Phonon-assisted entanglement between two quantum dots coupled to a plasmonic nanocavity

We theoretically investigate the influence of a phonon bath on entangled generation between two semiconductor quantum dots coupled to a plasmonic nanocavity and coherently driven by an external optical field. Our study reveals that phonons could be useful in a certain range of system parameters where a subradian state preparation is possible. We point out that this behavior is due to the fact that phonon environment modify the radiative properties of the two-qubit inducing different decay rates between the collective states of the system. The influence of the factors characteristic of quantum dot systems and plasmonic nanocavity such as energy mismatch in different dots, different plasmonic cavity losses as well as coupling to phonons is analyzed. The entanglement can be controlled via different external parameters such as the cavity detuning, the Rabi frequency of the driving field and the resonance frequency of the QDs

Entangling non planar molecules via inversion doublet transition with negligible spontaneous emission

We analyze theoretically the entanglement between two non-planar and light identical molecules (e.g., pyramidal NH3) that present inversion doubling due to the internal spatial inversion of their nuclear conformations by tunneling. The peculiarity of this system lies in the simplicity of this type of molecular system in which two near levels can be connected by an allowed electric dipole transition with considerable value of the dipole moment transition and negligible spontaneous emission because the transition is in the microwave or far-infrared range. These properties give place to entanglement states oscillating by free evolution with frequency determined by the dipole-dipole interaction and negligible spontaneous decay, which allows consideration of an efficient quantum Zeno effect by frequent measurements of one of the entangled states. If the molecules are initially both in the upper (or lower) eigenstate, the system evolves under an external radiation field, which can induce oscillations of the generated entangled states, with frequency of the order of the Rabi frequency of the field. For a certain detuning, a symmetric entangled state, which is an eigenstate of the collective system, can be populated, and given its negligible spontaneous emission, could be maintained for a time limited only by external decoherence processes, which could be minimized. Although the data used are those of the NH3 molecule, other molecules could present the same advantageous features

Absorption of a weak probe in singly charged n-doped quantum dots in the Voigt geometry

We analyze the absorption of a weak probe field in singly charged n-doped quantum dots in the presence of an external magnetic field in the Voigt geometry. The dots are modeled as double lambda systems. A laser field pumps two of the four active transitions while a probe field is applied along the two undriven transitions. It is shown that for both on resonance and out of resonance driving gain (without inversion) is obtained at two sidebands of the dressed QDs. We found that the peak value of gain can be maximized by a proper selection of both the external magnetic field and the Rabi frequency of the pump field for out of resonance driving. We also show that the coherent driving in combination with above-band excitation allows one to turn the absorption peaks into gain (with inversion) in the whole spectral range

Resonance fluorescence spectrum of a \Lambda-type quantum emitter close to a metallic nanoparticle

We theoretically study the resonance fluorescence spectrum of a three-level quantum emitter coupled to a spherical metallic nanoparticle. We consider the case that the quantum emitter is driven by a single laser field along one of the optical transitions. We show that the development of the spectrum depends on the relative orientation of the dipole moments of the optical transitions in relation to the metal nanoparticle. In addition, we demonstrate that the location and width of the peaks in the spectrum are strongly modified by the exciton-plasmon coupling and the laser detuning, allowing to achieve controlled strongly subnatural spectral line. A strong antibunching of the fluorescent photons along the undriven transition is also obtained. Our results may be used for creating a tunable source of photons which could be used for a probabilistic entanglement scheme in the field of quantum information processing

Nonlinear optical rectification and optical bistability in a coupled asymmetric quantum dot-metal nanoparticle hybrid

We study the optical response of a coupled asymmetric semiconductor quantum dot-spherical metal nanoparticle structure. The asymmetric quantum dot has permanent electric dipole moments that also interact with light. We derive the density matrix equations for the system including the modification of the electric field and the exciton-plasmon coupling. We emphasize on the effects of the nonlinear optical rectification and controlled optical bistability and analyze these phenomena for different values of the light intensity and different distances between the quantum dot and the metal nanoparticle. We show that when the system is set in a situation where optical bistability can be produced, the optical rectification of the hybrid system is bivalued. We also analyze the slow-down to reach the steady state when the system is driven close and far from the turning points

Squeezing in a Lambda-type three-level atom via spontaneously generated coherence

