Direct experimental characterization of the Bose-Einstein distribution of spatial fluctuations of spontaneous parametric down-conversion

We present, to the best of our knowledge, the first experimental demonstration by direct detection of the Bose-Einstein photon-number distribution of highly spatially multi-mode but temporally single mode spontaneous parametric down-conversion.Received: 20 November 2003, Published online: 6 January 2004PACS: 42.50.-p Quantum optics - 42.50.Ar Photon statistics and coherence theory - 42.65.-k Nonlinear optics - 42.65.Lm Parametric down conversion and production of entangled photons     

Generation of accurately synchronized pump source for optical parametric chirped pulse amplification

We demonstrate the generation of ultra-short pulses at 1064 nm by continuous-wave seeded non-collinear optical parametric amplification in a -barium borate crystal pumped by the second harmonics of a femtosecond Ti:sapphire laser. After two stages of seeded parametric amplifiers, the generated pulses at 1064 nm were accurately synchronized with the fundamental pump pulses, which could be used as seeding pulses for further amplification and then frequency doubling to produce an accurately synchronized pump source for optical parametric amplification of chirped pulses from the same Ti:sapphire laser. PACS 42.65.Re; 42.65.-k; 52.35.Mw     

A tentative approach to entanglement measures for a system of a three-level atom interacting with a quantized cavity-field

In this paper, an entanglement measure due to quasi-mutual entropy from initially entangled mixed states of a three-level atom interacting with a single cavity field is introduced. Detailed analytical and explicit expressions are given taking into account an arbitrary form of the intensity-dependent coupling. Despite its simplicity the model exhibits a very broad range of intricate physical effects and it is widely used in quantized theories of laser. We show that quantum revivals are possible for a broad continuous range of physical parameters in the case of initial coherent states. Entanglement degree effects are shown to be very sensitive to the initial state of the system. Numerical calculations under current experimental conditions are taken into account and it is found that the intensity-dependent coupling changes the general features dramatically.Received: 2 June 2003, Published online: 26 August 2003PACS: 03.65.Ud Entanglement and quantum nonlocality (e.g. EPR paradox, Bells inequalities, GHZ states, etc.) - 03.67.Hk Quantum communication - 42.65.-k Nonlinear optics - 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)     

Giant enhancement of optical nonlinearity in mixtures of graded particles with dielectric anisotropy

To investigate the effective linear dielectric constant and third-order nonlinear susceptibility of composite media, in which graded inclusions with radial dielectric anisotropy are randomly embedded in a linear isotropic matrix, we develop an nonlinear anisotropic differential effective dipole approximation (NADEDA). Alternatively, based on a first-principles approach, the exact expressions for and are also derived for the linear dielectric profiles with small slopes. Then, excellent agreement between the two methods is numerically demonstrated. As an application, we further apply the NADEDA to a nonlinear metal-dielectric composite, in which the metal particles possess spatially varying radial dielectric anisotropy, in an attempt to study the nonlinearity enhancement and the figure of merit of the composite. To this end, it is shown that the presence of gradation in the radial dielectric constant plays a crucial role in enhancing the optical nonlinearity as well as the figure of merit.Received: 16 August 2003, Published online: 30 January 2004PACS: 77.22.Ej Polarization and depolarization - 42.65.-k Nonlinear optics - 42.79.Ry Gradient-index (GRIN) devices - 77.84.Lf Composite materials     

Four-wave mixing based on cascaded second-order nonlinear processes in a femtosecond optical parametric oscillator operating near degeneracy

Multiple cascaded four-wave mixing (FWM) processes were observed simultaneously in a femtosecond optical parametric oscillator (OPO) based on periodically poled KTP, which produced, in addition to the OPO signal and idler, additional spectral components in the infrared. As a result, the tuning range of the OPO extended from about 1.25 m to 2.2 m with an unstable operation range from 1.56 to 1.64 m due to double resonance of the OPO near the degeneracy point. Further cascaded frequency mixing between the infrared spectra and the pump of the OPO produced a series of approximately equally spaced spectral components in the visible region. PACS 42.65.–k; 42.65.Yj; 42.65.Hw     

