A photon-photon quantum gate based on a single atom in an optical resonator

Two photons in free space pass each other undisturbed. This is ideal for the faithful transmission of information, but prohibits an interaction between the photons as required for a plethora of applications in optical quantum information processing. The long-standing challenge here is to realise a deterministic photon-photon gate. This requires an interaction so strong that the two photons can shift each others phase by pi. For polarisation qubits, this amounts to the conditional flipping of one photon's polarisation to an orthogonal state. So far, only probabilistic gates based on linear optics and photon detectors could be realised, as "no known or foreseen material has an optical nonlinearity strong enough to implement this conditional phase shift..." [Science 318, 1567]. Meanwhile, tremendous progress in the development of quantum-nonlinear systems has opened up new possibilities for single-photon experiments. Platforms range from Rydberg blockade in atomic ensembles to single-atom cavity quantum electrodynamics. Applications like single-photon switches and transistors, two-photon gateways, nondestructive photon detectors, photon routers and nonlinear phase shifters have been demonstrated, but none of them with the ultimate information carriers, optical qubits. Here we employ the strong light-matter coupling provided by a single atom in a high-finesse optical resonator to realise the Duan-Kimble protocol of a universal controlled phase flip (CPF, pi phase shift) photon-photon quantum gate. We achieve an average gate fidelity of F=(76.2+/-3.6)% and specifically demonstrate the capability of conditional polarisation flipping as well as entanglement generation between independent input photons. Our gate could readily perform most of the hitherto existing two-photon operations. It also discloses avenues towards new quantum information processing applications where photons are essential.Comment: 7 pages, 5 figure

Photon-Mediated Quantum Gate between Two Trapped Neutral Atoms in an Optical Cavity

Quantum logic gates are fundamental building blocks of quantum computers. Their integration into quantum networks requires strong qubit coupling to network channels, as can be realized with neutral atoms and optical photons in cavity quantum electrodynamics. Here we demonstrate that the long-range interaction mediated by a flying photon performs a gate between two stationary atoms inside an optical cavity from which the photon is reflected. This single step executes the gate in $2\,\mathrm{\mu s}$. We show an entangling operation between the two atoms by generating a Bell state with 76(2)% fidelity. The gate also operates as a CNOT. We demonstrate 74.1(1.6)% overlap between the observed and the ideal gate output, limited by the state preparation fidelity of 80.2(0.8)%. As the atoms are efficiently connected to a photonic channel, our gate paves the way towards quantum networking with multiqubit nodes and the distribution of entanglement in repeater-based long-distance quantum networks.Comment: 10 pages including appendix, 5 figure

Cavity Carving of Atomic Bell States

We demonstrate entanglement generation of two neutral atoms trapped inside an optical cavity. Entanglement is created from initially separable two-atom states through carving with weak photon pulses reflected from the cavity. A polarization rotation of the photons heralds the entanglement. We show the successful implementation of two different protocols and the generation of all four Bell states with a maximum fidelity of (90+-2)%. The protocol works for any distance between cavity-coupled atoms, and no individual addressing is required. Our result constitutes an important step towards applications in quantum networks, e.g. for entanglement swapping in a quantum repeater.Comment: 9 pages, 7 figures including Supplemen

Entwicklung eines schnellen optischen Quanten-Zufallsgenerators

This work reports on setup, characterisation and data processing of a true quantum random number generator. As a randomness source a pure quantum vacuum state of light is used, which is amplified by a laser beam. Performance and behaviour of the system as well as parasitic errors are investigated. For an optimized exploitation of the data they are Fourier-transformed and processed further as frequency amplitudes. The extractable entropy in that data is calculated to allow for elimination of non-random signal contributions by hashing. The system is able to produce true and unique random numbers at a rate of 25 Gbit/s and thus outperforms previous implementations considerably.Comment: Bachelor thesis at Friedrich-Alexander-Universit\"at Erlangen-N\"urnberg from September 22nd 2011 (in German

