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Practical quantum communication between remote quantum memories rely on single photons at telecom wavelengths. Although spin-photon entanglement has been demonstrated in atomic and solid-state qubit systems, the produced single photons at short wavelengths and with polarization encoding are not suitable for long-distance communication, because they suffer from high propagation loss and depolarization in optical fibres. Establishing entanglement between remote quantum nodes would further require the photons generated from separate nodes to be indistinguishable. Here, we report the observation of correlations between a quantum-dot spin and a telecom single photon across a 2-km fibre channel based on time-bin encoding and background-free frequency downconversion. The downconverted photon at telecom wavelengths exhibits two-photon interference with another photon from an independent source, achieving a mean wavepacket overlap of greater than 0.89 despite their original wavelength mismatch (900 and 911 nm). The quantum-networking operations that we demonstrate will enable practical communication between solid-state spin qubits across long distances.
The paper presents studies of Bose–Einstein Correlations (BEC) for pairs of like-sign charged particles measured in the kinematic range p\(_{T}\) > 100 MeV and |η| < 2.5 in proton collisions at centre-of-mass energies of 0.9 and 7 TeV with the ATLAS detector at the CERN Large Hadron Collider. The integrated luminosities are approximately 7 μb\(^{−1}\), 190 μb\(^{−1}\) and 12.4 nb\(^{−1}\) for 0.9 TeV, 7 TeV minimum-bias and 7 TeV high-multiplicity data samples, respectively. The multiplicity dependence of the BEC parameters characterizing the correlation strength and the correlation source size are investigated for charged-particle multiplicities of up to 240. A saturation effect in the multiplicity dependence of the correlation source size parameter is observed using the high-multiplicity 7 TeV data sample. The dependence of the BEC parameters on the average transverse momentum of the particle pair is also investigated.
The position of a single GaAs quantum dot (QD), which is optically active, grown by low-density droplet epitaxy (DE) (approximately 4 QDs/μm\(^{2}\)), was directly observed on the surface of a 45-nm-thick Al\(_{0.3}\)Ga\(_{0.7}\)As capping layer. The thin thickness of AlGaAs capping layer is useful for single photon sources with plasmonic optical coupling. A micro-photoluminescence for GaAs DE QDs has shown exciton/biexciton behavior in the range of 1.654 to 1.657 eV. The direct observation of positions of low-density GaAs DE QDs would be advantageous for mass fabrication of devices that use a single QD, such as single photon sources.
Search for W' → tb → qqbb decays in pp collisions at \(\sqrt {s}\) = 8 TeV with the ATLAS detector
(2015)
A search for a massive W′ gauge boson decaying to a top quark and a bottom quark is performed with the ATLAS detector in pp collisions at the LHC. The dataset was taken at a centre-of-mass energy of \(\sqrt {s}\) = 8 TeV and corresponds to 20.3 fb\(^{−1}\) of integrated luminosity. This analysis is done in the hadronic decay mode of the top quark, where novel jet substructure techniques are used to identify jets from high-momentum top quarks. This allows for a search for high-mass W′ bosons in the range 1.5–3.0 TeV. b-tagging is used to identify jets originating from b-quarks. The data are consistent with Standard Model background-only expectations, and upper limits at 95 % confidence level are set on the W′ → tb cross section times branching ratio ranging from 0.16 pb to 0.33 pb for left-handed W′ bosons, and ranging from 0.10 pb to 0.21 pb for W′ bosons with purely right-handed couplings. Upper limits at 95 % confidence level are set on the W′-boson coupling to tb as a function of the W′ mass using an effective field theory approach, which is independent of details of particular models predicting a W′ boson.
A search for the Standard Model Higgs boson produced in association with a top-quark pair, t\(\overline{t}\)H, is presented. The analysis uses 20.3 fb\(^{−1}\) of pp collision data at \(\sqrt {s}\) = 8 TeV, collected with the ATLAS detector at the Large Hadron Collider during 2012. The search is designed for the H→b\(\overline{b}\) decay mode and uses events containing one or two electrons or muons. In order to improve the sensitivity of the search, events are categorised according to their jet and b-tagged jet multiplicities. A neural network is used to discriminate between signal and background events, the latter being dominated by t\(\overline{t}\)+jets production. In the single-lepton channel, variables calculated using a matrix element method are included as inputs to the neural network to improve discrimination of the irreducible t\(\overline{t}\)+b\(\overline{b}\) background. No significant excess of events above the background expectation is found and an observed (expected) limit of 3.4 (2.2) times the Standard Model cross section is obtained at 95 % confidence level. The ratio of the measured t\(\overline{t}\)H signal cross section to the Standard Model expectation is found to be μ = 1.5 ± 1.1 assuming a Higgs boson mass of 125 GeV.
