Search Results (233 total)

A Mach-Zender interferometer with a Gaussian number-difference squeezed input state can exhibit sub-shot-noise phase resolution over a large phase interval. We derive the optimal level of squeezing for a given phase interval Δθ₀ and particle number N. We then propose an adaptive measurement sequence in which the amount of squeezing is increased with each measurement. With this scheme, any phase on (-Δθ₀,Δθ₀) can be measured with a precision of 3.5/N, requiring only 2–4 measurements, provided only that Ntan⁡(Δθ₀)<10⁴⁰. In a double-well Bose-Einstein condensate, the optimized input states can be created by adiabatic manipulation of the ground state.

Exact solutions are obtained for all the modes of wave propagation along an anisotropic cylindrical waveguide. Closed-form expressions for the energy flow on the waveguide are also derived. For extremely anisotropic waveguide where the transverse permittivity is negative (ε_⊥<0) while the longitudinal permittivity is positive (ε_∥>0), only transverse magnetic (TM) and hybrid modes will propagate on the waveguide. At any given frequency the waveguide supports an infinite number of eigenmodes. Among the TM modes, at most only one mode is forward wave. The rest of them are backward waves which can have very large effective index. At a critical radius, the waveguide supports degenerate forward- and backward-wave modes with zero group velocity. These waveguides can be used as phase shifters and filters, and as optical buffers to slow down and trap light.

By extending the concept of interaction-free imaging to the few-atom level, we show that asymptotically on-demand interaction- and measurement-free quantum logic gates can be realized for both single-atom and single-photon qubits. The interaction-free feature suppresses the possibility of qubit decoherence via atomic spontaneous decay, while the elimination of measurements can significantly reduce errors arising from detector inefficiency. We present a general theory of universal quantum Zeno gates, and discuss physical implementations for quantum-information processing with individual atoms and photons. In addition, we propose a loss-tolerant protocol for long-distance quantum communication using quantum Zeno gates incorporated into a Mach-Zehnder interferometer. The efficiency of our Zeno gates is limited primarily by the imprecise control of atom-photon scattering and the finite number of feedback cycles N due to the limited finesse of the optical ring cavity. We find that the success probability scales as 1−O(1/N), and for realistic parameters could be as high as 98.4%. Successful generation of atom-atom entanglement can be heralded by detection of the ancillary photon, upon which the fidelity scales as 1−O(1/N²), with an achievable fidelity of 99.994%, which comes at the cost of reducing the success probability by the detector efficiency.

Polarization-entangled photon pairs are easily perturbed in noisy channels. We propose an efficient strategy for sharing polarization-entangled photon pairs (PEPPs) using the additional frequency labels of polarization-frequency doubly entangled photon pairs (DEPPs). The DEPPs are used in transmission, followed by a two-step operation. In the first step, all the bit-flip noises are wiped off efficiently. In the second step, the frequency labels of the DEPPs are erased and the phase-flip noises are wiped off; thus PEPPs in the desired state are extracted. Theoretical analysis shows that this strategy has intrinsically high efficiency, which demonstrates that it has great potential in sharing entanglement via noisy transmission channels.

A cylindrical single-wall carbon nanotube (SWCNT) deforms in a nanotube bundle by van der Waals forces. The deformation is hard to measure, but is required in order to understand the properties of bundles. Here, we show that such deformations can be measured from changes in electron diffraction intensities. We demonstrate this for a bundle of two SWCNTs (d=2.05 and 1.56  nm). Deformation model predicted by atomistic simulations is scaled to fit the experiment. We show that the best fit gives flattening values of 0.7 and 0.35  Å at the middle of the binding surface for the two SWCNTs.

