arXiv subjects

C. K. Law

Publications and source records attributed to C. K. Law.

At least 19 recordsLinked to original sources

arXiv2018

Quantum radiation from a shaken two-level atom in vacuum

We present a non-relativistic theory of quantum radiation generated by shaking a two-level atom in vacuum. Such radiation has the same origin of photon emission in dynamical Casimir effect. By performing a time-dependent "dressing" transformation to the Hamiltonian, we derive an interaction term that governs the radiation. In particular, we show that photon pairs can be generated, not only by shaking the position of the atom, but also by changing the internal states of the atom. As applications of our theory, we calculate the emission rate from an oscillating atom, and the multi-photon state generated in a single-photon scattering process.

quant-ph↗

arXiv2017

Bound state in the continuum by spatially separated ensembles of atoms in a coupled-cavity array

We present an analytic solution of bound states in the continuum (BICs) for photons and atoms in a one-dimensional coupled cavity array. These bound states are formed by two ensembles of twolevel atoms confined in separated cavities of the array. We show that in the regime where the photon hopping rate between neighboring cavities is high compared with the collective Rabi frequency, the BIC corresponds to a subradiant collective atomic state in which the two ensembles of atoms are strongly entangled. On the other hand in the low photon hopping rate regime, the BIC behaves a quantum cavity in which photons can be trapped between the two ensemble of atoms.

quant-ph↗

arXiv2015

Enhancement of mechanical effects of single photons in modulated two-mode optomechanics

We propose an approach to enhance the mechanical effects of single photons in a two-mode optomechanical system. This is achieved by introducing a resonance-frequency modulation to the cavity fields. When the modulation frequency and amplitude satisfy certain conditions, the mechanical displacement induced by single photons could be larger than the quantum zero-point fluctuation of the oscillating resonator. This method can be used to create distinct mechanical superposition states.

quant-ph↗

arXiv2014

Phase-kicked control of counter-rotating interactions in the quantum Rabi model

We present an interaction scheme to control counter-rotating terms in the quantum Rabi model. We show that by applying a sequence of $π/2$ phase kicks to a two-level atom and a single mode quantized field, the natural dynamics of the Rabi model can be interrupted in a way that counter-rotating transitions can be significantly enhanced. This is achieved by a suitable timing of the phase kicks determined by a phase matching condition. If the time between successive kicks is sufficiently short, our scheme is turned into a dynamical decoupling problem in which the effects of counter-rotating terms can be strongly suppressed under ultrastrong coupling.

quant-ph↗

arXiv2013

Photon emission via vacuum-dressed intermediate states under ultrastrong coupling

We investigate theoretically quantum effects of a cavity-atom system in which the upper two levels of a cascade-type three-level atom interact with a cavity field mode in the ultrastrong coupling regime. By exploiting the virtual photons carried by atom-cavity dressed states, we indicate how two external driving fields induce Raman transitions such that a continuous conversion of virtual photons into real photons can be achieved with a high probability. We present analytical and numerical solutions of the system. In addition, we show that the converted photons exhibit a bunching behavior in the steady state. Our scheme demonstrates that atom-cavity dressed states in the ultrastrong coupling regime can serve as intermediate states to invoke nonlinear optical processes.

quant-ph↗

arXiv2013

Correlated two-photon scattering in cavity optomechanics

We present an exact analytical solution of the two-photon scattering in a cavity optomechanical system. This is achieved by solving the quantum dynamics of the total system, including the optomechanical cavity and the cavity-field environment, with the Laplace transform method. The long-time solution reveals detailed physical processes involved as well as the corresponding resonant photon frequencies. We characterize the photon correlation induced in the scattering process by calculating the two-photon joint spectrum of the long-time state. Clear evidence for photon frequency anti-correlation can be observed in the joint spectrum. In addition, we calculate the equal-time second-order correlation function of the cavity photons. The results show that the radiation pressure coupling can induce photon blockade effect, which is strongly modulated by the phonon sideband resonance. In particular, we obtain an explicit expression of optomechanical coupling strength determining these sideband modulation peaks based on the two-photon resonance condition.

quant-ph↗

arXiv2013

Ground state of a resonant two-qubit Rabi model in the ultrastrong coupling regime

We consider a generalized Rabi model formed by two identical qubits interacting with a common oscillator mode. In the near resonance configuration where the oscillator frequency is close to the transition frequency of the qubit, we determine the ground state of the model approximately by using a variational method and a transformation method. Both methods are shown to provide good agreement with the exact numerical ground state for a range of coupling strength in the ultrastrong regime. In addition, we indicate how the accuracy of the approximation can be further improved by using perturbation theory. We also examine the ground state entanglement between the two qubits. By calculating the negativity, we quantify the degree of entanglement as a function of coupling strength.

quant-ph↗

arXiv2013

Vacuum Rabi oscillation induced by virtual photons in the ultrastrong coupling regime

We present an interaction scheme that exhibits a dynamical consequence of virtual photons carried by a vacuum-field dressed two-level atom in the ultrastrong coupling regime. We show that, with the aid of an external driving field, virtual photons provide a transition matrix element that enables the atom to evolve coherently and reversibly to an auxiliary level accompanied by the emission of a real photon. The process corresponds to a type of vacuum Rabi oscillation, and we show that the effective vacuum Rabi frequency is proportional to the amplitude of a single virtual photon in the ground state. Therefore the interaction scheme could serve as a probe of ground state structures in the ultrastrong coupling regime.

