Coherent control of nuclear forward scattering

The possibility to control the coherent decay of resonant excitations in nuclear forward scattering is investigated. By changing abruptly the direction of the nuclear hyperfine magnetic field, the coherent scattering of photons can be manipulated and even completely suppressed via quantum interference effects between the nuclear transition currents. The efficiency of the coherent decay suppression and the dependence of the scattered light polarization on the specific switching parameters is analyzed in detail. Using a sophisticated magnetic switching sequence involving four rotations of the hyperfine magnetic field, two correlated coherent decay pulses with different polarizations can be generated out of one excitation, providing single-photon entanglement in the keV regime. The verification of the generated entanglement by testing a single-particle version of Bell's inequality in an x-ray optics experimental setup is put forward.Comment: 22 pages, 6 figures; revised to match the published version: added one figure, small modifications in tex

Far-Field Signatures of a Two-Body Bound State in Collective Emission from Interacting Two-Level Atoms on a Lattice

The collective emission from a one-dimensional chain of interacting two-level atoms is investigated. We calculate the light scattered by dissipative few-excitation eigenstates in the far-field, and in particular focus on signatures of a lattice two-body bound state. We present analytical results for the angle-resolved, temporal decay of the scattered light intensity. Moreover, we find that the steady-state emission spectrum that emerges when the system is probed by a weak, incoherent driving field exhibits a distinct signature for the existence of a bound state, and allows to determine the momentum distribution of the two-body relative wavefunction. Intriguingly, our study does not rely on single-atom addressability and/or manipulation techniques.Comment: 5 pages, 3 figures, supplemen

Controllable linear π\pi-phase modulation in a thermal atom vapor without diffraction or absorption

A scheme is proposed to achieve substantial controllable phase modulation for a probe field propagating through a thermal atomic vapor in double-$\Lambda$ configuration. The phase modulation is based on the linear susceptibility of the probe field, paraxial diffraction is eliminated by exploiting the thermal motion of atoms, and residual absorption is compensated via an incoherent pump field. As a result, a strong controllable uniform phase modulation without paraxial diffraction is achieved essentially independent of the spatial profile or the intensity of the probe field. This phase shift can be controlled via the intensities of the control or the incoherent pump fields. A possible proof-of-principle experiment in alkali atoms is discussed.Comment: 10 pages, 7 figure

Nonlocal nonlinear response of thermal Rydberg atoms and modulational instability in absorptive nonlinear media

Nonlinear and nonlocal effects are discussed in the interaction of laser fields with thermal Rydberg atoms in electromagnetically induced transparency configuration. We show that under the crucial approximation that the time variation in the dipole-dipole interactions due to atomic motions can be neglected in an ensemble average, an analytical form can be obtained for the nonlocal nonlinear atomic response of the thermal medium, and study it for different parameter cases. We further propose a generalized model to describe the modulational instability (MI) in absorptive nonlinear media, in order to understand the propagation dynamics in the thermal Rydberg medium. Interestingly, this model predicts that at short propagation distances, each wave component exhibits the MI effect in absorptive nonlinear media, unlike in the purely dispersive case.Comment: 15 pages, 11 figure

Ab initio few-mode theory for quantum potential scattering problems

Few-mode models have been a cornerstone of the theoretical work in quantum optics, with the famous single-mode Jaynes-Cummings model being only the most prominent example. In this work, we develop ab initio few-mode theory, a framework connecting few-mode system-bath models to ab initio theory. We first present a method to derive exact few-mode Hamiltonians for non-interacting quantum potential scattering problems and demonstrate how to rigorously reconstruct the scattering matrix from such few-mode Hamiltonians. We show that upon inclusion of a background scattering contribution, an ab initio version of the well known input-output formalism is equivalent to standard scattering theory. On the basis of these exact results for non-interacting systems, we construct an effective few-mode expansion scheme for interacting theories, which allows to extract the relevant degrees of freedom from a continuum in an open quantum system. As a whole, our results demonstrate that few-mode as well as input-output models can be extended to a general class of problems, and open up the associated toolbox to be applied to various platforms and extreme regimes. We outline differences of the ab initio results to standard model assumptions, which may lead to qualitatively different effects in certain regimes. The formalism is exemplified in various simple physical scenarios. In the process we provide proof-of-concept of the method, demonstrate important properties of the expansion scheme, and exemplify new features in extreme regimes.Comment: 41 pages, 14 figures, substantially extended version now also covering interacting and nonlinear problem