The problem of on-demand generation of entanglement between single-atom qubits via a common photonic channel is examined within the framework of optical interferometry. As expected, for a Mach-Zehnder interferometer with coherent laser beam as input, a high-finesse optical cavity is required to overcome sensitivity to spontaneous emission. We show, however, that with a twin-Fock input, useful entanglement can in principle be created without cavity enhancement. Both approaches require single-photon resolving detectors, and best results would be obtained by combining both cavity feedback and twin-Fock inputs. Such an approach may allow a fidelity of 0.99 using a two-photon input and currently available mirror and detector technology. In addition, we study interferometers based on NOON states, i.e., maximally entangled N-particle states, and show that they perform similarly to the twin-Fock states, yet without the need for high-precision photodetectors. The present interferometrical approach can serve as a universal, scalable circuit element for quantum information processing, from which fast quantum gates, deterministic teleportation, entanglement swapping, etc., can be realized with the aid of single-qubit operations.

The decays of J/ψ→ηϕf₀(980)[η→γγ,ϕ→K⁺K^-,f₀(980)→π⁺π^-] are analyzed using a sample of 5.8×10⁷ J/ψ events collected with the BESII detector at the Beijing Electron-Positron Collider. A structure at around 2.18  GeV/c² with about 5σ significance is observed in the ϕf₀(980) invariant mass spectrum. A fit with a Breit-Wigner function gives the peak mass and width of m=2.186±0.010(stat)±0.006(syst)  GeV/c² and Γ=0.065±0.023(stat)±0.017(syst)  GeV/c², respectively, which are consistent with those of Y(2175), observed by the BABAR Collaboration in the initial-state radiation process e⁺e^-→γ_ISRϕf₀(980). The production branching ratio is determined to be Br(J/ψ→ηY(2175))Br(Y(2175)→ϕf₀(980))Br(f₀(980)→π⁺π^-)=[3.23±0.75(stat)±0.73(syst)]×  10^-4, assuming that the Y(2175) is a 1^-- state.

The decays of J/ψ→ωKK̅ π and J/ψ→ϕKK̅ π are studied using 5.8×10⁷ J/ψ events collected with the Beijing Spectrometer (BESII) at the Beijing Electron-Positron Collider (BEPC). The K_S⁰K^±π^∓ and K⁺K^-π⁰ systems, produced in J/ψ→ωKK̅ π, have enhancements in the invariant mass distributions at around 1.44  GeV/c². However, there is no evidence for mass enhancements in the KK̅ π system in J/ψ→ϕKK̅ π. The branching fractions of J/ψ→ωK_S⁰K^±π^∓, ϕK_S⁰K^±π^∓, ωK^*K̅ +c.c., and ϕK^*K̅ +c.c. are obtained, and the J/ψ→ηK_S⁰K^±π^∓ branching fraction is measured for the first time.

We report upper critical field B_c2(T) measurements on a single-crystalline sample of the ferromagnetic superconductor UCoGe. B_c2(0) obtained for fields applied along the orthorhombic axes exceeds the Pauli limit for B∥a,b and shows a strong anisotropy B_c2^a≃B_c2^b≫B_c2^c. This provides evidence for an equal-spin pairing state and a superconducting gap function of axial symmetry with point nodes along the c axis, which is also the direction of the uniaxial ferromagnetic moment m₀=0.07μ_B. An unusual curvature or kink is observed in the temperature variation of B_c2, which possibly indicates UCoGe is a two-band ferromagnetic superconductor.

Two schemes of projection measurement are realized experimentally to demonstrate the de Broglie wavelength of three photons without the need for a maximally entangled three-photon state (the NOON state). The first scheme is based on a proposal by [Wang and Kobayashi, Phys. Rev. A 71, 021802(R) (2005)] that utilizes a couple of asymmetric beam splitters while the second one applies the general method of NOON-state projection measurement to a three-photon case. Quantum interference of three photons is responsible for projecting out the unwanted states, leaving only the NOON-state contribution in these schemes of projection measurement. A detailed multimode analysis is made to account for imperfect situations in the experiments.