Deterministic creation of entangled atom-light Schr\"odinger-cat states

Quantum physics allows for entanglement between microscopic and macroscopic objects, described by discrete and continuous variables, respectively. As in Schr\"odinger's famous cat gedanken experiment, a box enclosing the objects can keep the entanglement alive. For applications in quantum information processing, however, it is essential to access the objects and manipulate them with suitable quantum tools. Here we reach this goal and deterministically generate entangled light-matter states by reflecting a coherent light pulse with up to four photons on average from an optical cavity containing one atom. The quantum light propagates freely and reaches a remote receiver for quantum state tomography. We produce a plethora of quantum states and observe negative-valued Wigner functions, a characteristic sign of non-classicality. As a first application, we demonstrate a quantum-logic gate between an atom and a light pulse, with the photonic qubit encoded in the phase of the light field.Comment: includes Methods and Supplementary Informatio

The research into the Composition and Properties of Medieval plaster in the church of St. Mary at the Pond with Comparative analyses of commercial injection materials

U crkvi sv. Marije kod Lokve u istarskome gradiću Gologorici nalazi se zidna slika s kraja 14. stoljeća koja prikazuje scenu Poklonstvo kraljeva. Godine 2008. i 2009., imajući u vidu oštećenost oslika koja zahtijeva žurnu provedbu zaštitnih mjera, izvedena su sustavna istraživanja1 zidne slike u svrhu iznalaženja prihvatljive konzervatorsko-restauratorske koncepcije. Posebna pozornost posvećena je laboratorijskim istraživanjima sastava i svojstava srednjovjekovne žbuke (mokre kemijske analize, analize tankih presjeka polarizacijskim mikroskopom i metodom SEM/EDX, mjerenja maksimalnoga kapaciteta upijanja vode, raspodjele radijusa pora te dinamičkoga modula elastičnosti) koja potvrđuju da je riječ o višeslojnoj (arriccio i intonaco) vapnenoj žbuci s agregatom na bazi kalcita, odnosno agregatom povezanim kalcitom specifičnih svojstava, kao što su mala gustoća i visok udio finih pora. U svrhu odabira pogodnoga materijala za zaštitne radove, prije svega ispunjavanje šupljina, također su izmjerene vrijednosti statičkoga modula elastičnosti i biaksijano savojnovlačne čvrstoće izvornog arriccia te uspoređene s odgovarajućim vrijednostima danas uobičajenih, komercijalnih materijala za injektiranje. Materijali odabrani na temelju rezultata istraživanja primijenjeni su 2009. godine na jednoj testnoj osi zidne slike. Slijedeći uspostavljenu metodologiju, radovi u crkvi sv. Marije kod Lokve provode se od 2013. godine u sklopu restauratorske prakse studenata Odsjeka za konzerviranje i restauriranje umjetnina Akademije likovnih umjetnosti Sveučilišta u Zagrebu.The church of St. Mary at the Pond with a wall painting depicting The Adoration of the Magi I the most important art and cultural monument of the Istrian town of Gologorica. It is an east-facing church, approximately 7 m long and 5 m wide. Typologically, it belongs to a group of single-nave Romanesque churches with an inscribed apse. The wall painting, located on the north wall of the church interior (5m wide and ca. 2.2 m high), was discovered in the late 1950s by art historian Branko Fučić, who dated it to the turn of the 15th century. Around 1651, a major renovation of the church took place. The inscribed apse was removed and the floor level was raised with the building material obtained. Further alterations to the interior and exterior followed in the period between the 18th and the 20th century. The support for the multilayered plaster of the wall painting is rubble masonry made of lime sandstone, i.e. sand limestone. It is coated with a 1.5-cm-thick arriccio with a calcite-based aggregate, i.e. aggregate bound by calcite. The arriccio is covered with a 0.5-cm-thick layer of intonaco of almost identical composition. On top of it, various paint layers are applied over a toned lime wash. The paint layers are bound by lime that contains organic elements. The fragmentarily preserved wall painting shows many damages: static cracks, cavities, structural damages of the plaster, damages of the paint layers, and in some portions, the original plaster is completely missing. In 2008 and 2009 a systematic research of the wall painting was conducted in order to come up with an acceptable concept for conservation, while bearing in mind the damage to the painting which required a prompt implementation of protective measures. The research was part of Bastian Hacker’s diploma thesis at the Potsdam University of Applied Sciences (Fachhochschule Potsdam, Fachbereich Architectur und Sttebau, Studiengang Restaurierung). Special attention was paid to the laboratory analyses of the composition and properties of multilayered medieval plaster. In addition, an assessment was made of the most common commercial injection materials which were compared