Non-classical correlations between a C-band telecom photon and a stored spin-wave

Future ground-based quantum information networks will likely use single photons transmitted through optical fibers to entangle individual network nodes. To extend communication distances and overcome limitations due to photon absorption in fibers the concept of quantum repeaters has been proposed. For that purpose, it is required to achieve quantum correlations between the material nodes and photons at telecom wavelengths which can be sent over long distances in optical fibers. Here we demonstrate non-classical correlation between a frequency converted telecom C-band photon and a spin-wave stored in an atomic ensemble quantum memory. The photons emitted from the ensemble and heralding the spin-waves are converted from 780 nm to 1552 nm by means of an all-solid-state integrated waveguide non-linear device. We show ultra-low noise operation of the device enabling a high signal to noise ratio of the converted single photon, leading to a high spin-wave heralding efficiency. The presented work is an enabling step towards the practical entanglement of remote quantum memories and the entanglement of quantum systems operating at different wavelengths.Comment: 9 pages, 5 figure

Long-lived non-classical correlations for scalable quantum repeaters at room temperature

Heralded single-photon sources with on-demand readout are promising candidates for quantum repeaters enabling long-distance quantum communication. The need for scalability of such systems requires simple experimental solutions, thus favouring room-temperature systems. For quantum repeater applications, long delays between heralding and single-photon readout are crucial. Until now, this has been prevented in room-temperature atomic systems by fast decoherence due to thermal motion. Here we demonstrate efficient heralding and readout of single collective excitations created in warm caesium vapour. Using the principle of motional averaging we achieve a collective excitation lifetime of $0.27\pm 0.04$ ms, two orders of magnitude larger than previously achieved for single excitations in room-temperature sources. We experimentally verify non-classicality of the light-matter correlations by observing a violation of the Cauchy-Schwarz inequality with $R=1.4\pm 0.1>1$. Through spectral and temporal analysis we identify intrinsic four-wave mixing noise as the main contribution compromising single-photon operation of the source.Comment: 21 pages total, the first 17 pages are the main article and the remaining pages are supplemental materia

Quantum frequency conversion of quantum memory compatible photons to telecommunication wavelengths

We report an experiment demonstrating quantum frequency conversion of weak light pulses compatible with atomic quantum memories to telecommunication wavelengths. We use a PPLN nonlinear waveguide to convert weak coherent states at the single photon level with a duration of 30ns from a wavelength of 780nm to 1552nm. We measure a maximal waveguide internal (external) conversion efficiency eta_int = 0.41 (eta_ext = 0.25), and we show that the signal to noise ratio (SNR) is good enough to reduce the input photon number below 1. In addition, we show that the noise generated by the pump beam in the crystal is proportional to the spectral bandwidth of the device, suggesting that narrower filtering could significantly increase the SNR. Finally, we demonstrate that the quantum frequency converter can operate in the quantum regime by converting a time-bin qubit and measuring the qubit fidelity after conversion.Comment: 15 pages, 5 figures (To appear in Optics Express

