Fernando Rojas

National Autonomous University of Mexico | UNAM · Centre of Nanoscience and Nanotechnology

We study the geometric Uhlmann phase of mixed states at finite temperature in a system of two coupled spin-12 particles driven by a magnetic field applied to one of the spins. In the parameter space of temperature and coupling, we show the emergence of two topological Uhlmann phase transitions when the magnetic field evolves around the equator, whe...

We study the geometric Uhlmann phase of mixed states at finite temperature in a system of two coupled spin-$\frac 1 2$ particles driven by a magnetic field applied to one of the spins. In the parameter space of temperature and coupling, we show the emergence of two topological Uhlmann phase transitions when the magnetic field evolves around the equ...

We discover an environment-assisted quantum discord mechanism (ENAQD), similar to the phenomenon termed environment-assisted quantum transport (ENAQT), in the chromophores network of the Fenna–Matthews–Olson (FMO) pigment–protein complex from green sulfur bacteria Chlorobium tepidum. We use the Local Quantum Uncertainty (LQU), a discord-like quanti...

The hybrid atoms-cell site entanglement in a one-dimensional Su-Schrieffer-Heeger (SSH) topological insulator with first and second neighbor hopping in space representation of finite chains is analyzed. The geometrical phase is calculated by the Resta electric polarization and the entanglement in the atomic basis by the Schmidt number. A relation b...

We study the geometric Uhlmann phase of entangled mixed states in a composite system made of two coupled spin-12 particles with a magnetic field acting on one of them. Within a uniform noisy quantum channel setup, an exact analytical expression for this geometric phase in each subsystem is derived. We find an explicit connection to the concurrence...

We study the geometric Uhlmann phase of entangled mixed states in a composite system made of two coupled spin-$\frac 1 2$ particles with a magnetic field acting on one of them. Within a depolarizing channel setup, an exact analytical expression for such a phase in each subsystem is derived. We find an explicit connection to the concurrence of the d...

We study the hybrid atoms-cell site entanglement in one-dimensional Su-Schrieffer-Heeger (SSH) topological insulator with first, second neighbor-hopping interaction and intra-cell modulation, in space representation of finite chains. We determine the geometric phases by the Resta electric polarization and the entanglement in the atomic basis by the...

The generalization of the geometric phase to the realm of mixed states is known as Uhlmann phase. Recently, applications of this concept to the field of topological insulators have been made and an experimental observation of a characteristic critical temperature at which the topological Uhlmann phase disappears has also been reported. Surprisingly...

Green sulfur bacteria is a photosynthetic organism whose light-harvesting complex accommodates a pigment-protein complex called Fenna-Matthews-Olson (FMO). The FMO complex sustains quantum coherence and quantum correlations between the electronic states of spatially separated pigment molecules as energy moves with nearly a 100% quantum efficiency t...

In this work, we propose the generation of a hybrid entangled resource (HER) and its further application in a quantum teleportation scheme from an experimentally feasible point of view. The source for HER preparation is based on the four-wave mixing process in a photonic crystal fiber, from which one party of its output bipartite state is used to h...

In this work we propose the generation of a hybrid entangled resource (HER) and its further application in a quantum teleportation scheme from an experimentally feasible point of view. The source for HER preparation is based on the four wave mixing process in a photonic crystal fiber, from which one party of its output bipartite state is used to he...

We present a physical model of spin quantum teleportation protocol (QTP) in a triple quantum dot array using a genetic algorithm approach. The information to teleport is spin-coded in one electron confined in a single quantum dot (SQD). The remaining double quantum dot (DQD) system has just an electron with spin that includes spin–orbit interaction...

We study quantum correlations and discord in a bipartite continuous variable hybrid system formed by linear combinations of coherent states |α and single photon added coherent states (SPACS) of the form |ψ dp(pa) = N / √ 2(ˆ a † |α a |α b ± ˆ b † |α a |α b). We stablish a relationship between the quantum discord with a local observable (the quadrat...

