arXiv subjects

Nicolás Quesada

Publications and source records attributed to Nicolás Quesada.

69 records · Page 4Linked to original sources

arXiv2018

Gaussian functions are optimal for waveguided nonlinear-quantum-optical processes

Many nonlinear optical technologies require the two-mode spectral amplitude function that describes them---the \emph{joint spectral amplitude} (JSA)---to be separable. We prove that the JSA factorizes \emph{only} when the incident pump field and phase-matching function are Gaussian functions. We show this by mapping our problem to a known result, in continuous variable quantum information, that only squeezed states remain unentangled when combined on a beam splitter. We then conjecture that only a squeezed state minimizes entanglement when sent through a beam splitter with another pre-specified ket. This implies that to maximize JSA separability when one of the (pump or nonlinear medium) functions is non-Gaussian, the other function \emph{must} be Gaussian. This answers an outstanding question about optimal design of certain nonlinear processes, and is of practical interest to researchers using waveguide nonlinear optics to generate and manipulate quantum light.

quant-ph↗

arXiv2017

Why you should not use the electric field to quantize in nonlinear optics

We show that using the electric field as a quantization variable in nonlinear optics leads to incorrect expressions for the squeezing parameters in spontaneous parametric down-conversion and conversion rates in frequency conversion. This observation is related to the fact that if the electric field is written as a linear combination of bosonic creation and annihilation operators one cannot satisfy Maxwell's equations in a nonlinear dielectric.

quant-ph↗

arXiv2016

Physical Resources for Quantum-enhanced Phase Estimation

We study the role of quantum entanglement (particle entanglement and mode entanglement) in optical phase estimation by employing the first and second quantization formalisms of quantum mechanics. The quantum Fisher information (QFI) is expressed as a function of the first and second order optical coherence functions. The resulting form of the QFI elucidates the deriving metrological resources for quantum phase estimation: field intensity and photon detection correlations. In addition, our analysis confirms that mode entanglement is not required for quantum-enhanced interferometry, whereas particle entanglement is a necessary requirement.

quant-ph↗

arXiv2016

High efficiency in mode selective frequency conversion

Frequency conversion (FC) is an enabling process in many quantum information protocols. Recently, it has been observed that upconversion efficiencies in single-photon, mode-selective FC are limited to around 80%.In this letter we argue that these limits can be understood as time-ordering corrections (TOCs) that modify the joint conversion amplitude of the process. Furthermore we show, using a simple scaling argument, that recently proposed cascaded FC protocols that overcome the aforementioned limitations act as "attenuators" of the TOCs. This observation allows us to argue that very similar cascaded architectures can be used to attenuate TOCs in photon generation via spontaneous parametric down-conversion. Finally, by using the Magnus expansion, we argue that the TOCs, which are usually considered detrimental for FC efficiency, can also be used to increase the efficiency of conversion in partially mode selective FC.

quant-ph↗

arXiv2015

Quantum correlations in optical metrology: Heisenberg-limited phase estimation without mode entanglement

The quantum fisher information and quantum correlation parameters are employed to study the application of non-classical light to the problem of parameter estimation. It is shown that the optimal measurement sensitivity of a quantum state is determined by its inter-mode correlations (which depends of path-entanglement) and intra-mode correlations (which depends of the photon statistics). In light of these results, we consider the performance of quantum-enhanced optical interferometers. Furthermore, we propose a Heisenberg-limited metrology protocol involving standard elements from passive and active linear optics, for which the quantum Cramér-Rao bound is saturated with an intensity measurement. Interestingly, the quantum advantage for this scheme is derived solely from the non-classical photon statistics of the probe state and does not depend of entanglement. We study the performance of this scheme in the presence of realistic losses and consequently predict a substantial enhancement over the shot-noise limit with current technological capabilities.

quant-ph↗

arXiv2014

Time ordering effects in the generation of entangled photons using nonlinear optical processes

We study the effects of time ordering in photon generation processes such as spontaneous parametric down-conversion (SPDC) and four wave mixing (SFWM). The results presented here are used to construct an intuitive picture that allows us to predict when time ordering effects significantly modify the joint spectral amplitude (JSA) of the photons generated in SPDC and SFWM. These effects become important only when the photons being generated lie with the pump beam that travels through the non-linear material for a significant amount of time. Thus sources of spectrally separable photons are ideal candidates for the observation of modifications of the JSA due to time ordering.

quant-ph↗

arXiv2014

Effects of time ordering in quantum nonlinear optics

We study time ordering corrections to the description of spontaneous parametric down-conversion (SPDC), four wave mixing (SFWM) and frequency conversion (FC) using the Magnus expansion. Analytic approximations to the evolution operator that are unitary are obtained. They are Gaussian preserving, and allow us to understand order-by-order the effects of time ordering. We show that the corrections due to time ordering vanish exactly if the phase matching function is sufficiently broad. The calculation of the effects of time ordering on the joint spectral amplitude of the photons generated in spontaneous SPDC and SFWM are reduced to quadrature.

