Separability of entangled q-bit pairs

Abstract

The state of an entangled q-bit pair is specified by 15 numerical parameters that are naturally regarded as the components of two 3-vectors and a 3 × 3 dyadic. There are easy-to-use criteria to check whether a given pair of 3-vectors plus a dyadic specify a 2-q-bit state; and if they do, whether the state is entangled; and if it is, whether it is a separable state. Some progress has been made in the search for analytical expressions for the degree of separability. We report, in particular, the answer in the case of vanishing 3-vectors.     

Theoretical investigation of quasi-three-level longitudinally pumped continuous wave lasers

A new theoretical derivation that makes it possible to compute the performance of a quasi-three-level longitudinally pumped cw laser is proposed. This theory, which takes into account the laser wave amplification and the pump wave absorption saturation coupling, seems simpler and more accurate than what has been previously published. The model accounts for single-pass, double-pass, and resonant pumping and can be used to characterize the thermal effect in the gain medium. It is also well suited for designing and optimizing such lasers. As an example, the theory is applied to the well-known Yb:YAG gain medium.     

Generation of motional entangled coherent state in an optomechanical system in the single photon strong coupling regime

The single-photon strong coupling in the deep-resolved sideband of the optomechanical system induces photon blockade (PB) effect. For the PB cavity, an initial mechanical coherent state evolves into superposition of phonon cat states entangled with the cavity Fock states. Measurement of the cavity photon number states produces phonon even and odd cat states. The information leakage effect of two photon states on the fidelity of cat states is calculated, it is shown that for low average phonon number this effect is negligible and decreases by increasing the two photon cavity state. The Lindblad equation is solved numerically to obtain the environmental effects on the fidelity of cat states.     

Efficient upconversion-pumped continuous wave Er3+:LiLuF4 lasers

We report on detailed spectroscopic investigations and efficient visible upconversion laser operation of Er³⁺:LiLuF₄. This material allows for efficient resonant excited-state-absorption (ESA) pumping at 974 nm. Under spectroscopic conditions without external feedback, ESA at the laser wavelength of 552 nm prevails stimulated emission. Under lasing conditions in a resonant cavity, the high intracavity photon density bleaches the ESA at 552 nm, allowing for efficient cw laser operation.We obtained the highest output power of any room-temperature crystalline upconversion laser. The laser achieves a cw output power of 774 mW at a slope efficiency of 19% with respect to the incident pump power delivered by an optically-pumped semiconductor laser. The absorption efficiency of the pump radiation is estimated to be below 50%.To exploit the high confinement in waveguides for this laser, we employed femtosecond-laser pulses to inscribe a cladding of parallel tracks of modified material into Er³⁺:LiLuF₄ crystals. The core material allows for low-loss waveguiding at pump and laser wavelengths. Under Ti:sapphire pumping at 974 nm, the first crystalline upconversion waveguide laser has been realized. We obtained waveguide-laser operation with up to 10 mW of output power at 553 nm.     

Transferring the spatial state of an entangled photon pair to a single photon by photon detection

We show theoretically that the spatial state of entangled photons generated by parametric down-conversion can be transferred to the spatial state of an idler photon by signal photon detection. This study considered the general condition with an arbitrary pump field profile and the detection of a signal photon at an arbitrary distance from a nonlinear crystal where the entangled photons are generated. Upon the detection of a signal photon, the two-photon state function of the entangled state can be transferred to a single-photon state function of the idler field due to the EPR type correlation between the signal and idler fields. The spatial state of the idler field contains more information on the original two-photon state.     

Generation of arbitrary two-qubit entangled states in cavity QED

Abstract

We present a scheme for the generation of arbitrary two-qubit entangled states between two cavity fields via interaction of an atom resonantly interacting with classical and quantized cavity fields. The controlling parameters are the interaction times with these fields.     

