Correlated photon emission from multiatom Rydberg dark States

We consider three-level atoms driven by two resonant light fields in a ladder scheme where the upper level is a highly excited Rydberg state. We show that the dipole-dipole interactions between Rydberg excited atoms prevents the formation of single particle dark states and leads to strongly correlated photon pairs from atoms separated by distances large compared to the emission wavelength. For a pair of atoms, this enables realization of an efficient photon-pair source with on average one pair every 30 μs.     

On the distribution of 1550-nm photon pairs efficiently generated using a periodically poled lithium niobate waveguide

We report on efficient generation of 1550-nm photon pairs in a periodically poled lithium niobate waveguide using the spontaneous parametric down-conversion process. Such photon pairs are expected to find applications in fiber-based long-distance quantum communication. Pumping the waveguide with a pulsed semiconductor laser with a pulse rate of 800 kHz and a maximum average pump power of 50 μW, we obtain a coincidence rate of 600 s⁻¹. Despite only two single-photon detectors are used, we gain some information about the photon-number distribution. Our measurements are found to be in agreement with a Poissonian photon-pair distribution, but clearly differ from the expected outcomes for both conventional and two-mode squeezed states, the latter corresponding to a thermal photon-pair distribution. The Poissonian photon-pair distribution is also explained by comparing the coherence time of the pump light and of the detected photons. An average of 0.9 generated photon pairs per pulse can thus be inferred.     

Effects of multiple pairs on visibility measurements of entangled photons generated by spontaneous parametric processes

Entangled photon-pair sources based on spontaneous parametric processes are widely used in photonic quantum information experiments. In this paper, we clarify the relationship between average photon-pair number and the visibility of two-photon interference (TPI) using those entanglement sources. We consider sources that generate distinguishable and indistinguishable entangled photon pairs, assuming coincidence measurements that use threshold detectors. We present formulas for the TPI visibility of a polarization entanglement that take account of all the high-order multi-pair emission events. Moreover, we show that the formulas can be approximated with simple functions of the average pair number when the photon collection efficiency is small. As a result, we reveal that an indistinguishable entangled pair provides better visibility than a distinguishable one.     

Spontaneous parametric down-conversion and quantum walks in arrays of quadratic nonlinear waveguides

We analyze the process of photon-pair generation with simultaneous quantum walks in a quadratic nonlinear waveguide array. We demonstrate that the spontaneous parametric down-conversion in the array allows for creating quantum states with strongly pronounced spatial correlations, which are qualitatively different from those possible in bulk crystals or through quantum walks in linear waveguide arrays. Most importantly, the photon correlations can be controlled entirely classically by varying the spatial profile of the pump beam or the phase-matching conditions.     

PPLN waveguide for quantum communication

We report on energy-time and time-bin entangled photon-pair sources based on a periodically poled lithium niobate (PPLN) waveguide. Degenerate twin photons at 1 314 nm wavelength are created by spontaneous parametric down-conversion and coupled into standard telecom fibers. Our PPLN waveguide features a very high conversion efficiency of about 10^-6, roughly 4 orders of magnitude more than that obtained employing bulk crystals [#!Tanzilli01a!#]. Even if using low power laser diodes, this engenders a significant probability for creating two pairs at a time - an important advantage for some quantum communication protocols. We point out a simple means to characterize the pair creation probability in case of a pulsed pump. To investigate the quality of the entangled states, we perform photon-pair interference experiments, leading to visibilities of 97% for the case of energy-time entanglement and of 84% for the case of time-bin entanglement. Although the last figure must still be improved, these tests demonstrate the high potential of PPLN waveguide based sources to become a key element for future quantum communication schemes. Received 13 July 2001     

Demonstration of a quantum controlled-NOT gate in the telecommunications band

We present the first quantum controlled-not (cnot) gate realized using a fiber-based indistinguishable photon-pair source in the 1.55 microm telecommunications band. Using this free-space cnot gate, all four Bell states are produced and fully characterized by performing quantum-state tomography, demonstrating the gate's unambiguous entangling capability and high fidelity. Telecom-band operation makes this cnot gate particularly suitable for quantum-information-processing tasks that are at the interface of quantum communication and linear optical quantum computing.     

