arXiv subjects

Carlos Tejedor

Publications and source records attributed to Carlos Tejedor.

16 recordsLinked to original sources

arXiv2025

Cavity Control of Topological Qubits: Fusion Rule, Anyon Braiding, and Majorana-Schrödinger Cat States

We investigate the effects of coupling a local electromagnetic cavity to a segment of a topological Kitaev chain (KC), with particular emphasis on the interplay between photons and Majorana zero modes (MZMs). In addition to the well-known {\it scissor effect}-which effectively partitions the chain and isolates free MZMs in the bulk-we provide evidence of non-trivial fusion rules and braiding operations, hallmark signatures of non-Abelian anyons, enabled by spatially selective ultrastrong KC-cavity coupling. We propose that these distinctive MZM properties can be experimentally probed via fermionic parity measurements and photon-induced Berry phases. Furthermore, we demonstrate that, in the so-called sweet-spot regime, the coupled system can be mapped onto a Rabi-like model with a homodyne-rotated quadrature, offering a simplified yet powerful theoretical description. Exploiting the symmetry of fermionic modes within a two-site cavity configuration, we also show the feasibility of generating hybrid MZM-polariton Schrödinger cat states. Our findings offer a novel approach to manipulating topological quantum matter through local light-matter interactions and provide theoretical tools for future experimental realizations in platforms such as quantum materials or superconducting circuits.

quant-ph↗

arXiv2017

Filtering Multiphoton Emission from State-of-the-Art Cavity QED

Engineering multiphoton states is an outstanding challenge with applications in multiple fields, such as quan- tum metrology, quantum lithography or even biological systems. State-of-the-art methods to obtain them rely on post-selection, multi-level systems or Rydberg atomic ensembles. Recently, it was shown that a strongly driven two-level system interacting with a detuned cavity mode can be engineered to continuously emit n-photon states. In the present work, we show that spectral filtering of its emission relaxes considerably the requirements on the system parameters even to the more accessible bad-cavity situation, opening up the possibility of implementing this protocol in a much wider landscape of different platforms. This improvement is based on a key observation: in the imperfect case where only a certain fraction of emission is composed of n-photon states, these have a well defined energy separated from the rest of the signal, which allows to reveal and purify multiphoton emission just by frequency filtering. We demonstrate these results by obtaining analytical expressions for relevant figures of merit of multiphoton emission, such as the n-photon coupling rate between cavity and emitter, the fraction of light emitted as n-photon states, and n-photon emission rates. This allows us to make a systematic study of such figures of merit as a function of the system parameters and demonstrate the viability of the protocol in several relevant types of cavity QED setups, where we take into account the impact of their respective experimental limitations.

quant-ph↗

arXiv2015

Enhanced two-photon emission from a dressed biexciton

Radiative two-photon cascades from biexcitons in semiconductor quantum dots under resonant two-photon excitation are promising candidates for the generation of photon pairs. In this work, we propose a scheme to obtain two-photon emission that allows to operate under very intense driving fields. This approach relies on the Purcell enhancement of two-photon virtual transitions between states of the biexciton dressed by the laser. The richness provided by the biexcitonic level structure allows to reach a variety of regimes, from antibunched and bunched photon pairs with polarization orthogonal to the driving field, to polarization entangled two-photon emission. This evidences that the general paradigm of two-photon emission from a ladder of dressed states can find interesting, particular implementations in a variety of systems.

quant-ph↗

arXiv2014

Violation of classical inequalities by photon frequency-filtering

The violation of the Cauchy-Schwarz and Bell inequalities ranks among the major evidences of the genuinely quantum nature of an emitter. We show that by dispensing from the usual approximation of mode correlations and studying directly correlations between the physical reality -the photons- these violations can be optimized. This is achieved by extending the concept of photon correlations to all frequencies in all the possible windows of detections, with no prejudice to the supposed origin of the photons. We identify the regions of quantum emission as rooted in collective de-excitation involving virtual states instead of, as previously assumed, cascaded transitions between real states.

quant-ph↗

arXiv2012

Theory of frequency-filtered and time-resolved N-photon correlations

A theory of correlations between N photons of given frequencies and detected at given time delays is presented. These correlation functions are usually too cumbersome to be computed explicitly. We show that they are obtained exactly through intensity correlations between two-level sensors in the limit of their vanishing coupling to the system. This allows the computation of correlation functions hitherto unreachable. The uncertainties in time and frequency of the detection, which are necessary variables to describe the system, are intrinsic to the theory. We illustrate the formalism with the Jaynes--Cummings model, showing how correlations of various peaks at zero or finite time delays bring new insights into the dynamics of open quantum systems.

