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The unitary description of beam splitters (BSs) and optical parametric amplifiers (OPAs) in terms of the dynamical Lie groups $SU(2)$ and $SU(1,1)$ has a long history. Recently, an inherent duality has been proposed that relates the unitaries of both optical devices. At the physical level, this duality relates the linear nature of a lossless BS to the nonlinear Parametric Down-Conversion (PDC) process exhibited by an OPA. Here, we argue that the duality between BS and PDC can instead be naturally interpreted by analyzing the geometrical properties of both Lie groups, an approach that explicitly connects the dynamical group description of the optical devices with the aforementioned duality. Furthermore, we show that the BS-PDC duality can be represented through tensor network diagrams, enabling the implementation of a PDC as a circuit on a standard quantum computing platform. Thus, it is feasible to simulate nonlinear processes by using single-qubit unitaries that can be implemented on currently available digital quantum processors.

Employing currently available quantum technology, we design and implement a non-classically correlated SWAP heat engine that allows to achieve an efficiency above the standard Carnot limit. Such an engine also boosts the amount of extractable work, in a wider parameter window, with respect to engine's cycle in the absence of initial quantum correlations in the working substance. The boosted efficiency arises from a trade-off between the entropy production and the consumption of quantum correlations during the full thermodynamic cycle. We derive a generalized second-law limit for the correlated cycle and implement a proof-of-principle demonstration of the engine efficiency enhancement by effectively tailoring the thermal engine on a cloud quantum processor.

The so-called contemporary AI revolution has reached every corner of the social, human and natural sciences -- physics included. In the context of quantum many-body physics, its intersection with machine learning has configured a high-impact interdisciplinary field of study; with the arise of recent seminal contributions that have derived in a large number of publications. One particular research line of such field of study is the so-called Neural-Network Quantum States, a powerful variational computational methodology for the solution of quantum many-body systems that has proven to compete with well-established, traditional formalisms. Here, a systematic review of literature regarding Neural-Network Quantum States is presented., Comment: 9 pages, 4 figures

Solar cells based on organic compounds are a proven emergent alternative to conventional electrical energy generation. Here, we provide a computational study of power conversion efficiency optimization of BODIPY-derivatives by means of their associated open circuit voltage, short-circuit density, and fill factor. In so doing, we compute for the derivatives' geometrical structures, energy levels of frontier molecular orbitals, absorption spectra, light collection efficiencies, and exciton binding energies, via density functional theory (DFT) and time dependent (TD)--DFT calculations. We fully-characterize four D--$\pi$--A (BODIPY) molecular systems of high efficiency and improved $J_{sc}$ that are well suited for integration into bulk heterojunction (BHJ) organic solar cells as electron-donor materials in the active layer. Our results are two-fold: We found that molecular complexes with an structural isoxazoline ring exhibit a higher power conversion efficiency (PCE), a useful result for improving the BHJ current, and, on the other hand, by considering the molecular systems as electron-acceptor materials, with P3HT as the electron-donor in the active layer, we found a high PCE compound favorability with a pyrrolidine ring in its structure, in contrast to the molecular systems built with an isoxazoline ring. The theoretical characterization of the electronic properties of the BODIPY-derivatives here provided, computed with a combination of ab-initio methods and quantum models, can be readily applied to other sets of molecular complexes in order to hierarchize optimal power conversion efficiency., Comment: 17 pages, 8 figures

Molecular systems containing donor-bridge-acceptor sites or molecular antennas constitute promising candidates for organic photovoltaic device implementation. Photo-induced electron transfer in multi-chromophore molecular systems is defined by a subtle interaction between the donor and the molecular bridge, and by the system-solvent coupling. Here, we address the computation of quantum properties such as population inversion and electron transfer in molecular photo-systems composed of fulleroisoxazoline, fulleropyrrolidine, BODIPY and Zn-porphyrin, as well as their system-solvent ultrafast dynamics. The molecular complexes are modelled as two- and three-site systems, and we use the density functional theory (DFT) for obtaining the site energies required in the construction of the open system diabatic Hamiltonians relevant to the computation of the electron transfer. The site energies and electronic couplings are calculated by using a continuous polarizable model that allow for the analysis of different solvent environments, and the site-to-site couplings are computed by means of the generalized Mulliken Hush method at the DFT level. We find that the stabilization energy of the charge transfer states exhibit a significant variation for a compound embedded in different polar environments, and thus the effect due to the solvent has been analyzed for the specific cases of Methanol, THF and Toluene. We show that the incorporation of a molecular bridge generates the creation of an intermediate state that plays a crucial role in the charge transfer process by defining $\Lambda$ or cascaded-type energy schemes; this affects the asymptotic value of the transfer rate and favors the cascaded-type configuration., Comment: 14 pages, 12 figures

