C. Tejedor

Violation of classical inequalities by photon frequency filtering

The violation of the Cauchy-Schwarz and Bell inequalities ranks among the major evidence of the genuinely quantum nature of an emitter. The conventional theoretical approaches associate operators with spectral lines to study correlations between photons from real-state transitions. Instead, we use a formalism that studies directly correlations between the physical reality---the photons---with no prejudice as to their origin. This allows us to extend photon correlations to all frequencies in all the possible windows of detection and to reveal landscapes of two-photon correlations that delineate regions of quantum emission, i.e., where classical inequalities are violated. We show that quantum correlations are rooted in the joint emission of two photons involving virtual states of the emitter instead of, as previously assumed, cascaded transitions between real states. As a result, correlations can be optimized in a process akin to distillation by keeping only the emission which is quantum and filtering out that which is not.

Two-photon spectra of quantum systems

Summary form only given. We present our recent theory [1] of frequency-filtered and time-resolved N-photon correlations or N-photon spectra, g^(N)_Γ (ω₁, T₁;... ; ω_N T_N). There are an extension of the standard Nth order temporal correlation functions, g^(N) (T₁;... ;T_N) (that measure the probability of emission of N photons at times T₁,..., T_N), into the frequency domain. Heisenberg uncertainty principle requires the introduction of the detector or filter frequency resolution, Γ, in the theory.

Dephasing of strong coupling in the non‐linear regime

We study the effect of pure dephasing on the strong‐coupling of a two‐level system (typically, a quantum dot) with the single mode of a microcavity. Whereas dephasing merely broadens the line in the linear regime, it results in a new spectral shape when transitions between higher manifolds of the Jaynes‐Cummings ladder are involved, namely, a triplet, that we show does not imply weak‐coupling. Higher excitation brings the system into lasing. Description of this intermediate triplet allows to demonstrate quantum nonlinearities even in noisy systems and to measure the magnitude of dephasing.

Buildup and decay of the coherence in a polariton condensate

We report the dynamics of a polariton condensate, created by a nonresonant light pulse, and the determination of its coherence buildup and decay. The time‐resolved experiments show clearly the interplay between the relevant quantities of polariton condensates: population, linewidth and polarization. A comparison with the results of a Lindblad master equation, describing the dynamics of the system coupled to a reservoir, demonstrates the importance of both the pulse shape and the flow of particles between the ground‐state and the reservoir in the build‐up of the coherence.

Quantitative Description of Strong‐Coupling of Quantum Dots in Microcavities

We have recently developed a self‐consistent theory of Strong‐Coupling in the presence of an incoherent pumping [arXiv:0807.3194] and shown how it could reproduce quantitatively the experimental data [PRL, 101 (2008)]. Here, we summarize our main results, provide the detailed analysis of the fitting of the experiment and discuss how the field should now evolve beyond merely qualitative expectations, that could well be erroneous even when they seem to be firmly established.

Observation of Quantum Hydrodynamic Effects in Microcavity Polaritons

We study the hydrodynamics of a coherent polariton fluid lasting for hundreds of picoseconds and moving inside of a semiconductor microcavity. The momentum of the polariton droplets can be well controlled by selecting the angle and energy of two excitation laser beams. We observe unique behaviours due to the quantum nature of the polariton states, such as the linearization of the polariton dispersion around the energy of the signal states, and the propagation at constant speed of the polariton fluids. The experiments have been realized making use of a new technique for the detection of images simultaneously resolved in time, energy and real‐ or reciprocal‐ space.

Quantum regression formula and luminescence spectra of two coupled modes under incoherent continuous pumping

We study the quantum regression formula for two coupled dissipative modes in the steady state under incoherent continuous pumping. We analyze the equations for one and two‐time correlators, needed to compute the spectra of emission of the system, for two coupled harmonic oscillators (linear model), on the one hand, and two coupled two‐level systems, on the other hand. We present a comparison between them, on the basis of fully analytical results.

Cavity quantum electrodynamics for two quantum dots

We analyse the decay rate of one and two quantum dots coupled to a cavity mode in all coupling regimes. In the weak coupling regime, the system undergoes a superradiant cascade in case of identical detunings with the cavity. We study how adding inhomogeneous detuning induces oscillations which can enhace or destroy the cooperative effect.