Quantum fluctuations in holographic teleportation of optical images

We investigate in detail the quantum fluctuations in the quantum holographic teleportation protocol that we recently proposed [11]. This protocol implements a continuous variable teleportation scheme that enables the transfer of the quantum state of spatially multimode electromagnetic fields, preserving their quantum correlations in space-time, and can be used to perform teleportation of 2D optical images. We derive a characteristic functional, which provides any arbitrary spatio-temporal correlation function of the teleported field, and calculate the fidelity of the teleportation scheme for multimode Gaussian input states. We show that for multimode light fields one has to distinguish between a global and a reduced fidelity. While the global fidelity tends to vanish for teleportation of fields with many degrees of freedom, the reduced fidelity can be made close to unity by choosing properly the number of essential degrees of freedom and the spatial bandwidth of the EPR beams used in the teleportation scheme.Received: 16 March 2004, Published online: 11 May 2004PACS: 03.67.-a Quantum information - 03.65.Bz Foundations, theory of measurement, miscellaneous theories (including Aharonov-Bohm effect, Bell inequalities, Berrys phase) - 42.50.Dv Nonclassical states of the electromagnetic field, including entangled photon states; quantum state engineering and measurements     

Level Dynamics and Universality of Spectral Fluctuations

The spectral fluctuations of quantum (or wave) systems with a chaotic classical (or ray) limit are mostly universal and faithful to random-matrix theory. Taking up ideas of Pechukas and Yukawa we show that equilibrium statistical mechanics for the fictitious gas of particles associated with the parametric motion of levels yields spectral fluctuations of the random-matrix type. Previously known clues to that goal are an appropriate equilibrium ensemble and a certain ergodicity of level dynamics. We here complete the reasoning by establishing a power law for the dependence of the mean parametric separation of avoided level crossings. Due to that law universal spectral fluctuations emerge as average behavior of a family of quantum dynamics drawn from a control parameter interval which becomes vanishingly small in the classical limit; the family thus corresponds to a single classical system. We also argue that classically integrable dynamics cannot produce universal spectral fluctuations since their level dynamics resembles a nearly ideal Pechukas–Yukawa gas.     

Dielectric resonance,local field distribution,and optical response in 3D resonant composites

We extend the Greens function formalism in a binary 2D composite to 3D. Using the formalism, we investigate the dielectric resonances, local-field distribution, and effective linear optical responses for one-bond, two-bond and three-bond clusters, as well as for various disordered composites. Due to the different values of Greens function in 2D and 3D, for the same cluster, the values of the dielectric resonances in 3D are smaller than those in 2D, but the fields are more localized than those in 2D. The sum rule of dielectric resonance in three-component composites is extended to d dimensions. For the same resonance, the intensity of the local-field in 3D is also weaker than that in the 2D case, but the fields are more localized than those in 2D. For the disordered composites in 2D and 3D, inverse participation ratios (IPR) with q = 2 are used to represent the localization of the field. When we increase the concentration of impurity bonds, a blue shift of IPR peaks occurs in 3D, while in 2D, these peaks are very stable. Finally, both for 2D and 3D disordered composites, the absorption range broadens with increasing impurity concentration, and a red shift of the absorption peak is observed in 3D.Received: 19 August 2004, Published online: 14 December 2004PACS: 77.84.Lf Composite materials - 42.65.-k Nonlinear optics     

Bismuth triborate (BiB3O6) optical parametric oscillators

We report the phase matching of parametric frequency conversion in the nonlinear material BiB₃O₆ (BiBO) and on an investigation of optical parametric oscillators (OPOs) of this new crystal. Based on the calculation of collinear type I and type II phase matching within the refractive-index planes, the most favorable directions for phase matching are identified for OPOs pumped by the fundamental or the harmonics of 1064-nm Nd-doped lasers. Based on these results, pulsed 532-nm-pumped ns OPOs are realized. The pump source is either a Q-switched high repetition rate (10 kHz) Nd:YVO₄ laser (with a pulse energy of 24 J) or a low repetition rate (10 Hz), high pulse energy (120 mJ) Nd:YAG laser system. The BiBO OPO pumped by the Nd:YVO₄ laser showed a very low threshold of 0.047 J/cm². At an average pump power of 2.4 W the total OPO output power was 630 mW. By changing the phase-matching angle within the yz plane from 0 to 11.6° the signal wavelength was tuned from 735 nm to 970 nm, while the spectral width changed from 0.2 nm to 1.4 nm. By pumping the OPO with the Nd:YAG laser, the OPO had a threshold of 0.12 J/cm², a steep slope (59%) and a high total efficiency (of up to 48%). Due to divergence broadening the spectral width changes from 8.5 nm at 800 nm to 70 nm near degeneracy. The properties of BiBO determined from the experimental results are compared with those of well-known nonlinear materials such as BBO, LBO and KTP. PACS 42.65.-k; 42.65.Yj; 42.70.-a; 42.70.Mp     

