Spectral line elimination and spontaneous emission cancellation via quantum interference

Coherent control of spatial and angular Goos-Hänchen shifts with spontaneously generated coherence and incoherent pumping

We propose an efficient scheme to manipulate the Goos-Hänchen (GH) shift of a reflected beam from a metal-clad waveguide, where a coherent atomic medium with a Λ-type configuration is employed as the substrate. Using experimentally achievable parameters, we identify the conditions under which spontaneously generated coherence (SGC) allows us to enhance the spatial and angular GH shifts of the reflected beam. With the help of SGC, the relative phases of the probe and control fields can alter the absorption gain and refractive index of the atomic medium, thereby manipulating the magnitudes, signs, and positions of the spatial and angular shifts. Furthermore, the spatial and angular GH shifts can be coherently controlled via adjusting the incoherent pumping rate and the intensity of the control field. Our proposal provides an avenue for the manipulation of spatial and angular GH shifts and potential applications in optical switching and optical steering.

Enhanced magneto-optical rotation of probe field in thermal medium via spontaneous generated coherence

A four-level double lambda closed atomic configuration is considered to study the polarization plane rotation of the probe beam through cold as well as thermal Rb\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{87}$$\end{document}87 atomic medium by varying the spontaneously generated coherence (SGC). Magnetic field and strong coupling field are applied to the atomic configuration. The light-matter interaction leads to enhanced the magneto-optical rotation. The intensity of the applied fields plays promising role in the generation and enhancement of birefringence. It ultimately enhances the polarization plane rotation of the probe beam in the Doppler medium. In the presence of both SGC and Doppler broadening effects, the optical rotation and transmission of the weak light beam are modified and controlled as well, which have potential applications in magnetometery and laser frequency stabilization.

Intermittent decoherence blockade in a chiral ring environment

It has long been recognized that emission of radiation from atoms is not an intrinsic property of individual atoms themselves, but it is largely affected by the characteristics of the photonic environment and by the collective interaction among the atoms. A general belief is that preventing full decay and/or decoherence requires the existence of dark states, i.e., dressed light-atom states that do not decay despite the dissipative environment. Here, we show that, contrary to such a common wisdom, decoherence suppression can be intermittently achieved on a limited time scale, without the need for any dark state, when the atom is coupled to a chiral ring environment, leading to a highly non-exponential staircase decay. This effect, that we refer to as intermittent decoherence blockade, arises from periodic destructive interference between light emitted in the present and light emitted in the past, i.e., from delayed coherent quantum feedback.

Control of quantum electrodynamical processes by shaping electron wavepackets

Fundamental quantum electrodynamical (QED) processes, such as spontaneous emission and electron-photon scattering, encompass phenomena that underlie much of modern science and technology. Conventionally, calculations in QED and other field theories treat incoming particles as single-momentum states, omitting the possibility that coherent superposition states, i.e., shaped wavepackets, can alter fundamental scattering processes. Here, we show that free electron waveshaping can be used to design interferences between two or more pathways in a QED process, enabling precise control over the rate of that process. As an example, we show that free electron waveshaping modifies both spatial and spectral characteristics of bremsstrahlung emission, leading for instance to enhancements in directionality and monochromaticity. The ability to tailor general QED processes opens up additional avenues of control in phenomena ranging from optical excitation (e.g., plasmon and phonon emission) in electron microscopy to free electron lasing in the quantum regime.

Transparency and tunable slow-fast light in a hybrid cavity optomechanical system

We theoretically investigate the optomechanically induced transparency (OMIT) phenomenon in a hybrid optomechanical system composing of an optomechanical cavity and a traditional one. A Kerr medium is inserted in the optomechanical cavity and the other traps the atomic ensemble. We demonstrate the appearance of electromagnetically and optomechanically induced transparency when there is only Kerr medium or atoms in the system. We give an explicit explanation for the mechanism of the transparency. Moreover, we set up new scheme for the measurement of Kerr coefficient and the single atom-photon coupling strength. It is shown that Kerr nonlinearity can inhibit the normal mode splitting (NMS) when the tunnel strength is strong coupling. Furthermore, in the output field, slow light and fast light are converted to realize the tunable switch from slow light to fast light. This study has some important guiding significance in the fields of the high precision measurement and quantum information processing.

