arXiv subjects

A. Vidiella-Barranco

Publications and source records attributed to A. Vidiella-Barranco.

At least 19 recordsLinked to original sources

arXiv2026

Fock-state filtering of squeezed states: enhanced nonclassicality and effects of a thermal bath

The ability to tailor photon-number statistics is central to the generation and control of nonclassical states of light. Here, we investigate how Fock-state filtering of squeezed coherent states can be used to engineer their photon statistics. We consider the selective removal of the $m=0$ and $m=1$ Fock components and show that this process can enhance the sub-Poissonian character of the field, depending on the coherent amplitude. We then analyze the evolution of these engineered states in a thermal environment. Although all states eventually relax toward thermal statistics, the signatures of the filtering process may persist over a significant part of the evolution, while the purity of the filtered states is rapidly degraded by incoherent population redistribution. Our results illustrate both the potential and the limitations of photon-number engineering in realistic, dissipative settings.

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arXiv2025

Phase-space approach to cavity field dynamics in a squeezed thermal reservoir

Quantum systems, such as a single-mode cavity field coupled to a thermal bath, typically experience destructive effects due to interactions with their noisy environment. When the bath combines both thermal fluctuations and a nonclassical feature like quadrature squeezing, forming a squeezed thermal reservoir, the system's behaviour can change substantially. In this work, we study the evolution of the cavity field in this generalized environment using an alternative phase-space approach based on the Glauber-Sudarshan $P$-function. We derive a compact analytical expression for the time-dependent $P$-function for arbitrary initial cavity field states and demonstrate its utility through specific examples. Additionally, we obtain analytical expressions, as a function of time, for some statistical properties of the cavity field, as well as for the nonclassical depth, $τ_m$, a nonclassicality measure calculated directly from the $P$-function.

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arXiv2024

A note on the emission spectrum and trapping states in the Jaynes-Cummings model

The emission of light from an atom represents a fundamental process that provides valuable insights into the atom-light interaction. The Jaynes-Cummings model is one of the simplest fully quantized models to deal with these interactions, allowing for an analytical solution, while exhibiting notable non-trivial effects. We explore new features in the fluorescence emission spectrum for initial "trapping states", which suppress the atomic population inversion. Despite the seemingly dormant activity of the atom, the resulting emission spectra exhibit rich features, and using a dressed-state coordinates formalism, we are able to quantitatively explain the different profiles in the spectrum. We generalize the trapping conditions for non-zero atom-field detuning and also unveil two types of trapping states that lead to spectra with three peaks, in contrast to previously known states: a center peak and one secondary peak on each side. These are a trapping state formed by a Schrödinger cat state with Poissonian statistics (Yurke-Stoler state) and also a different type of "perfect trapping state".

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arXiv2023

Enhancing nonclassical properties of quantum states of light using linear optics

In this letter, we present a simple and versatile scheme for enhancing the nonclassical properties of light states using only linear optics and photodetectors. By combining a coherent state $|α\rangle$ and an arbitrary pure state of light $|ϕ\rangle$ (excluding coherent states) at two beam splitters, we show that the amplitude $α$ of the coherent state can be tuned to filter out specific Fock components and generate states of light with enhanced nonclassical features. We provide two examples of input states and demonstrate the effectiveness of our scheme in enhancing the sub-Poissonian statistics or the quadrature squeezing of the output states.

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arXiv2023

Parametrically driving a quantum oscillator into exceptionality

The mathematical objects employed in physical theories do not always behave well. Einstein's theory of space and time allows for spacetime singularities and Van Hove singularities arise in condensed matter physics, while intensity, phase and polarization singularities pervade wave physics. Within dissipative systems governed by matrices, singularities occur at the exceptional points in parameter space whereby some eigenvalues and eigenvectors coalesce simultaneously. However, the nature of exceptional points arising in quantum systems described within an open quantum systems approach has been much less studied. Here we consider a quantum oscillator driven parametrically and subject to loss. This squeezed system exhibits an exceptional point in the dynamical equations describing its first and second moments, which acts as a borderland between two phases with distinctive physical consequences. In particular, we discuss how the populations, correlations, squeezed quadratures and optical spectra crucially depend on being above or below the exceptional point. We also remark upon the presence of a dissipative phase transition at a critical point, which is associated with the closing of the Liouvillian gap. Our results invite the experimental probing of quantum resonators under two-photon driving, and perhaps a reappraisal of exceptional and critical points within dissipative quantum systems more generally.

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arXiv2023

Two coupled qubits under the influence of a minimal, phase-sensitive environment

In this work, we investigate the influence of a minimal, phase-sensitive environment on a system of two coupled qubits. The environment is constituted by a single-mode field initially prepared in a type of Schrödinger cat state, a quantum superposition of two squeezed coherent states. We present an analytical solution to the model and investigate the degradation of the quantum features of the system due to the action of the environment. In particular, we find that the time-averaged linear entropy for long times, $\bar{S}_T$, has approximately a linear dependence on Mandel's $Q$ parameter as well as on the variance of the $\hat{X}$ quadrature of the initial state of the environment.