We investigate the behaviour of quantum CHSH-nonlocality, $\rm F_3$-steering, and usefulness for teleportation in an interacting two-qubit dissipative system. We show regimes where these three quantum correlations can be extracted by means of local filtering operations, despite them not being displayed in the bare natural time evolution. Moreover, we show the existence of local hidden state (LHS) and local hidden variable (LHV) models for some states during the dynamics and thus, showing that apparently-useless physical systems could still exhibit quantum correlations, which are hidden from us, but that can still be revealed by means of local filtering operations and therefore, displaying the phenomenon of \emph{hidden} quantum correlations. We furthermore show that there actually exists a trade-off between the amount of quantum correlations which can be extracted and the filtering probability with which such protocol can be implemented. From a theoretical point of view, the existence of such trade-off imposes a fundamental limit to the extraction of quantum correlations by local filtering operations. From a practical point of view on the other hand, the results here presented determine the amount of resources that should be invested in order to extract such maximal hidden quantum correlations., Comment: 15 pages; 5 Figures; Discussion on the trade-off between hidden correlations and their probability of occurrence has been included and improved

A question of global concern regarding the sustainable future of humankind stems from the effect due to aerosols on the global climate. The quantification of atmospheric aerosols and their relationship to climatic impacts are key to understanding the dynamics of climate forcing and to improve our knowledge about climate change. Due to its response to precipitation, temperature, topography and human activity, one of the most dynamical atmospheric regions is the atmospheric boundary layer (ABL): ABL aerosols have a sizable impact on the evolution of the radiative forcing of climate change, human health, food security, and, ultimately, on the local and global economy. The identification of ABL pattern behaviour requires constant monitoring and the application of instrumental and computational methods for its detection and analysis. Here, we show a new method for the retrieval of ABL top arising from light detection and ranging (LiDAR) signals, by training a convolutional neural network in a supervised manner; forcing it to learn how to retrieve such a dynamical parameter on real, non-ideal conditions and in a fully automated, unsupervised way. Our findings pave the way for a full integration of LiDAR elastic, inelastic, and depolarisation signal processing, and provide a novel approach for real-time quantitative sensing of aerosols., Comment: 8 pages, 6 figures

Emergent technologies that make use of novel materials and quantum properties of light states are at the forefront in the race for the physical implementation, encoding and transmission of information. Photonic crystals (PCs) enter this paradigm with optical materials that allow the control of light propagation and can be used for optical communication, and photonics and electronics integration making use of materials ranging from semiconductors, to metals, metamaterials, and topological insulators, to mention but a few. In particular, here we show how designer superconductor materials integrated into PCs fabrication allow for an extraordinary reduction of electromagnetic waves damping and possibilitate their optimal propagation and tuning through the structure, below critical superconductor temperature. We experimentally demonstrate, for the first time, a successful integration of ferroelectric and superconductor materials into a one-dimensional (1D) PC composed of (BTO/YBCO)N/STO bilayers that work in the whole visible spectrum, and below (and above) critical superconductor temperature (measured in the 10-300 K range). Theoretical calculations support, for different number of bilayers N, the effectiveness of the produced 1D PCs and pave the way for novel optoelectronics integration and information processing in the visible spectrum at low temperature, while preserving their electric and optical properties., Comment: 12 pages, 5 figures