quant-ph↗

arXiv2013

Bound entanglement in the Jaynes-Cummings model

We study in detail entanglement properties of the Jaynes-Cummings model assuming a two-level atom (qubit) interacting with the first $N$ levels of an electromagnetic field mode (qudit) in a cavity. In the Jaynes-Cummings model, the number operator is the conserved quantity that allows for the exact diagonalization of the Hamiltonian and thus we study states that commute with this conserved quantity and whose structure is preserved under the Jaynes-Cummings dynamics. Contrary to the common belief, we show that there are bound entangled states that satisfy the symmetries imposed by the conservation of the number of excitations when $N>3$. Furthermore we show that \emph{the Jaynes-Cummings interaction can be used to generate bound-entanglement} between the atom and the mode.

quant-ph↗

arXiv2013

Self-calibrating tomography for multi-dimensional systems

We present a formalism for self-calibrating tomography of arbitrary dimensional systems. Self-calibrating quantum state tomography was first introduced in the context of qubits, and allows the reconstruction of the density matrix of an unknown quantum state despite incomplete knowledge of the unitary operations used to change the measurement basis. We show how this can be generalized to qudits, i.e. d-level systems, and provide a specific example for a V-type three-level atomic system whose transition dipole moments are not known. We show that it is always possible to retrieve the unknown state and process parameters, except for a set of zero measure in the state-parameter space.

quant-ph↗

arXiv2012

Quantum properties and dynamics of X states

X states are a broad class of two-qubit density matrices that generalize many states of interest in the literature. In this work, we give a comprehensive account of various quantum properties of these states, such as entanglement, negativity, quantum discord and other related quantities. Moreover, we discuss the transformations that preserve their structure both in terms of continuous time evolution and discrete quantum processes.

quant-ph↗

arXiv2012

Strong coupling of two quantum emitters to a single light mode: the dissipative Tavis-Cummings ladder

A criterion for strong coupling between two quantum emitters and a single resonant light mode in a cavity is presented. The criterion takes into account the escape of cavity photons and the spontaneous emission of the emitters, which are modeled as two level systems. By using such criterion, the dissipative Tavis-Cummings ladder of states is constructed, and it is shown that the inclusion of one more emitter with respect to the Jaynes-Cummings (single emitter) case increases the effective parameter region in which $n^{\text{th}}$ order Rabi splitting is observed.

quant-ph↗

arXiv2011

Strong coupling of two interacting excitons confined in a nanocavity-quantum-dot system

We present a study of the strong coupling between radiation and matter, considering a system of two quantum dots, which are in mutual interaction and interacting with a single mode of light confined in a semiconductor nanocavity. We take into account dissipative mechanisms such as the escape of the cavity photons, decay of the quantum dot excitons by spontaneous emission, and independent exciton pumping. It is shown that the mutual interaction between the dots can be measured off-resonance, only if the strong coupling condition is reached. Using the quantum regression theorem, a reasonable definition of the dynamical coupling regimes is introduced in terms of the complex Rabi frequency. Finally, the emission spectrum for relevant conditions is presented and compared with the above definition, demonstrating that the interaction between the excitons does not affect the Strong Coupling.

cond-mat.str-el↗

arXiv2010

Density operator of a system pumped with polaritons: A Jaynes-Cummings like approach

We investigate the effects of considering two different incoherent pumpings over a microcavity-quantum dot system modelled using the Jaynes-Cummings Hamiltonian. When the system is incoherently pumped with polaritons it is able to sustain a large number of photons inside the cavity with Poisson-like statistics in the stationary limit, and also leads to a separable exciton-photon state. We also investigate the effects of both types of pumpings (Excitonic and Polaritonic) in the emission spectrum of the cavity. We show that the polaritonic pumping as considered here is unable to modify the dynamical regimes of the system as the excitonics pumping does. Finally, we obtain a closed form expression for the negativity of the density matrices that the quantum master equation considered here generates.

cond-mat.mes-hall↗

arXiv2010

Melting of Lennard-Jones rare gas clusters doped with a single impurity atom

Single impurity effect on the melting process of magic number Lennard-Jones, rare gas, clusters of up to 309 atoms is studied on the basis of Parallel Tempering Monte Carlo simulations in the canonical ensemble. A decrease on the melting temperature range is prevalent, although such effect is dependent on the size of the impurity atom relative to the cluster size. Additionally, the difference between the atomic sizes of the impurity and the main component of the cluster should be considered. We demonstrate that solid-solid transitions due to migrations of the impurity become apparent and are clearly differentiated from the melting up to cluster sizes of 147 atoms.

physics.atm-clus↗

arXiv2010

Comment on: "Luminescence spectra of quantum dots in microcavities."

In this comment we show that there is a direct connection between coherent exchange of energy among light and matter and the emission spectrum of a microcavity quantum dot system as modeled in Phys. Rev. B 79, 235325 (2009) [arXiv:0807.3194] by F. P. Laussy, E. del Valle, and C. Tejedor. To do so, we show that in their model the necessary and sufficient conditions for having eigenvalues with non-zero imaginary parts in the propagator of the bare mode populations, are the same as for having strong coupling in the emission spectrum. This amounts to saying that, whenever there is strong coupling there will be oscillating frequencies in the dynamics of the populations. These conditions are valid both for the case where matter is treated as bosonic or fermionic, in the spontaneous emission case.

cond-mat.mes-hall↗