Colloidal quantum dots

The applications and physical properties of colloidal quantum dots are briefly reviewed and contrasted with those of Stransky–Krastanov grown quantum dots. To cite this article: P. Guyot-Sionnest, C. R. Physique 9 (2008).     

Tsallis' quantum q-fields

We generalize several well known quantum equations to a Tsallis' q-scenario, and provide a quantum version of some classical fields associated with them in the recent literature. We refer to the q-Schro¨dinger, q-KleinGordon, q-Dirac, and q-Proca equations advanced in, respectively, Phys. Rev. Lett. 106, 140601(2011), EPL 118,61004(2017) and references therein. We also introduce here equations corresponding to q-Yang-Mills fields, both in the Abelian and non-Abelian instances. We show how to define the q-quantum field theories corresponding to the above equations, introduce the pertinent actions, and obtain equations of motion via the minimum action principle.These q-fields are meaningful at very high energies(Te V scale) for q = 1.15, high energies(Ge V scale) for q = 1.001,and low energies(Me V scale) for q =1.000001 [Nucl. Phys. A 955(2016) 16 and references therein].(See the ALICE experiment at the LHC). Surprisingly enough, these q-fields are simultaneously q-exponential functions of the usual linear fields' logarithms.     

Accurate modelling of InGaN quantum wells

The internal field, the band structure and the oscillator strengths of the optical transitions of wurtzite strained InGaN quantum wells are accurately computed by a self-consistent solution of the Poisson equation and an eight-band k · p Schrödinger equation taking into account charges due to polarisation fields, doping and free carriers. The results are used to compare luminescence and gain spectra for single and triple quantum well structures and to elucidate the effect of the polarisation fields.     

Semiconductor heterostructures for spintronics and quantum information

A selection of optical experiments is presented, demonstrating the utility of semiconductors in two novel areas of research: spintronics and quantum information. First we show examples of spin manipulation in semiconductor quantum wells. The light is used to generate a spin polarization and to detect it. Next we discuss application of optical methods in studies of carrier-induced ferromagnetism in quantum wells. Finally, we present examples of single quantum dot spectroscopy related to perspectives of application of quantum dots in quantum information, and, in particular, the use of photon correlation measurements as a tool to study the quantum dot excitation mechanisms. To cite this article: J.A. Gaj et al., C. R. Physique 8 (2007).     

Cavity QED effects with single quantum dots

A single quantum dot embedded in a photonic crystal defect cavity allows for the investigation of cavity quantum electrodynamics effects in a solid-state environment. We present experiments demonstrating the quantum nature of this fundamental system in the strong coupling regime. Photon correlation measurements are used to characterize the fundamental properties of this unique system: through these experiments, we identify an unexpected, efficient sustaining mechanism that ensures strong cavity emission and is quantum correlated with the exciton resonance, even when all the quantum dot resonances are far detuned from the cavity mode. To cite this article: A. Badolato et al., C. R. Physique 9 (2008).     

A classical model of EPR experiment with quantum mechanical correlations and bell inequalities

A simple model of a classical break-up process is given in which the correlation E(a,b) of the components A and B of the spins of the two subsystems along directions a and b gives precisely the quantum mechanical result $\text{?cos(}\text{a·b}\text{)}$. The model is “local”, but the normalization procedure of correlation functions in terms of “hidden variables” is different from that used in deriving Bell's inequalities. A discretization procedure of the classical spins is then given which reproduces fully the dichotomous quantum mechanical results both for probabilities and for correlation functions. This procedure illustrates particularly clearly the difference between quantum and classical spins and provides a possible intuitive picture for the notion of the “reduction of the wave function”.     

On Noether's theorem in quantum field theory

Extending the construction of local generators of symmetries in (S. Doplicher, Commun. Math. Phys.85 (1982), 73; S. Doplicher and R. Longo, Commun. Math. Phys.88 (1983), 399) to space-time and supersymmetries, we establish a weak form of Noether's theorem in quantum field theory. We also comment on the physical significance of the “split property,” underlying our analysis, and discuss some local aspects of superselection rules following from our results.     

