Showing papers in "Journal of Modern Optics in 2002"

A quantum Rosetta stone for interferometry

Abstract: Heisenberg-limited measurement protocols can be used to gain an increase in measurement precision over classical protocols. Such measurements can be implemented using, for example, optical Mach—Zehnder interferometers and Ramsey spectroscopes. We address the formal equivalence between the Mach—Zehnder interferometer, the Ramsey spectroscope and a generic quantum logic circuit. Based on this equivalence we introduce the 'quantum Rosetta stone', and we describe a projective-measurement scheme for generating the desired correlations between the interferometric input states in order to achieve Heisenberg-limited sensitivity. The Rosetta stone then tells us that the same method should work in atom spectroscopy.

SLOW AND STOPPED LIGHT 'Slow' and 'fast' light in resonator-coupled waveguides

Abstract: We describe a device constructed of a sequence of microresonators coupled to an optical waveguide. The influence of these resonators is to enhance nonlinearities and to induce strong dispersive effects, leading to exotic optical properties including slow and superluminal group velocities of propagation.

The asymmetric lossy near-perfect lens

Abstract: We extend the ideas of the perfect lens recently proposed [J.B. Pendry, Phys. Rev. Lett. 85, 3966 (2000)] to an alternative structure. We show that a slab of a medium with negative refractive index bounded by media of different positive refractive index also amplifies evanescent waves and can act as a near-perfect lens. We examine the role of the surface states in the amplification of the evanescent waves. The image resolution obtained by this asymmetric lens is more robust against the effects of absorption in the lens. In particular, we study the case of a slab of silver, which has a negative dielectric constant, with air on one side and other media such as glass or GaAs on the other side as an ‘asymmetric’ lossy near-perfect lens for p-polarized waves. It is found that retardation has an adverse effect on the imaging due to the positive magnetic permeability of silver, but we conclude that subwavelength image resolution is possible in spite of it.

Reflections upon separability and distillability

Abstract: An abstract formulation is presented of the so-called Innsbruck-Hannover programme that investigates quantum correlations and entanglement in terms of convex sets. A unified description is given of optimal decompositions of quantum states and the optimization of witness operators that detect whether a given state belongs to a given convex set. The abstract formulation is illustrated with several examples, and relations between optimal entanglement witnesses and n -copy non-distillable states with non-positive partial transpose are discussed.

Theory of the radiation pressure on dielectric surfaces

Abstract: The radiation pressures exerted by light beams incident normally on dielectric surfaces are calculated by evaluation of the quantum-mechanical Lorentz force. The free-space surface of an almost transparent dielectric and the interface of a lossy dielectric with a transparent medium are treated in detail. Light beams in the forms of a short single-photon pulse and a continuous-mode narrow-band coherent excitation are considered. The use of a pulse excitation enables discrimination of the surface and bulk contributions to the force. It is shown that the surface force is directed inwards to the dielectric for entrance and exit of the pulse from and to free space, contrary to the conclusions of some earlier work. For the interface of lossy and transparent dielectrics, it is shown that the high-reflectivity mirror modelled by an appropriate limit of the lossy dielectric experiences a force enhanced by the refractive index of the transparent medium, in agreement with experiments on a mirror suspended in liquid ...

Cold trapped ions as quantum information processors

Abstract: In this tutorial we review the physical implementation of quantum computing using a system of cold trapped ions. We discuss systematically all the aspects for making the implementation possible. Firstly, we go through the loading and confining of atomic ions in the linear Paul trap, then we describe the collective vibrational motion of trapped ions. Further, we discuss interactions of the ions with a laser beam. We treat the interactions in the travelling-wave and standing-wave configuration for dipole and quadrupole transitions. We review different types of laser cooling techniques associated with trapped ions. We address Doppler cooling, sideband cooling in and beyond the Lamb-Dicke limit, sympathetic cooling and laser cooling using electromagnetically induced transparency. After that we discuss the problem of state detection using the electron shelving method. Then quantum gates are described. We introduce single-qubit rotations, two-qubit controlled-NOT and multi-qubit controlled-NOT gates. We also co...

The one-way quantum computer - a non-network model of quantum computation

Abstract: A one-way quantum computer (QC C ) works by performing a sequence of one-qubit measurements on a particular entangled multi-qubit state, the cluster state. No non-local operations are required in the process of computation. Any quantum logic network can be simulated on the QC C . On the other hand, the network model of quantum computation cannot explain all ways of processing quantum information possible with the QC C . In this paper, two examples of the non-network character of the QC C are given. First, circuits in the Clifford group can be performed in a single time step. Second, the QC C -realization of a particular circuit—the bit-reversal gate—has no network interpretation.

