arXiv subjects

Edgar A. Rojas-González

Publications and source records attributed to Edgar A. Rojas-González.

6 recordsLinked to original sources

arXiv2026

Transport regimes of biphoton-state quantum walks in ordered and disordered arrays of nonlinear waveguides

A quantum walk in an ordered medium exhibits ballistic propagation. A related process is the driven quantum walk, in which the number of walkers varies along the propagation. In this work, we show that a driven quantum walk of biphoton states in an array of nonlinear waveguides does not propagate ballistically, but instead presents two transport regimes: superballistic and superdiffusive. We also study the effects of off-diagonal and diagonal disorder. In addition to the observation of Anderson localization, both types of disorder lead to the disappearance of the superballistic regime.

quant-ph↗

arXiv2025

Analytic solution to degenerate biphoton states generated in arrays of nonlinear waveguides

The arrays of nonlinear waveguides are a powerful integrated photonics platform for studying and manipulating quantum states of light. Also, they are a valuable resource for various quantum technologies. In this work, we employed a supermode approach to obtain an analytic solution to the evolution of degenerate biphoton states in arrays of nonlinear waveguides. The solution accounts for arrays of an arbitrary number of waveguides and coupling profiles. In addition, it provides an explicit analytic expression without the need of computationally-expensive steps. Actually, it only relies on the calculation of the eigenvalues and eigenvectors of the coupling matrix. In general, this needs to be performed numerically, but there are relevant instances in which analytic expressions are available. Thus, in certain cases, the procedure proposed here does not require the use of any numerical method. We analyze the general properties of the solution and show some application examples. Particularly, simple solutions that can be obtained by special symmetric injection profiles, the results for small arrays, and the properties of the propagation when only the center waveguide is pumped in a symmetric odd array. In addition, we present a proof-of-principle example on how to use the analytic solution to tackle inversion problems. That is, obtaining the initial conditions required to achieve a desired quantum state -- which is a valuable technological application. A relevant aspect of the method presented here is its scalability for large arrays due to the lack of resource-intensive steps -- both for the direct and inverse problems.

quant-ph↗

arXiv2025

Impacto del desorden en los estados cuánticos de dos fotones generados en arreglos de guías de onda no lineales

In an array of nonlinear waveguides, quantum states can be generated from classical states through spontaneous parametric down-conversion of photons. This work simulates and analyzes the effect of disorder on light propagation and its quantum correlations, implementing disorder in three system parameters: coupling, injection amplitude, and injection phase. It is determined that when light is injected into only one waveguide, disorder in the coupling increases localization and reduces dispersion. Moreover, in this case, propagation tends to remain ballistic, a characteristic feature of waveguide arrays. Conversely, disorder in the injection amplitude and phase tends to delocalize the wave function, with the latter having a more pronounced effect. Finally, it is observed that quantum correlations, obtained from the correlation matrix, are robust in the presence of disorder, particularly the null elements. -- En un arreglo de guías de onda no lineales se pueden generar estados cuánticos a partir de estados clásicos mediante la conversión paramétrica descendente espontánea de fotones. En este trabajo se simula y analiza el efecto del desorden en la propagación de la luz y sus correlaciones cuánticas, implementando desorden en tres perfiles del sistema: acoplamiento, amplitud de inyección y fase de inyección. Se determina que, cuando se inyecta solamente una guía de onda, el desorden en el acoplamiento aumenta la localización y disminuye la dispersión. Además, en este caso se preserva la propagación con tendencia a ser balística, lo cual es característico de arreglos de guías de onda. Contrariamente, el desorden en la amplitud y fase de inyección tienden a deslocalizar la función de onda, siendo el efecto mayor en el último caso. Finalmente, se observa que las correlaciones cuánticas, obtenidas a partir de la matriz de correlación, son robustas ante la presencia de desorden, en particular los elementos nulos.

quant-ph↗

arXiv2020

Charge coloration dynamics of electrochromic amorphous tungsten oxide studied by simultaneous electrochemical and color impedance measurements