A search for the associated production of the Higgs boson with a top quark pair is performed in multilepton final states using 20.3 fb\(^{−1}\) of proton–proton collision data recorded by the ATLAS experiment at \(\sqrt {s}\)=8 TeV at the Large Hadron Collider. Five final states, targeting the decays H→WW\(^{*}\), ττ, and ZZ\(^{*}\), are examined for the presence of the Standard Model (SM) Higgs boson: two same-charge light leptons (e or μ) without a hadronically decaying τ lepton; three light leptons; two same-charge light leptons with a hadronically decaying τ lepton; four light leptons; and one light lepton and two hadronically decaying τ leptons. No significant excess of events is observed above the background expectation. The best fit for the t\(\overline{t}\)H production cross section, assuming a Higgs boson mass of 125 GeV, is 2.1\(^{+1.4}_{-1.2}\) times the SM expectation, and the observed (expected) upper limit at the 95% confidence level is 4.7 (2.4) times the SM rate. The p-value for compatibility with the background-only hypothesis is 1.8σ; the expectation in the presence of a Standard Model signal is 0.9σ.
Two searches for supersymmetric particles in final states containing a same-flavour opposite-sign lepton pair, jets and large missing transverse momentum are presented. The proton–proton collision data used in these searches were collected at a centre-of-mass energy \(\sqrt {s}\) = 8 TeV by the ATLAS detector at the Large Hadron Collider and corresponds to an integrated luminosity of 20.3 fb\(^{−1}\). Two leptonic production mechanisms are considered: decays of squarks and gluinos with Z bosons in the final state, resulting in a peak in the dilepton invariant mass distribution around the Z-boson mass; and decays of neutralinos (e.g. \(\tilde{χ}\)\(^{0}_{2}\) → ℓ\(^{+}\)ℓ\(^{−}\)\(\tilde{χ}\)\(^{0}_{1}\)), resulting in a kinematic endpoint in the dilepton invariant mass distribution. For the former, an excess of events above the expected Standard Model background is observed, with a significance of three standard deviations. In the latter case, the data are well-described by the expected Standard Model background. The results from each channel are interpreted in the context of several supersymmetric models involving the production of squarks and gluinos.
This Letter presents a search at the LHC for s-channel single top-quark production in proton–proton collisions at a centre-of-mass energy of 8 TeV. The analyzed data set was recorded by the ATLAS detector and corresponds to an integrated luminosity of 20.3 fb\(^{−1}\). Selected events contain one charged lepton, large missing transverse momentum and exactly two b-tagged jets. A multivariate event classifier based on boosted decision trees is developed to discriminate s-channel single top-quark events from the main background contributions. The signal extraction is based on a binned maximum-likelihood fit of the output classifier distribution. The analysis leads to an upper limit on the s-channel single top-quark production cross-section of 14.6 pb at the 95% confidence level. The fit gives a cross-section of σs=5.0±4.3 pb, consistent with the Standard Model expectation.
This paper reports on a search for narrow resonances in diboson production in the ℓℓq\(\overline{q}\) final state using pp collision data corresponding to an integrated luminosity of 20 fb\(^{−1}\) collected at \(\sqrt {s}\) = 8 TeV with the ATLAS detector at the Large Hadron Collider. No significant excess of data events over the Standard Model expectation is observed. Upper limits at the 95 % confidence level are set on the production cross section times branching ratio for Kaluza–Klein gravitons predicted by the Randall–Sundrum model and for Extended Gauge Model W′ bosons. These results lead to the exclusion of mass values below 740 and 1590 GeV for the graviton and W′ boson respectively.
A search is presented for narrow diboson resonances decaying to WW or WZ in the final state where one W boson decays leptonically (to an electron or a muon plus a neutrino) and the other W/Z boson decays hadronically. The analysis is performed using an integrated luminosity of 20.3 fb\(^{−1}\) of pp collisions at \(\sqrt {s}\) = 8 TeV collected by the ATLAS detector at the large hadron collider. No evidence for resonant diboson production is observed, and resonance masses below 700 and 1490 GeV are excluded at 95 % confidence level for the spin-2 Randall–Sundrum bulk graviton G\(^{*}\) with coupling constant of 1.0 and the extended gauge model W′ boson respectively.