We present calculation on the azimuthal spin asymmetries for pion pair production in semi-inclusive deep inelastic scattering (SIDIS) process at both HERMES and COMPASS kinematics, with transversely polarized proton, deuteron, and neutron targets. We calculate the asymmetry by adopting a set of parametrization of the interference fragmentation functions and two different models for the transversity. We find that the result for the proton target is insensitive to the approaches of the transversity but more helpful to understand the interference fragmentation functions. However, for the neutron target, which can be obtained through using deuteron and ³He targets, we find different predictions for different approaches to the transversity. Thus probing the two pion interference fragmentation from the neutron can provide us more interesting information on the transversity.

We use in-plane tunneling spectroscopy to study the temperature dependence of the local superconducting gap Δ(T) in electron-doped copper oxides with various T_c’s and Ce-doping concentrations. We show that the temperature dependence of Δ(T) follows the expectation of the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity, where Δ(0)∕k_BT_c≈1.72±0.15 and Δ(0) is the average superconducting gap across the Fermi surface, for all the doping levels investigated. These results suggest that the electron-doped superconducting copper oxides are weak-coupling BCS superconductors.

After years of research into colossal magnetoresistant (CMR) manganites using bulk techniques, there has been a recent upsurge in experiments directly probing the electronic states at or near the surface of the bilayer CMR materials La_2−2xSr_1+2xMn₂O₇ using angle-resolved photoemission or scanning probe microscopy. Here, we report temperature-dependent, angle-resolved photoemission data from single crystals with a doping level of x=0.36. The first important result is that there is no sign of a pseudogap in the charge channel of this material for temperatures below the Curie temperature T_C. The data show unprecedented sharp spectral features, enabling the unambiguous identification of clear, resolution-limited quasiparticle features from the bilayer split 3d_x²−y²-derived Fermi surfaces both at the zone-face and zone diagonal k_F locations. The data show that these low temperature Fermi surfaces describe closed shapes in k_∥, centered at the (π∕a,π∕a) points in the two dimensional Brillouin zone, and are not open and arclike in nature. The second important result concerns the temperature dependence of the electronic states. The spectra display strong incoherent intensity at high binding energies and a very strong temperature dependence, both characteristics reminiscent of polaronic systems. However, the clear and strong quasiparticle peaks at low temperatures are difficult to place within a polaronic scenario. A careful analysis of the temperature-dependent changes in the Fermi surface spectra both at the zone face and zone diagonal regions in k space indicates that the coherent quasiparticle weight disappears for temperatures significantly above T_C and that the k dependence of the T-induced changes in the spectra invalidates an interpretation of these data in terms of the superposition of a “universal” metallic spectrum and an insulating spectrum whose relative weight changes with temperature. In this sense, our data are not compatible with a phase separation scenario.

In this paper, photonic entanglement and interference are described and analyzed with the language of quantum information processing. Correspondingly, a photon state involving several degrees of freedom is represented in an expression based on the permutation symmetry of bosons. In this expression, each degree of freedom of a single photon is regarded as a qubit and operations on photons as qubit gates. The two-photon Hong-Ou-Mandel interference is well interpreted with it. Moreover, the analysis reveals the entanglement between different degrees of freedom in a four-photon state from parametric down conversion, even if there is no entanglement between them in the two-photon state. The entanglement will decrease the state purity and photon interference visibility in the experiments on a four-photon polarization state.

There are very few materials that exhibit zero thermal expansion (ZTE), and of these even fewer are appropriate for electronic and optoelectronic applications. We find that a multifunctional crystalline hybrid inorganic-organic semiconductor, β-ZnTe(en)_0.5 (en denotes ethylenediamine), shows uniaxial ZTE in a very broad temperature range of 4–400 K, and concurrently possesses superior electronic and optical properties. The ZTE behavior is a result of compensation of contraction and expansion of different segments along the inorganic-organic stacking axis. This work suggests an alternative route to designing materials in a nanoscopic scale with ZTE or any desired positive or negative thermal expansion (PTE or NTE), which is supported by preliminary data for ZnTe(pda)_0.5 (pda denotes 1,3-propanediamine) with a larger molecule.