Quantum interference in optical fields and atomic radiation

Abstract: We discuss the connection between quantum interference effects in optical beams and radiation fields emitted from atomic systems. We illustrate this connection by a study of the first- and second-order correlation functions of optical fields and atomic dipole moments. We explore the role of correlations between the emitting systems and present examples of practical methods to implement two systems with non-orthogonal dipole moments. We also derive general conditions for quantum interference in a two-atom system and for a control of spontaneous emission. The relation between population trapping and dark states is also discussed. Moreover, we present quantum dressed-atom models of cancellation of spontaneous emission, amplification on dark transitions, fluorescence quenching and coherent population trapping.

Entanglement transfer from continuous variables to qubits

Abstract: We show that two qubits can be entangled by local interactions with an entangled two-mode continuous variable state This is illustrated by the evolution of two two-level atoms interacting with a two-mode squeezed state Two modes of the squeezed field are injected respectively into two spatially separate cavities and the atoms are then sent into the cavities to interact resonantly with the cavity field We find that the atoms may be entangled even by a two-mode squeezed state which has been decohered while penetrating into the cavity

The a, b, cs of oceanographic lidar predictions: a significant step toward closing the loop between theory and experiment

Abstract: Small-angle phase functions (0.05° < θ < 20°) have been measured for turbid samples that were then used in a Monte-Carlo theoretical light-scattering model. The measured phase function yields excellent agreement between model predictions and long-path tank measurements.

Linear and nonlinear optical properties of plasma-enhanced chemical-vapour deposition grown silicon nanocrystals

Abstract: We provide a systematic study on the linear and nonlinear optical properties of silicon nanocrystals (Si-nc) grown by plasma-enhanced chemical vapour deposition (PECVD). Linear optical properties, namely absorption, emission and refractive indices are reported. The sign and magnitude of both real and imaginary parts of third-order nonlinear susceptibility X(3) of Si-nc are measured by the Z-scan method. Closed aperture Z-scan reveals a positive nonlinearity for all the samples. From the open aperture measurements, nonlinear absorption coefficients are evaluated and attributed to two-photon absorption. Absolute values of X(3) are in the order of 10-9 esu and show systematic correlation with the Si-nc size, due to quantum confinement related effects. A correlation has been made between X(3), nanocrystalline size, linear refractive index and optical band gap.

PHOTONIC MATERIALS AND DEVICES Progress in relating rare-earth ion 4f and 5d energy levels to host bands in optical materials for hole burning, quantum information and phosphors

Abstract: Rare-earth ions play an important role in modern technology as optically active elements in solid-state luminescent materials. In many of these materials, interactions between the electronic band states of the host crystal and the rare-earth ion's localized 4f N and 4f N−1 5d states influence the material's optical properties. The importance of these interactions is discussed for material applications in photon-gated hole burning, quantum information and phosphors. Material dependent trends in the relative binding energies of the 4f N states and the host bands have been observed and are summarized. An empirical model for the ion dependence of the 4f electron binding energies is formulated in terms of atomic number and compared with previous models. These models are extended to describe the 4f N−1 5d states with one additional parameter. Improved estimates for the free-ion ionization potentials used in the model are also presented and discussed.

A transfer matrix approach to local field calculations in multiple scattering problems

Abstract: We present a new recursive transfer matrix method for calculating local electromagnetic fields in systems of spheres subject to strong dependent scattering. Local field information is often lost or discarded in recursive transfer matrix approaches. In order to preserve the local field information, and to avoid problems associated with the dimensional cut-off of the translation matrices, we calculate the scatterer-centred transfer matrices. Our technique permits systematic studies of local field eiects for all possible incident field directions, and configurations (including orientation averages). Illustrative calculations are presented.

Temperature dependence of the Brillouin linewidth in water

Abstract: In the frequency spectrum of light that is scattered in liquid water there is a central elastically scattered peak that is due mainly to scattering by suspended particles; and, there is a peak on each side of the central peak that is displaced by the Brillouin frequency shift. The Brillouin shift is a direct measure of sound speed. The linewidth of the Brillouin shifted lines is dependent on the bulk and shear viscosity of water as well as its density, thermal conductivity, and specific heat. The linewidth of the Brillouin peaks has been investigated in laboratory experiments over a range 1°C to 35°C. The frequency spectrum of back-scattered laser light was analysed using a scanning Fabry-Perot etalon. A strong dependence of the linewidth of the Brillouin shifted lines on temperature was found. In particular, for low temperatures in the range 10°C down to 1°C the linewidth shows an increase from 750 MHz to 1.4 GHz.