Understanding electron transfer in organic molecules is of great interest in quantum materials for light harvesting, energy conversion, and integration of molecules into solar cells. This, however, poses the challenge of designing specific optimal molecular structure for which the processes of ultrafast quantum coherence and electron transport are not so well understood. In this work, we investigate subpicosecond time scale quantum dynamics and electron transfer in an efficient electron acceptor Rhodanine chromophoric complex. We consider an open quantum system approach to model the complex-solvent interaction and compute the crossover from weak to strong dissipation on the reduced system dynamics for both a polar (Methanol) and a non-polar solvent (Toluene). We show that the electron transfer rates are enhanced in the strong chromophore-solvent coupling regime, being the highest transfer rates those found at room temperature. Even though the computed dynamics are highly non-Markovian, and they may exhibit a quantum character up to hundreds of femtoseconds, we show that quantum coherence does not necessarily optimize the electron transfer in the chromophore., Comment: 10 pages, 6 figures

The monitoring of the impact of aerosols in Latin America on a local scale is usually limited due to the infrastructure and instrumentation available. In Colombia, there are two international ground surface monitoring networks, the AErosol RObotic NETwork (AERONET) and the Latin American LIDAR NETwork (LALINET). However, the AERONET performance relies on only one sun photometer which makes measurements distributed among five ground-based stations in different cities such as Bogot\'a and Medell\'in. On the other hand, LALINET has only one ground-based station formed by an elastic LiDAR system located at Medell\'in. Although Cali is the largest city of Colombian southwestern, with an accelerated grown rate of both urban and vehicular fleet, and counts with the third largest population of this country, is not reached by these networks. Here, we report on the implementation of a monostatic-coaxial multispectral LiDAR system using a pulsed Nd:YAG laser with 450 mJ of average energy at 1064 nm. To perform the atmospheric measurements, this system is capable of spatially resolving elastic backscatter down to 3.75 m with a Pulse Repetition Frequency (PRF) of 10 Hz. We have developed a hybrid algorithm for data analysis by combining the Fitting and Gradient method and the Klett-Fernald algorithm to estimate the Planetary Boundary Layer (PBL) Top and the optical properties of aerosols. This work constitutes the first quantitative atmospheric exploration to study the aerosols dynamics and the PBL in the northwest of South America.

Arrays of covalently bound organic molecules possess potential for light-harvesting and energy transfer applications due to the strong coherent dipole-dipole coupling between the transition dipole moments of the molecules involved. Here, we show that such molecular systems, based on perylene-molecules, can be considered as arrays of qubits that are amenable for laser-driven quantum coherent control. The perylene monomers exhibit dephasing times longer than four orders of magnitude a typical gating time, thus allowing for the execution of a large number of gate operations on the sub-picosecond timescale. Specifically, we demonstrate quantum logic gates and entanglement in bipartite (dimer) and tripartite (trimer) systems of perylene-based arrays. In dimers, naturally entangled states with a tailored degree of entanglement can be produced. The nonlocality of the molecular trimer entanglement is demonstrated by testing Mermin's (Bell-like) inequality violation., Comment: 14 pages, 8 figures, comments are welcome

Push-Pull functional compounds consisting of dicyanorhodanine derivatives have attracted a lot of interest because their optical, electronic, and charge transport properties make them useful as building blocks for organic photovoltaic implementations. The analysis of the frontier molecular orbitals shows that the vertical transitions of electronic absorption are characterized as intramolecular charge transfer; furthermore, we show that the analyzed compounds exhibit bathochromic displacements when comparing the presence (or absence) of solvent as an interacting medium. In comparison with materials defined by their energy of reorganization of electrons (holes) as electron (hole) transporters, we find a transport hierarchy whereby the molecule (Z)-2-((1,1-Dicyanomethylene)-5-(4-dimethylamino)benzylidene)-1,3-thiazol-4 is better at transporting holes than molecule (Z)-2-((1,1-Dicyanomethylene)-5-(tetrathiafulvalene-2-ylidene)-1,3-thiazol-4., Comment: 24 pages, 6 figures

