Strongly-coupled Quantum Research Articles

Holographic influence functional and its application to decoherence induced by quantum critical theories

The dynamics of a particle influenced by strongly-coupled quantum critical theories is studied by the holographic approach. A real-time prescription for the AdS/CFT correspondence in the context of nonequilibrium physics is proposed from mainly the field theoretic consideration, and the associated holographic influence functional is obtained. We then study the decoherence dynamics of a particle induced by the quantum critical theories with a dynamical exponent $z$. We find that as $z$ increases, the decoherence effect becomes less significant. The similar behavior is found as we heat up the environment. However, in this case the decoherence effect is enhanced not only by its strong coupling constant but also from the finite temperature effect. Finally a comparison is made with the result of a particle influenced by the free field.

Understanding the electric field control of the electronic and optical properties of strongly-coupled multi-layered quantum dot molecules.

Strongly-coupled quantum dot molecules (QDMs) are widely employed in the design of a variety of optoelectronic, photovoltaic, and quantum information devices. An efficient and optimized performance of these devices demands engineering of the electronic and optical properties of the underlying QDMs. The application of electric fields offers a way to realise such a control over the QDM characteristics for a desired device operation. We performed multi-million-atom atomistic tight-binding calculations to study the influence of electric fields on the electron and hole wave function confinements and symmetries, the ground-state transition energies, the band-gap wavelengths, and the optical transition modes. Electrical fields parallel (Ep) and anti-parallel (Ea) to the growth direction were investigated to provide a comprehensive guide for understanding the electric field effects. The strain-induced asymmetry of the hybridized electron states is found to be weak and can be balanced by applying a small Ea electric field, of the order of 1 kV cm(-1). The strong interdot couplings completely break down at large electric fields, leading to single QD states confined at the opposite edges of the QDM. This mimics a transformation from a type-I band structure to a type-II band structure for the QDMs, which is a critical requirement for the design of intermediate-band solar cells (IBSCs). The analysis of the field-dependent ground-state transition energies reveals that the QDM can be operated both as a high dipole moment device by applying large electric fields and as a high polarizability device under the application of small electric field magnitudes. The quantum confined Stark effect (QCSE) red shifts the band-gap wavelength to 1.3 μm at the 15 kV cm(-1) electric field; however the reduced electron-hole wave function overlaps lead to a decrease in the interband optical transition strengths by roughly three orders of magnitude. The study of the polarisation-resolved optical modes indicates the benefits of applying small electric fields, which leads to an isotropic polarisation response, a desirable property for semiconductor optical amplifiers (SOAs).

Angular momentum generation from holographic Chern-Simons models

We study parity-violating effects, particularly the generation of angular momentum density and its relation to the parity-odd and dissipationless transport coefficient Hall viscosity, in strongly-coupled quantum fluid systems in 2+1 dimensions using holographic method. We employ a class of 3+1-dimensional holographic models of Einstein-Maxwell system with gauge and gravitational Chern-Simons terms coupled to a dynamical scalar field. The scalar can condensate and break the parity spontaneously. We find that when the scalar condensates, a non-vanishing angular momentum density and an associated edge current are generated, and they receive contributions from both gauge and gravitational Chern-Simons terms. The angular momentum density does not satisfy a membrane paradigm form because the vector mode fluctuations from which it is calculated are effectively massive. On the other hand, the emergence of Hall viscosity is a consequence of the gravitational Chern-Simons term alone and it has membrane paradigm form. We present both general analytic results and numeric results which take back-reactions into account. The ratio between Hall viscosity and angular momentum density resulting from the gravitational Chern-Simons term has in general a deviation from the universal 1/2 value obtained from field theory and condensed matter physics.

Nonclassical higher-order photon correlations with a quantum dot strongly coupled to a photonic-crystal nanocavity

We use the third- and fourth-order autocorrelation functions $g^{(3)}(\tau_1,\tau_2)$ and $g^{(4)}(\tau_1,\tau_2, \tau_3)$ to detect the non-classical character of the light transmitted through a photonic-crystal nanocavity containing a strongly-coupled quantum dot probed with a train of coherent light pulses. We contrast the value of $g^{(3)}(0, 0)$ with the conventionally used $g^{(2)}(0)$ and demonstrate that in addition to being necessary for detecting two-photon states emitted by a low-intensity source, $g^{(3)}$ provides a more clear indication of the non-classical character of a light source. We also present preliminary data that demonstrates bunching in the fourth-order autocorrelation function $g^{(4)}(\tau_1,\tau_2, \tau_3)$ as the first step toward detecting three-photon states.