quant-ph↗

arXiv2012

Dark states of a moving mirror in the single-photon strong-coupling regime

We investigate an optomechanical system in which a cavity with a moving mirror is driven by two external fields. When the field frequencies match resonance conditions, we show that there exists a class of dark states of the moving mirror in the single-photon strong-coupling regime. These dark states, which cause the cavity to be decoupled from the external fields, is a manifestation of quantum coherence associated with the mirror's mechanical degrees of freedom. We discuss the properties of the dark states and indicate how they can be generated by optical pumping due to the decay of cavity field.

quant-ph↗

arXiv2012

Optomechanical coupling between a moving dielectric sphere and radiation fields: a Lagrangian-Hamiltonian formalism

We present a Lagrangian-Hamiltonian formalism of a moving dielectric sphere interacting with radiation fields. By including the interaction up to the first order in the speed of the sphere, we derive the Hamiltonian and perform quantization of both the field and the mechanical motion of the sphere. In particular, we show how independent degrees of freedom can be consistently identified under the generalized radiation gauge via instantaneous mode projection. Our Hamiltonian indicates the form of coupling due to velocity-dependent interactions beyond adiabatic approximation. In addition, the Hamiltonian predicts that a geometrical quantum phase can be gained by the sphere moving in a light field.

quant-ph↗

arXiv2012

Rotational optomechanical coupling of a spinning dielectric sphere

We formulate a non-relativistic Hamiltonian in order to describe how the rotational degrees of freedom of a dielectric sphere and quantized light fields are coupled. Such an interaction is shown to take a form of angular momentum coupling governed by the field angular momentum inside the dielectric. As a specific example, we show that the coupling due to a single whispering gallery mode can lead to precession dynamics and frequency shifts of light.

quant-ph↗

arXiv2012

Spectrum of single-photon emission and scattering in cavity optomechanics

We present an analytic solution describing the quantum state of a single photon after interacting with a moving mirror in a cavity. This includes situations when the photon is initially stored in a cavity mode as well as when the photon is injected into the cavity. In addition, we obtain the spectrum of the output photon in the resolved-sideband limit, which reveals spectral features of the single-photon strong-coupling regime in this system. We also clarify the conditions under which the phonon sidebands are visible and the photon-state frequency shift can be resolved.

quant-ph↗

arXiv2011

Cooling of a mirror in cavity optomechanics with a chirped pulse

We investigate the response of a harmonically confined mirror to an optical pulse in cavity optomechanics. We show that when the pulsed coupling strength takes the form of a chirped pulse, thermal fluctuations of the mirror can be significantly transferred to the cavity field. In addition, the frequency modulation of the pulse could enable a better cooling performance by suppressing the sensitivity of the dependence of detuning and pulse areas. Using numerical investigations, we find that the pulsed-cooling is mainly limited by the cavity-field decay rate.

quant-ph↗

arXiv2011

Non-adiabatic optomechanical Hamiltonian of a moving dielectric membrane in a cavity

We formulate a non-relativistic Hamiltonian in order to describe the interaction between a moving dielectric membrane and radiation pressure. Such a Hamiltonian is derived without making use of the single-mode adiabatic approximation, and hence it enables us to incorporate multi-mode effects in cavity optomechanics. By performing second quantization, we show how a set of generalized Fock states can be constructed to represent quantum states of the membrane and cavity field. In addition, we discuss examples showing how photon scattering among different cavity modes would modify the interaction strengths and the mechanical frequency of the membrane.

quant-ph↗

arXiv2011

Parametric generation of quadrature squeezing of mirrors in cavity optomechanics

We propose a method to generate quadrature squeezed states of a moving mirror in a Fabry-Perot cavity. This is achieved by exploiting the fact that when the cavity is driven by an external field with a large detuning, the moving mirror behaves as a parametric oscillator. We show that parametric resonance can be reached approximately by modulating the driving field amplitude at a frequency matching the frequency shift of the mirror. The parametric resonance leads to an efficient generation of squeezing, which is limited by the thermal noise of the environment.

quant-ph↗

arXiv2011

Fast electrochemical doping due to front instability in organic semiconductors

The electrochemical doping transformation in organic semiconductor devices is studied in application to light-emitting cells. It is shown that the device performance can be significantly improved by utilizing new fundamental properties of the doping process. We obtain an instability, which distorts the doping fronts and increases the doping rate considerably. We explain the physical mechanism of the instability, develop theory, provide experimental evidence, and perform numerical simulations. We further show how improved device design can amplify the instability thus leading to a much faster doping process and device kinetics.

cond-mat.mtrl-sci↗

arXiv2011

Two-polariton bound states in the Jaynes-Cummings-Hubbard model

We examine the eigenstates of the one-dimensional Jaynes-Cummings-Hubbard (JCH) model in the two-excitation subspace. We discover that two-excitation bound states emerge when the ratio of vacuum Rabi frequency to the tunneling rate between cavities exceeds a critical value. We determine the critical ratio as a function of the quasi-momentum quantum number, and indicate that the bound states carry a strong correlation in which the two polaritons appear to be spatially confined together.

quant-ph↗

arXiv2010

Correlated two-photon transport in a one-dimensional waveguide side-coupled to a nonlinear cavity

We investigate the transport properties of two photons inside a one-dimensional waveguide side-coupled to a single-mode nonlinear cavity. The cavity is filled with a nonlinear Kerr medium. Based on the Laplace transform method, we present analytic solution of quantum states of the transmitted and reflected two photons, which are initially prepared in a Lorentzian wave packet. The solution reveals how quantum correlation between the two photons emerge after the scattering by the nonlinear cavity. In particular, we show that the output wave function of the two photons in position space can be localized in the relative coordinates, which is a feature that may be interpreted as a two-photon bound state in this waveguide-cavity system.

quant-ph↗