We report on some quantum properties of physical systems, namely, entanglement, nonlocality, $k$-copy nonlocality (superactivation of nonlocality), hidden nonlocality (activation of nonlocality through local filtering) and the activation of nonlocality through tensoring and local filtering. The aim of this work is two-fold. First, we provide a review of the numerical procedures that must be followed in order to calculate the aforementioned properties, in particular, for any two-qubit system, and reproduce the bounds for two-qudit Werner states. Second, we use such numerical tools to calculate new bounds of these properties for two-qudit Isotropic states and two-qubit Hirsch states., Comment: 23 pages, 7 figures

Game theory is a well established branch of mathematics whose formalism has a vast range of applications from the social sciences, biology, to economics. Motivated by quantum information science, there has been a leap in the formulation of novel game strategies that lead to new (quantum Nash) equilibrium points whereby players in some classical games are always outperformed if sharing and processing joint information ruled by the laws of quantum physics is allowed. We show that, for a bipartite non zero-sum game, input local quantum correlations, and separable states in particular, suffice to achieve an advantage over any strategy that uses classical resources, thus dispensing with quantum nonlocality, entanglement, or even discord between the players' input states. This highlights the remarkable key role played by pure quantum coherence at powering some protocols. Finally, we propose an experiment that uses separable states and basic photon interferometry to demonstrate the locally-correlated quantum advantage., Comment: 10 pages, 7 figures

The Pusey-Barrett-Rudolph (PBR) theorem deals with the realism of the quantum states. It establishes that every pure quantum state is real, in the context of quantum ontological models. Specifically, by guaranteeing the property of not-Post-Peierls ($\neg$PP) compatibility (or antidistinguishability) for a particular set of states $P$, together with the ad hoc postulate known as Preparation Independence Postulate (PIP), the theorem establishes that these two properties imply the $\psi$-onticity (realism) of the set of all pure states. This PBR result has triggered two particular lines of research: On the one hand, it has been possible to derive similar results without the use of the PIP, although at the expense of implying weaker properties than $\psi$-onticity. On the other hand, it has also been proven that the property of $\neg$PP compatibility alone is an explicit witness of usefulness for the task known as conclusive exclusion of states. In this work, we explore the $\neg$PP compatibility of the set of states $P$, when $P$ is under the interaction of some noisy channels, which would consequently let us identify some noisy scenarios where it is still possible to perform the task of conclusive exclusion of states. Specifically, we consider the set $P$ of $n$-qubit states in interaction with an environment by means of i) individual and ii) collective couplings. In both cases, we analytically show that the phenomenon of achieving $\neg$PP compatibility, although reduced, it is still present. Searching for an optimisation of this phenomenon, we report numerical experiments up to $n=4$ qubits., Comment: 19 pages, 5 figures

Most works on open quantum systems generally focus on the reduced physical system by tracing out the environment degrees of freedom. Here we show that the qubit distributions with the environment are essential for a thorough analysis, and demonstrate that the way that quantum correlations are distributed in a quantum register is constrained by the way in which each subsystem gets correlated with the environment. For a two-qubit system coupled to a common dissipative environment $\mathcal{E}$, we show how to optimise interqubit correlations and entanglement via a quantification of the qubit-environment information flow, in a process that, perhaps surprisingly, does not rely on the knowledge of the state of the environment. To illustrate our findings, we consider an optically-driven bipartite interacting qubit $AB$ system under the action of $\mathcal{E}$. By tailoring the light-matter interaction, a relationship between the qubits early stage disentanglement and the qubit-environment entanglement distribution is found. We also show that, under suitable initial conditions, the qubits energy asymmetry allows the identification of physical scenarios whereby qubit-qubit entanglement minima coincide with the extrema of the $A\mathcal{E}$ and $B\mathcal{E}$ entanglement oscillations., Comment: 4 figures, 9 pages

We report on quantum, classical, and total correlations in a set of distant quantum emitters coupled via their interaction with the plasmon modes of a one-dimensional waveguide driven by an external laser field. The coupling of the emitters with the plasmonic modes and its influence on the collective decay rate suggest that entanglement does not play a significant role in the qubit dynamics. Rather, discord is the quantity that matters and should be harnessed as a resource., Comment: Published version is in double column and has 7 pp, 3 figures