Spin(7) duality for N $$ \mathcal{N} $$ = 1 CS-matter theories

In this paper we propose a duality for non-holomorphic $$ \mathcal{N} $$ = 1 CS-matter theories living on M2 branes probing Spin(7) cones. We call this duality Spin(7) duality. Two theories are named Spin(7) dual if they have the same moduli space: a real Spin(7) cone with base a weak G2 manifold, and they are hence holographic dual to the same AdS4 × G2 M theory solution. We provide a systematic way to generate these dualities, derived by combining toric duality for $$ \mathcal{N} $$ = 2 CS-matter theories and generalized non-holomorphic orientifold projections to $$ \mathcal{N} $$ = 1. Brane construction, AdS/CFT correspondence, and the computation of the moduli space support our proposal at the classical level and provide some arguments at the quantum strong coupling regime. The relation with Seiberg-like duality is also analyzed.

Effective field theory, three-loop perturbative expansion, and their experimental implications in graphene many-body effects

Many-body electron-electron interaction effects are theoretically considered in monolayer graphene from a continuum effective field-theoretic perspective by going beyond the standard leading-order perturbative renormalization group (RG) analysis. Given that the bare fine structure constant in graphene is of order unity, which is neither small to justify a perturbative expansion nor large enough for strong-coupling theories to be applicable, the problem is a difficult one, with some similarity to 2+1-dimensional strong-coupling quantum electrodynamics (QED). In this work, we take a systematic and comprehensive analytical approach, working primarily at the Dirac point (intrinsic graphene), by going up to three loops in the diagrammatic expansion to both ascertain the validity of perturbation theory and to estimate quantitatively higher-order renormalization effects. While no direct signatures for non-Fermi liquid behavior at the Dirac point have yet been observed experimentally, there is ample evidence for the interaction-induced renormalization of the graphene velocity as the carrier density approaches zero. We provide a critical comparison between theory and experiment, using both bare- and screened-interaction (RPA) calculations. We find that while the one-loop RG analysis gives reasonable agreement with the experimental data, especially when screening and finite-density effects are included through the RPA, the two-loop analysis reveals a strong-coupling critical point in suspended graphene, signifying either a quantum phase transition or a breakdown of the weak-coupling RG approach. We show that the latter is more likely by adapting Dyson's argument for the breakdown of perturbative QED to the case of graphene. We propose future experiments and theoretical directions to make further progress on this important and difficult problem.

Cavity-Enhanced Real-Time Monitoring of Single-Charge Jumps at the Microsecond Time Scale

We use fast coherent reflectivity measurements, in a strongly-coupled quantum dot-micropillar device, to monitor in real-time single-charge jumps at the microsecond timescale. Thanks to the strong enhancement of light-matter interaction inside the cavity, the measurement rate is five orders of magnitude faster than with previous experiments of direct single-charge sensing with quantum dots. The monitored transitions, identified at any given time with a less than 0.2% error probability, correspond to a carrier being captured and then released by a single material defect. This high-speed technique opens the way for the real-time monitoring of other rapid single quantum events, such as the quantum jumps of a single spin.

Quantum Ising dynamics and Majorana-like edge modes in the Rabi lattice model

The atomic dipoles in the Rabi lattice model exhibit quantum Ising dynamics in the limit of strong atom-photon interaction. It governs the paraelectric to ferroelectric phase transition in the ground state. On an open chain, it implies the existence of two Majorana-like, albeit topologically unprotected, edge modes in the ordered phase. The relation ${\ensuremath{\rho}}_{1L}^{x}={p}^{8}$ between the end-to-end dipole correlation ${\ensuremath{\rho}}_{1L}^{x}$ and the spontaneous polarization $p$ is proposed as an observable signature of these edge modes. The density-matrix renormalization-group calculations on the one-dimensional Rabi lattice support the strong-coupling quantum Ising behavior and correctly yield the proposed end-to-end dipole correlation. The conditions that protect the edge modes against the adverse perturbations are also identified.