Indistinguishable photon pairs from independent true chaotic sources

Indistinguishability of events in quantum mechanics is manifested by interference between their probability amplitudes. We report a unique kind of interference occurring between indistinguishable events of photon-pair emission, where each photon of the pair is emitted from a distinct true chaotic light source and has a different energy. The indistinguishability results in an interference which is observed as an ultrafast modulation of the second-order coherence function, measured on a femtosecond time scale by two-photon absorption in a semiconductor photomultiplier tube.     

Analysis of elliptically polarized maximally entangled states for bell inequality tests

When elliptically polarized maximally entangled states are considered, i.e., states having a non random phase factor between the two bipartite polarization components, the standard settings used for optimal violation of Bell inequalities are no longer adapted. One way to retrieve the maximal amount of violation is to compensate for this phase while keeping the standard Bell inequality analysis settings. We propose in this paper a general theoretical approach that allows determining and adjusting the phase of elliptically polarized maximally entangled states in order to optimize the violation of Bell inequalities. The formalism is also applied to several suggested experimental phase compensation schemes. In order to emphasize the simplicity and relevance of our approach, we also describe an experimental implementation using a standard Soleil-Babinet phase compensator. This device is employed to correct the phase that appears in the maximally entangled state generated from a type-II nonlinear photon-pair source after the photons are created and distributed over fiber channels.     

Absorption and emission of terahertz radiation in doped GaAs/AlGaAs quantum wells

Spontaneous emission of terahertz radiation from structures with GaAs/AlGaAs quantum wells in a longitudinal magnetic field has been studied. It is shown that some bands in the emission spectrum can be related to radiative electron transitions between resonant and localized impurity states, as well as to the transitions with participation of subband states. The temperature dependence of the equilibrium intraband absorption of terahertz radiation and its modulation in a longitudinal electric field in GaAs/AlGaAs quantum wells has been investigated.     

Spontaneous emission suppression via quantum path interference in coupled microcavities

We examine theoretically the spontaneous emission rate in optical microstructures with cavity resonances that overlap in both position and frequency. Using projection techniques, we show that the spontaneous emission in such structures can be accurately described by the direct emission and quantum path interference of emission into a few discrete resonant modes, even though the exact infinite-dimensional problem involves a coupling to the continuum of radiation states. Moreover, we obtain an efficient numerical time-domain method for determining the spontaneous emission rate that incorporates these effects, including the suppression of spontaneous emission into some modes.     

Robust Ghost Imaging Based on Degenerate Spontaneous Parametric Down-Conversion

In traditional ghost imaging, the entangled photon pairs produced from the spontaneous parametric down conversion(SPDC) process are used. There is an intrinsic disadvantage that the utilization efficiency of the photon pairs is very low. Inasmuch as all the correlated photon pairs produced by the degenerate SPDC process can be used to record the image of an object, the ghost imaging scheme we present here has a higher utilization efficiency of the photon pairs. We also investigate the robustness of our experimental scheme. The experimental results show that, no matter whether the photon-pair source is two light cones or two beam-like spots, the clear image of the object can be obtained. The slight rotation of the nonlinear crystal has no influence on the imaging quality.Our experimental results also demonstrate that when the part of the photon-pair source in the signal path or the idler path is blocked by unwanted things, the clear ghost image of the object can still be recorded.     

Generalized coherent states and the statistics of two-photon lasers

Stimulated two-photon emission is proposed as a possible mechanism for the generation of “generalized coherent states”, which have useful quantum properties not available in ordinary coherent states.     

First-principles density-functional calculations on the field emission properties of BN nanocones

The first-principles density-functional theory is used to study the geometrical structures and field emission properties of different boron nitride nanocones with 240 disclination. It is found that the nanocones can be stable under applied electric field and the emission current is sensitively dependent on the tips of nanocones. The nanocones with homonuclear bonds at the tip can introduce additional energy states near Fermi level, which can reduce the ionization potential and increase the emission current of these boron nitride nanocones. This investigation indicates that the boron nitride nanocone can be a promising candidate as a field emission electron source.     

Photo-field-emission: A new probe of electron states between the Fermi and vacuum levels

A new experiment, combining photo and field emission techniques, is suggested. It should be particularly useful as a probe of bulk and surface electron states in the energy range between the Fermi and the vacuum levels.     