Solution of scaling quantum networks

We show that all scaling quantum graphs are explicitly solvable, i.e., that any one of their spectral eigenvalues E_n is computable analytically, explicitly, and individually for any given n. This is surprising, since quantum graphs are excellent models of quantum chaos (see, e.g., T. Kottos and H. Schanz, Physica E 9, 523 (2001)).     

Thermal tripartite quantum correlations: quantum discord and entanglement perspectives

We investigate thermal tripartite quantum correlations for a spin star network and for a new extended version of it. In a spin star network, three peripheral spins interact with the central spin identically while in extended spin star network, three peripheral spins interact with two central spatially separated spins in the same way. We exploit the method of [C.C. Rulli, M.S. Sarandy, Phys. Rev. A 84, 042109 (2011)] to evaluate the tripartite quantum discord (TQD) and the method of [M. Li, S. Fei, Z. Wang, Rep. Math. Phys 65, 289 (2010)] called as lower bound of tripartite concurrence (LBTC) to evaluate the tripartite entanglement (TE) of the the peripheral parties in both systems. It is found that thermal TQD is much more robust than thermal TE as a function of temperature T. Also, the peripheral parties of the extended spin star network, in comparison with those of the spin star one, can exhibit higher values of TQD at T > 0. This, indeed, motivates us to realise improved quantum information and quantum computation tasks at finite temperatures.     

Optical properties of GaN/AlN quantum dots

We present here a review of the peculiar optical properties of GaN/AlN quantum dots. These systems show unusually large exciton binding energies and band-offsets. Moreover, when grown along the (0001) axis in the wurtzite phase, the optical properties are dominated by huge on-axis internal electric fields, leading to a very low oscillator strength and complex dynamical behavior. It is also possible to grow GaN quantum dots in the cubic phase or along nonpolar axis of the wurtzite cell. We discuss properties of ensembles of quantum dots, as well as of single quantum dots studied by micro-photoluminescence. To cite this article: P. Lefebvre, B. Gayral, C. R. Physique 9 (2008).     

Giant quantum oscillations of acoustoelectric current in Bi

In single crystals of Bi with resistivity ratio R₂₉₃/R_4.2 ≈ 300 by a helium temperature at frequencies 500 MHz of longitudinal sonic wave detected and investigated the longitudinal acoustoelectric effect.In strong magnetic fields if the condition $\text{h}\text{?}$ Ω ? kT,ql ? 1 Ω — cyclotron frequency, k — Boltzman constant) for directions H 6 q 6 y (y— a bissector crystal axis) and H 6 q 6 z (z — a triagonal crystal axis) is fulfilled, giant quantum oscillations of longitudinal acoustoelectric current are detected. Sign of current oscillations is determined by the type of current carriers.     

Photoreflectance study on the photovoltaic effect in InAs/GaAs quantum dot solar cell

To investigate the effect of quantum dot (QD) layers on the photovoltaic process of InAs/GaAs QD solar cell (QDSC), QD layers were embedded in conventional GaAs p-n junction SC (GaAs SC) structures. The photoreflectance (PR) was examined at different temperatures (T) and excitation light intensities (I_ex) to investigate the photovoltaic effects through observation of the Franz-Keldysh oscillations (FKOs) in the PR spectra. The evaluated the p-n junction electric fields (F_pn) of the InAs QDSC was different from that of the GaAs SC. Moreover, InAs QDSC show that the different photovoltaic behaviors compared with GaAs SC by varying I_ex and T. From these considerations, we suggest that the different photovoltaic behaviors are caused by the effect of the additional photo-carrier generation in InAs QD layers resulting in enhancement of the field screening effect in F_pn.     

