arXiv subjects

L. Sanz

Publications and source records attributed to L. Sanz.

At least 19 recordsLinked to original sources

arXiv2025

Quantum entanglement response to pulsed gate modulation

We examine the impact of time-dependent gate voltages on entanglement generation in two capacitively coupled charge qubits, with single-electron injection triggered on demand. The gate voltage modulates the tunnel coupling between the qubits and electronic reservoirs, initiating charge transport into the system. The formation of entangled states arises from the competition between inter-qubit Coulomb interactions and electron hopping processes. Particular attention is paid to the temporal structure of the gate pulse, which plays a pivotal role in shaping the entanglement dynamics. By exploring a variety of pulse profiles, we uncover regimes of enhanced entanglement and identify optimal driving conditions. Additionally, we investigate how environmental dephasing deteriorates entanglement formation. Within the framework of the density matrix formalism, we calculate fidelity, linear entropy, and negativity to identify robust operational windows. These results provide insights into controlling quantum correlations in mesoscopic systems and underscore the importance of error mitigation strategies in realizing high-performance electronic quantum devices.

quant-ph↗

arXiv2024

Schrödinger cats coupled with cavities losses: the effect of finite and structured reservoirs

We discuss the generation of a Schrödinger cat in a nanocavity created by the coupling of an electromagnetic mode with an exciton in a quantum dot considering the dispersive limit of the Jaynes-Cummings model. More than the generation itself, we focus on the effects of the environment over the bosonic state in the nanocavity, which has losses simulated by coupling with two different kind of reservoirs. In the first case, the interaction between the system with a finite reservoir shows that fragments of different sizes of the reservoir deliver the same amount of information about the physical system in the dynamics of the birth and death of the Schrödinger cat. The second case considers a structured reservoir, whose spectral density varies significantly with frequency. This situation becomes relevant in solid-state devices where quantum channels are embedded, as memory effects generally cannot be neglected. Under these circumstances, it is observed that the dynamics can differ substantially from the Markovian, presenting oscillations related to the average number of photons. These oscillations influence the information flow between the system and the environment, evidenced here by the measurement of non-Markovianity.

quant-ph↗

arXiv2024

A Cooper-pair beam splitter as a feasible source of entangled electrons

We investigate the generation of an entangled electron pair emerging from a system composed of two quantum dots attached to a superconductor Cooper pair beam splitter. We take into account three processes: Crossed Andreev Reflection, cotuneling, and Coulomb interaction. Together, these processes play crucial roles in the formation of entangled electronic states, with electrons being in spatially separated quantum dots. By using perturbation theory, we derive an analytical effective model that allows a simple picture of the intricate process behind the formation of the entangled state. Several entanglement quantifiers, including quantum mutual information, negativity, and concurrence, are employed to validate our findings. Finally, we define and calculate the covariance associated with the detection of two electrons, each originating from one of the quantum dots with a specific spin value. The time evolution of this observable follows the dynamics of all entanglement quantifiers, thus suggesting that it can be a useful tool for mapping the creation of entangled electrons in future applications within quantum information protocols.

quant-ph↗

arXiv2022

Enhanced solar photocurrent using a quantum dot molecule

In this paper we make a detailed study of the role of coherent tunneling, on the photocurrent and power delivered by a quantum dot molecule (QDM) in the presence of solar light. We focus our analysis on the coherence driven by tunneling and its impact on the photovoltaic properties of the QDM. The coherence developed by the system raises as a resource from the interplay between the strength of the tunneling coupling, the QDM band alignment, and the coupling rates with the reservoirs of thermal phonons. Our results show that a QDM can deliver up to 30% more power than a single quantum dot, and our calculations of efficiency show coherence effects, which are consistent with the Second Law of Thermodynamics.

quant-ph↗

arXiv2021

Photovoltaic efficiency at maximum power of a quantum dot molecule

In this work, it is investigated the behavior of the efficiency at maximum power of a quantum dot molecule, acting as a device for photovoltaic conversion. A theoretical approach using a master equation, considering the effect of the energy offsets, and the width of the quantum barrier, identifies realistic physical conditions that enhance the photovoltaic response of the photocell. The results show the potentiality of increasing the gain in 30\% of maximum power delivered per molecule if compared with a single quantum dot. Also, the system exhibits gain when compared to the Chambadal-Novikov efficiency at maximum power, without exceeding Carnot's efficiency, as expected from the second law of thermodynamics.

cond-mat.mes-hall↗

arXiv2021

Dynamic generation of GHZ states with coupled charge qubits

In this paper, we present a proof-of-principle of the formation of pure maximally entangled states from the Greenberger-Horne-Zeilinger class, in the experimental context of charged quantum dots. Each qubit must be identified as a pair of quantum dots, sharing an excess electron, coupled by tunneling. The electron-electron interaction is accounted for and is responsible for the coupling between the qubits. The interplay between coherent tunneling events and many-body interaction gives rise to the formation of highly entangled states. We begin by treating the problem of encoding three-qubits in a system with three pairs of quantum dots, and the numerical analysis of the exact quantum dynamics to find the conditions for the generation of the GHZ states. An effective two-level model sheds light on the role of a high-order tunneling process behind the dynamics. The action of the main decoherence process, the charge dephasing, is quantified in the process. We then evaluate the physical requirements for the dynamical generation of GHZ states in a $N$ qubit scenario, and its challenges.