Resumen: Presentamos una simulación del espín en el experimento de Stern-Gerlach utilizando el modelo del autómata cuántico implementado en un robot Vex con un estado interno representado por qubits y cambios de estados dependientes de las señales de sensores externos con los que se realizan las mediciones y acciones correspondientes. De esta forma...

We present a quantum teleportation protocol based on a hybrid entangled resource (HER) obtained with heralded photons and coherent states. High efficiency of teleportation can be obtained by adjusting spectral detection windows.

We present a quantum teleportation protocol based on a hybrid entangled resource (HER) obtained with heralded photons and coherent states. High efficiency of teleportation can be obtained by adjusting spectral detection windows.

We study the degree of entanglement of arbitrary superpositions of $m,n$ photon-added coherent states (PACS) $|\psi\rangle \propto u |{\alpha},m\rangle |{\beta},n \rangle+ v |{\beta},n\rangle |{\alpha},m\rangle$ using the concurrence, and obtain the general conditions for maximal entanglement. We show that photon addition process can be identified...

We present a physical example model of how Quantum Control with genetic algorithms is applied to implement the quantum superdense code protocol. We studied a model consisting of two quantum dots with an electron with spin, including spin-orbit interaction. The electron and the spin get hybridized with the site acquiring two degrees of freedom, spin...

We have studied the thermal entanglement as a function of the temperature for a two-qubits Heisenberg spins system; we have included Dzyaloshinskii–Moriya interaction (DM), an external magnetic field (EMF) and hyperfine interaction due to the nuclear field of the surrounding nuclei. A critical value for the EMF was found, around B(c) ext,z ∼ 39 mT,...

We study the H3×N hybrid entanglement between charge and vibrational modes in a triple-quantum-dot shuttle system. Three quantum dots are linearly connected, with the outer dots fixed and the central dot oscillating, described as a quantum harmonic oscillator with oscillation modes that are entangled with the electronic states of the quantum dots....

En este trabajo se utilizó la aproximación LCAO (“Linear Combination of Atomic Orbitals” por sus siglas en inglés) y el Modelo de Hückel [1] para aproximar la función de onda y el Hamiltoniano de una molécula polyene abierta de 2 átomos de carbono. Las moléculas polyene son la base de las moléculas pigmento presentes en los complejos colectores de...

We investigate the electronic and transport properties of circular graphene structures (quantum dots) that include a pentagonal defect. In our calculations, we employ a tight-binding model determining total and local density of states, transmission function and participation number.For the closed structure, we observe that the effect of the defect...

The aim of this work was to design and control, using genetic algorithm (GA) for parameter optimization, one-charge-qubit quantum logic gates sigmax, sigmay, and sigmaz, using two bound states as a qubit space, of circular graphene quantum dots in a homogeneous magnetic field. The method employed for the proposed gate implementation is through the...

We study the effect of Rashba spin-orbit coupling on the Hofstadter spectrum of a two-dimensional tight-binding electron system in a perpendicular magnetic field. We obtain the generalized coupled Harper spin-dependent equations which include the Rashba spin-orbit interaction and solve for the energy spectrum and spin polarization. We investigate t...

We study the energy spectrum of an electron in a two-dimensional periodic lattice in a perpendicular magnetic field in the tight-binding approximation including the Rashba spin-orbit coupling. We have investigated how the fractal spectrum known as Hofstadter's butterfly is modified if one includes the spin-orbit coupling. We find a set of decoupled...

We study the properties of the charge distribution (polarization) of a 4-quantum dot square cell with two electrons, induced or modified by the coupling to a single quantized mode field. The effect of the relevant parameters (tunneling between dots, the amplitude of the electron-cavity interaction, cavity frequency) is investigated in the propertie...

Bloch oscillations arise when electrons are in a one-dimensional linear chain of atoms under a constant electric field. In this paper we show numerically that electrons in different types of carbon nanotubes show oscillations with a Bloch frequency proportional to the constant electric field applied along the nanotube axis. We show these oscillatio...