Abrupt turn-on and hysteresis in a VCSEL with frequency-selective optical feedback

The emission characteristics of a vertical-cavity surface-emitting laser (VCSEL) operated in a single-transverse mode and coupled to an external cavity with a diffraction grating as a frequency-selective element are analyzed experimentally, numerically and analytically. The experiments yield a rather abrupt turn-on of the VCSEL to a high-amplitude emission state and hysteresis phenomena. The experimental results are explained by numerical simulations and analytical calculations demonstrating the possibility of bistability between lasing and non-lasing states close to threshold. Hence, the scheme might be useful in all-optical photonic switching applications. A detailed bifurcation analysis near threshold is given by superimposing the numerical results with analytical steady-state curves. The mode selection and switching behavior obtained in the simulations can be interpreted from the point of view of the preference of states with the minimal total losses.     

Emission and energy relaxation of excitons in quantum dots with disordered interfaces grown in a low-permittivity matrix

Samples of borosilicate glasses doped by CdS with concentrations smaller than 1% are studied. It is shown that, due to a disorder at interfaces of quantum dots, the main channels of emission of excitons by quantum dots are the annihilation of excitons in quantum and localized surface states, while the efficiency of interaction between the channels largely depends on the radius of quantum dots. It is found for the first time that states that form the second emission channel are not discrete energy levels in the band gap, as is usually assumed in some experimental and theoretical works, but rather form a quasi-continuous tail of the density of localized states. These localized states appear as a result of dangling bonds of outer atoms of quantum dots. Energy relaxation of carriers via localized states is the reason for a long response time of these structures to an external action and can be enhanced due to a polarization effect caused by different dielectric constants of materials of quantum dots and matrix.     

An Optimum Scheme to Generate Entangled Photon Pairs

An optimum scheme is proposed to generate a strong entangled photon-pair light source by combining two incident laser lights. Such entangled photon source should have various potential applications in many fields such as quantum metrology, quantum information, and quantum lithography, etc.     

Multichannel polarization-entangled photonpair source for entanglement distribution

A multichannel polarization-entangled photon-pair source in an Mg O-doped periodically poled lithium niobate(Mg O:PPLN) waveguide is proposed. Based on type I quasi-phase-matched spontaneous parametric down conversion in a single Mg O: PPLN waveguide placed inside a Sagnac interferometer and pumped by monochromatic light, a source capable of supporting tens to hundreds of channels of polarization-entangled photon pairs in fiber communication bands simultaneously can be achieved. An inherent channel switch of this source is investigated,which will be significant for future entanglement distribution networks.     

Study of photoluminescence and electronic states in nanophase strontium titanate

Nanophase strontium titanate specimens were prepared by a stearic acid sol-gel process. The phase structures, photoluminescence, and electron states were investigated by X-ray diffraction and fluorescence spectroscopy. A visible emission band centered at 500 nm was observed in the nanosized particles under an excitation with energy lower than the band gap at room temperature. The peak intensity of the visible emission band increases with decreasing particle size. This visible emission is attributed to the recombination of the long-lived self-trapped excitons with forceful binding-energy formed in the nanosized SrTiO₃ mediated by the localized levels which are correlated to the intrinsic surface states and defect centers via a strong electron-phonon interaction. The dependence of the visible emission band on excitation wavelength was examined under both room and liquid nitrogen temperatures where there exist a series of levels of the self-trapped excitons within the forbidden gap and the properties of the self-trapped states are closely related to temperature.     

Direct emission and Auger decay from multiplet final states in Zr observed by ultraviolet photoemission CFS techniques

Photoemission techniques have been used to investigate the structures accompanying excitation of shallow p-core states in crystalline Zr. The continuum nature of synchrotron radiation has been exploited between hv = 20 and 45 eV to determine the Constant Final-state-energy Spectra (CFS) over a range of kinetic energies of the emitted electrons. The results reveal strong direct emission from the multiplet final states and indicate that the final states are broadened by configuration interaction. Auger processes appear to be a probable mode of decay of the p-hole. Secondaries resulting from the Auger processes give rise to structures in the CFS's which compare well to structures in the optical absorption coefficient.