to the original plaster. The discovery of fragments of the multilayered plaster withoat the paint layer beneath the present-day floor, enabled various laboratory investigations into its composition and properties. Since the content of components soluble in hydrochloric acid was high in the tested samples (90.89w% – 93.34w%), and these were interpreted as being lime binder, the results of the wet chemical analyses proved irrelevant. For this reason, it was impossible to give a more specific estimate of the binder and aggregate ratio or of the composition and shape of the aggregate. As a result, thin sections of the samples of the original arriccio layer were prepared, in which the binder and aggregate ratio was calculated by counting the presence of aggregate, pores and binder. Since the share of fine pores was disregarded in calculation, a very high binder con tent was obtained. However, by measuring the maximum water absorption capacity, the share of fine pores was obtained, which enabled the binder and aggregate ratio to finally be defined as 2 : 1. In addition, in the thin section, lime lumps were spotted in the binder, as well as aggregates of marble, limestone, calcite in the form of mineral and lime sandstone i.e. sand limestone. Measurement of the dynamic modulus of elasticity indicated a relatively low density of the arriccio plaster samples. The conclusion thereof could be made that the share of pores was large. To acquire more detailed information of the size and volume of pores in the arriccio, an analysis of the maximum water absorption capacity and the pore radius distribution was conducted. These parameters largely influence the properties of arriccio and therefore had to be considered in the making of replacement plaster. The share of maximum water absorption capacity was 35w%, little above the usual values for brick. A large share of pores smaller than 3μm was determined. Furthermore, imaging was done by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) that determined the presence of calcium, aluminium, magnesium, iron, potassium, silicon and niobium in the medieval plaster. Analyses of the plaster samples indicated that the components of multilayered plaster – joint mortar, arriccio and intonaco – are lime plasters in composition, with specific characteristics such as a high binder and fine pores content, and that they contain calcite-based aggregates, i.e. aggregates bound by calcite. Given that the filling of cavities was assessed to be the protection priority, in addition to the fact that the treatment itself represents an irreversible intrusion into the core of the wall painting, it was important to choose an appropriate injection material. For that purpose, standardized samples of the most common injection grouts were prepared, whose static modulus of elasticity and biaxial flexural-tensile strength were compared to the values of the original arriccio. Values of these parameters of the injection grouts should be lower than those of the original plaster. Three standardized samples of the medieval arriccio were tested alongside samples of the materials KSE 500 STE-Modul-System, CalXnova, PLM-AL, PLMA and Ledan. The injection materials mostly showed higher values of static modulus of elasticity than the medieval plaster. Lower values than those of the original plaster were found only in CalXnova modified with hollow glass microspheres and PLM-AL prepared according to the manufacturer’s instructions. These materials were therefore used in the applying of protective measures on the wall painting. In order to consolidate the particularly endangered portions of the wall painting, appropriate replacement plasters for arriccio and intonaco needed to be prepared. As replacement plasters must have properties similar to those of the original plaster, in order for their behavior to be similar, local slaked lime was used for their preparation, in addition to the calcite-based aggregates, i.e. aggregates buond by calcite: marble, calcite in the form of mineral and local stone from the church surroundings, i.e. lime sandstone. In order to obtain softer replacement plasters, the binder content was reduced to 1 (binder) : 2 (aggregate) for the arriccio and 1 : 3 for the intonaco plaster. In one particularly endangered area of the wall painting, protection measures were applied in 2009. The consolidation consisted of the structural strengthening of the plaster, the filling of cavities and replacing the plaster layers with materials that were selected based on the results of research. By following the previously established methodology, the work in the church of St. Mary at the Pond has been under way since 2013, as part of practical training in conservation for students of the Department for Conservation and Restoration of Works of Art (OKIRU) of the Academy of Fine Arts (ALU), University of Zagreb

Remote sensing of geomorphodiversity linked to biodiversity — part III: traits, processes and remote sensing characteristics