The coloration mechanisms in electrochromic (EC) systems can be probed by comparing the dynamics of the electrical and optical responses. In this paper, the linear frequency-dependent electrical and optical responses of an amorphous tungsten oxide thin film were measured simultaneously by a combination of two techniques$\text{---}$that is, electrochemical impedance spectroscopy (EIS) and the so-called color impedance spectroscopy (CIS). This was done at different bias potentials, which can be associated with different intercalation levels. Equivalent circuit fitting to the EIS spectra was used to extract the Faradaic components participating in the total impedance response. The latter were assigned to an intermediate adsorption step before the intercalation and to the diffusion of the electron-ion couple in the film. A quantity denoted complex optical capacitance is compared to the complex electrical capacitance$\text{---}$in particular, their expressions are related to the Faradaic processes. The coloration at low intercalation levels followed both the adsorption and diffusion phenomena. Conversely, the diffusion contribution was dominant at high intercalation levels and the adsorption one seemed to be negligible in this case. For perfectly synchronized electrical and optical responses, their complex spectra are expected to differ only by a multiplying factor. This was the case at low intercalation levels, apart from small deviations at high frequencies. A clear departure from this behavior was observed as the intercalation level increased. A combination of frequency-dependent techniques, as presented in this work, can help in the understanding of the dynamics of the coloration mechanisms in EC materials at various conditions$\text{---}$for example, at different intercalation levels and optical wavelengths.

physics.app-ph↗

arXiv2020

Differential coloration efficiency of electrochromic amorphous tungsten oxide as a function of intercalation level: Comparison between theory and experiment

Optical absorption in amorphous tungsten oxide ($\textit{a}\mathrm{WO}_{3}$), for photon energies below that of the band gap, can be rationalized in terms of electronic transitions between localized states. For the study of this phenomenon, we employed the differential coloration efficiency concept, defined as the derivative of the optical density with respect to the inserted charge. We also made use of its extension to a complex quantity in the context of frequency-resolved studies. Combined $\textit{in situ}$ electrochemical and optical experiments were performed on electrochromic $\textit{a}\mathrm{WO}_{3}$ thin films for a wide lithium intercalation range using an optical wavelength of $810~\mathrm{nm}$ ($1.53~\mathrm{eV}$). Quasi-equilibrium measurements were made by chronopotentiometry (CP). Dynamic frequency-dependent measurements were carried out by simultaneous electrochemical and color impedance spectroscopy (SECIS). The differential coloration efficiency obtained from CP changes sign at a critical intercalation level. Its response exhibits an excellent agreement with a theoretical model that considers electronic transitions between $\mathrm{W}^{4+}$, $\mathrm{W}^{5+}$, and $\mathrm{W}^{6+}$ sites. For the SECIS experiment, the low-frequency limit of the differential coloration efficiency shows a general trend similar to that from CP. However, it does not change sign at a critical ion insertion level. This discrepancy could be due to degradation effects occurring in the films at high $\mathrm{Li}^+$ insertion levels. The methodology and results presented in this work can be of great interest both for the study of optical absorption in disordered materials and for applications in electrochromism.

physics.app-ph↗

arXiv2019

Setup for simultaneous electrochemical and color impedance measurements of electrochromic films: theory, assessment, and test measurement

Combined frequency-resolved techniques are suitable to study electrochromic (EC) materials. We present an experimental setup for simultaneous electrochemical and color impedance studies of EC systems in transmission mode and estimate its frequency-dependent uncertainty by measuring the background noise. We define the frequency-dependent variables that are relevant to the combined measurement scheme, and a special emphasis is given to the complex optical capacitance and the complex differential coloration efficiency, which provide the relation between the electrical and optical responses. Results of a test measurement on amorphous $\mathrm{WO}_3$ with LED light sources of peak wavelengths of $470$, $530$, and $810~\mathrm{nm}$ are shown and discussed. In this case, the amplitude of the complex differential coloration efficiency presented a monotonous increase down to about $0.3~\mathrm{Hz}$ and was close to a constant value for lower frequencies. We study the effect of the excitation voltage amplitude on the linearity of the electrical and optical responses for the case of amorphous $\mathrm{WO}_3$ at $2.6~\mathrm{V}~\mathrm{vs.}~\mathrm{Li/Li}^+$, where a trade-off should be made between the signal-to-noise ratio (SNR) of the optical signal and the linearity of the system. For the studied case, it was possible to increase the upper accessible frequency of the combined techniques (defined in this work as the upper threshold of the frequency region for which the SNR of the optical signal is greater than $5$) from $11.2~\mathrm{Hz}$ to $125.9~\mathrm{Hz}$ while remaining in the linear regime with a tolerance of less than $5\%$.

physics.app-ph↗