quant-ph↗

arXiv2010

Dissipative dynamics of a solid-state qubit coupled to surface plasmons: from non-Markov to Markov regimes

We theoretically study the dissipative dynamics of a quantum emitter placed near the planar surface of a metal supporting surface plasmon excitations. The emitter-metal coupling regime can be tuned by varying some control parameters such as the qubit-surface separation and/or the detuning between characteristic frequencies. By using a Green's function approach jointly with a time-convolutionless master equation, we analyze the non-Markovian dissipative features on the qubit time evolution in two cases of interest: i) an undriven qubit initially prepared in its excited state and ii) the evolution towards a steady-state for a system driven by a laser field. For weak to moderate qubit-metal coupling strength, and on timescales large compared to the surface plasmon oscillation time, a Markovian approximation for the master equation results to be adequate to describe the qubit main optical properties: surface enhancements of rate emission, optical spectra and time-dependent photon-photon correlation functions. The qubit decay shows a crossover passing from being purely dissipative for small qubit-surface distances to plasmon emission for larger separations.

cond-mat.mes-hall↗

arXiv2010

Two-photon lasing by a single quantum dot in a high-Q microcavity

We investigate theoretically two-photon processes in a microcavity containing one quantum dot in the strong coupling regime. The cavity mode can be tuned to resonantly drive the two-photon transition between the ground and the biexciton states, while the exciton states are far-off resonance due to the biexciton binding energy. We study the steady state of the quantum dot and cavity field in presence of a continuous incoherent pumping. We identify the regime where the system acts as two-photon emitter and discuss the feasibility and performance of realistic single quantum dot devices for two-photon lasing.

cond-mat.mes-hall↗

arXiv2009

Luminescence Spectra of Quantum Dots in Microcavities. II. Fermions

We discuss the luminescence spectra of coupled light-matter systems realized with semiconductor heterostructures in microcavities in the presence of a continuous, incoherent pumping, when the matter field is Fermionic. The linear regime--which has been the main topic of investigation both experimentally and theoretically--converges to the case of coupling to a Bosonic material field, and has been amply discussed in the first part of this work. We address here the nonlinear regime, and argue that, counter to intuition, it is better observed at low pumping intensities. We support our discussion with particular cases representative of, and beyond, the experimental state of the art. We explore the transition from the quantum to the classical regime, by decomposing the total spectrum into individual transitions between the dressed states of the light-matter coupling Hamiltonian, reducing the problem to the positions and broadenings of all possible transitions. As the system crosses to the classical limit, rich multiplet structures mapping the quantized energy levels melt and turn to cavity lasing and to an incoherent Mollow triplet in the direct exciton emission for very good structure. Less ideal figures of merit can still betray the quantum regime, with a proper balance of cavity versus electronic pumping.

cond-mat.mes-hall↗

arXiv2008

Luminescence Spectra of Quantum Dots in Microcavities. I. Bosons

We provide a unified theory of luminescence spectra of coupled light-matter systems realized with semiconductor heterostructures in microcavities, encompassing: i) the spontaneous emission case, where the system decays from a prepared (typically pure) initial state, and ii) luminescence in the presence of a continuous, incoherent pump. While the former case has been amply discussed in the literature (albeit mainly for the case of resonance), no consideration has been given to the influence of the incoherent pump. We show how, by provoking a self-consistent quantum state, the pump considerably alters the emission spectra, even at vanishing intensities. The main outcome of our analysis is to unambiguously identify strong-coupling in situations where it appears in disguise or only seems to appear. Here, we consider bosonic matter fields, in which case fully analytical solutions can be obtained. This describes the case of quantum wells or large quantum dots, or the limit of low excitation where the average populations remain much smaller than one.

cond-mat.mes-hall↗

arXiv2008

Strong-coupling of quantum dots in microcavities

We show that strong-coupling (SC) of light and matter as it is realized with quantum dots (QDs) in microcavities differs substantially from the paradigm of atoms in optical cavities. The type of pumping used in semiconductors yields new criteria to achieve SC, with situations where the pump hinders, or on the contrary, favours it. We analyze one of the seminal experimental observation of SC of a QD in a pillar microcavity [Reithmaier et al., Nature (2004)] as an illustration of our main statements.