Remote sensing (RS) enables a cost-effective, extensive, continuous and standardized monitoring of traits and trait variations of geomorphology and its processes, from the local to the continental scale. To implement and better understand RS techniques and the spectral indicators derived from them in the monitoring of geomorphology, this paper presents a new perspective for the definition and recording of five characteristics of geomorphodiversity with RS, namely: geomorphic genesis diversity, geomorphic trait diversity, geomorphic structural diversity, geomorphic taxonomic diversity, and geomorphic functional diversity. In this respect, geomorphic trait diversity is the cornerstone and is essential for recording the other four characteristics using RS technologies. All five characteristics are discussed in detail in this paper and reinforced with numerous examples from various RS technologies. Methods for classifying the five characteristics of geomorphodiversity using RS, as well as the constraints of monitoring the diversity of geomorphology using RS, are discussed. RS-aided techniques that can be used for monitoring geomorphodiversity in regimes with changing land-use intensity are presented. Further, new approaches of geomorphic traits that enable the monitoring of geomorphodiversity through the valorisation of RS data from multiple missions are discussed as well as the ecosystem integrity approach. Likewise, the approach of monitoring the five characteristics of geomorphodiversity recording with RS is discussed, as are existing approaches for recording spectral geomorhic traits/ trait variation approach and indicators, along with approaches for assessing geomorphodiversity. It is shown that there is no comparable approach with which to define and record the five characteristics of geomorphodiversity using only RS data in the literature. Finally, the importance of the digitization process and the use of data science for research in the field of geomorphology in the 21st century is elucidated and discussed

Large expert-curated database for benchmarking document similarity detection in biomedical literature search

Document recommendation systems for locating relevant literature have mostly relied on methods developed a decade ago. This is largely due to the lack of a large offline gold-standard benchmark of relevant documents that cover a variety of research fields such that newly developed literature search techniques can be compared, improved and translated into practice. To overcome this bottleneck, we have established the RElevant LIterature SearcH consortium consisting of more than 1500 scientists from 84 countries, who have collectively annotated the relevance of over 180 000 PubMed-listed articles with regard to their respective seed (input) article/s. The majority of annotations were contributed by highly experienced, original authors of the seed articles. The collected data cover 76% of all unique PubMed Medical Subject Headings descriptors. No systematic biases were observed across different experience levels, research fields or time spent on annotations. More importantly, annotations of the same document pairs contributed by different scientists were highly concordant. We further show that the three representative baseline methods used to generate recommended articles for evaluation (Okapi Best Matching 25, Term Frequency-Inverse Document Frequency and PubMed Related Articles) had similar overall performances. Additionally, we found that these methods each tend to produce distinct collections of recommended articles, suggesting that a hybrid method may be required to completely capture all relevant articles. The established database server located at https://relishdb.ict.griffith.edu.au is freely available for the downloading of annotation data and the blind testing of new methods. We expect that this benchmark will be useful for stimulating the development of new powerful techniques for title and title/abstract-based search engines for relevant articles in biomedical research.Peer reviewe

A Search for Pulsars in Ultracompact X-ray Binary Systems

We present a search for periodic pulsations in the X-ray emission of seven Ultracompact X-ray Binary (UCXB) systems, in which the coherent time domain resampling method was used to recover signals distorted by the target's orbital motion. While other UCXBs have been observed as X-ray pulsars, our targets still lack such a detection. We demonstrated the superior sensitivity of our search method over standard approaches, enabling us to find very faint signals at the cost of a large computing effort. However no pulsations were detected. Therefore we deduced stringent upper limits for the pulsed fraction in each observation, which are on the order of 0.5%.Validerat; 20130613 (global_studentproject_submitter

Phase-locking an interferometer with single-photon detections

We report on a novel phase-locking technique for fiber-based Mach-Zehnder interferometers based on discrete single-photon detections, and demonstrate this in a setup. Our interferometer decodes relative-phase-encoded optical pulse pairs for quantum key distribution applications and requires no locking laser in addition to the weak received signal. Our new simple locking scheme is shown to produce an Ornstein-Uhlenbeck dynamic and achieve optimal phase noise for a given count rate. In case of wavelength drifts that arise during the reception of Doppler-shifted satellite signals, the arm-length difference gets continuously readjusted to keep the interferometer phase stable.Comment: 13 page