We address the problem of optically controlling and quantifying the dissipative dynamics of quantum and classical correlations in a set-up of individual quantum emitters under external laser excitation. We show that both types of correlations, the former measured by the quantum discord, are present in the system's evolution even though the emitters may exhibit an early stage disentanglement. In the absence of external laser pumping,we demonstrate analytically, for a set of suitable initial states, that there is an entropy bound for which quantum discord and entanglement of the emitters are always greater than classical correlations, thus disproving an early conjecture that classical correlations are greater than quantum correlations. Furthermore, we show that quantum correlations can also be greater than classical correlations when the system is driven by a laser field. For scenarios where the emitters' quantum correlations are below their classical counterparts, an optimization of the evolution of the quantum correlations can be carried out by appropriately tailoring the amplitude of the laser field and the emitters' dipole-dipole interaction. We stress the importance of using the entanglement of formation, rather than the concurrence, as the entanglement measure, since the latter can grow beyond the total correlations and thus give incorrect results on the actual system's degree of entanglement., Comment: 11 pages, 10 figures, this version contains minor modifications; to appear in Phys. Rev. A

We report on the dissipative dynamics of an entangled, bipartite interacting system. We show how to induce and control the so-called early stage disentanglement (and the delayed entanglement generation) dynamics by means of a driving laser field. We demonstrate that some of the features currently associated with pure non-Markovian effects in such entanglement behavior can actually take place in Markovian environments if background noise QED fluctuations are considered. We illustrate this for the case of a dimer interacting molecular system for which emission rates, interaction strength, and radiative corrections have been previously measured. We also show that even in the absence of collective decay mechanisms and qubit-qubit interactions, the entanglement still exhibits collapse-revival behavior. Our results indicate that zero point energy fluctuations should be taken into account when formulating precise entanglement dynamics statements., Comment: 9 pages, 10 figures

We study the entanglement dynamics in the externally-driven single-mode Dicke model in the thermodynamic limit, when the field is in resonance with the atoms. We compute the correlations in the atoms-field ground state by means of the density operator that represents the pure state of the universe and the reduced density operator for the atoms, which results from taking the partial trace over the field coordinates. As a measure of bipartite entanglement, we calculate the linear entropy, from which we analyze the entanglement dynamics. In particular, we found a strong relation between the stability of the dynamical parameters and the reported entanglement., Comment: Contribution to the SLAFES XIX. This version to appear in J. Phys.: Conference Series

We propose a molecular setup for the physical implementation of a barrier global quantum computation scheme based on the electron-doped pi-conjugated copolymer architecture of nine blocks PPP-PDA-PPP-PA-(CCH-acene)-PA-PPP-PDA-PPP (where each block is an oligomer). The physical carriers of information are electrons coupled through the Coulomb interaction, and the building block of the computing architecture is composed by three adjacent qubit systems in a quasi-linear arrangement, each of them allowing qubit storage, but with the central qubit exhibiting a third accessible state of electronic energy far away from that of the qubits' transition energy. The third state is reached from one of the computational states by means of an on-resonance coherent laser field, and acts as a barrier mechanism for the direct control of qubit entanglement. Initial estimations of the spontaneous emission decay rates associated to the energy level structure allow us to compute a damping rate of order 10^{-7} s, which suggest a not so strong coupling to the environment. Our results offer an all-optical, scalable, proposal for global quantum computing based on semiconducting pi-conjugated polymers., Comment: To appear in J. Phys.: Conf. Series (2009)

We perform a theoretical study of composite superconducting qubit systems for the case of a coupled qubit configuration based on a hybrid qubit circuit made of both charge and phase qubits, which are coupled via a sigma(x)xsigma(z) interaction. We compute the system's eigen-energies in terms of the qubit transition frequencies and the strength of the inter-qubit coupling, and describe the sensitivity of the energy crossing/anti-crossing features to such coupling. We compute the hybrid system's dissipative dynamics for the cases of i) collective and ii) independent decoherence, whereby the system interacts with one common and two different baths of harmonic oscillators, respectively. The calculations have been performed within the Bloch-Redfield formalism and we report the solutions for the populations and the coherences of the system's reduced density matrix. The dephasing and relaxation rates are explicitly calculated as a function of the heat bath temperature., Comment: To appear in J. Phys.: Conf. Series (2009)