We investigate numerically the dynamical properties of electrons in cavities of nanotubes under the influence of an external electric field. In particular, the dynamical behaviour of electrons is studied when the nanostructure is connected to two electrodes at different electric potential. Here it is shown how the phenomenon of Bloch oscillations i...

We demonstrate theoretical stationary and dynamical generation of Bell states in a system of two parallel double quantum dots with one mobile electron each (proposed as two charge qubits) driven by an external potential difference applied to the second of the double dots. For coherent dynamics, it is shown that each one of the four Bell states is o...

Several experiments have been carried out to observe and control spin properties of electrons in quantum dots subject to the hyperfine interaction due to nuclear spins. In this work, we study the manifestation of the hyperfine interaction through current calculations in one and two quantum dots. We use the density matrix master equation approach in...

The exchange coupling between two spins has been proposed as a control parameter over the states of the two-qubit space, so that it can be used to produce quantum gate operations, and in particular, the (SWAP)α two-qubit quantum gate. In this work, we study the effect that the Dzyaloshinski-Moriya term (DM) has on the (SWAP)α gate operation. By con...

Entanglement has become an important resource in quantum information processing, it is important to quantify the degree of entanglement between two qubits at a finite temperature [1]. As charge qubits realized in double quantum dots are promising solid state candidates for both qubit and entangled states realization, we explore theoretically the no...

We study the static and dynamic formation of entanglement in charge states of a two double quantum dot array with two mobile electrons under the effect of an external driving field. We include dissipation via contact with a phonon bath. By using the density matrix formalism and an open quantum system approach, we describe the dynamical behaviour of...

We show that the tunable hybridization between two lateral quantum dots connected to a nonmagnetic current leads in a “hanging-dot” configuration that can be used to implement a bipolar spin filter. The competition between Zeeman, exchange interaction, and interdot tunneling (molecular hybridization) yields a singlet-triplet transition of the doubl...

A bipolar spin filter has been proposed using a few-electron double quantum dot (DQD) in a `hanging-dot' configuration [1], and tunable by controlling the molecular hybridization (interdot tunneling) between dots. The bipolar nature of the electron spin current arises from a singlet-triplet transition in the ground state of the two-electron DQD. Th...

The role of spin and charge distribution in a planar semiconductor quantum dot array (square geometry) with two electrons is studied in the presence of a driver cell and magnetic nanoparticles located near two quantum dots providing a local magnetic field. We use an extended Hubbard model to describe the electrons in the cell, taking into account i...

We present a theoretical study of the spectral and the spin-dependent transport properties of a few electron semiconductor parallel double quantum dot (DQD) in the presence of local induced Zeeman splittings at the quantum dots. Working in an extended Hubbard model and treating the coupled QD as a single coherent system, the linear response spin-de...

Quantum dot arrays or quantum-dot cellular automata (QCA) have been proposed as elements capable to encode, process and transmit logical information based on quantum effects in terms of charge distributions in specific geometries. and the basis for the charge qubits. Quantum Entanglement is a resource to encode information in a completely new way m...

The electron dynamics in one-dimensional periodic crystals presents important effects such as ballistic motion, Bloch oscillations and dynamic localization. We are interested in evaluate how these effects are modified if ones includes the spin degree of freedom. In this work, we study the electron dynamics with spin orbit interaction (SOI) in one-d...

We study the dynamic evolution of the charge distribution (polarization) of a 2×2 quantum-dot cell with two electrons in the presence of a time-dependent driver cell and a magnetic field. We describe the effects of the magnetic flux on the response of the basic dot cell, for fixed, and linear switching of the driver polarization. In the static case...

We study the spin-dependent electron transport through a structure consisting of a coupled semiconductor double quantum dot in the presence of two spin magnetic impurities localized in the vicinity of the quantum dots. Electrical gates on a two-dimensional electron gas in a narrow gap semiconductor heterojunction define the quantum dot system. A si...

We study the role of dissipation and structural defects on the time evolution of quantum dot arrays with mobile charges under external driving fields. These structures, proposed as quantum dot cellular automata, exhibit interesting quantum dynamics which we describe in terms of equations of motion for the density matrix. Using an open system approa...