Quantum control of single spin excitations in cold atomic quantum memories

Optical quantum memories are important devices in quantum information science. In particular, they are building blocks of quantum repeater architectures that have been proposed to increase the range of quantum communication beyond the limits set by losses in optical fibers. In this thesis, we report experiments with a quantum memory based on cold atoms. We focus on two important aspects relevant for using the memories as quantum repeater nodes: the connectivity to the optical fiber network, and the ability to operate in a time-multiplexed fashion. The core of the work presented in this thesis was the implementation of a quantum memory based on spontaneous Raman scattering, following the protocol of Duan, Lukin, Cirac and Zoller (DLCZ). The memory is implemented with a cold ensemble of 87Rb atoms loaded in a magneto optical trap. Single collective atomic spin excitations (spin-waves) are created in a heralded manner, before being retrieved by conversion into strongly non-classically correlated single photons. Our system showed measured second-order cross-correlation function values up to 200, an inferred intrinsic retrieval efficiency inside the science chamber up to 44%, and a memory lifetime up to 55 µs. Current realizations of DLCZ quantum memories present several limitations, reducing the maximum practical distance achievable for quantum repeaters based on these systems. We partially addressed two of them. The first one originates from high absorption in optical fibers at the operating wavelength of 780 nm. The second one is that current demonstrations only allow the creation of spin-waves in single temporal modes, limiting the entanglement generation rates in quantum repeaters protocols. A good solution to alleviate the first limitation is to translate the wavelength of the single photons to the telecom C-band, where absorption is minimal, while preserving their quantum characteristics. For this, we demonstrated an ultra-low-noise solid state photonic quantum interface based on an integrated-waveguide in a non-linear PPLN crystal. We converted heralded single photons emitted by the DLCZ quantum memory at 780 nm to the telecommunication wavelength of 1552 nm. We achieved a maximum signal-to-noise ratio of 80 for a mean input photon number of 1, allowing us to show significant non-classical correlations between the heralding and converted photons via the violation of the Cauchy-Schwarz inequality. To address the second limitation, we demonstrated the first experimental steps towards the realization of a temporally multiplexed DLCZ-type quantum repeater node. We showed active control of the spin-waves created in our quantum memory by means of an external magnetic field gradient inducing an inhomogeneous broadening of the atomic hyperfine levels. Acting on this gradient allows active dephasing and rephasing of individual spin-waves, enabling spin-wave creation in multiple temporal modes and read out a specific time-bin only. We showed that the active rephasing technique preserves the non-classical statistics of the heralded photons via the observation of anti-bunching. We then created spin-waves in two temporal modes and demonstrated selective read-out of only one of them with a selectivity up to 92%. All these results pave the way towards the realization of future temporally multiplexed quantum repeater nodes based on the DLCZ protocol.Les memòries quàntiques òptiques son dispositius importants en el camp científic de la informació quàntica. En particular, són peces fonamentals de les estructures de repetidors quàntics, les quals han estat proposades per tal d’incrementar la distància en la comunicació quàntica més enllà dels límits imposats per les pèrdues en fibres òptiques. En aquesta tesi mostrem experiments duts a terme amb una memòria quàntica basada en àtoms freds. Ens hem centrat en dos aspectes importants que són rellevants a l’hora d’usar les memòries com a nodes de repetidors quàntics: la connectivitat cap a la xarxa de fibres òptiques i l’habilitat d’operar amb multiplexació temporal. La part central del treball presentat en aquesta tesi és la implementació d’una memòria quàntica basada en la dispersió Raman espontània, seguint el protocol de Duan, Lukin, Cirac i Zoller (DLCZ). La memòria és implementada en un conjunt d’àtoms de 87Rb en una trampa òptico-magnètica. Excitacions individuals col·lectives d’espins atòmics (ones d’espín) són creades de manera anunciada, abans de ser recuperades en una conversió cap a fotons individuals amb fortes correlacions no-clàssiques. En el nostre sistema vam mostrar mesures de la funció de correlació creuada de segon ordre amb valors de fins a 200, una eficiència de recuperació intrínseca dins la cambra experimental de fins a un 44% i un temps de vida de la memòria de 55 µs. Les realitzacions actuals de memòries quàntiques DLCZ presenten varies limitacions, les quals redueixen la distància màxima que els repetidors quàntics basats en aquests sistemes poden assolir. Nosaltres n’hem adreçat parcialment dues d’elles. La primera és originada per l’alta absorció en fibres òptiques de la longitud d’ona de 780 nm. La segona té a veure amb el fet que altres experiments actuals només permeten la creació d’ones d’espín en un únic mode temporal, limitant el ritme de la generació d’entrellaçament en protocols de repetidors quàntics. Una bona solució per mitigar la primera limitació és traslladar la longitud d’ona dels fotons individuals, cap a la banda C de telecomunicacions en la que l’absorció és mínima, preservant les seves característiques quàntiques. Per això, vam demostrar l’operació d’una interfície fotònica quàntica d’estat sòlid amb un soroll ultra-baix basada en una guia d’ones integrada en un cristall PPLN. Vam convertir fotons individuals anunciats emesos per la memòria quàntica DLCZ a 780 nm cap a la longitud d¿ona de telecomunicacions de 1552 nm. Vam aconseguir una relació senyal-soroll màxima de 80 per a un nombre mitjà incident de fotons de 1, permetent-nos mostrar correlacions no-clàssiques significatives entre el fotó anunciat i el convertit, mitjançant la violació de la desigualtat de Cauchy-Schwarz. Per tal d’adreçar la segona limitació, vam demostrar els primers passos experimentals cap a la realització d’un node de repetidor quàntic de tipus DLCZ amb multiplexació temporal. Vam mostrar el control actiu d’ones d’espín creades a la nostra memòria quàntica, utilitzant un gradient de camp magnètic extern que indueix un eixamplament inhomogeni dels nivells atòmics hiperfins. Actuar en aquest gradient permet el desfasament i refasament actiu d’ones d’espín individuals, permetent crear ones d’espín en múltiples modes temporals i llegir només un mode temporal específic. Vam mostrar que la tècnica de refasament actiu preserva les estadístiques no-clàssiques dels fotons anunciats a través de l’observació d’anti-agrupament. Seguidament vam crear ones d'espín en dos modes temporals i vam demostrar la lectura selectiva de només un mode amb una selectivitat de fins a un 92%. Tots aquest resultats obren la porta a la realització de futurs nodes de repetidors quàntics amb multiplexació temporal basats en el protocol DLCZ