Second-harmonic generation in strongly scattering porous gallium phosphide

Second-harmonic generation in a strongly scattering macroporous GaP layer with typical sizes of GaP nanocrystals ranging from 200 to 500 nm is studied using nanosecond pulses of an optical parametric oscillator. As the pump wavelength decreases from 1.6 to 1.0 m the second-harmonic yield from porous GaP increases more than one order of magnitude relative to the second-harmonic yield from crystalline GaP. This enhancement of the second-harmonic generation for shorter wavelengths correlates with the increase in the efficiency of light scattering, indicating the importance of scattering effects in nonlinear-optical processes in strongly scattering random media . PACS 42.25.Fx; 42.65.Ky     

Constructing Four-Photon States for Quantum Communication and Information Processing

We provide a method for constructing a set of four-photon states suitable for quantum communication applications. Among these states is a set of concatenated quantum code states that span a decoherence-free subspace that is robust under collective-local as well as global dephasing noise. This method requires only the use of spontaneous parametric down-conversion, quantum state post-selection, and linear optics. In particular, we show how this method can be used to produce all sixteen elements of the second-order Bell gem , which includes these codes states and is an orthonormal basis for the Hilbert space of four qubits composed entirely of states that are fully entangled under the four-tangle measure.     

Tunable generation of entangled photons in a nonlinear directional coupler

The on‐chip integration of quantum light sources has enabled the realization of complex quantum photonic circuits. However, for the practical implementation of such circuits in quantum information applications, it is crucial to develop sources delivering entangled quantum photon states with on‐demand tunability. Here we propose and experimentally demonstrate the concept of a widely tunable quantum light source based on spontaneous parametric down‐conversion in a simple nonlinear directional coupler. We show that spatial photon‐pair correlations and entanglement can be reconfigured on‐demand by tuning the phase difference between the pump beams and the phase mismatch inside the structure. We experimentally demonstrate the generation of split states, robust N00N states, various intermediate regimes and biphoton steering on a single chip. Furthermore we theoretically investigate other regimes allowing all‐optically tunable generation of all Bell states and flexible control of path‐energy entanglement. Such wide‐range capabilities of a structure comprised of just two coupled nonlinear waveguides are attributed to the intricate interplay between linear coupling and nonlinear phase matching. This scheme provides an important advance towards the realization of reconfigurable quantum circuitry.

     

Low temperature nonequilibrium dynamics in transverse Ising spin glass

The real part of the time-dependent ac susceptibility of the short-range Ising spin glass in a transverse field has been investigated at very low temperatures. We have used the quantum linear response theory and domain coarsening ideas of quantum droplet scaling theory. It is found that after a temperature quench to a temperature T ₁ (lower than the spin glass transition temperature T _g ) the ac susceptibility decreases with time approximately in a logarithmic way as the system tends to the equilibrium. It is shown that the transverse field of tunneling has unessential effect on the nonequilibrium dynamical properties of the magnetic droplet system. The role of quantum fluctuations in the behavior of the ac susceptibility is discussed.Received: 26 February 2004, Published online: 18 June 2004PACS: 75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.) - 75.10.Nr Spin-glass and other random models - 75.50.Lk Spin glasses and other random magnets     

Long-term frequency stability and linewidth properties of continuous-wave pump-resonant optical parametric oscillators

We present a frequency stability and linewidth analysis of two different setups of continuous-wave pump and signal-resonant optical parametric oscillators (pump resonant, singly resonant OPO, PR-SRO). Both designs, a common-cavity and a dual-cavity version, utilize a frequency-stable and narrow-linewidth pump laser and are stabilized without using an external reference. A long-term frequency stability better than ±30 MHz is reached over more than 30 minutes for both designs. The frequency jitter on a one-second time-scale is 56 kHz for the common-cavity PR-SRO and about 10 MHz for the dual-cavity PR-SRO. The short-term linewidths were measured using an external high-finesse cavity and are (9±2)kHz and (6±1)kHz in 20 s, respectively. To our knowledge, these are the lowest values demonstrated so far for a widely continuously wavelength-tunable all-solid-state laser source. PACS 42.65.Yj; 42.72.Ai; 42.62.Fi     