Topologically Protected Quantum Coherence in a Superatom

Exploring the properties and applications of topological quantum states is essential to better understand topological matter. Here, we theoretically study a quasi-one-dimensional topological atom array. In the low-energy regime, the atom array is equivalent to a topological superatom. Driving the superatom in a cavity, we study the interaction between light and topological quantum states. We find that the edge states exhibit topology-protected quantum coherence, which can be characterized from the photon transmission. This quantum coherence helps us to find a superradiance-subradiance transition, and we also study its finite-size scaling behavior. The superradiance-subradiance transition also exists in symmetry-breaking systems. More importantly, it is shown that the quantum coherence of the subradiant edge state is robust to random noises, allowing the superatom to work as a topologically protected quantum memory. We suggest a relevant experiment with three-dimensional circuit QED. Our study may have applications in quantum computation and quantum optics based on topological edge states.

Quantum Interference Effects on Information Phase Space and Entropy Squeezing

Wehrl entropy and its density are used to investigate the dynamics of loss of coherence and information in a phase space for an atomic model of two-photon two-level atom coupled to different radiation reservoirs (namely, normal vacuum (NV), thermal field (TF) and squeezed vacuum (SV) reservoirs). Particularly, quantum interference (QI) effect, due to the 2-photon transition decay channels, has a paramount role in: (i) the atomic inversion decay in the NV case, which behaves as quantum Zeno and anti-Zeno decay effect; (ii) the coherence and information loss in the phase space; and (iii) identifying temporal information entropy squeezing. Results are also sensitive to the initial atomic state.

Spontaneous feedback for the simultaneous narrowing and elevation of fluorescence spectral lines

Narrow linewidth and high intensity of the fluorescence spectra are two important elements to improve the accuracy and efficiency of related practical measurements, but so far they have not been achievable simultaneously. We propose a new approach to sharpen the linewidth and to heighten the intensity simultaneously. Rather than double coherent mechanisms, it uses the spontaneous emission of inverted atoms in a cavity below the threshold as a resource for feedback control. The spontaneous feedback is simpler in principle and easier to realize technologically, and represents a kind of new way of controlling spontaneous emission.

Two-dimensional atom localization based on coherent field controlling in a five-level M-type atomic system

We study two-dimensional sub-wavelength atom localization based on the microwave coupling field controlling and spontaneously generated coherence (SGC) effect. For a five-level M-type atom, introducing a microwave coupling field between two upper levels and considering the quantum interference between two transitions from two upper levels to lower levels, the analytical expression of conditional position probability (CPP) distribution is obtained using the iterative method. The influence of the detuning of a spontaneously emitted photon, Rabi frequency of the microwave field, and the SGC effect on the CPP are discussed. The two-dimensional sub-half-wavelength atom localization with high-precision and high spatial resolution is achieved by adjusting the detuning and the Rabi frequency, where the atom can be localized in a region smaller thanλ/10×λ/10. The spatial resolution is improved significantly compared with the case without the microwave field.

Dark state with counter-rotating dissipative channels

Dark state as a consequence of interference between different quantum states has great importance in the fields of chip-scale atomic clock and quantum information. For the Λ-type three-level system, this dark state is generally regarded as being dissipation-free because it is a superposition of two lowest states without dipole transition between them. However, previous studies are based on the rotating-wave approximation (RWA) by neglecting the counter-rotating terms in the system-environment interaction. In this work, we study non-Markovian quantum dynamics of the dark state in a Λ-type three-level system coupled to two bosonic baths and reveal the effect of counter-rotating terms on the dark state. In contrast to the dark state within the RWA, leakage of the dark state occurs even at zero temperature, as a result of these counter-rotating terms. Also, we present a method to restore the quantum coherence of the dark state by applying a leakage elimination operator to the system.

Multiple spontaneously generated coherence and phase control of optical bistability and multistability in a tripod four-level atomic medium

Optical bistability (OB) and optical multistability (OM) behaviors induced by multiple spontaneously generated coherence (SGC) are investigated theoretically in a tripod four-level atomic scheme. It is found that OB or OM is sensitive to the SGC effects, and the thresholds of OB can be controlled via changing the strength of multiple SGC or the relative phases of the applied fields. In addition, we can switch OB to OM by adjusting the twofold relative phases of the applied fields or vice versa.

