arXiv subjects

P. Grünwald

Publications and source records attributed to P. Grünwald.

12 recordsLinked to original sources

arXiv2019

Radiation pressure in finite Fabry-Pérot cavities

We study the effect of finite size and misalignment on a fundamental optomechanical setup: a Fabry-Pérot cavity with one fixed and one moveable mirror. We describe in detail light confinement under these real world imperfections and compare the behaviour of the intracavity and output fields to the well-known ideal case. In particular, we show that it is possible to trace the motion of the movable mirror itself by measuring intensity changes in the output field even in the presence of fabrication shortcomings and thermal noise. Our result might be relevant to the transition from high precision research experiments to everyday commercial applications of optomechanics; such as high-precission stepmotor or actuator positioning.

physics.optics↗

arXiv2017

Rydberg excitons in the presence of an ultralow-density electron-hole plasma

We use two-color pump-probe spectroscopy to study Rydberg excitons in Cu$_2$O in the presence of free carriers injected by above-band-gap excitation. Already at plasma densities $ρ_\text{eh}$ below one hundredth electron-hole pair per \textmu m$^{3}$, the Rydberg exciton absorption lines are bleached while their energies remain constant, until they finally disappear, starting from the highest observed principal quantum number $n_\text{max}$. As confirmed by calculations, the band gap is reduced by many-particle effects caused by free carriers scaling as $ρ_\text{eh}^{1/2}$. An exciton line looses oscillator strength when the band edge approaches the exciton energy vanishing completely at the crossing point. We quantitatively describe this plasma blockade by introducing an effective Bohr radius that determines the energy distance to the shifted band edge. In combination with the negligible associated decoherence this opens the possibility to control the Rydberg exciton absorption through the plasma-induced band gap modulation.

cond-mat.mtrl-sci↗

arXiv2016

Signatures of Quantum Coherence in Rydberg Excitons

Coherent optical control of individual particles has been demonstrated both for atoms and semiconductor quantum dots. Here we demonstrate the emergence of quantum coherent effects in semiconductor Rydberg excitons in bulk Cu$_2$O. Due to the spectral proximity between two adjacent Rydberg exciton states, a single-frequency laser may pump both resonances with little dissipation from the detuning. As a consequence, additional resonances appear in the absorption spectrum that correspond to dressed states consisting of two Rydberg exciton levels coupled to the excitonic vacuum, forming a V-type three-level system, but driven only by one laser light source. We show that the level of pure dephasing in this system is extremely low. These observations are a crucial step towards coherently controlled quantum technologies in a bulk semiconductor.

cond-mat.mes-hall↗

arXiv2016

Enhanced squeezing by absorption

Absorption is usually expected to be detrimental to quantum coherence effects. However, the situation for complex absorption spectra has been little studied yet. We consider the resonance fluorescence of excitons in a semiconductor quantum well. The creation of excitons requires absorption of the incoming pump-laser light. Thus, the absorption spectrum of the medium acts as a spectral filter for the emitted light. Surprisingly, absorption can even improve quantum effects, as is demonstrated for the squeezing of the resonance fluorescence of the quantum-well system. This effect can be explained by an improved phase matching due to absorption.

quant-ph↗

arXiv2015

Nonclassical light from an incoherently pumped quantum dot in a microcavity

Semiconductor microcavities with artificial single-photon emitters have become one of the backbones of semiconductor quantum optics. In many cases however, technical and physical issues limit the study of optical fields to incoherently excited systems. We analyze the model of a two-level system in a single-mode cavity, where the former is incoherently driven. The specific structure of the applied master equation yields a recurrence relation for the steady-state values of correlations of the intracavity field and the emitter. We provide boundary conditions, that permit a systematical, easy to implement solution, which is numerically less demanding than standard methods. Different cavity systems from previous experiments are analyzed. The derived boundary conditions also allow us direct analytical statements about the overall quantum state and its higher order moments. With this we can give very good approximations for the full quantum state of the field and show, that for every physically reasonable set of system parameters, the state of the intracavity field is nonclassical.