Combined wide pump tuning and high power of a continuous-wave, singly resonant optical parametric oscillator

A new singly resonant, single-frequency optical parametric oscillator (OPO) has been developed for the 2.6–4.7 m infrared wavelength region, using a high power (>20 W), widely tunable (1024–1034 nm) Yb:YAG pump source. With the OPO frequency stabilized with an intracavity etalon, the OPO achieved an idler output power of 3 W at 2.954 m. Tuning of the idler frequency was achieved by longitudinal mode-hop tuning of the pump source (FSR 100 MHz). In this way an idler frequency scan of 100–150 GHz could be obtained, after which the signal frequency hops ahead over the FSR of the intracavity etalon of the OPO (207 GHz). Due to un-optimized mirror coatings for the OPO cavity and PPLN crystal, the frequency stability was limited to 90 MHz over 1 s, with an unaffected long-term frequency stability of 250 MHz over 200 seconds. PACS 42.65.Yj; 82.80.Kq; 42.62.Fi     

Experimental investigation of the spectro-temporal dynamics of the light pulses of Q-switched Nd:YAG lasers and nanosecond optical parametric oscillators

We report an experimental investigation of the spectro-temporal dynamics of the pulse formation in Q-switched Nd:YAG lasers and in nanosecond optical parametric oscillators (OPOs). The temporal evolution of the spectral intensity distribution of the light pulses was measured with a 1-m Czerny–Turner spectrometer in combination with a fast streak camera. This detection system allows the analysis of temporal changes in the spectrum of single nanosecond pulses. The measurements were performed for a flashlamp-pumped, Q-switched Nd:YAG laser and for an unseeded as well as for a seeded singly-resonant nanosecond OPO. The laser output spectrum varies strongly from pulse to pulse and even within a single pulse due to mode beating. In an unseeded OPO, individual spectral modes start to oscillate statistically from the parametric noise for pump powers close to the OPO threshold. With increasing pump power a strong modulation in the spectral formation of the pulse is observed, resulting from a strong interaction of parametric conversion and back conversion of signal and idler radiation into pump radiation. By means of injection seeding, the starting condition was controlled for a single mode. Due to the seed radiation, the seeded mode starts sooner than the unseeded modes. These are suppressed completely in the case of sufficient seed power and moderate pump power. The observations are in good agreement with results of corresponding numerical simulations. PACS 42.65.Sf, 42.65.Yj     

Photon entanglement in the phase-space Q representation of quantum optics

Several examples of photon entanglement are studied in the Q representation of quantum optics. In particular, the entangled states produced in parametric downconversion are studied in detail, and we determine the conditions for the violation of Bell's inequality. Our approach shows that photon entanglement is related to the existence of correlations between the quantum fluctuations of the electromagnetic field associated to different modes. Received 10 August 2002 / Received in final form 7 November 2002 Published online 4 February 2003     

Statistical mechanics of semiflexible polymers

We present the statistical-mechanical theory of semiflexible polymers based on the connection between the Kratky-Porod model and the quantum rigid rotator in an external homogeneous field, and treatment of the latter using the quantum mechanical propagator method. The expressions and relations existing for flexible polymers can be generalized to semiflexible ones, if one replaces the Fourier-Laplace transform of the end-to-end polymer distance, 1/(k ²/3 + p), through the matrix , where D and M are related to the spectrum of the quantum rigid rotator, and considers an appropriate matrix element of the expression under consideration. The present work provides also the framework to study polymers in external fields, and problems including the tangents of semiflexible polymers. We study the structure factor of the polymer, the transversal fluctuations of a free end of the polymer with fixed tangent of another end, and the localization of a semiflexible polymer onto an interface. We obtain the partition function of a semiflexible polymer in half space with Dirichlet boundary condition in terms of the end-to-end distribution function of the free semiflexible polymer, study the behaviour of a semiflexible polymer in the vicinity of a surface, and adsorption onto a surface.Received: 23 March 2004, Published online: 23 July 2004PACS: 36.20.-r Macromolecules and polymer molecules - 61.41. + e Polymers, elastomers, and plastics - 82.35.Gh Polymers on surfaces; adhesion