The squeezing spectrum of the fluorescent light is investigated for a laser-driven three-level atom of the Lambda configuration when quantum interference of the decay channels is accounted for. We show that when the two atomic transitions contribute to the detected fluorescence field, squeezing at certain frequency intervals is obtained in both the weak- and the high-Rabi-frequency regimes even for equally decay rates of the transitions. Unlike in two-level atoms in free space, squeezing can be obtained in both the in-phase and out-of-phase quadrature spectra although in different spectral regions. We also show that the squeezing spectrum can be controlled by an adequate selection of the Rabi frequencies and atomic detunings. Another remarkable effect is that squeezing can be achieved with proper relative phases of the driving fields. We provide an analytical description in the dressed basis which accounts for the main features of the squeezing spectra obtained from the numerical work

Optical and microwave control of resonance fluorescence and squeezing spectra in a polar molecule

A two-level quantum emitter with broken inversion symmetry simultaneously driven by an optical field and a microwave field that couples to the permanent dipole's moment is presented. We focus to a situation where the angular frequency of the microwave field is chosen such that it closely matches the Rabi frequency of the optical field, the so-called Rabi resonance condition. Using a series of unitary transformations we obtain an effective Hamiltonian in the double-dressed basis which results in easily solvable Bloch equations which allow us to derive analytical expressions for the spectrum of the scattered photons. We analyze the steady-state population inversion of the system which shows a distinctive behavior at the Rabi resonance with regard to an ordinary two-level nonpolar system. We show that saturation can be produced even in the case that the optical field is far detuned from the transition frequency, and we demonstrate that this behavior can be controlled through the intensity and the angular frequency of the microwave field. The spectral properties of the scattered photons are analyzed and manifest the emergence of a series of Mollow-like triplets which may be spectrally broadened or narrowed for proper values of the amplitude and/or frequency of the low-frequency field. We also analyze the phase-dependent spectrum which reveals that a significant enhancement or suppression of the squeezing at certain sidebands can be produced. These quantum phenomena are illustrated in a recently synthesized molecular complex with high nonlinear optical response although they can also occur in other quantum systems with broken inversion symmetry

Photon statistics of a quantum emitter close to a lattice of plasmonic nanoparticles

We study theoretically the statistics of photons generated by a quantum emitter located in the vicinity of a periodic plasmonic nanostructure. The presented formalism is based on a macroscopic QED formalism in conjunction with a density-matrix approach in order to obtain the second-order correlation function of the emitted photons accounting for the influence of the plasmonic environment. The metallic reservoir coupling is computed using Green's-function theory, which, for a periodic lattice of scatterers, is calculated by a multiple-scattering method. We show that the photon statistics and the antibunching of emitted photons depend very strongly on the orientation of the quantum emitter relative to the lattice, on the transition frequency of the emitter, on the intensity of the applied field, and on the geometrical parameters of the nanoparticles, such as the shell thickness

Phase tunability of group velocity by modulated-pump-forced coherent population oscillations

We propose a technique to obtain slow and fast light propagations based on coherent population oscillations forced by a modulated pump. This mechanism produces an enhancement of 1 order of magnitude of the delay or advancement of light signals. The relative phase between the pumps to the signal fields is used as a knob for changing light propagation from ultraslow group velocities to negative group velocities. The experimental realization of the phenomenon was carried out in an erbium-doped fiber amplifier at room temperature

Optical pumping of a single hole spin in a p-doped quantum dot coupled to a metallic nanoparticle

The preparation of quantum states with a defined spin is analyzed in a hybrid system consisting of a p-doped semiconductor quantum dot (QD) coupled to a metallic nanoparticle. The quantum dot is described as a four-level atom-like system using the density matrix formalism. The lower levels are Zeeman-split hole spin states and the upper levels correspond to positively charged excitons containing a spin-up, spin-down hole pair and a spin electron. A metallic nanoparticle with spheroidal geometry is placed in close proximity to the quantum dot, and its effects are considered in the quasistatic approximation. A linearly polarized laser field drives two of the optical transitions of the QD and produces localized surface plasmons in the nanoparticle which act back upon the QD. The frequencies of these localized plasmons are very different along the two principal axes of the nanoparticle, thus producing an anisotropic modification of the spontaneous emission rates of the allowed optical transitions which is accompanied by local-field corrections. This effect translates into a preferential acceleration of some of the optical pathways and therefore into a fast initialization of the QD by excitation with a short optical pulse. The population transfer between the lower levels of the QD and the fidelity is analyzed as a function of the nanoparticle's aspect ratio, the external magnetic field, and the Rabi frequency of the driving field. It is also shown that the main effect of the local-field corrections is a lengthening of the time elapsed to reach the steady-state. The hole spin is predicted to be successfully cooled from 5 to 0.04 K at a magnetic field of 4.6 T applied in the Voigt geometry