Generation of photon number states on demand

Abstract

The widely discussed applications in quantum information and quantum cryptography require radiation sources capable of producing a fixed number of photons. This paper reviews the work performed in our laboratory to produce these fields on demand. Two different methods are discussed. The first is based on the one-atom maser or micromaser operating under the conditions of the so-called trapping states. In this situation the micromaser stabilises to a photon number state. Recently, we also succeeded in determining the Wigner function of a single-photon state. The second device, recently realized in our laboratory, uses a single trapped ion in an optical cavity.     

Technique for continuous tuning of optical fiber lasers

For the first time to the authors' knowledge, we have demonstrated how thermally controlled overcoupled fused fiber couplers and fiber loop mirrors based on these couplers can be used as broadband tuning elements in a fiber laser cavity. No bulk optical elements play any role in this technique. Temperature tuning the coupler results in a shift in the coupling ratio or in the effective output coupler transmission. For a fixed pump source, and for a given laser cavity, this shift causes the lasing wavelength to shift. We have continuously tuned an Er silica fiber laser in this manner over the range of 1527-1570 nm in a ring configuration, and, using a fiber loop mirror with these couplers in a linear Tm silica fiber laser cavity, we have achieved more than 50-nm broadband tuning over the range of 1850-1910 nm. The tuning range and the sensitivity to temperature depend on the degree of overcoupling of the loop mirror coupler.     

Simple way for achieving passive all-optical switching of continuous waves lasers using pure nematic liquid crystal

We have examined pure nematic liquid crystal (LC), 4'-pentyl-4-biphenylcarbonitrile (5-CB), with a 90° twisted alignment within a cell made of two cross-polarized absorptive plastic polarizers, and investigated the nonlinear transmission properties using cw (532 nm) lasers. We observed optically self-activated polarization switching with a factor of three lower switching power than a dye-doped LC cell with similar linear transmittance using glass substrates. We also studied the dynamics of the switching processes and observed millisecond switching time. These studies have demonstrated a simpler but more efficient way for fabricating broadband, low switching power, millisecond time scale switching, and optical limiting devices.     

Generation of high-power ultrashort optical pulses by semiconductor lasers

Fiber-coupled semiconductor lasers have been studied when pumped by high-power short electrical pulses of 5 ns width and leading front duration below 1 ns. In this pumping regime, it is possible to ensure significant sharpening of output pulses, the duration of which decreases below 80 ps for a single-mode laser and below 120 ps for a broad aperture multimode laser at an output peak optical power as high as 1.5 and 27 W, respectively.     

Generation of quantum photon states in an active microcavity trap

Abstract

The active microcavity is adopted as an efficient source of non-classical light. By this device, excited by a mode-locked laser at a rate of 100 MHz, single photons are generated over a single field mode with a non-classical sub-Poissonian distribution. The process of adiabatic recycling within a multistep Franck–Condon molecular optical-pumping mechanism, characterized in our case by a quantum efficiency close to one, implies a pump self-regularization process leading to a striking n-squeezing effect. Moreover, the microactivity has been adopted as active beam splitter in a novel Hanbury-Brown–Twiss configuration for the radiation taking place over the two output mirrors. By a replication of the basic single-atom excitation process a beam of quantum photon |n〉 states (Fock states) can be created. The new process may represent a significant advance in the modern fields of basic quantum-mechanical investigation, quantum communication and quantum cryptography.     

Resonance Raman spectroscopy of photolabile transition metal carbonyls: controlled photoalteration with continuous wave lasers to record spectra of reactants or transient species

A method has been developed that enables resonance Raman spectra of photolabile species in solution to be recorded under conditions where the level of photoalteration is controlled: a low level enables reactant spectra to be recorded, whereas a high level enables the spectra of short-lived transient species to be recorded in real time using continuous-wave (CW) lasers and standard Raman detection equipment. The design includes a sealed flow system, enabling air-sensitive species to be studied under an inert atmosphere. A simple theoretical model has been developed to aid the interpretation of experimental results, and its applicability is demonstrated. Controlled photoalteration and its theory are demonstrated with 413.1-nm excitation of carbonmonoxymyoglobin (MbCO), which generates deoxymyoglobin (deoxy-Mb) on photolysis, and for which the spectra of both species are well established. The methods have also been applied to two air-sensitive, photolabile transition metal carbonyls using 514.5-nm wavelength excitation: for Cp2Mo2(CO)6 (Cp = eta5-C5H5), increasing levels of photoalteration result only in a decrease in the parent band intensities, relative to the solvent bands; for Cp2Fe2(CO)4, increasing levels of photoalteration result in the appearance of additional bands that are assigned to the transient species CpFe(mu-CO)3FeCp, formed following the loss of a CO ligand.     

