Transport of quantum noise through random media

We present an experimental study of the propagation of quantum noise in a multiple scattering random medium. Both static and dynamic scattering measurements are performed: the total transmission of noise is related to the mean free path for scattering, while the noise frequency correlation function determines the diffusion constant. The quantum noise observables are found to scale markedly differently with scattering parameters compared to classical noise observables. The measurements are explained with a full quantum model of multiple scattering.     

Geometrical spin dephasing in quantum dots

We study spin-orbit mediated relaxation and dephasing of electron spins in quantum dots. We show that higher order contributions provide a relaxation mechanism that dominates for low magnetic fields and is of geometrical origin. In the low-field limit relaxation is dominated by coupling to electron-hole excitations and possibly 1/f noise rather than phonons.     

Resonance fluorescence in transport through quantum dots: noise properties

We study a two-level quantum dot embedded in a phonon bath and irradiated by a time-dependent ac field, and develop a method that allows us to extract simultaneously the full counting statistics of the electronic tunneling and relaxation (by phononic emission) events as well as their correlation. We find that the quantum noise of both the transmitted electrons and the emitted phonons can be controlled by the manipulation of external parameters such as the driving field intensity or the bias voltage.     

Interface phonons in semiconductor nanostructures with quantum dots

The vibrational spectra of structures with InAs quantum dots in an AlGaAs matrix and AlAs quantum dots in an InAs matrix are investigated experimentally and theoretically. The Raman spectra exhibit features that correspond to transverse-optical (TO), longitudinal-optical (LO), and interface phonons. The frequencies of interface phonons in InAs and AlAs quantum dots and in an AlGaAs matrix with various concentrations of aluminum are calculated with the use of experimental values of transverse-and longitudinal-optical phonons in the approximation of a dielectric continuum. It is shown that the model of a dielectric continuum adequately describes the behavior of interface phonons in structures with quantum dots under the assumption that the quantum dots are spheroidal.     

Phonon Superfluids in Sets of Trapped Ions

We show that transverse phonons in a set of trapped ions under the action of lasers are described by an interacting boson model whose parameters can be externally adjusted. If the radial trapping frequency is large enough, the system is described by a Bose–Hubbard model, in which hopping of the phonons between different ions is provided by the Coulomb interaction. On the other hand, the non-linear terms in the interaction of the ions with a standing-wave provide us with the phonon–phonon interaction. We investigate the possibility of observing several quantum many—body phenomena, including (quasi)Bose–Einstein Condensation as well as a superfluid-Mott insulator quantum phase transition.     

Surface enhanced Raman scattering by CdS quantum dots

Surface enhanced Raman scattering is studied in nanostructures with CdS quantum dots formed using the Langmuir-Blodgett technology. Features due to quantum dot longitudinal optical phonons are observed in the Raman spectra of both free CdS quantum dots and such dots distributed in an organic matrix. The surface enhanced Raman scattering by nanostructures with CdS quantum dots covered by an Ag cluster film is observed experimentally. Applying Ag clusters onto the nanostructure surfaces results in a sharp (40-fold) increase in the intensity of Raman scattering by optical phonons in the quantum dots. It is shown that the dependence of surface enhanced Raman scattering on the excitation energy is resonant with a maximum at the energy corresponding to the maximum absorption coefficient of Ag clusters.     

Putting mechanics into circuit quantum electrodynamics

We review the use of mechanical oscillators in circuit quantum electrodynamics. The capacitive coupling of nano-electromechanical systems with quantum bits and superconducting microwave resonators gives rise to a rich quantum physics involving electrons, photons and phonons. We focus in particular on the linear coupling between a mechanical oscillator and a microwave resonator and present the quantum dynamics that stems from the phonotonic Josephson junction. The microwave cavity turns out to be a powerful device to detect quantum phonon states and manipulate entangled states between phonons and photons.     

Chaos-assisted tunneling and 1/falpha spectral fluctuations in the order-chaos transition

It has been shown that the spectral fluctuations of different quantum systems are characterized by 1/falpha noise, with 1< or =alpha< or =2, in the transition from integrability to chaos. This result is not well understood. We show that chaos-assisted tunneling gives rise to this power-law behavior. We develop a random matrix model for intermediate quantum systems, based on chaos-assisted tunneling, and we discuss under which conditions it displays 1/falpha noise in the transition from integrability to chaos. We conclude that the variance of the elements that connect regular with chaotic states must decay with the difference of energy between them. We compare the characteristics of the transition modeled in this way with what is obtained for the Robnik billiard.     

Geometric aspects of phonon polarization transport

We study the polarization transport of transverse phonons by adopting a new approach based on the quantum mechanics of spin-orbit interactions. This approach has the advantage of being apt for incorporating fluctuations in the system. The formalism gives rise to Berry effect terms manifested as the Rytov polarization rotation law and the polarization-dependent Hall effect. We derive the distribution of the Rytov rotation angle in the presence of thermal noise and show that the rotation angle is robust against fluctuations.     

Asymmetrical shapes of optical line profiles in individual quantum dots

The experimental data on individual quantum dots show that the optical line can have the form of a very narrow spike accompanied by a shoulder. So far the shoulder has been found at the lower energy side of the narrow peak. In the present work, we study theoretically the origin of such a lineshape. We shall use a simple model of quantum dot and a simple approximation to the electronic excitation. The electronic system will be assumed to be coupled to the longitudinal optical phonons. We will show that the electronic multiple scattering on the optical phonons can then give us an explanation of the observed optical lineshape.     