Intersublevel transitions in self-assembled quantum dots

Intersublevel transitions in semiconductor quantum dots are transitions of a charge carrier between quantum dot confined states. In InAs/GaAs self-assembled quantum dots, optically active intersublevel transitions occur in the mid-infrared spectral range. These transitions can provide a new insight on the physics of semiconductor quantum dots and offer new opportunities to develop mid-infrared devices. A key feature characterizing intersublevel transitions is the coupling of the confined carriers to phonons. We show that the effect of the strong coupling regime for the electron–optical phonon interaction and the formation of mixed electron–phonon quasi-particles called polarons drastically affect and control the dynamical properties of quantum dots. The engineering of quantum dot relaxation rates through phonon coupling opens the route to the realization of new devices like mid-infrared polaron lasers. We finally show that the measurement of intersublevel absorption is not limited to quantum dot ensembles and that the intersublevel ultrasmall absorption of a single quantum dot can be measured with a nanometer scale resolution by using phonon emission as a signature of the absorption. To cite this article: P. Boucaud et al., C. R. Physique 9 (2008).     

Comments on the Kolmogorov-Sinai-Sąsiada entropy and the quantum information theory

Commenting the recent generalization by S$\text{a}\text{?}$siada of the Kolmogorov-Sinai entropy to the quantum case (KSSentropy), it is remarked that this entropy refers to the process of evolution as a whole and to the initial state (t = 0), not to the state at any time (t ? 0). Therefore, the KSS entropy has no direct relation to the von Neumann entropy or A-entropy at time t. Secondly, the proof of the no-increase theorem of S$\text{a}\text{?}$siada (referring to the initial time) is valid only for the Markov type of time evolution, while the KSS entropy can be generalized to time evolution with arbitrary time correlations. Some important consequences of the new concept for the formulation of the quantum information theory are also presented.     

Probing spin physics in the quantum Hall regime by heat capacity and magnetotransport measurements

We review recent heat capacity and magnetotransport experiments on GaAs/AlGaAs heterostructures containing multilayer two-dimensional electron systems (2DESs) in the quantum Hall regime. Emphasis in this article is on the study of the heat capacity near Landau level filling factor ν=1. We also present a detailed survey of the development of the quantum Hall effect in tilted-magnetic fields for ν≲2. Among the novel phenomena we address is the strong coupling between the nuclear spins and the electrons associated with the spin phase transitions of the 2DES at ν=4/3 and near ν=1. To cite this article: S. Melinte et al., C. R. Physique 3 (2002) 667–676.     

Quantum Cascade Lasers: the quantum technology for semiconductor lasers in the mid-far-infrared

The quantum cascade laser is a new light source based on resonant tunnelling and optical transitions between quantised conduction band states. In these semiconductor devices the principles of operation arise from the quantum engineering of electronic energy levels and tailoring of their wavefunctions. In recent years the performance of these devices has improved markedly and this semiconductor technology is now an attractive choice for the fabrication of mid-far infrared lasers in a very wide spectral range (3–80 μm). At present, quantum cascade lasers are capable of continuous-wave room temperature operation and can deliver 200–300 mW of average power (at λ∼9 μm) operating on a Peltier cooler. To cite this article: C. Sirtori, J. Nagle, C. R. Physique 4 (2003).     

Two-way quantum communication: Generalization of secure quantum information exchange to quantum network

The idea of secure quantum information exchange (SQIE) [J. Phys. B: At. Mol. Opt. Phys.44, 115504 (2011)] is introduced for the secure exchange of single qubit information states between two legitimate users, Alice and Bob. In the present paper, we extend this original SQIE protocol by presenting a scheme, which enables the secure exchange of n-single qubit information states among the n nodes of a quantum network, with the aid of a special kind of 4n-qubit entangled state and the classical assistance of an extra participant Charlie. For experimental realization of our extended SQIE protocol, we suggest an efficient scheme for the generation of a special kind of 4n-qubit entangled state using the interaction between highly detuned Λ-type three-level atoms and optical coherent field. Further, by discussing the various experimental parameters, we show that the special kind 4n-qubit entangled state can be generated with the presently available technology.