We show that quantum coherence of biomolecular excitons is maintained over exceedingly long times due to the constructive role of their non-Markovian protein-solvent environment. Using a numerically exact approach, we demonstrate that a slow quantum bath helps to sustain quantum entanglement of two pairs of Forster coupled excitons, in contrast to a Markovian environment. We consider the crossover from a fast to a slow bath and from weak to strong dissipation and show that a slow bath can generate robust entanglement. This persists to surprisingly high temperatures, even higher than the excitonic gap and is absent for a Markovian bath., Comment: online-published version, minor modifications

We present a general strategy that allows a more flexible method for the construction of fully additive multipartite entanglement monotones than the ones so far reported in the literature of axiomatic entanglement measures. Within this framework we give a proof of a conjecture of outstanding implications in information theory: the full additivity of the Entanglement of Formation., Comment: Paper withdrawn. The result regarding the additivity of EoF was not as general as initially considered. Apologies to our colleagues

We generalize the strategy presented in Refs. [1, 2], and propose general conditions for a measure of total correlations to be an entanglement monotone using its pure (and mixed) convex-roof extension. In so doing, we derive crucial theorems and propose a concrete candidate for a total correlations measure which is a fully additive entanglement monotone., Comment: 8 pages, 3 figures. Title changed, new results

Based on the idea of measuring the factorizability of a given density matrix, we propose a pairwise analysis strategy for quantifying and understanding multipartite entanglement. The methodology proves very effective as it immediately guarantees, in addition to the usual entanglement properties, additivity and strong super additivity. We give a specific set of quantities that fulfill the protocol and which, according to our numerical calculations, make the entanglement measure an LOCC non-increasing function. The strategy allows a redefinition of the structural concept of global entanglement., Comment: 4 pages, new version contains corrected typos. Title changed, proofs added, new results reported. Additivity revisited

Within the framework of constructions for quantifying entanglement, we build a natural scenario for the assembly of multipartite entanglement measures based on Hopf bundle-like mappings obtained through Clifford algebra representations. Then, given the non-factorizability of an arbitrary two-qubit density matrix, we give an alternate quantity that allows the construction of two types of entanglement measures based on their arithmetical and geometrical averages over all pairs of qubits in a register of size N, and thus fully characterize its degree and type of entanglement. We find that such an arithmetical average is both additive and strongly super additive., Comment: This is the updated, finally published, version

We propose a method for quantum information processing using molecules coupled to an external laser field. This utilizes molecular interactions, control of the external field and an effective energy shift of the doubly-excited state of two coupled molecules. Such a level shift has been seen in the two-photon resonance experiments recently reported in Ref. [1]. Here we show that this can be explained in terms of the QED Lamb shift. We quantify the performance of the proposed quantum logic gates in the presence of dissipative mechanisms. The unitary transformations required for performing one- and two-qubit operations can be implemented with present day technology., Comment: 4 pages, 3 figures, Updated to correct important missing reference

The possibility of using strongly and continuously interacting spins for quantum computation has recently been discussed. Here we present a simple optical scheme that achieves this goal while avoiding the drawbacks of earlier proposals. We employ a third state, accessed by a classical laser field, to create an effective barrier to information transfer. The mechanism proves to be highly efficient both for continuous and pulsed laser modes; moreover it is very robust, tolerating high decay rates for the excited states. The approach is applicable to a broad range of systems, in particular dense structures such as solid state self-assembled (e.g., molecular) devices. Importantly, there are existing structures upon which `first step' experiments could be immediately performed., Comment: 5 pages including 3 figures. Updated to published version

We consider two coupled generic quantum dots, each modelled by a simple potential which allows the derivation of an analytical expression for the inter-dot Foerster coupling, in the dipole-dipole approximation. We investigate the energy level behaviour of this coupled two-dot system under the influence of an external applied electric field and predict the presence of anticrossings in the optical spectra due to the Foerster interaction., Comment: 13 pages, 7 figures. Published version. Substantially revised, new sections on decay rates, absorption spectra, and tunneling