Error corrections of quantum key distribution of the quantum codes via optical wireless link

Quantum key generated by using a fiber optic Mach Zehnder interferometer incorporating a fiber optic ring resonator is proposed. The generated quantum key is distributed via an optical wireless link system, which is available for the free space link. The random quantum bits can be formed and used as the secret codes in quantum communication system. By adding an ancillary photon after the signal photon within the correlation time of the fiber noise and by performing quantum parity checking, the high fidelity to the noiseless quantum in free space is achieved by the technique known as the quantum error correction.     

Nuclear spin dynamics in the quantum regime of a single-molecule magnet

We show that the nuclear spin dynamics in the single-molecule magnet Mn12-ac below 1 K is governed by quantum tunneling fluctuations of the cluster spins, combined with intercluster nuclear spin diffusion. We also obtain the first experimental proof that-surprisingly-even deep in the quantum regime the nuclear spins remain in good thermal contact with the lattice phonons. We propose a simple model for how T-independent tunneling fluctuations can relax the nuclear polarization to the lattice that may serve as a framework for more sophisticated theories.     

Quantum Statistical Properties of a Nanoelectromechanical System

In the present work, we devised an interaction model between a quantum nanoelectromechanical system (QNEMS) and a quantum bit (QBit), such that the effective Hamiltonian of the model results in a parametric oscillator. The implication is that the statistical properties of the QNEMS modes are dynamical, leading to bunching and antibunching of phonons. We also present two situations in which we obtain sub- and super-poissonian distributions.     

Heating and cooling of a two-dimensional electron gas by terahertz radiation

The absorption of terahertz radiation by free charge carriers in n-type semiconductor quantum wells accompanied by the interaction of electrons with acoustic and optical phonons is studied. It is shown that intrasubband optical transitions can cause both heating and cooling of the electron gas. The cooling of charge carriers occurs in a certain temperature and radiation frequency region where light is most efficiently absorbed due to intrasubband transitions with emission of optical phonons. In GaAs quantum wells, the optical cooling of electrons occurs most efficiently at liquid nitrogen temperatures, while cooling is possible even at room temperature in GaN heterostructures.     

Telegraph noise and fractional statistics in the quantum Hall effect

We study theoretically nonequilibrium noise in the fractional quantum Hall regime for an Aharonov-Bohm ring with a third contact in the middle of the ring. Because of their fractional statistics the tunneling of Laughlin quasiparticles between the inner and outer edges of the ring changes the effective Aharonov-Bohm flux experienced by quasiparticles going around the ring, leading to a change in the conductance across the ring. A small current in the middle contact, therefore, gives rise to fluctuations in the current flowing across the ring which resemble random telegraph noise. We analyze this noise using the chiral Luttinger liquid model. At low frequencies the telegraph noise varies inversely with the tunneling current and can be much larger than the shot noise. We propose that combining the Aharonov-Bohm effect with a noise measurement provides a direct method for observing fractional statistics.     

Distribution of Resonances for Open Quantum Maps

We analyze a simple model of quantum chaotic scattering system, namely the quantized open baker’s map. This model provides a numerical confirmation of the fractal Weyl law for the semiclassical density of quantum resonances. The fractal exponent is related to the dimension of the classical repeller. We also consider a variant of this model, for which the full resonance spectrum can be rigorously computed, and satisfies the fractal Weyl law. For that model, we also compute the shot noise of the conductance through the system, and obtain a value close to the prediction of random matrix theory.     

Intrasubband light absorption in a parabolic quantum well with consideration of scattering on the three-dimensional optical phonons

Within the frameworks of the second order perturbation theory the light absorption by free carriers in a parabolic quantum well (QW) is investigated taking into account the scattering by the three-dimensional optical phonons. An analytical expression for the absorption coefficient is obtained taking into the consideration two processes, with initial absorption of a photon and further scattering by optical phonons, and with the initial scattering by the optical phonons and the subsequent absorption of the photon. Frequency characteristics and dependences on the temperature and the width of QW are examined.     

Loss of atom interference due to a random phase

We suggest a model where the influence of an environment on the atom interference is associated with a random phase. The model consists of sending a two-level state atom through two cavities, both containing a standing wave field in the Bragg regime and Raman–Nath regime, respectively. In view of this model, we can visualize the loss of interference fringes if the randomness of the phase increases, and the restoration of the pattern when it decreases, as which-path information. Then, the controllable random noise acts as a decoherence that would destroy the quantum features.     

Intersubband relaxation of hot carriers in quantum well systems

We use an ensemble Monte Carlo simulation of coupled electrons, holes and nonequilibrium polar optical phonons in multiple quantum well systems to model the intersubband relaxation of hot carriers measured in ultra-fast optical experiments. We have investigated the effect of various models of confined photon modes on the energy relaxation and intersubband transition rate in single quantum well and coupled well systems. In particular, the symmetry of the atomic displacement with respect to the quantum well has a marked effect on the relative intersubband versus intrasubband scattering rates, depending on whether one considers electrostatic boundary conditions(slab modes) or mechanical boundary conditions(guided modes). In single quantum wells systems, the overall intersubband relaxation time is not found to be strongly dependent on the confined mode model used due to competing effects of hot phonons and the relative intrasubband scattering rates. For coupled well systems, the relaxation rate is much more dependent on the exact nature of the phonon amplitude. Large effects are found associated with localized AlAs interface modes which dominate the intersubband relaxation time.