Patersonia rosea (Iridaceae, Patersonioideae) a new species from the New South Wales central and lower north coast regions

Patersonia rosea Branwhite sp. nov. is described and illustrated, and notes provided on distribution, conservation status and habitat. Morphological differences that distinguish it from similar species of Patersonia are discussed and molecular data indicating relationships presente

Quantum geometry and mock modularity

In previous work, we used new mathematical relations between Gopakumar-Vafa (GV) invariants and rank 0 Donaldson-Thomas (DT) invariants to determine the first few terms in the generating series of Abelian D4-D2-D0 indices for a class of compact one-parameter Calabi-Yau threefolds. This allowed us to obtain striking checks of S-duality, namely the prediction that these series should be vector-valued weakly holomorphic modular forms under $SL(2,\mathbb{Z})$. In this work, we extend this analysis to the case of D4-D2-D0 indices with two units of D4-brane charge, where S-duality instead predicts that the corresponding generating series should be mock modular with a specific shadow. For the degree 10 hypersurface in weighted projective space $\mathbb{P}_{5,2,1,1,1}$, and the degree 8 hypersurface in $\\mathbb{P}_{4,1,1,1,1}$, where GV invariants can be computed to sufficiently high genus, we find that the first few terms indeed match a unique mock modular form with the required properties, which we determine explicitly. Turning the argument around, we obtain new boundary conditions on the holomorphic ambiguity of the topological string amplitude, which in principle allow to determine it completely up to genus 95 and 112, respectively, i.e. almost twice the maximal genus obtainable using gap and ordinary Castelnuovo vanishing conditions.Comment: 34 page

Deflationary Universe Scenario

We show that it is possible to realize an inflationary scenario even without conversion of the false vacuum energy to radiation. Such cosmological models have a deflationary stage in which $Ha^2$ is decreasing and radiation produced by particle creation in an expanding Universe becomes dominant. The preceding inflationary stage ends since the inflaton potential becomes steep. False vacuum energy is finally (partly) converted to the inflaton kinetic energy , the potential energy rapidly decreases and the Universe comes to the deflationary stage with a scale factor $a(t) \propto t^{1/3}$. Basic features and observational consequences of this scenario are indicated.Comment: 18p, KUNS-1201 Plain TeX, phyzzx. Reference file is included which previously was misse

Learning Colour Representations of Search Queries

Image search engines rely on appropriately designed ranking features that capture various aspects of the content semantics as well as the historic popularity. In this work, we consider the role of colour in this relevance matching process. Our work is motivated by the observation that a significant fraction of user queries have an inherent colour associated with them. While some queries contain explicit colour mentions (such as 'black car' and 'yellow daisies'), other queries have implicit notions of colour (such as 'sky' and 'grass'). Furthermore, grounding queries in colour is not a mapping to a single colour, but a distribution in colour space. For instance, a search for 'trees' tends to have a bimodal distribution around the colours green and brown. We leverage historical clickthrough data to produce a colour representation for search queries and propose a recurrent neural network architecture to encode unseen queries into colour space. We also show how this embedding can be learnt alongside a cross-modal relevance ranker from impression logs where a subset of the result images were clicked. We demonstrate that the use of a query-image colour distance feature leads to an improvement in the ranker performance as measured by users' preferences of clicked versus skipped images.Comment: Accepted as a full paper at SIGIR 202