Superposition Of Ground States Research Articles

Detecting the relative phase between different frequency components of a photon using a three-level Λ atom coupled to a waveguide

The authors study the scattering of a two-tone photon with a three-level $\mathrm{\ensuremath{\Lambda}}$ atom in a superposition of ground states, when the atom is coupled to a waveguide. The system can be used to detect both the relative phase between the photon frequencies and the relative phase of the atomic ground states as the output scattering depends on both quantities.

The Hartree functional in a double well

We consider a non-linear Hartree energy for bosonic particles in a symmetric double-well potential. In the limit where the wells are far apart and the potential barrier is high, we prove that the ground state and first excited state are given to leading order by an even, respectively odd, superposition of ground states in single wells. The corresponding energies are separated by a small tunneling term that we evaluate precisely.

Improvement of harmonic spectra from superposition of initial state driven by homogeneous and inhomogeneous combined field

A larger harmonic cutoff with a higher harmonic yield is a necessary requirement for producing intense ultrashort extreme ultraviolet (XUV) and X-ray sources. In this paper, we present a potential method to improve the harmonic cutoff and harmonic yield from the superposition of initial states in the He atom driven by the low-intensity homogeneous and inhomogeneous combined field. The results can be separated into two parts, that is, (i) the extension of harmonic cutoff via the optimization of three-color homogeneous and inhomogeneous field (covering the carrier envelope phase, delay time, intensity ratio, and inhomogeneous effect); and (ii) the enhancement of harmonic yield via the superposition of ground state and excited state as the initial state. Consequently, a water window harmonic plateau with an intensity enhancement of six orders of magnitude can be obtained. Moreover, the harmonic plateau is contributed by a single harmonic emission peak. Finally, through the Fourier transformation of some selected harmonics on the spectral continuum, a number of sub-50 as water window attosecond pulses with the intensity enhancement of six orders of magnitude can be obtained.

Driving to the steady ground-state superposition assisted by spontaneous emission

Driving to the steady ground-state superposition assisted by spontaneous emission

Thermal corpuscular black holes

We study the corpuscular model of an evaporating black hole consisting of a specific quantum state for a large number $N$ of self-confined bosons. The single-particle spectrum contains a discrete ground state of energy $m$ (corresponding to toy gravitons forming the black hole), and a gapless continuous spectrum (to accommodate for the Hawking radiation with energy $\omega>m$). Each constituent is in a superposition of the ground state and a Planckian distribution at the expected Hawking temperature in the continuum. We first find that, assuming the Hawking radiation is the leading effect of the internal scatterings, the corresponding $N$-particle state can be collectively described by a single-particle wave-function given by a superposition of a total ground state with energy $M=N\,m$ and a Planckian distribution for $E>M$ at the same Hawking temperature. From this collective state, we compute the partition function and obtain an entropy which reproduces the usual area law with a logarithmic correction precisely related with the Hawking component. By means of the horizon wave-function for the system, we finally show the backreaction of modes with $\omega>m$ reduces the Hawking flux. Both corrections, to the entropy and to the Hawking flux, suggest the evaporation properly stops for vanishing mass, if the black hole is in this particular quantum state.

Reflections on biocatalysis involving phosphorus

Early studies on chemical synthesis of biological molecules can be seen to progress to preparation and biological evaluation of phosphonates as analogues of biological phosphates, with emphasis on their isosteric and isopolar character. Work with such mimics progressed into structural studies with a range of nucleotide-utilising enzymes. The arrival of metal fluorides as analogues of the phosphoryl group, PO(3)(-), for transition state (TS) analysis of enzyme reactions stimulated the symbiotic deployment of (19)F NMR and protein crystallography. Characteristics of enzyme transition state analogues are reviewed for a range of reactions. From the available MF(x) species, trifluoroberyllate gives tetrahedral mimics of ground states (GS) in which phosphate is linked to carboxylate and phosphate oxyanions. Tetrafluoroaluminate is widely employed as a TS mimic, but it necessarily imposes octahedral geometry on the assembled complexes, whereas phosphoryl transfer involves trigonal bipyramidal (tbp) geometry. Trifluoromagnesate (MgF(3)(-)) provides the near-ideal solution, delivering tbp geometry and correct anionic charge. Some of the forty reported tbp structures assigned as having AlF(3)(0) cores have been redefined as trifluoromagnesate complexes. Transition state analogues for a range of kinases, mutases, and phosphatases provide a detailed description of mechanism for phosphoryl group transfer, supporting the concept of charge balance in their TS and of concerted-associative pathways for biocatalysis. Above all, superposition of GS and TS structures reveals that in associative phosphoryl transfer, the phosphorus atom migrates through a triangle of three, near-stationary, equatorial oxygens. The extension of these studies to near attack conformers further illuminates enzyme catalysis of phosphoryl transfer.