Coherence dynamics and quantum-to-classical crossover in an exciton–cavity system in the quantum strong coupling regime

Interaction between light and matter generates optical nonlinearities, which are particularly pronounced in the quantum strong coupling regime. When a single bosonic mode couples to a single fermionic mode, a Jaynes–Cummings (JC) ladder is formed, which we realize here using cavity photons and quantum dot excitons. We measure and model the coherent anharmonic response of this strongly coupled exciton–cavity system at resonance. Injecting two photons into the cavity, we demonstrate a larger polariton splitting with respect to the vacuum Rabi splitting. This is achieved using coherent nonlinear spectroscopy, specifically four-wave mixing, where the coherence between the ground state and the first (second) rung of the JC ladder can be interrogated for positive (negative) delays. With increasing excitation intensity and thus rising average number of injected photons, we observe spectral signatures of the quantum-to-classical crossover of the strong coupling regime.

Photoluminescence of a microcavity quantum dot system in the quantum strong-coupling regime

The Jaynes-Cummings model, describing the interaction between a single two-level system and a photonic mode, has been used to describe a large variety of systems, ranging from cavity quantum electrodynamics, trapped ions, to superconducting qubits coupled to resonators. Recently there has been renewed interest in studying the quantum strong-coupling (QSC) regime, where states with photon number greater than one are excited. This regime has been recently achieved in semiconductor nanostructures, where a quantum dot is trapped in a planar microcavity. Here we study the quantum strong-coupling regime by calculating its photoluminescence (PL) properties under a pulsed excitation. We discuss the changes in the PL as the QSC regime is reached, which transitions between a peak around the cavity resonance to a doublet. We particularly examine the variations of the PL in the time domain, under regimes of short and long pulse times relative to the microcavity decay time.

Jaynes-Cummings treatment of superconducting resonators with dielectric loss due to two-level systems

We perform a quantum mechanical analysis of superconducting resonators subject to dielectric loss arising from charged two-level systems. We present numerical and analytical descriptions of the dynamics of energy decay from the resonator within the Jaynes-Cummings model. Our analysis allows us to distinguish the strong and weak coupling regimes of the model and to describe within each regime cases where the two-level system is unsaturated or saturated. We find that the quantum theory agrees with the classical model for weak coupling. However, for strong coupling the quantum theory predicts lower loss than the classical theory in the unsaturated regime. Also, in contrast to the classical theory, the photon number at which saturation occurs in the strong coupling quantum theory is independent of the coupling between the resonator and the two-level system.

Landauer’s principle in the quantum regime

We demonstrate the validity of Landauer's erasure principle in the strong coupling quantum regime by treating the system-reservoir interaction in a thermodynamic way. We show that the initial coupling to the reservoir modifies both the energy and the entropy of the system, and provide explicit expressions for the latter for a damped quantum harmonic oscillator. These contributions are related to the Hamiltonian of mean force and dominate in the strong damping limit. They need therefore to be fully taken into account in any low temperature thermodynamic analysis of quantum systems.

Exciton–polariton light–semiconductor coupling effects

The integrated absorption of an excitonic resonance is a measure of a semiconductor's coupling to an optical field. The concept of an exciton–polariton expresses the non-perturbative coupling between the electromagnetic field and the optically induced matter polarization. Ways to alter this coupling include confining the light in optical cavities and localizing the excitonic wavefunction in quantum wells and dots, which is illustrated by quantum strong coupling between a single dot and an optical nanocavity. Positioning quantum wells in periodic or quasiperiodic lattices with spacing close to a half wavelength results in pronounced modifications to the light transmission. Light–matter coupling can also be used to generate and interrogate an exciton population, for example by the recently developed technique of absorbing terahertz radiation.

Galileon Fifth Forces

The Galileon is a proposal to explain the current accelerated expansion of the universe through a scalar field with non-linear dynamics. The form of these non-linearities is protected by the Galileon symmetry and the theory is ghost free. I review the way in which the Galileon scalar field hides from experimental probes of gravity dynamically, showing that the theory is less constrained than previously supposed. I discuss the implications of this for quantum strong coupling scale.