quant-ph↗

arXiv2021

Spin-Polarized Initialization and Readout for Single-Qubit State Tomography

We propose a theoretical protocol for reconstructing the density matrix of a single-electron spin qubit using spin-polarized transport. The system consists of a quantum dot coupled to ferromagnetic reservoirs and subject to a magnetic field lying in the $xy$ plane of the Bloch sphere. Spin-dependent tunneling events measured along the $x\pm$, $y\pm$, and $z\pm$ quantization axes give rise to probability distributions that encode the quantum state of the qubit. The open-system dynamics are described using a Lindblad master equation, which captures the time evolution of the spin under continuous coupling to the reservoirs. By counting tunneling events for four different magnetic alignments, we formulate a scheme for reconstructing the full density matrix of the qubit. The resulting simulation data are analyzed using machine-learning techniques to process the measured probability distributions and infer the corresponding density matrix elements. The proposed model enables complete access to the open-system density matrix, including both population probabilities and relative phase information. Successful state reconstruction demonstrates the validity and robustness of the approach, highlighting its applicability to experimentally accessible spin-transport platforms.

quant-ph↗

arXiv2019

Quantum entanglement driven by electron-vibrational mode coupling

In this work, we provided a proof-of-principle of efficient production of maximally entangled states using charged quantum dots coupled to vibrational modes. The physical system consists of two pairs of quantum dots, each pair with a single electron able to tunnel between the dots, thus encoding a qubit. The electrons, initially not coupled, interact with two bosonic vibrational modes. It is demonstrated that the electron-vibrational mode coupling drives to an effective electron-electron interaction, which is the main mechanism behind the formation of maximally quantum entangled electronic states. The effect of this coupling follows a non-monotonic behavior, which is explained through an effective hamiltonian which takes into account high order transition processes.

cond-mat.mes-hall↗

arXiv2018

Autler-Townes Doublet observation via a Cooper-Pair Beam Splitter

We present a proof-of-principle of how electronic transport measurements permits the observation of the Autler-Townes doublet, an optical property of nanodevices. The quantum physical system consists of one optically pumped quantum dot, a second auxiliary quantum dot, and a superconductor lead which provides an effective coupling between the dots via crossed Andreev reflection. Electrodes, working as source and drain, acts as nonequilibrium electronic reservoirs. Our calculations of the photocurrent at both, transient and stationary regimes, obtained using a density matrix formalism for open quantum systems, shows signatures of the formation of the Autler-Townes doublet, caused by the interplay between the optical pumping and the crossed Andreev reflection.

cond-mat.mes-hall↗

arXiv2017

Lindblad Formalism based on Fermion-to-Qubit mapping for Non-equilibrium Open-Quantum Systems

We present an alternative form of master equation, applicable on the analysis of non-equilibrium dynamics of fermionic open quantum systems. The formalism considers a general scenario, composed by a multipartite quantum system in contact with several reservoirs, each one with a specific chemical potential and in thermal equilibrium. With the help of Jordan-Wigner transformation, we perform a fermion-to-qubit mapping to derive a set of Lindblad superoperators that can be straightforwardly used on a wide range of physical setups.To illustrate our approach, we explore the effect of a charge sensor, acting as a probe, over the dynamics of electrons on coupled quantum molecules. The probe consists on a quantum dot attached to source and drain leads, that allows a current flow. The dynamics of populations, entanglement degree and purity show how the probe is behind the sudden deaths and rebirths of entanglement, at short times. Then, the evolution leads the system to an asymptotic state being a statistical mixture. Those are signatures that the probe induces dephasing, a process that destroys the coherence of the quantum system.

quant-ph↗

arXiv2017

Coherent control of the dynamics of single quantum-dot exciton qubit in a cavity

In this work we demonstrate theoretically how to use external laser field to control the population inversion of a single quantum dot exciton qubit in a nanocavity. We consider the Jaynes-Cummings model to describe the system, and the incoherent losses were take into account by using Lindblad operators. We have demonstrated how to prepare the initial state in a superposition of the exciton in the ground state and the cavity in a coherent state. The effects of exciton-cavity detuning, the laser-cavity detunings, the pulse area and losses over the qubit dynamics are analyzed. We also show how to use a continuous laser pumping in resonance with the cavity mode to sustain a coherent state inside the cavity, providing some protection to the qubit against cavity loss.