Conditional control of quantum beats in a cavity QED system

We probe a ground-state superposition that produces a quantum beat in the intensity correlation of a two-mode cavity QED system. We mix drive with scattered light from an atomic beam traversing the cavity, and effectively measure the interference between the drive and the light from the atom. When a photon escapes the cavity, and upon detection, it triggers our feedback which modulates the drive at the same beat frequency but opposite phase for a given time window. This results in a partial interruption of the beat oscillation in the correlation function, that then returns to oscillate.

From quantum feedback to probabilistic error correction: manipulation of quantum beats in cavity QED

It is shown how one can implement quantum feedback and probabilistic error correction in an open quantum system consisting of a single atom, with ground- and excited-state Zeeman structure, in a driven two-mode optical cavity. The ground-state superposition is manipulated and controlled through conditional measurements and external fields, which shield the coherence and correct quantum errors. Modeling an experimentally realistic situation demonstrates the robustness of the proposal for realization in the laboratory.

Where are the degrees of freedom responsible for black-hole entropy?

Considering the entanglement between quantum field degrees of freedom inside and outside the horizon as a plausible source of black-hole entropy, we address the question: where are the degrees of freedom that give rise to this entropy located? When the field is in ground state, the black-hole area law is obeyed and the degrees of freedom near the horizon contribute most to the entropy. However, for excited state, or a superposition of ground state and excited state, power-law corrections to the area law are obtained, and more significant contributions from the degrees of freedom far from the horizon are shown.PACS Nos.: 04.60.–m, 04.62., 04.70.–s, 03.65.Ud

Ultrahigh Harmonic Generation from an Atom with Superpositionof Ground State and Highly Excited States

We investigate the high-order harmonic generation from an atomprepared in a superposition of ground state and highly excited state.When the atom is irradiated by an ultrashort pulse, the cutoff positionof the plateau in the harmonic spectrum is largely extended comparedwith the case that the atom is initially in the ground state. Thephysics of the extension of the high-order harmonic plateau can beinterpreted by the spatial structure of the atomic initial wave packet.We can optimize the generation of high-order harmonics by substitutingthe excited state for a particular coherent superposition of somehighly excited states to form a spatially localized excited wavepacket.

Coherent pulsed excitation of degenerate multistate systems: Exact analytic solutions

We show that the solution of a multistate system composed of $N$ degenerate lower (ground) states and one upper (excited) state can be reduced by using the Morris-Shore transformation to the solution of a two-state system involving only the excited state and a (bright) superposition of ground states. In addition, there are $N\ensuremath{-}1$ dark states composed of ground states. We use this decomposition to derive analytical solutions for degenerate extensions of the most popular exactly soluble models: the resonance solution, the Rabi, Landau-Zener, Rosen-Zener, Allen-Eberly, and Demkov-Kunike models. We suggest various applications of the multistate solutions, for example, as tools for creating multistate coherent superpositions by generalized resonant $\ensuremath{\pi}$ pulses. We show that such generalized $\ensuremath{\pi}$ pulses can occur even when the upper state is far off resonance, at specific detunings, which makes it possible to operate in the degenerate ground-state manifold without populating the (possibly lossy) upper state, even transiently.

Optically driven qubits in artificial molecules

We present novel models of quantum gates based on coupled quantum dots in which a qubit is regarded as the superposition of ground states in each dot. Coherent control on the qubit is performed by both a frequency and a polarization of a monochromatic light pulse illuminated on the quantum dots. We also show that a simple combination of two single qubit gates functions as a controlled NOT gate resulting from an electron-electron interaction. To examine the decoherence of quantum states, we discuss electronic relaxation contributed mainly by LA phonon processes.

Relaxation processes in coherent-population trapping.

The role played by a buffer gas in the ground-state superposition created in coherent-population-trapping experiments is investigated theoretically. The density matrix equations are solved on the basis of the coupled-noncoupled states. The velocity-changing collisions are examined in the limit of strong collisions. The relaxation rate for transfer of the ground-state superposition between atoms with different velocities is derived from a fit of available experimental results. {copyright} {ital 1996 The American Physical Society.}

Stimulated desorption of neutrals: quantum effects in the Antoniewicz model

The quantum theory of stimulated desorption of a neutral van der Waals bound adsorbate via excitation of an ionized intermediate state is studied. The asymmetric decay introduced by quantum motion in a strongly varying imaginary potential leads to a different dependence of coordinate and velocity on time. The position xcrit which has to be reached, so that the adsorbate has enough kinetic energy to be desorbed is shifted. Both these effects have a tendency to cancel each other. On the other hand the broadening of the wave packet leads to a corresponding broadening of the energy distribution and a certain spread of possible xcrit values. The final result is an increase of the intensity of neutrals combined with a shift of the peak of the intensity distribution to higher energies. The agreement with experiment is partly better, partly worse as compared to classical theory. To obtain full agreement one probably has to use a better adjustment of the real part of the excited state potential: A simple superposition of groundstate plus image potential does not seem to be good enough.