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 ,...
A sequence of single photons is emitted on demand from a single three-level atom strongly coupled to a high-finesse optical cavity. The photons are generated by an adiabatically driven stimulated Raman transition between two atomic ground states, with the vacuum field of the cavity stimulating one branch of the transition, and laser pulses deterministically driving the other branch. This process is unitary and therefore intrinsically reversible, which is essential for quantum communication and networking, and the photons should be appropriate for all-optical quantum information processing.PACS numbers: 03.67.Hk, 42.55.Ye, 42.65.Dr A future quantum network connecting remote quantum processors and memories has several advantages in processing quantum information as compared to a local quantum computer, since it combines scalability with modularity. Different kinds of networks have been proposed [1]: one is an all-optical network [2], where the nodes are linear optical components, with quantum information encoded in the number of photons flying from node to node. The nodes perform gate operations based on quantum interference effects between indistinguishable photons. In another, more general, network the nodes also serve as quantum memories storing information, e.g., in long-lived states of atoms located in an optical cavity [3]. The key requirement for such a network is its ability to interconvert stationary and flying qubits and to transmit flying qubits between specified locations [4]. The atom-cavity system, in particular, must be able to transfer quantum information between atoms and photons in a coherent manner [5,6]. It must also act as an emitter and a receiver of single-photon states. These states must therefore be generated by a reversible process. However, all deterministic single-photon emitters demonstrated so far [7,8,9,10,11,12,13,14,15] do not meet this essential requirement. The reason is that the emission process, namely an electronic excitation of the system followed by spontaneous emission, cannot be described by a Hamiltonian evolution and, hence, is irreversible.This letter describes the realization of an intrinsically reversible single-photon source [3,16,17,18,19], which is based on a stimulated Raman process driving an adiabatic passage [20] (STIRAP) between two ground states of a single atom strongly coupled to a single mode of a high-finesse optical cavity [21,22]. A laser beam illuminating the atom excites one branch of the Raman transition, while the cavity vacuum stimulates the emission of the photon on the other branch. STIRAP is slow compared to the photon lifetime in the cavity, so that the field generated inside the cavity is instantaneously mapped to the outside world. Moreover, it employs a dark state, which has two important consequences: first, any electronic excitation is avoided, so that irreversible spontaneous processes do not occur. Second, the scheme allows one to continuously tune the frequency of the photon within a range that is only limited by the atom-cavity cou...
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...
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