Collection selection has been a research issue for years. Typically, in related work, precomputed statistics are employed in order to estimate the expected result quality of each collection, and subsequently the collections are ranked accordingly. Our thesis is that this simple approach is insufficient for several applications in which the collections typically overlap. This is the case, for example, for the collections built by autonomous peers crawling the web. We argue for the extension of existing quality measures using estimators of mutual overlap among collections and present experiments in which this combination outperforms CORI, a popular approach based on quality estimation. We outline our prototype implementation of a P2P web search engine, coined MINERVA 1 , that allows handling large amounts of data in a distributed and self-organizing manner. We conduct experiments which show that taking overlap into account during collection selection can drastically decrease the number of collections that have to be contacted in order to reach a satisfactory level of recall, which is a great step toward the feasibility of distributed web search.

We report on the realization of a free-space single-photon absorber, which deterministically absorbs exactly one photon from an input pulse. Our scheme is based on the saturation of an optically thick medium by a single photon due to Rydberg blockade. By converting one absorbed input photon into a stationary Rydberg excitation, decoupled from the light fields through fast engineered dephasing, we blockade the full atomic cloud and change our optical medium from opaque to transparent. We show that this results in the subtraction of one photon from the input pulse over a wide range of input photon numbers. We investigate the change of the pulse shape and temporal photon statistics of the transmitted light pulses for different input photon numbers and compare the results to simulations. Based on the experimental results, we discuss the applicability of our single-photon absorber for number resolved photon detection schemes or quantum gate operations.The elementary operation of subtracting exactly one photon from an arbitrary light pulse is of great interest for testing fundamental concepts of quantum optics [1,2] as well as for the preparation of non-classical states of light for quantum information [3][4][5][6], simulation [7][8][9], and metrology protocols [10]. Heralded single-photon subtraction has been realized by monitoring the weak reflection of a highly imbalanced beam splitter, where a single detection event corresponds to subtraction of a photon from the transmitted pulse [1,11]. For sufficiently low reflectivity such that the subtraction of two or more photons becomes negligible, this procedure implements the photon annihilation operatorα [1]. This operation is inherently probabilistic, with the success rate depending on the number of incoming photons. In contrast, deterministic single-photon subtraction, where always exactly one photon is removed independent of the input photon state, can be implemented by sending the light through a medium saturable by a single absorption event. One realization of such a single-photon absorber is a single 3-level quantum emitter strongly coupled to an optical resonator [12,13], as recently demonstrated by Rosenblum et al. using a single atom coupled to a microsphere resonator [14].Here we demonstrate the experimental realiziation of a deterministic free-space single-photon absorber [15], which is based on the saturation of an optically thick free-space medium by a single photon due to Rydberg blockade [16]. Single-photon subtraction adds a new component to the growing Rydberg quantum optics toolbox [17][18][19][20], which already contains photonic logic building-blocks such as single-photon sources [21], switches [22], transistors [23][24][25], and conditional π-phase shifts [26]. Our approach is scalable to multiple cascaded absorbers, essential for preparation of non-classical light states for quantum information and metrology applications [5,11,27], and, in combination with the singlephoton transistor, high-fidelity number-resolved photon detection [15,28,29].Any pr...

Producing positronium (Ps) in the metastable 2 3 S state is of interest for various applications in fundamental physics. We report here on an experiment in which Ps atoms are produced in this long-lived state by spontaneous radiative decay of Ps excited to the 3 3 P level manifold. The Ps cloud excitation is obtained with a UV laser pulse in an experimental vacuum chamber in presence of guiding magnetic field of 25 mT and an average electric field of 300 V cm −1 . The evidence of the 2 3 S state production is obtained to the 3.6σ level of statistical significance using a novel analysis technique of the single-shot positronium annihilation lifetime spectra. The dynamic of the Ps population on the involved levels has been studied with a rate equation model.

Peer-to-Peer (P2P) search requires intelligent decisions for query routing: selecting the best peers to which a given query, initiated at some peer, should be forwarded for retrieving additional search results. These decisions are based on statistical summaries for each peer, which are usually organized on a per-keyword basis and managed in a distributed directory of routing indices. Such architectures disregard the possible correlations among keywords. Together with the coarse granularity of per-peer summaries, which are mandated for scalability, this limitation may lead to poor search result quality. This paper develops and evaluates two solutions to this problem, sk-STAT based on single-key statistics only, and mk-STAT based on additional multi-key statistics. For both cases, hash sketch synopses are used to compactly represent a peer's data items and are efficiently disseminated in the P2P network to form a decentralized directory. Experimental studies with Gnutella and Web data demonstrate the viability and the trade-offs of the approaches.

In this work we describe a high-resolution position-sensitive detector for positronium. The detection scheme is based on the photoionization of positronium in a magnetic field and the imaging of the freed positrons with a Microchannel Plate assembly. A spatial resolution of ± (88 5) μm on the position of the ionized positronium -in the plane perpendicular to a 1.0 T magnetic field-is obtained. The possibility to apply the detection scheme for monitoring the emission into vacuum of positronium from positron/positronium converters, imaging posihttps://doi.

Positronium in the 2 3 S metastable state exhibits a low electrical polarizability and a long lifetime (1140 ns) making it a promising candidate for interferometry experiments with a neutral matterantimatter system. In the present work, 2 3 S positronium is produced -in absence of electric field -via spontaneous radiative decay from the 3 3 P level populated with a 205 nm UV laser pulse. Thanks to the short temporal length of the pulse, 1.5 ns full-width at half maximum, different velocity populations of a positronium cloud emitted from a nanochannelled positron/positronium converter were selected by delaying the excitation pulse with respect to the production instant. 2 3 S positronium atoms with velocity tuned between 7 · 10 4 m s −1 and 10 · 10 4 m s −1 were thus produced. Depending on the selected velocity, a 2 3 S production efficiency ranging from ∼ 0.8% to ∼ 1.7%, with respect to the total amount of emitted positronium, was obtained. The observed results give a branching ratio for the 3 3 P-2 3 S spontaneous decay of (9.7 ± 2.7)%. The present velocity selection technique could allow to produce an almost monochromatic beam of ∼ 1 · 10 3 2 3 S atoms with a velocity spread < 10 4 m s −1 and an angular divergence of ∼ 50 mrad.

We describe a multi-step "rotating wall" compression of a mixed cold antiproton-electron non-neutral plasma in a 4.46 T Penning-Malmberg trap developed in the context of the AEḡIS experiment at CERN. Such traps are routinely used for the preparation of cold antiprotons suitable for antihydrogen production. A tenfold antiproton radius compression has been achieved, with a minimum antiproton radius

Antihydrogen atoms with K or sub-K temperature are a powerful tool to precisely probe the validity of fundamental physics laws and the design of highly sensitive experiments needs antihydrogen with controllable and well defined conditions. We present here experimental results on the production of antihydrogen in a pulsed mode in which the time when 90% of the atoms are produced is known with an uncertainty of ~250 ns. The pulsed source is generated by the charge-exchange reaction between Rydberg positronium atoms—produced via the injection of a pulsed positron beam into a nanochanneled Si target, and excited by laser pulses—and antiprotons, trapped, cooled and manipulated in electromagnetic traps. The pulsed production enables the control of the antihydrogen temperature, the tunability of the Rydberg states, their de-excitation by pulsed lasers and the manipulation through electric field gradients. The production of pulsed antihydrogen is a major landmark in the AE$$\bar{g}$$ ḡ IS experiment to perform direct measurements of the validity of the Weak Equivalence Principle for antimatter.