cond-mat.mes-hall↗

arXiv2015

Excitonic entanglement of protected states in quantum dot molecules

The entanglement of an optically generated electron-hole pair in artificial quantum dot molecules is calculated considering the effects of decoherence by interaction with environment. Since the system evolves into a mixed states and due to the complexity of energy level structure, we use the negativity as entanglement quantifier, which is well defined in $d \otimes d^\prime$ composite vector spaces. By a numerical analysis of the non-unitary dynamics of the exciton states, we establish the feasibility of producing protected entangled superpositions by an appropriate tuning of bias electric field, $F$. A stationary state with a high value of negativity (high degree of entanglement) is obtained by fine tuning of $F$ close to a resonant condition between indirect excitons. We also found that when the optical excitation is set approximately equal to the electron tunneling coupling, $Ω/T_e \sim 1$, the entanglement reaches a maximum value. In front of the experimental feasibility of the specific condition mentioned before, our proposal becomes an useful strategy to find robust entangled states in condensed matter systems.

cond-mat.mes-hall↗

arXiv2015

Implementation of quantum logic gates using coupled Bose-Einstein condensates

In this work, we are interested on the implementation of single-qubit gates on coupled Bose-Einstein condensates (BECs). The system, a feasible candidate for a qubit, consists on condensed atoms of different hyperfine levels coupled by a two-photon transition. It is well established that the dynamics of coupled BECs can be described by the two-mode Hamiltonian which takes into account the confinement potential of the trap and the effects of collisions associated with each condensate. Other effects, as collisions between atoms belonging to different BECs and detuning are included in this approach. We demonstrate how to implement two types of quantum logic gates: \textit{transfer-population} gates (NOT, $\hat{Y}$, and Hadamard), which require inversion of population between hyperfine levels, and \textit{phase} gates ($\hat{Z}$, $\hat{S}$, and $\hat{T}$), which require self-trapping. We also discuss the experimental feasibility, checking the robustness of quantum gates against variations of physical parameters out of the ideal conditions for implementation of each quantum logic gate.

quant-ph↗

arXiv2015

Bell states and entanglement dynamics on two coupled quantum molecules

This work provides a complete description of entanglement properties between electrons inside coupled quantum molecules, nanoestructures which consist of two quantum dots. Each electron can tunnel between the two quantum dots inside the molecule, being also coupled by Coulomb interaction. First, it is shown that Bell states act as a natural basis for the description of this physical system, defining the characteristics of the energy spectrum and the eigenstates. Then, the entanglement properties of the eigenstates are discussed, shedding light on the roles of each physical parameters on experimental setup. Finally, a detailed analysis of the dynamics shows the path to generate states with a high degree of entanglement, as well as physical conditions associated with coherent oscillations between separable and Bell states.

quant-ph↗

arXiv2013

Quantum interference and control of the optical response in quantum dot molecules

We discuss the optical response of a quantum molecule under the action of two lasers fields. Using a realistic model and parameters, we map the physical conditions to find three different phenomena reported in the literature: the tunneling induced transparency, the formation of Autler-Townes doublets, and the creation of a Mollow-like triplet. We found that the electron tunneling between quantum dots is the responsible for the different optical regime. Our results not only explain the experimental results in the literature but also give insights for future experiments and applications in optics using quantum dots molecules.

cond-mat.mes-hall↗

arXiv2011

Atom-mediated effective interactions between modes of a bimodal cavity

We show a procedure for engineering effective interactions between two modes in a bimodal cavity. Our system consists of one or more two-level atoms, excited by a classical field, interacting with both modes. The two effective Hamiltonians have a similar form of a beam-splitter and quadratic beam-splitter interactions, respectively. We also demonstrate that the nonlinear Hamiltonian can be used to prepare an entangled coherent state, also known as multidimensional entangled coherent state, which has been pointed out as an important entanglement resource. We show that the nonlinear interaction parameter can be enhanced considering N independent atoms trapped inside a high-finesse optical cavity.

quant-ph↗

arXiv2010

Robust states in semiconductor quantum dot molecules

Semiconductor quantum dots coherently driven by pulsed laser are fundamental physical systems which allow studying the dynamical properties of confined quantum states. These systems are attractive candidates for a solid-state qubit, which open the possibility for several investigations in quantum information processing. In this work we study the effects of a specific decoherence process, the spontaneous emission of excitonic states, in a quantum dot molecule. We model our system considering a three-level Hamiltonian and solve the corresponding master equation in the Lindblad form. Our results show that the spontaneous emission associated with the direct exciton helps to build up a robust indirect exciton state. This robustness against decoherence allows potential applications in quantum memories and quantum gate architectures. We further investigate several regimes of physical parameters, showing that this process is easily controlled by tuning of external fields.

cond-mat.mes-hall↗

arXiv2008

Phonon modulation of the spin-orbit interaction as a spin relaxation mechanism in InSb quantum dots

We calculate the spin relaxation rates in a parabolic InSb quantum dots due to the spin interaction with acoustical phonons. We considered the deformation potential mechanism as the dominant electron-phonon coupling in the Pavlov-Firsov spin-phonon Hamiltonian. By studying suitable choices of magnetic field and lateral dot size, we determine regions where the spin relaxation rates can be practically suppressed. We analyze the behavior of the spin relaxation rates as a function of an external magnetic field and mean quantum dot radius. Effects of the spin admixture due to Dresselhaus contribution to spin-orbit interaction are also discussed.

cond-mat.mes-hall↗