Up on the Jaynes–Cummings ladder of a quantum-dot/microcavity system

In spite of their different natures, light and matter can be unified under the strong-coupling regime, yielding superpositions of the two, referred to as dressed states or polaritons. After initially being demonstrated in bulk semiconductors and atomic systems, strong-coupling phenomena have been recently realized in solid-state optical microcavities. Strong coupling is an essential ingredient in the physics spanning from many-body quantum coherence phenomena, such as Bose-Einstein condensation and superfluidity, to cavity quantum electrodynamics. Within cavity quantum electrodynamics, the Jaynes-Cummings model describes the interaction of a single fermionic two-level system with a single bosonic photon mode. For a photon number larger than one, known as quantum strong coupling, a significant anharmonicity is predicted for the ladder-like spectrum of dressed states. For optical transitions in semiconductor nanostructures, first signatures of the quantum strong coupling were recently reported. Here we use advanced coherent nonlinear spectroscopy to explore a strongly coupled exciton-cavity system. We measure and simulate its four-wave mixing response, granting direct access to the coherent dynamics of the first and second rungs of the Jaynes-Cummings ladder. The agreement of the rich experimental evidence with the predictions of the Jaynes-Cummings model is proof of the quantum strong-coupling regime in the investigated solid-state system.

Dispersive collective charge modes in an incommensurately modulated cuprate Mott insulator

We report measurement of collective charge modes of insulating Sr_(14)Cu_(24)O_(41) using inelastic resonant x-ray scattering over the complete Brillouin zone. Our results show that the intense excitation modes at the charge gap edge predominantly originate from ladder-containing planar substructures. The observed modes are found to be dispersive for momentum transfers along the “legs” (ℏQ ∥∥c ) but nearly localized along the “rungs” (ℏQ ∥∥a ). We show that the dispersion and peak width characteristics of the modes can be understood in the strong-coupling quantum limit (Hubbard U⪢t_(ladder)>t_(chain), where t is the hopping parameter). Quite generally, we demonstrate that the momentum tunability (Q resolution) of inelastic x-ray scattering can be utilized to resolve mode contributions in multicomponent incommensurate quantum electron systems.

Can quantum Brownian motion be Markovian?

The question whether a dissipative quantum system can approximately be described by a Markov process is reexamined using Brownian motion in a field of force as an example. The exactly solvable model of a damped harmonic oscillator allows for a precise identification of the range of validity of Markovian evolution equations. Such equations are valid both in the limits of weak and strong damping if the temperature is not too low. From these findings the parameter domain within which the weak coupling quantum master equation and the strong coupling quantum Smoluchowski equation can be employed to describe nonlinear quantum systems is outlined.

Collision of Fano resonances: An exactly solvable model

We examine new effects arising from the collision of two Fano dipoles formed in the transmission of an Aharonov–Bohm ring with strongly-coupled quantum dots. A new quasi-particle is formed, which behaves as a coupled object called a Fano-quadrupole, giving new possibilities for controlling the transmission as a function of flux.

Thermodynamics of quantum Brownian motion with internal degrees of freedom: the role of entanglement in the strong-coupling quantum regime

We study the influence of entanglement on the relation between the statistical entropy of an open quantum system and the heat exchanged with a low temperature environment. A model of quantum Brownian motion of the Caldeira-Leggett type - for which a violation of the Clausius inequality has been stated by Th.M. Nieuwenhuizen and A.E. Allahverdyan [Phys. Rev. E 66, 036102 (2002)] - is reexamined and the results of the cited work are put into perspective. In order to address the problem from an information theoretical viewpoint a model of two coupled Brownian oscillators is formulated that can also be viewed as a continuum version of a two-qubit system. The influence of an additional internal coupling parameter on heat and entropy changes is described and the findings are compared to the case of a single Brownian particle.

DBI in the sky: Non-Gaussianity from inflation with a speed limit

We analyze the spectrum of density perturbations generated in models of the recently discovered ``D-cceleration'' mechanism of inflation. In this scenario, strong coupling quantum field theoretic effects sum to provide a Dirac-Born-Infeld--like action for the inflaton. We show that the model has a strict lower bound on the non-Gaussianity of the cosmic microwave background radiation power spectrum at an observable level, and is thus falsifiable. This, in particular, observationally distinguishes this mechanism from traditional slow-roll inflation generated by weakly interacting scalar fields. The model also favors a large observable tensor component to the cosmic microwave background radiation spectrum.