We present calculations for the static and dynamical properties of two parallel double dots, with one extra electron each, constituting a four-dot QCA basic cell. When a potential difference is applied to one of the double dots, causing the electron to switch, we find that the electron on the other double dot switches accordingly, in response to in...

We investigate the role of spin-orbit interaction on the energy spectra, charge and spin configurations of a planar semiconductor quantum dot cell array in a square geometry. The quantum dot array is assumed to be formed by electrical gate confinement, for instance, of a two dimensional electron gas on a semiconductor heterojunction. Hence, it is e...

We study the phase-ordering dynamics of the O(n) model with a conserved order parameter for systems with topological defects. We present results from both cell dynamical simulations and predictions of a Gaussian auxiliary field (GAF) approximation for the XY (n = 2) model in two and three dimensions, and the Heisenberg (n = 3) model in three dimens...

We study the dynamics of the charge distribution (polarization) in an array of four quantum dots located at the corners of a square. The array has two extra electrons in a neutralizing background. We take into account electron-phonon interaction effects using a quantum Markovian master equation approach. Results show that dissipative effects are de...

We study the properties of the charge distribution (polarization) of a 4-quantum dot square cell with two electrons, induced or modified by the coupling to a single quantized mode field. The effect of the relevant parameters (tunneling between dots, the amplitude of the electron-cavity interaction, cavity frequency) is investigated in the propertie...

We consider the dynamic evolution of the charge distribution (polarization) of chains of 4- and 5-quantum dot cells with two electrons each, in response to the time dependent polarization of a driver cell. Coulomb interaction between quantum dots and intra-cell tunneling are taken into account. For linear and periodic switching of the driver cell,...

We study the dynamic evolution of 4-and 5-quantum dot cells in the presence of a time-dependent driver cell. We analyze the eeects of imperfections, tunneling intensity and switching times on the response of the basic cell, for linear and periodic switching of the driver polarization. We ÿnd that the eeects of the imperfections have strong conseque...

To sensitively test scaling in the two-dimensional XY model quenched from high temperatures into the ordered phase, we study the difference between measured correlations and the (scaling) results of a Gaussian-closure approximation. We also directly compare various length scales. All of our results are consistent with dynamical scaling and an asymp...

Inelastic effects on resonant tunnelling are considered for a model consisting of a double- delta -function potential with the region in between oscillating with amplitude V1 and frequency omega . In the limit of small V1/ omega , the authors obtain analytic expressions for the transmission coefficient for the elastic and the two nearest inelastic...

Using a cell-dynamic system simulation scheme, we investigate the phase-ordering dynamics of nonconserved O(n) models without topological defects, i.e., for n>~d+1 where d is the spatial dimensionality. In particular, we consider zero-temperature quenches for d=2, n=4,5 and for d=1, n=3,4,5. We find, in agreement with previous simulations using fix...

The zero-temperature ordering kinetics of conserved XY models in spatial dimensions $d=2$ and 3 is studied using cell dynamical simulations. The growth of the characteristic length scale $L(t)$ is fully consistent with recent theoretical predictions: $L(t) \sim t^{1/4}$ for $d=2$ and $L(t) \sim (t\ln t)^{1/4}$ for $d=3$. A gaussian closure approxim...

We study the dynamic scaling properties of systems with a conserved order parameter with O(n) symmetry, using the Gaussian auxiliary field approach of Mazenko [Phys. Rev. Lett. 63, 1605 (1989); Phys. Rev. B 42, 4487 (1990); 43, 5747 (1990)]. Results valid in the limit of large n, and for finite n, are presented. An explicit numerical solution to th...

We present preliminary results on the problem of single-electron quantum transmission through time-periodic oscillating potential barriers. This work is motivated by recent experiments in semiconducting quantum dots with RF modulated tunnel barriers. The potential barriers are modeled by the time-dependent potential P(x,t) = ΣnV(n)(t) δ(x-xn), with...