High-dimensional atom localization via spontaneously generated coherence in a microwave-driven atomic system

We investigate the two-dimensional (2D) and three-dimensional (3D) atom localization behaviors via spontaneously generated coherence in a microwave-driven four-level atomic system. Owing to the space-dependent atom-field interaction, it is found that the detecting probability and precision of 2D and 3D atom localization behaviors can be significantly improved via adjusting the system parameters, the phase, amplitude, and initial population distribution. Interestingly, the atom can be localized in volumes that are substantially smaller than a cubic optical wavelength. Our scheme opens a promising way to achieve high-precision and high-efficiency atom localization, which provides some potential applications in high-dimensional atom nanolithography.

Plasmon-enhanced Kerr nonlinearity via subwavelength-confined anisotropic Purcell factors

We theoretically investigate the enhancement of Kerr nonlinearity through anisotropic Purcell factors provided by plasmon nanostructures. In a three-level atomic system with crossing damping, larger anisotropism of Purcell factors leads to more enhanced Kerr nonlinearity in electromagnetically induced transparency windows. While for fixed anisotropic Purcell factors, Kerr nonlinearity with orthogonal dipole moments increases with the decrease of its crossing damping, and Kerr nonlinearity with nonorthogonal dipole moments is very sensitive to both the value of crossing damping and the orientation of the dipole moments. We design the non-resonant gold nanorods array, which only provides subwavelength-confined anisotropic Purcell factors, and demonstrate that the Kerr nonlinearity of cesium atoms close to the nanorods array can be modulated at the nanoscale. These findings should have potential application in ultracompact quantum logic devices.

Transparency and tunable slow and fast light in a nonlinear optomechanical cavity

We investigate theoretically the optical response of the output field and the tunable slow and fast light in a nonlinear optomechanical cavity with a degenerate optical parametric amplifier (OPA) and a higher order excited atomic ensemble. Studies show that the higher-order-excitation atom which is similar to the degenerate OPA that acts as a nonlinear medium, induces an additional dip in absorption spectrum of the probe field. The coherence of the mechanical oscillator leads to split the peak in absorption in the probe field spectrum so that the phenomenon of optomechanically induced transparency (OMIT) is generated from the output probe field. In particular, the presence of nonlinearities with the degenerate OPA and the higher order excited atoms can affect significantly the width of the transparency windows, providing an additional flexibility for controlling optical properties. Furthermore, in the presence of the degenerate OPA, the optical-response properties for the probe field become phase-sensitive so that a tunable switch from slow to fast light can be realized.

Quantum-beat based dissipation for spin squeezing and light entanglement

We show an engineered dissipation for the spin squeezing and the light entanglement in a quantum beat system, in which two bright fields interact with an ensemble of three-level atoms in V configuration. The dissipation is based on the atom-field nonlinear interaction that is controlled by the atomic coherence between the excited states off two-photon resonance. Physical analysis and numerical verification are presented for the symmetrical parameters by using the dressed atomic states. It is shown that for particular parameters, the engineered dissipation induces almost perfect two-mode squeezing and entanglement both for the bright fields and for the dressed spins. The excited-state spin has squeezing of near 40% below the standard quantum limit although there remains the spontaneous emission from the involved excited states.

Coherent Control of the Optical Absorption in a Plasmonic Lattice Coupled to a Luminescent Layer

We experimentally demonstrate the coherent control, i.e., phase-dependent enhancement and suppression, of the optical absorption in an array of metallic nanoantennas covered by a thin luminescent layer. The coherent control is achieved by using two collinear, counterpropagating, and phase-controlled incident waves with wavelength matching the absorption spectrum of dye molecules coupled to the array. Symmetry arguments shed light on the relation between the relative phase of the incident waves and the excitation efficiency of the optical resonances of the system. This coherent control is associated with a phase-dependent distribution of the electromagnetic near fields in the structure which enables a significant reduction of the unwanted dissipation in the metallic structures.

Dephasing-Induced Control of Interference Nature in Three-Level Electromagnetically Induced Tansparency Systems

The influence of the dephasing on interference is investigated theoretically and experimentally in three-level electromagnetically induced transparency systems. The nature of the interference, constructive, no interference or destructive, can be controlled by adjusting the dephasing rates. This new phenomenon is experimentally observed in meta-atoms. The physics behind the dephasing-induced control of interference nature is the competing between stimulated emission and spontaneous emission. The random phase fluctuation due to the dephasing will result in the correlation and anti-correlation between the two dressed states, which will enhance and reduce the stimulated emission, respectively.