By combining experimental measurements of the quasiparticle and dynamical magnetic properties of optimally electron-doped Pr_0.88LaCe_0.12CuO₄ with theoretical calculations, we demonstrate that the conventional fermiology approach cannot possibly account for the magnetic fluctuations in these materials. In particular, we perform tunneling experiments on the very same sample for which a dynamical magnetic resonance has been reported recently and use photoemission data by others on a similar sample to characterize the fermionic quasiparticle excitations in great detail. We subsequently use this information to calculate the magnetic response within the conventional fermiology framework as applied in a large body of work for the hole-doped superconductors to find a profound disagreement between the theoretical expectations and the measurements: this approach predicts a steplike feature rather than a sharp resonance peak, it underestimates the intensity of the resonance by an order of magnitude, it suggests an unreasonable temperature dependence of the resonance, and most severely, it predicts that most of the spectral weight resides in incommensurate wings which are a key feature of the hole-doped cuprates but have never been observed in the electron-doped counterparts. Our findings strongly suggest that the magnetic fluctuations reflect the quantum-mechanical competition between antiferromagnetic and superconducting orders.

We have studied the effect of resonant electronic-state coupling on the formation of ultracold ground-state ⁸⁵Rb₂. Ultracold Rb₂ molecules are formed by photoassociation (PA) to a coupled pair of 0_u⁺ states, 0_u⁺(P_1∕2) and 0_u⁺(P_3∕2), in the region below the 5S+5P_1∕2 limit. Subsequent radiative decay produces high vibrational levels of the ground state, X  ¹Σ_g⁺. The population distribution of these X-state vibrational levels is monitored by resonance-enhanced two-photon ionization through the 2  ¹Σ_u⁺ state. We find that the populations of vibrational levels v^″=112–116 are far larger than can be accounted for by the Franck-Condon factors for 0_u⁺(P_1∕2)→X  ¹Σ_g⁺ transitions with the 0_u⁺(P_1∕2) state treated as a single channel. Further, the ground-state molecule population exhibits oscillatory behavior as the PA laser is tuned through a succession of 0_u⁺ state vibrational levels. Both of these effects are explained by a calculation of transition amplitudes that includes the resonant character of the spin-orbit coupling of the two 0_u⁺ states. The resulting enhancement of more deeply bound ground-state molecule formation will be useful for future experiments on ultracold molecules.

We report the coexistence of ferromagnetic order and superconductivity in UCoGe at ambient pressure. Magnetization measurements show that UCoGe is a weak ferromagnet with a Curie temperature T_C=3  K and a small ordered moment m₀=0.03μ_B. Superconductivity is observed with a resistive transition temperature T_s=0.8  K for the best sample. Thermal-expansion and specific-heat measurements provide solid evidence for bulk magnetism and superconductivity. The proximity to a ferromagnetic instability, the defect sensitivity of T_s, and the absence of Pauli limiting, suggest triplet superconductivity mediated by critical ferromagnetic fluctuations.

We use a new set of Collins functions to update a previous prediction on the azimuthal asymmetries of pion production in a semi-inclusive deep-inelastic scattering process on a transversely polarized nucleon target. We find that the calculated results can give a good explanation to the HERMES experiment with the new parametrization, and this can enrich our knowledge of the fragmentation process. Furthermore, with two different approaches of distribution and fragmentation functions, we present a prediction on the azimuthal asymmetries of pion and kaon production at the kinematics region of the experiments E06010 and E06011 planned at Jefferson Lab. It is shown that the results are insensitive to the models for the pion case. However, the results for kaon production are sensitive to different approaches of distribution and fragmentation functions. This is helpful to clarify some points in the study of the azimuthal spin asymmetries and fragmentation functions in hadronization processes.