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arXiv2022

Generation and transfer of entangled states between two connected microtoroidal cavities: analysis of different types of coupling

We investigate the generation and transfer of entangled states between two coupled microtoroidal cavities considering two different types of couplings, namely i) via a bridge qubit and ii) via evanescent fields. The cavities support two counter-propagating whispering-gallery modes (WGMs) that may also interact with each other. We firstly show that it is possible to transfer, with high fidelity, a maximally entangled state between the two modes of the first cavity (cavity 1) to the two modes of the second cavity (cavity 2), independently of the type of coupling. Interesting differences, though, arise concerning the generation of entangled states from initial product states; if the cavities are coupled via a bridge qubit, we show that it is possible to generate a 4-partite entangled state involving all four cavity modes. On the other hand, contrarily to what happens in the qubit coupling case, it is possible to generate bipartite maximally entangled states between modes of different cavities from initial separable states for cavities coupled by evanescent waves. Besides, we show that different entangled states between the propagating and counter-propagating modes of distinct cavities may be generated by tuning the interaction between modes belonging to the same cavity (intra-cavity couplings). Again, this is possible only for the couplings via evanescent waves. For the completion of our work, we discuss the effects of losses on the dynamics of the system.

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arXiv2022

Generation of nonclassical states of light via truncation of mixed states

A possible way of generating nonclassical states of light, especially non-Gaussian states, is via the truncation of a given state in the Fock basis. In recent work, we presented an alternative scheme for such quantum scissors [Phys. Rev. A 104, 033715 (2021)], employing a nondegenerate parametric amplifier, a beam splitter and photodetectors. An advantage of this setup is that it does not require the generation of Fock states beforehand, as in previous proposals. Here we extend this treatment to mixed input states. We show the possibilities of generating truncated states with either a maximum Fock number N or states having a minimum Fock number N. We discuss two specific examples of states to be truncated: i) the thermal state, and ii) the phase-diffused coherent state. In both cases, we show that the generated states can have significant sub-Poissonian statistics as well as non-Gaussian character. The degree of such nonclassical properties, as well as the success probabilities, can be changed by adjusting the parametric amplifier strength and the beam splitter transmittance.

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arXiv2021

Performance analysis of continuous-variable quantum key distribution using non-Gaussian states

In this study, we analyze the efficiency of a protocol with discrete modulation of continuous variable non-Gaussian states, the coherent states having one photon added and then one photon subtracted (PASCS). We calculate the secure key generation rate against collective attacks using the fact that Eve's information can be bounded based on the protocol with Gaussian modulation, which in turn is unconditionally secure. Our results for a four-state protocol show that the PASCS always outperforms the equivalent coherent states protocol under the same environmental conditions. Interestingly, we find that for the protocol using discrete-modulated PASCS, the noisier the line, the better will be its performance compared to the protocol using coherent states. Thus, our proposal proves to be advantageous for performing quantum key distribution in non-ideal situations.

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arXiv2021

Quantum entanglement in a four-partite hybrid system containing three macroscopic subsystems

The combination of different quantum systems may allow the exploration of the distinctive features of each system for the investigation of fundamental phenomena as well as for quantum technologies. In this work we consider a setup consisting of an atomic ensemble enclosed within a laser-driven optomechanical cavity, having the moving mirror further (capacitively) coupled to a low frequency LC circuit. This constitutes a four-partite optoelectromechanical quantum system containing three macroscopic quantum subsystems of a different nature, viz, the atomic ensemble, the massive mirror and the LC circuit. The quantized cavity field plays the role of an auxiliary system that allows the coupling of two other quantum subsystems. We show that for experimentally achievable parameters, it is possible to generate steady state bipartite Gaussian entanglement between pairs of macroscopic systems. In particular, we find under which conditions it is possible to obtain a reasonable amount of entanglement between the atomic ensemble and the LC circuit, systems that might be suitable for constituting a quantum memory and for quantum processing, respectively. For complementarity, we discuss the effect of the environmental temperature on quantum entanglement.

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arXiv2021

Quantum state truncation using an optical parametric amplifier and a beamsplitter

We present a scheme of quantum state truncation in the Fock basis (quantum scissors), based on the combined action of a nondegenerate optical parametric amplifier and a beamsplitter. Differently from previously proposed linear-optics-based quantum scissors devices, which depend on reliable Fock states sources, our scheme requires only readily available Gaussian states, such as coherent states inputs (vacuum state included). A truncated state is generated after performing photodetections in the global state. We find that, depending on which output ports each of the two photodetectors is positioned, different types of truncated states may be produced: i) states having a maximum Fock number of $N$, or ii) states having a minimum Fock number $N$. In order to illustrate our method, we discuss an example having as input states a coherent state in the beamsplitter and vacuum states in the amplifier, and show that the resulting truncated states display nonclassical properties, such as sub-Poissonian statistics and squeezing. We quantify the nonclassicality degree of the generated states using the Wigner-Yanase skew information measure. For complementarity, we discuss the efficiency of the protocol, e.g., generation probability as well as the effects of imperfections such as the detector's quantum efficiency and dark counts rate.