A travelling wave rotary motor driven by three pairs of Langevin transducers

In this work, numerical simulations and experimental measurements carried out on a high-power ultrasonic motor are presented. The proposed motor is composed of an annular shaped stator and two cone-shaped rotors. The rotors are pressed into contact with the edges of the inner surface of the stator by means of a pre-stress system. A traveling rotating wave is generated in the stator by three pairs of Langevin transducers suitably shifted both in space and time. Each transducer is designed to excite radial nonaxisymmetric modes in the ring. The motor has been effectively clamped to the housing by using two flanges passing through the middle plane of 2 transducers. Finite element analysis was employed to evaluate two different configurations of the motor, both using 6 driving transducers, and to analyze the effect of the flange on the design and on the performance of the transducer. Experimental measurements carried out on a manufactured prototype show that the proposed motor exhibits greatly improved performance in terms of output torque and mechanical power compared with similar previous prototypes which use fewer driving transducers and a different clamping system. The motor weighed 0.67 kg, and had a working frequency of 23.6 kHz, maximum rotational speed of 116 rpm, and static torque of 0.94 Nm.     

The machining of oxides using gas lasers

The use of CO₂-N₂-He gas lasers in the cutting and engraving of oxide materials is demonstrated. The materials investigated were vitreous silica, zirconia, alumina, magnesium aluminate spinel, magnesia and aluminous porcelain.     

Optimization of pump absorption in MOF lasers via multi-long-period gratings: design strategies

Different strategies for designing optical couplers, optimized to enhance the pump absorption in the rare-earth-doped core of microstructured fiber lasers, are illustrated. Three kinds/configurations of optical couplers have been designed and compared as examples of the different design strategies which can be followed. Their effectiveness to enhance the performance of an ytterbium-doped, double cladding, microstructured optical fiber laser has been accurately simulated. They consist of a suitable cascade of multiple long-period gratings (MLPGs) inscribed in the fiber core region. The characteristics of the MLPG couplers have been simulated via a homemade computer code based on both rate equations and an extended coupled mode theory. The proposed MLPG couplers seem particularly useful in the case of low rare-earth concentration but, even for a middle-high ytterbium concentration, as N(Yb)=5×10(25) ions/m(3), the slope efficiency S can be increased up to 20%, depending on the fiber length.     

连续波金属探测器电磁场模型的理论分析

介绍了连续波金属探测器的工作原理，提出一种新的连续波金属探测器电磁场模型，采用基于电磁场和电路的混合方法对该模型进行了详细的理论分析，推导出在空气及海水中金属探测器接收感应电压的计算公式，并给出了简要的设计实例．文中理论分析结果对于连续波金属探测器设计与分析具有指导意义，对其它类型的金属探测器设计也有参考价值．     

Direct measurement of photon number statistics at telecom wavelengths using a charge integration photon detector

In optical quantum information technology, a photon number resolving detector (PNRD) is the basic device for developing photonic quantum computers. The demands for the PNRD are high quantum efficiency and wide dynamic range. We have developed a charge integration photon detector (CIPD) with a quantum efficiency of 80% at telecom wavelengths. The repetition rate of the CIPD is as low as 40 Hz at present, but it can be applied for measurement of short pulses. We report the capability of the CIPD for multiphoton counting over 10 photons, its responsivity to the short pulses, and its high linearity using a binary intensity modulated short pulse (2 ns) train and simultaneous irradiation of two kinds of pulses.