cond-mat.mes-hall↗

arXiv2007

Electrostatic control of quantum dot entanglement induced by coupling to external reservoirs

We propose a quantum transport experiment to prepare and measure charge-entanglement between two electrostatically defined quantum dots. Coherent population trapping, as realized in cavity quantum electrodynamics, can be carried out by using a third quantum dot to play the role of the optical cavity. In our proposal, a pumping which is quantum mechanically indistinguishable for the quantum dots drives the system into a state with a high degree of entanglement. The whole effect can be switched on and off by means of a gate potential allowing both state preparation and entanglement detection by simply measuring the total current.

cond-mat.mes-hall↗

arXiv2007

Tuning the conductance of a molecular switch

The ability to control the conductance of single molecules will have a major impact in nanoscale electronics. Azobenzene, a molecule that changes conformation as a result of a trans/cis transition when exposed to radiation, could form the basis of a light-driven molecular switch. It is therefore crucial to clarify the electrical transport characteristics of this molecule. Here, we investigate theoretically charge transport in a system in which a single azobenzene molecule is attached to two carbon nanotubes. In clear contrast to gold electrodes, the nanotubes can act as true nanoscale electrodes and we show that the low-energy conduction properties of the junction may be dramatically modified by changing the topology of the contacts between the nanotubes and the molecules, and/or the chirality of the nanotubes (that is, zigzag or armchair). We propose experiments to demonstrate controlled electrical switching with nanotube electrodes.

cond-mat.mes-hall↗

arXiv2006

Scaling of the conductance in gold nanotubes

A new form of gold nanobridges has been recently observed in ultrahigh-vacuum experiments, where the gold atoms rearrange to build helical nanotubes, akin in some respects to carbon nanotubes. The good reproducibility of these wires and their unexpected stability will allow for conductance measurements and make them promising candidates for future applications . We present here a study of the transport properties of these nanotubes in order to understand the role of chirality and of the different orbitals in quantum transport observables. The conductance per atomic row shows a light decreasing trend as the diameter grows, which is also shown through an analytical formula based on a one-orbital model.

cond-mat.mes-hall↗

arXiv2004

Defective transport properties of three-terminal carbon nanotube junctions

We investigate the transport properties of three terminal carbon based nanojunctions within the scattering matrix approach. The stability of such junctions is subordinated to the presence of nonhexagonal arrangements in the molecular network. Such "defective" arrangements do influence the resulting quantum transport observables, as a consequence of the possibility of acting as pinning centers of the correspondent wavefunction. By investigating a fairly wide class of junctions we have found regular mutual dependencies between such localized states at the carbon network and a strikingly behavior of the conductance. In particular, we have shown that Fano resonances emerge as a natural result of the interference between defective states and the extended continuum background. As a consequence, the currents through the junctions hitting these resonant states might experience variations on a relevant scale with current modulations of up to 75%.

cond-mat.mes-hall↗

arXiv2000

Low temperature transport in AC-driven Quantum Dots in the Kondo regime

We present a fully nonequilibrium calculation of the low temperature transport properties of a quantum dot in the Kondo regime when an AC potential is applied to the gate voltage. We solve a time dependent Anderson model with finite on-site Coulomb interaction. The interaction self-energy is calculated up to second order in perturbation theory in the on-site interaction, in the context of the Keldysh non-equilibrium technique, and the effect of the AC voltage is taken into account exactly for all ranges of AC frequencies and AC intensities. The obtained linear conductance and time-averaged density of states of the quantum dot evolve in a non trivial way as a function of the AC frequency and AC intensity of the harmonic modulation.

cond-mat.str-el↗

arXiv1998

Kondo effect in AC transport through Quantum dots

A theory of the Kondo effect in quantum dots at zero temperature in the presence of arbitrary intense AC potentials is presented.We generalize the Friedel-Langreth sum rule to take care of charge conservation and propose a consistent procedure to study a time dependent Anderson Hamiltonian. The effect of the AC potential on both the quantum dot density of states and the linear conductance shows the importance of using a theory which describes intradot finite interaction and nonperturbative effects.

cond-mat.str-el↗