Collective dynamics in a laser-pumped mixture of two atomic ensembles

We investigate the quantum dynamics of an atomic mixture composed of two multi-atom ensembles. Each ensemble is driven separately by a coherent laser field, respectively, and dampens via the interactions with the environmental vacuum electromagnetic field reservoir. We find that, due to the photon exchange among the two components, long-time excitation oscillations appear, which may be significantly longer than the inverse lifetime of a single emitter. Furthermore, few-atom "jumps" to the excited state occur as a function of the parameter characterizing the inter-component interactions around a certain working point.Comment: 6 pages, 5 figure

Numerical Optical Centroid Measurements

Optical imaging methods are typically restricted to a resolution of order of the probing light wavelength $\lambda_p$ by the Rayleigh diffraction limit. This limit can be circumvented by making use of multiphoton detection of correlated $N$-photon states, having an effective wavelength $\lambda_p/N$. But the required $N$-photon detection usually renders these schemes impractical. To overcome this limitation, recently, so-called optical centroid measurements (OCM) have been proposed which replace the multi-photon detectors by an array of single-photon detectors. Complementary to the existing approximate analytical results, we explore the approach using numerical experiments by sampling and analyzing detection events from the initial state wave function. This allows us to quantitatively study the approach also beyond the constraints set by the approximate analytical treatment, to compare different detection strategies, and to analyze other classes of input states.Comment: 15 pages, 18 figure

Tailoring superradiance to design artificial quantum systems

Cooperative phenomena arising due to the coupling of individual atoms via the radiation field are a cornerstone of modern quantum and optical physics. Recent experiments on x-ray quantum optics added a new twist to this line of research by exploiting superradiance in order to construct artificial quantum systems. However, so far, systematic approaches to deliberately design superradiance properties are lacking, impeding the desired implementation of more advanced quantum optical schemes. Here, we develop an analytical framework for the engineering of single-photon superradiance in extended media applicable across the entire electromagnetic spectrum, and show how it can be used to tailor the properties of an artificial quantum system. This "reverse engineering" of superradiance not only provides an avenue towards non-linear and quantum mechanical phenomena at x-ray energies, but also leads to a unified view on and a better understanding of superradiance across different physical systems.Comment: 6 pages + supplemental materia

Coherent versus incoherent excitation dynamics in dissipative many-body Rydberg systems

We study the impact of dephasing on the excitation dynamics of a cloud of ultracold two-level Rydberg atoms for both resonant and off-resonant laser excitation, using the wave function Monte Carlo (MCWF) technique. We find that while for resonant laser driving, dephasing mainly leads to an increase of the Rydberg population and a decrease of the Mandel Q parameter, at off-resonant driving strong dephasing toggles between direct excitation of pairs of atoms and subsequent excitation of single atoms, respectively. These two excitation mechanisms can be directly quantified via the pair correlation function, which shows strong suppression of the two-photon resonance peak for strong dephasing. Consequently, qualitatively different dynamics arise in the excitation statistics for weak and strong dephasing in off-resonant excitation. Our findings show that time-resolved excitation number measurements can serve as a powerful tool to identify the dominating process in the system's excitation dynamics.Comment: 10 pages, 10 figure

Loading atom lasers by collectivity-enhanced optical pumping

The effect of collectivity on the loading of an atom laser via optical pumping is discussed. In our model, atoms in a beam are laser-excited and subsequently spontaneously decay into a trapping state. We consider the case of sufficiently high particle density in the beam such that the spontaneous emission is modified by the particle interaction. We show that the collective effects lead to a better population of the trapping state over a wide range of system parameters, and that the second order correlation function of the atoms can be controlled by the applied laser field.Comment: 5 pages, 7 figure