.