The quantitative interface resistance between polycrystalline ferromagnetic Co and Nb_xTi_1−x, with x=1, 0.6, and 0.4, is measured and analyzed at 4.2  K. Both the superconducting and normal states of Nb_xTi_1−x, respectively, above and below the superconducting critical thickness, are studied with current flowing perpendicular to the interface. A one-band series-resistance model is used to analyze our data. The interface transparencies in terms of the ratio between interface resistance and various physical quantities are discussed.

By performing an experiment on stimulated emission by two photons in the parametric amplification process and comparing it to a three-photon interference scheme, we present evidence in support of the idea that the underlying physics of stimulated emission is simply the constructive interference due to photon indistinguishability. So the observed signal enhancement upon the input of photons can be interpreted as a result of multiphoton interference of the input photons and the otherwise spontaneously emitted photon from the amplifier.

We have investigated the ionic relaxations, electronic structures, and optical properties for Cd_1−xZn_xTe alloys using density functional theory. The quasi-zinc-blende structure is used with special emphasis on the relaxation behaviors of Te²⁻ around either Cd²⁺ or Zn²⁺. Our calculations confirm that the relaxations of the anion rather than the cation contribute primarily to the alloying process as predicated by the experiments. The differences in the ionicity of Cd²⁺ and Zn²⁺ and their configurations around Te²⁻ are responsible for the different relaxation behaviors of Te²⁻. A striking result is the relevance of the relaxation behaviors of Te²⁻ with the alloying effect on the electronic states. This result supports the electronic features of Cd_0.5Zn_0.5Te alloy reported by the systematic analyses with quasirandom structure. The band structures obtained here are used to determine the optical functions. The comparison with the available experimental and theoretical results suggests an overall topological resemblance in the present dielectric function spectra when the band-gap correction is included.

We show that with an appropriate surface modification, a slab of photonic crystal can be made to allow wave transmission within the photonic band gap. Furthermore, negative refraction and all-angle negative refraction (AANR) can be achieved by this surface modification in frequency windows that were not realized before in two-dimensional photonic crystals [C. Luo , Phys. Rev. B 65, 201104 (2002)]. This approach to AANR leads to different applications in flat lens imaging. Previous flat lens using photonic crystals requires object-image distance u+v less than or equal to the lens thickness d, u+v∼d. Our approach can be used to design a flat lens with u+v=σd with σ≫1, thus being able to image large and/or far away objects. Our results are confirmed by finite-difference time-domain simulations.

Annealing effects on the implanted vicinal Si(111) were analyzed by reflective second-harmonic generation (RSHG). The phenomena of impurity diffusion and precipitation were observed through the anisotropic contribution of the C_3V component in the RSHG rotational anisotropy experiments for a series of rapid thermal annealing (RTA) times. The surface reconstruction of the implanted vicinal Si(111) was clearly observed due to the contribution of the C_1V symmetry which is raised from the step structure on the vicinal surface. The enhanced value of the C_1V component originates because P atoms participate in the surface reconstruction. The phase difference between the C_3V and C_1V components has large variations at lower RTA temperature because the reconstruction situation near the surface was not completed until the RTA time of 30  s and was influenced by the precipitation of P atoms. With the assistance of step structure on vicinal Si(111), the reconstruction of the implanted Si(111) reveals more physical information.

A highly efficient photonic band edge dye-doped cholesteric liquid crystal (CLC) laser is demonstrated. By sandwiching an active CLC cell within a resonator consisting of two passive CLC reflectors, the lasing efficiency is dramatically enhanced. Theoretical analysis using the improved 4×4 transfer matrix and scattering matrix shows that the band edge laser mode can be supported by the external CLC resonator and its optimal output can be achieved by a relatively thin active CLC layer and thin passive CLC reflectors. Theoretical analysis agrees well with the experimental results.