Cavity-based architecture to preserve quantum coherence and entanglement

Quantum technology relies on the utilization of resources, like quantum coherence and entanglement, which allow quantum information and computation processing. This achievement is however jeopardized by the detrimental effects of the environment surrounding any quantum system, so that finding strategies to protect quantum resources is essential. Non-Markovian and structured environments are useful tools to this aim. Here we show how a simple environmental architecture made of two coupled lossy cavities enables a switch between Markovian and non-Markovian regimes for the dynamics of a qubit embedded in one of the cavity. Furthermore, qubit coherence can be indefinitely preserved if the cavity without qubit is perfect. We then focus on entanglement control of two independent qubits locally subject to such an engineered environment and discuss its feasibility in the framework of circuit quantum electrodynamics. With up-to-date experimental parameters, we show that our architecture allows entanglement lifetimes orders of magnitude longer than the spontaneous lifetime without local cavity couplings. This cavity-based architecture is straightforwardly extendable to many qubits for scalability.

Dynamically controlled resonance fluorescence spectra from a doubly dressed single InGaAs quantum dot

We report the first experimental demonstration of the interference-induced spectral line elimination predicted by Zhu and Scully [Phys. Rev. Lett. 76, 388 (1996)] and Ficek and Rudolph [Phys. Rev. A 60, R4245 (1999)]. We drive an exciton transition of a self-assembled quantum dot in order to realize a two-level system exposed to a bichromatic laser field and observe the nearly complete elimination of the resonance fluorescence spectral line at the driving laser frequency. This is caused by quantum interference between coupled transitions among the doubly dressed excitonic states, without population trapping. We also demonstrate a multiphoton ac Stark effect with shifted subharmonic resonances and dynamical modifications of resonance fluorescence spectra by using double dressing.

Polarized linewidth-controllable double-trapping electromagnetically induced transparency spectra in a resonant plasmon nanocavity

Surface plasmons with ultrasmall optical mode volume and strong near field enhancement can be used to realize nanoscale light-matter interaction. Combining surface plasmons with the quantum system provides the possibility of nanoscale realization of important quantum optical phenomena, including the electromagnetically induced transparency (EIT), which has many applications in nonlinear quantum optics and quantum information processing. Here, using a custom-designed resonant plasmon nanocavity, we demonstrate polarized position-dependent linewidth-controllable EIT spectra at the nanoscale. We analytically obtain the double coherent population trapping conditions in a double-Λ quantum system with crossing damping, which give two transparent points in the EIT spectra. The linewidths of the three peaks are extremely sensitive to the level spacing of the excited states, the Rabi frequencies and detunings of pump fields, and the Purcell factors. In particular the linewidth of the central peak is exceptionally narrow. The hybrid system may have potential applications in ultra-compact plasmon-quantum devices.

Electromagnetically induced grating based on the giant Kerr nonlinearity controlled by spontaneously generated coherence

We propose a scheme for realizing electromagnetically induced grating via the giant Kerr nonlinearity in a coherently driven four-level system with spontaneously generated coherence. In the presence of spontaneously generated coherence, Kerr nonlinearity can be enhanced with vanishing linear absorption. Thus, with a standing-wave coupling field, one can achieve a pure absorption grating, which leads the probe field to gather the zero-order direction when the detuning of the coupling field is on resonance. Moreover, we can obtain a pure phase grating, which diffracts a weak probe light into the first-order direction and the second-order direction when the detuning of the coupling field is off resonance.

Surface-plasmon-induced modification on the spontaneous emission spectrum via subwavelength-confined anisotropic Purcell factor

The mechanism of using the anisotropic Purcell factor to control the spontaneous emission linewidths in a four-level atom is theoretically demonstrated; if the polarization angle bisector of the two dipole moments lies along the axis of large/small Purcell factor, destructive/constructive interference narrows/widens the fluorescence center spectral lines. Large anisotropy of the Purcell factor, confined in the subwavelength optical mode volume, leads to rapid spectral line narrowing of atom approaching a metallic nanowire, nanoscale line width pulsing following periodically varying decay rates near a periodic metallic nanostructure, and dramatic modification on the spontaneous emission spectrum near a custom-designed resonant plasmon nanostructure. The combined system opens a good perspective for applications in ultracompact active quantum devices.