quant-ph↗

arXiv2014

Quantum Measurement of Broadband Nonclassical Light Fields

Based on the measurement of quantum correlation functions, the quantum statistical properties of spectral measurements are studied for broadband radiation fields. The spectral filtering of light before its detection is compared with the direct detection followed by the spectral analysis of the recorded photocurrents. As an example, the squeezing spectra of the atomic resonance fluorescence are studied for both types of filtering procedures. The conditions for which the detection of the nonclassical signatures of the radiation is possible are analyzed. For the considered example, photocurrent filtering appears to be the superior option to detect nonclassicality, due to the vacuum-noise effects in the optical filtering.

quant-ph↗

arXiv2014

Quantum-entangled light from localized emitters

Localized radiation sources are analyzed with respect to the relation of nonclassicality and quantum entanglement of the emitted light. The source field parts of the radiation emitted in different directions are closely related to each other. As a consequence, nonclassicality of the light fields in one direction directly implies entanglement of the field modes in different directions. This implication can be extended to multipartite-entanglement and multi-time quantum correlations. Given that a nonclassical effect is observed, our approach explicitly yields the multipartite entanglement witnesses. Two examples are considered, the fluorescence radiation of a system of two-level atoms and of excitons in a semiconductor quantum well.

quant-ph↗

arXiv2013

Superfluorescence spectra of excitons in quantum wells

We study the fluorescence light emitted from GaAs excitons in semiconductor quantum wells. The excitons are modeled as interacting bosons. By combining quantum optical methods for the excitonic emission spectrum with many particle descriptions of the transmission through the medium, we can evaluate the spectra outside the well. Comparing with experimental spectra, we get a very good agreement. The method helps explaining the main features of the observed spectra. It is demonstrated that the observed spectra show clear evidence of superfluorescent emission.

quant-ph↗

arXiv2013

Optimal Squeezing in the Resonance Fluorescence of Single Photon Emitters

Annual Review of Astronomy and Astrophysics, Volume 51, page 511e studied for coherently driven single photon sources, such as atoms and quantum dots. Maximal squeezing is realized, if the electronic subsystem of the emitter is in a pure quantum state. The purification is achieved by using a cavity as a second decay channel, besides the incoherent coupling to the electromagnetic vacuum. For realistic cavities this yields a purity of the electronic state of more than 99%. Aside from numerical calculations, we also derive approximate analytical results. Based on the approximations, effects are studied which originate from the environment of the emitter, including radiationless dephasing and incoherent pumping of the emitter and the cavity mode. The fragility of squeezing against decoherence is substantially reduced, so that squeezing persists even under hostile conditions. The measurement of squeezing from such light sources is also considered.

quant-ph↗

arXiv2012

Optimal Squeezing in Resonance Fluorescence via Atomic-State Purification

Squeezing of atomic resonance fluorescence is shown to be optimized by a properly designed environment, which can be realized by a quasi-resonant cavity. Optimal squeezing is achieved if the atomic coherence is maximized, corresponding to a pure atomic quantum state. The atomic-state purification is achieved by the backaction of the cavity field on the atom, which increases the atomic coherence and decreases the atomic excitation. For realistic cavities, the coupling of the atom to the cavity field yields a purity of the atomic state of more than 99%. The fragility of squeezing against dephasing is substantially reduced in this scenario, which may be important for various applications.

quant-ph↗

arXiv2011

Raman-assisted Rabi resonances in two-mode cavity QED

The dynamics of a vibronic system in a lossy two-mode cavity is studied, with the first mode being resonant to the electronic transition and the second one being nearly resonant due to Raman transitions. We derive analytical solutions for the dynamics of this system. For a properly chosen detuning of the second mode from the exact Raman resonance, we obtain conditions that are closely related to the phenomenon of Rabi resonance as it is well known in laser physics. Such resonances can be observed in the spontaneous emission spectra, where the spectrum of the second mode in the case of weak Raman coupling is enhanced substantially.

quant-ph↗

arXiv2010

Entanglement in atomic resonance fluorescence

The resonance fluorescence from regular atomic systems is shown to represent a continuous source of non-Gaussian entangled radiation propagating in two different directions. For a single atom entanglement occurs under the same conditions as squeezing. For more atoms, the entanglement can be more robust against dephasing than squeezing, hence providing a useful continuous source for various applications of entangled radiation.

quant-ph↗