On the geometry of entangled states

Abstract: The basic question that is addressed in this paper is finding the closest separable state for a given entangled state, measured with the Hilbert-Schmidt distance While this problem is in general very difficult, we show that the following strongly related problem can be solved: find the Hilbert-Schmidt distance of an entangled state to the set of all partially transposed states We prove that this latter distance can be expressed as a function of the negative eigenvalues of the partial transpose of the entangled state, and show how it is related to the distance of a state to the set of positive, partially transposed (PPT) states We illustrate this by calculating the closest biseparable state to the W state and give a simple and very general proof for the fact that the set of W-type states is not of measure zero Next we show that all surfaces with states whose partial transposes have constant minimal negative eigenvalue are similar to the boundary of PPT states We illustrate this with some examples on bipartite qubit states, where contours of constant negativity are plotted on two-dimensional intersections of the complete state space

Generation of pulsed polarization-entangled two-photon state via temporal and spectral engineering

Abstract: The quantum state of the photon pair generated from type-II spontaneous parametric downconversion pumped by an ultrafast laser pulse exhibits strong decoherence in its polarization entanglement, an effect which can be attributed to the clock effect of the pump pulse or, equivalently, to distinguishing spectral information in the two-photon state. Here, we propose novel temporal and spectral engineering techniques to eliminate these detrimental decoherence effects. The temporal engineering of the two-photon wavefunction results in a universal Bell-state synthesizer that is independent of the choice of pump source, crystal parameters, wavelengths of the interacting photons and the bandwidth of the spectral filter. In the spectral engineering technique, the distinguishing spectral features of the two-photon state are eliminated through modifications to the two-photon source. In addition, spectral engineering also provides a means for the generation of polarization-entangled states with novel spectral charact...

Concurrence in arbitrary dimensions

Abstract: We argue that a complete characterization of quantum correlations in bipartite systems of many dimensions may require a quantity which, even for pure states, does not reduce to a single number. Subsequently, we introduce multidimensional generalizations of concurrence and find evidence that they may provide useful tools for the analysis of quantum correlations in mixed bipartite states. We also introduce biconcurrence that leads to a necessary and sufficient condition for separability.

Quantum nonlocality for a mixed entangled coherent state

Abstract: Quantum nonlocality is tested for an entangled coherent state, interacting with a dissipative environment. A pure entangled coherent state violates Bell's inequality regardless of its coherent amplitude. The higher the initial nonlocality, the more rapidly quantum nonlocality is lost. The entangled coherent state can also be investigated in the framework of 2 X 2 Hilbert space. The quantum nonlocality persists longer in 2 X 2 Hilbert space. When it decoheres it is found that the entangled coherent state fails the nonlocality test, which contrasts with the fact that the decohered entangled state is always entangled.

Transparency and parametric generation in a four-level system † Reviewing of this paper was handled by a member of the Editorial Board.

Abstract: We analyse two distinct nonlinear phenomena in a four-level quantum system having a tripod configuration. First, we show that the system can become transparent at two different frequencies of a probe laser field. The potential for parametric generation in this medium is also investigated. We show that, if the system is prepared in a coherent superposition of two or three of the lower levels, one or two new fields can be generated.

Saturable bragg reflector-based continuous-wave mode locking of yb : kgd(wo4)(2) laser

Abstract: We demonstrate the saturable Bragg reflector (SBR)-based stable self-starting continuous-wave mode locking of a Yb:KGd(WO4)2 laser. With a double quantum well SBR structure the shortest pulses observed had durations of 169 fs, average powers of 18 mW and an oscillating wavelength centred on 1028 nm.

Fabrication of photonic nanostructures in nonlinear optical polymers

Abstract: Two experimental techniques have been developed for creating photonic structures in nonlinear optical (NLO) polymers with precisions down to nanoscale. The first technique uses nanoimprinting technology to directly pattern the guest-host NLO polymers. It can be applied to the fabrication of photonic bandgap structures in NLO materials, as well as many other photonic structures in both linear and nonlinear polymers. The second technique utilizes self-assembly of NLO polymer monolayers onto a nanostructured template. This approach provides a highly effective means to implement practical waveguide devices using high performance self-assembled polymers with large electro-optic activity and inherent long-term stability. An optical switching device is proposed, based on nanofabricated NLO polymers.

Iterative Fourier transform algorithm: comparison of various approaches

Abstract: In this paper, a convergence of various approaches within the iterative Fourier transform algorithm (IFTA) is analysed for the case of phase-only diffractive optical elements with quantized phase levels (either binary or multilevel structures). Firstly, the general scheme of the IFTA iterative approach with partial quantization is briefly presented and discussed. Then, the special assortment of the general IFTA scheme is given with respect to quantization constraint strategies. Based on such a special classification, the three practically interesting approaches are chosen, further analysed and compared with each other. The performance of these algorithms is compared in detail in terms of the development of the signal-to-noise ratio characteristic with respect to the number of iterations, for various input diffusive-type objects chosen. Also, the performance of the complex spectra developments is documented for typical computer reconstruction results. The advantages and drawbacks of all approaches are disc...