Coherent frequency conversion in a superconducting artificial atom with two internal degrees of freedom

By adding a large inductance in a dc-SQUID phase qubit loop, one decouples the junctions' dynamics and creates a superconducting artificial atom with two internal degrees of freedom. In addition to the usual symmetric plasma mode (s mode) which gives rise to the phase qubit, an antisymmetric mode (a mode) appears. These two modes can be described by two anharmonic oscillators with eigenstates |ns> and |na> for the s and a mode, respectively. We show that a strong nonlinear coupling between the modes leads to a large energy splitting between states |0s,1a> and |2s,0a>. Finally, coherent frequency conversion is observed via free oscillations between the states |0s,1a> and |2s,0a>.

Control of spontaneous emission from a micro-wave driven atomic system

We demonstrate coherent control of spontaneous emission from an indirectly coupled transition in a microwave driven four-level atomic system. The transition of concern is not directly coupled by any laser fields, while the ground state is coupled to another ground state by a microwave field. We show that the coupling of the microwave field produces interesting features such as double narrow lines in the emission spectrum. The heights, widths and positions of the emission peaks can be controlled by modifying the Rabi frequency and detuning of the microwave field. We discuss the spectra in the dressed states basis.

Coherence generation and population transfer by stimulated Raman adiabatic passage and π pulse in a four-level ladder system

We propose a new scheme for achieving the complete population transfer and the optimal coherence generation between the ground state and the Rydberg state in a four-level ladder system by combining the STIRAP or fractional STIRAP technique and the π pulse technique. We consider, in particular, two different situations where spontaneous emission from the two highest states are neglected or not. Our numerical calculations show that the time width and the delay time of the π pulse are two critical parameters for attaining the maximal population transfer and coherence generation in this scheme.

High-precision metrology of highly charged ions via relativistic resonance fluorescence

Resonance fluorescence of laser-driven highly charged ions is investigated with regard to precisely measuring atomic properties. For this purpose an ab initio approach based on the Dirac equation is employed that allows for studying relativistic ions. These systems provide a sensitive means to test correlated relativistic dynamics, quantum electrodynamic phenomena and nuclear effects by applying x-ray lasers. We show how the narrowing of sidebands in the x-ray fluorescence spectrum by interference due to an additional optical driving can be exploited to determine atomic dipole or multipole moments to unprecedented accuracy.

Superconducting qubit with Purcell protection and tunable coupling

We present a superconducting qubit for the circuit quantum electrodynamics architecture that has a tunable qubit-resonator coupling strength g. This coupling can be tuned from zero to values that are comparable with other superconducting qubits. At g = 0, the qubit is in a decoherence-free subspace with respect to spontaneous emission induced by the Purcell effect. Furthermore, we show that in this decoherence-free subspace, the state of the qubit can still be measured by either a dispersive shift on the resonance frequency of the resonator or by a cycling-type measurement.

Dynamically induced double photonic bandgaps in the presence of spontaneously generated coherence

We study a four-level double-Lambda system with spontaneously generated coherence driven by a standing-wave coupling field. It is found that two well-developed photonic bandgaps with reflectivities of about 90% can be generated on the probe resonance in the presence of maximal spontaneously generated coherence. The induced double photonic bandgaps become, however, severely malformed when spontaneously generated coherence vanishes. Dynamic control of the double photonic bandgaps may be exploited to achieve a novel two-port double-channel routing scheme for weak light signals in quantum networks.

Plasmon-induced enhancement of quantum interference near metallic nanostructures

We show that the quantum interference between two spontaneous emission channels can be greatly enhanced when a three-level V-type atom is placed near plasmonic nanostructures such as metallic slabs, nanospheres, or periodic arrays of metal-coated spheres. The spontaneous emission rate is calculated by a rigorous first-principles electromagnetic Green's tensor technique. The enhancement of quantum interference is attributed to the strong dependence of the spontaneous emission rate on the orientation of an atomic dipole relative to the surface of the nanostructure at the excitation frequencies of surface plasmons.

Coherence transfer between atomic ground states by the technique of stimulated Raman adiabatic passage

We experimentally and theoretically demonstrate that the atomic coherence can be completely transferred or arbitrarily contributed among the different levels in a four-level atomic (tripod) scheme by a group of coupled pulse sequences. This technique can be applied to the information conversion in slow-light storage, quantum logical gates, and so on, which is based on the atomic coherence effect.