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arXiv2021

Quantum phase estimation using squeezed quasi-Bell states

In this paper we present a study of the quantum phase estimation problem employing continuous-variable, entangled squeezed coherent (quasi-Bell) states as probe states. We show that their inherent squeezing and entanglement properties might bring advantages, increasing the precision of phase estimation compared to protocols which employ other continuous variable states e.g., two-mode, entangled coherent states or single-mode, squeezed states. We also analyze the phase estimation process considering: i) a linear (unitary) perturbation, and ii) dissipation, and conclude that the use of entangled squeezed coherent states as probe states may still be advantageous even under non-ideal conditions.

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arXiv2020

Evolution of the quantized field coupled to a thermal bath: a phase space approach

We present an alternative method for describing the evolution of a mode of the quantized electromagnetic field in contact with a finite temperature thermal bath. We use the expansion of the field density operator in terms of coherent states and the related Glauber-Sudarshan $P$-function in phase space. The method allows us to obtain analytical expressions of the system's time-evolved $P$-function without needing to solve the corresponding master equation.

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arXiv2020

Sudden death of entanglement induced by a minimal thermal environment

We study the dynamics of two interacting two-level systems (qubits) having one of them isolated and the other coupled to a single mode electromagnetic field in a thermal state. The field plays the role of a small environment, in contrast to the usual approach of modeling an environment via a thermal reservoir with many degrees of freedom. We find the analytical solution of the proposed model, which allows us to investigate the consequences of the coupling to the small environment on characteristic quantum features of the two-qubit system. We study the time evolution of quantum entanglement and coherence, verifying the dependence on the relevant coupling constants as well as the influence of the effective temperature of the environment. Interestingly, we find that both sudden death and sudden birth of entanglement may occur in such a simple system. We also discuss a different partition, in which the isolated qubit is considered to be coupled to a composite environment, constituted by the other qubit plus the field mode.

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arXiv2018

Deviations from reversible dynamics in a qubit-oscillator system coupled to a very small environment

In this contribution it is considered a simple and solvable model consisting of a qubit in interaction with an oscillator exposed to a very small "environment" (a second qubit). An isolated qubit-oscillator system having the oscillator initially in one of its energy eigenstates exhibits Rabi oscillations, an evidence of coherent quantum behaviour. It is shown here in which way the coupling to a small "environment" disrupts such regular behaviour, leading to a quasi-periodic dynamics for the qubit linear entropy. In particular, it is found that the linear entropy is very sensitive to the amount of mixedness of the "environment". For completeness, fluctuations in the oscillator energy are also taken into account.

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arXiv2018

A simple model for a minimal environment: the two-atom Tavis-Cummings model revisited

Individual quantum systems may be interacting with surrounding environments having a small number of degrees of freedom. It is therefore relevant to understand the extent to which such small (but uncontrollable) environments could affect the quantum properties of the system of interest. Here we discuss a simple system-environment toy model, constituted by a two-level atom (atom 1) interacting with a single mode cavity field. The field is also assumed to be (weakly) coupled to an external noisy subsystem, the small environment, modeled as a second two-level atom (atom 2). We investigate the action of the minimal environment on the dynamics of the linear entropy (state purity) and the atomic dipole squeezing of atom 1, as well as the entanglement between atom 1 and the field. We also obtain the full analytical solution of the two atom Tavis-Cummings model for both arbitrary coupling strengths and frequency detunings, necessary to analyze the influence of the field-environment detuning on the evolution of the above mentioned quantum properties. For complementarity, we discuss the role of the degree of mixedness of the environment by analyzing the time-averaged linear entropy of atom 1.

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arXiv2018

Generation of hybrid entanglement between a mirror and a trapped ion

We present a scheme for cavity-assisted generation of hybrid entanglement between a moving mirror belonging to an optomechanical cavity and a single trapped ion located inside a second cavity. Due to radiation pressure, it is possible to entangle the moving mirror with one of the cavity fields. Also, by tuning the second cavity field with the internal degrees of freedom of the ion, an entangled state of the cavity field/ion can be independently generated. The fields leaking from each cavity are then combined in a beam-splitter, and following the detection of the outgoing photons by conveniently placed photodetectors, we show that it is possible to generate entangled states of the moving mirror and the single trapped ion subsystems. In our scheme the generated states are hybrid entangled states, in the sense that they are constituted by discrete (Fock) states and continuous variable (coherent) states.

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arXiv2017

Journeys from Quantum Optics to Quantum Technology

Sir Peter Knight is a pioneer in quantum optics which has now grown to an important branch of modern physics to study the foundations and applications of quantum physics. He is leading an effort to develop new technologies from quantum mechanics. In this collection of essays, we recall the time we were working with him as a postdoc or a PhD student and look at how the time with him has influenced our research.

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