Cloning a qutrit

Abstract: Several classes of state-dependent quantum cloners for three-level systems are investigated. These cloners optimally duplicate some of the four maximally-conjugate bases with an equal fidelity, thereby extending the phasecovariant qubit cloner to qutrits. Three distinct classes of qutrit cloners can be distinguished, depending on whether two, three, or four maximally-conjugate bases are cloned as well (the latter case simply corresponds to the universal qutrit cloner). These results apply to symmetric as well as asymmetric cloners, so that the balance between the fidelity of the two clones can also be analysed.

Soliton self-frequency shift effects in photonic crystal fibre

Abstract: Using a femtosecond Ti:sapphire laser operating at a wavelength of 810nm we have demonstrated infrared generation in photonic crystal fibre at distinct wavelengths which can be attributed to the soliton self-frequency shift effect. The maximum observed shift produced spectra centred at 1260nm and the frequency-shifted light accounted for up to 80% of the fibre output power. We show that the shifts can be explained by the dispersion properties of the fundamental and higher-order waveguide modes of the fibre.

Quantum theory of preparation and measurement

Abstract: The conventional postulate for the probabilistic interpretation of quantum mechanics is asymmetric in preparation and measurement, making retrodiction reliant on inference by use of Bayes' theorem. Here, a more fundamental symmetric postulate is presented, from which both predictive and retrodictive probabilities emerge immediately, even where measurement devices more general than those usually considered are involved. It is shown that the new postulate is perfectly consistent with the conventional postulate.

Atoms in strong laser fields: challenges in relativistic quantum mechanics

Abstract: We address the challenges raised by the question of how to produce quantitative data in order to reproduce and to interpret the results of recent and future experiments performed with ultra-intense laser pulses. The main challenges lie in the problem of implementing reliable numerical codes for describing quantum processes experienced by electrons brought in the relativistic regime in the presence of the field.

Acousto-optic displacement-measuring interferometer: a new heterodyne interferometer with Ångstrom-level periodic error

Abstract: A concept for a new displacement-measuring interferometer has been developed. The motivation behind this work is minimization of the periodic error caused by unwanted leakage between the two frequencies in commercially available heterodyne systems. Typically, the two frequencies are carried on a single beam and separated by polarization-dependent optics. Imperfect optics, non-ideal laser heads and mechanical misalignment may cause each frequency to be leaked into both the reference and the measurement paths, ideally consisting of a single light frequency. The new polarization-independent interferometer described here uses an acousto-optic modulator as the beam splitter and eliminates the frequency leakage by spatially separating the two light beams. Bench-top experiments have shown a reduction in periodic error to 0.18 nm (1.8 A). The device description, as well as suggestions for simple architecture implementations, are presented.

Quantum fast Fourier transform using multilevel atoms

Abstract: We propose an implementation of the quantum fast Fourier transform algorithm in an entangled system of multilevel atoms. The Fourier transform occurs naturally in the unitary time evolution of energy eigenstates and is used to define an alternative wave-packet basis for quantum information in the atom. A change of basis from energy levels to wave packets amounts to a discrete quantum Fourier transform within each atom. The algorithm then reduces to a series of conditional phase transforms between two entangled atoms in mixed energy and wave-packet bases. We show how to implement such transforms using wave-packet control of the internal states of the ions in the linear ion-trap scheme for quantum computing.

Mechanisms for laser control of chemical reactions

Abstract: During the past several years, a new technique for realistic simulations of the interaction of light with matter has been developed and employed. Recent simulations of laser pulses interacting with molecules clearly demonstrate the potential for control of chemical reactions through various mechanisms, which include the following: (i) excitation of electrons to states that have different bonding properties; (ii) control of electron populations through a coherent pump-pulse, control-pulse sequence; and (iii) control of molecular vibrations through a pump-control sequence. Significant chemical insights are gained when one can watch a realistic animation of species interacting and reacting. One can monitor the time evolution of electronic states and their occupancy, as well as the motion of the atoms. One can also observe the evolution from reactants to products through transition states. Finally, one can determine how this evolution is affected by the various properties of the laser pulses, including intens...

Demonstration of a radio-frequency spectrum analyser based on spectral hole burning

Abstract: We demonstrate application of spectral hole burning to the spectral analysis of broad-band rf signals. In quite the same way as an acousto-optic spectrometer, the device operates on an optically carried rf signal and achieves angular separation of the signal spectral components. An instantaneous bandwidth of 2.5 GHz has been achieved, with a power dynamic range of 35 dB, limited by the detector. Extension to greater than 10 GHz instantaneous bandwidth with greater than 1000 channels is consistent with the active material capabilities.