Quantum interference enhancement with left-handed materials

The quantum interference between two spontaneous emission transitions, which are orthogonal, in a three-level system, can be greatly enhanced by using left-handed materials due to the focusing and phase compensation effects. The influence of the dispersion and dissipation of the materials on the quantum interference enhancement is investigated, and the enhancement of quantum interference is still significant. The quantum interference enhancement will result in a large quenching of the spontaneous emission, even if the distance between the materials and systems is larger than 10 wavelengths.

Quantum interference enforced by time-energy complementarity

The interplay of the concepts of complementarity and interference in the time-energy domain are studied. In particular, we theoretically investigate the fluorescence light from a J = 1/2 to J= 1/2 transition that is driven by a monochromatic laser field. We find that the spectrum of resonance fluorescence exhibits a signature of vacuum-mediated interference effects, whereas the total intensity is not affected by interference. We demonstrate that this result is a consequence of the principle of complementarity, applied to time and energy. Since the considered level scheme can be found, e.g., in (198)Hg(+) ions, our model system turns out to be an ideal candidate to provide evidence for as yet experimentally unconfirmed vacuum-induced atomic coherences.

Control of spontaneous emission in the presence of collisions

We explore the coherent suppression of spontaneous emission processes in the presence of decoherence due to collisions with an external bath.

Stimulated and spontaneous optical generation of electron spin coherence in charged GaAs quantum dots

We report on the coherent optical excitation of electron spin polarization in the ground state of charged GaAs quantum dots via an intermediate charged exciton (trion) state. Coherent optical fields are used for the creation and detection of the Raman spin coherence between the spin ground states of the charged quantum dot. The measured spin decoherence time, which is likely limited by the nature of the spin ensemble, approaches 10 ns at zero field. We also show that the Raman spin coherence in the quantum beats is caused not only by the usual stimulated Raman interaction but also by simultaneous spontaneous radiative decay of either excited trion state to a coherent combination of the two spin states.

Active control of the dynamics of atoms and molecules

There has been much progress in the control of chemical reactions since methods of active control were first proposed by Brumer & Shapiro and by Tannor & Rice ten years ago. This chapter reviews both theoretical and experimental advances in the field. Control schemes based on quantum mechanical interference between competing paths and the manipulation of wave packets with tailored laser pulses are discussed. The theory of optimal control, the limitations of control theory applied to many-body dynamics, and the effects of constraints on the trajectory of the controlled observable are presented. Experimental progress in controlling the population of specific quantum states, in manipulating the dynamics of bound wave packets, and in the control of chemical reactions are reviewed, and current problems in the field are summarized.

Laser control of collective spontaneous emission

The collective spontaneous emission of a pair of two coupled three-level radiators in vacuum is investigated in the presence of a possibly intense laser field. The parameters describing the collective interaction along with the population and decay rates of all involved dressed states are shown to be controllable by the applied laser field. In particular, all populations of the collective system may be transferred at will in a reversible way into a subradiant state, allowing effective storage and manipulation of the quantum system.

Spontaneous-emission suppression on arbitrary atomic transitions

We propose a very simple scheme to slow down the usual exponential decay of upper state population in an atomic two-level system considerably. The scheme uses an additional possibly intense field with frequency lower than the total decay width of the atomic transition. This allows for additional decay channels with the exchange of one or more low-frequency photons during an atomic transition. These channels may then interfere with each other. The intensity and frequency of the low-frequency field are shown to act as two control parameters modifying duration and amount of the population trapping. An extension of the scheme to include transitions to more than one lower state is straightforward.

Inhibition of decoherence due to decay in a continuum

We propose a scheme for slowing down decay into a continuum. We make use of a sequence of ultrashort 2pi pulses applied on an auxiliary transition of the system so that there is a destructive interference between the two transition amplitudes--one before the application of the pulse and the other after the application of the pulse. We give explicit results for a structured continuum. Our scheme can also inhibit unwanted transitions.

Anisotropic vacuum-induced interference in decay channels

We demonstrate how the anisotropy of the vacuum of the electromagnetic field can lead to quantum interferences among the decay channels of close lying states. Our key result is that interferences are given by the scalar formed from the antinormally ordered electric field correlation tensor for the anisotropic vacuum and the dipole matrix elements for the two transitions. We present results for emission between two conducting plates as well as for a two photon process involving fluorescence produced under coherent cw excitation.