Resonant energy transfer mechanisms have been observed in the sensitized luminescence of solids, in quantum dots and in molecular nanostructures, and they also play a central role in light harvesting processes in photosynthetic organisms. We demonstrate that such mechanisms, together with the exciton-exciton binding energy shift typical of these nanostructures, can be used to perform universal quantum logic. In particular, we show how to generate controlled exciton entanglement and identify two different regimes of quantum behaviour., Comment: 4 pages, 4 separate figures, REVTeX 4 style

The dynamical evolution of a quantum register of arbitrary length coupled to an environment of arbitrary coherence length is predicted within a relevant model of decoherence. The results are reported for quantum bits (qubits) coupling individually to different environments (`independent decoherence') and qubits interacting collectively with the same reservoir (`collective decoherence'). In both cases, explicit decoherence functions are derived for any number of qubits. The decay of the coherences of the register is shown to strongly depend on the input states: we show that this sensitivity is a characteristic of $both$ types of coupling (collective and independent) and not only of the collective coupling, as has been reported previously. A non-trivial behaviour ("recoherence") is found in the decay of the off-diagonal elements of the reduced density matrix in the specific situation of independent decoherence. Our results lead to the identification of decoherence-free states in the collective decoherence limit. These states belong to subspaces of the system's Hilbert space that do not get entangled with the environment, making them ideal elements for the engineering of ``noiseless'' quantum codes. We also discuss the relations between decoherence of the quantum register and computational complexity based on the new dynamical results obtained for the register density matrix., Comment: Typos corrected. Discussion and references added. 1 figure + 3 tables added. This updated version contains 13 (double column) pages + 8 figures. PRA in press

A major question for condensed matter physics is whether a solid-state quantum computer can ever be built. Here we discuss two different schemes for quantum information processing using semiconductor nanostructures. First, we show how optically driven coupled quantum dots can be used to prepare maximally entangled Bell and Greenberger-Horne-Zeilinger states by varying the strength and duration of selective light pulses. The setup allows us to perform an all-optical generation of the quantum teleportation of an excitonic state in an array of coupled quantum dots. Second, we give a proposal for reliable implementation of quantum logic gates and long decoherence times in a quantum dots system based on nuclear magnetic resonance (NMR), where the nuclear resonance is controlled by the ground state transitions of few-electron QDs in an external magnetic field. The dynamical evolution of these systems in the presence of environmentally-induced decoherence effects is also discussed., Comment: 27 pages, 12 figures. Invited chapter for the Proceedings of the ISI-Accademia dei Lincei Conference ``Conventional and non Conventional Computing (Quantum and DNA)", to be published by Springer Verlag

We propose a nanostructure switch based on nuclear magnetic resonance (NMR) which offers reliable quantum gate operation, an essential ingredient for building a quantum computer. The nuclear resonance is controlled by the magic number transitions of a few-electron quantum dot in an external magnetic field., Comment: 4 pages, 2 separate PostScript figures. Minor changes included. One reference added

We show how optically-driven coupled quantum dots can be used to prepare maximally entangled Bell and Greenberger-Horne-Zeilinger states. Manipulation of the strength and duration of the selective light-pulses needed for producing these highly entangled states provides us with crucial elements for the processing of solid-state based quantum information. Theoretical predictions suggest that several hundred single quantum bit rotations and Controlled-Not gates could be performed before decoherence of the excitonic states takes place., Comment: 3 separate PostScript Figures + 7 pages. Typos corrected. Minor changes added. This updated version is to appear in PRA

We propose a practical solid-state system capable of demonstrating quantum teleportation. The set-up exploits recent advances in the optical control of excitons in coupled quantum dots, in order to produce maximally-entangled Bell and Greenberger-Horne-Zeilinger (GHZ) states. Only two unitary transformations are then required: a quantum Controlled-Not gate and a Hadamard gate. The laser pulses necessary to generate the maximally-entangled states, and the corresponding unitary transformations, are given explicitly., Comment: LaTeX file (10 pages) + 3 separate figures