arXiv subjects

Christian Schneider

Publications and source records attributed to Christian Schneider.

At least 55 records · Page 3Linked to original sources

arXiv2025

Constraining atmospheric composition from the outflow: helium observations reveal the fundamental properties of two planets straddling the radius gap

TOI-836 is a ~2-3 Gyr K dwarf with an inner super Earth ($R=1.7 R_\oplus$, $P=3.8$ d) and an outer mini Neptune ($R=2.6 R_\oplus$, $P=8.6$ d). JWST/NIRSpec 2.8--5.2 $μ$m transmission spectra are flat for both planets. We present Keck/NIRSPEC observations of escaping helium for super-Earth b, which shows no excess absorption in the 1083 nm triplet to deep limits (<0.2%), and mini-Neptune c, which shows strong (0.7%) excess absorption in both visits. These results demonstrate that planet c retains at least some primordial atmosphere, while planet b is consistent with having lost its entire primordial envelope. Self-consistent 1D radiative-hydrodynamic models of planet c reveal that the helium excess absorption signal is highly sensitive to metallicity: its equivalent width collapses by a factor of 13 as metallicity increases from 10x to 100x solar, and by a further factor of 12 as it increases to 200x solar. The observed equivalent width is 88% the model prediction for 100x metallicity, suggesting an atmospheric metallicity similar to K2-18b and TOI-270d, the first two mini-Neptunes with detected absorption features in JWST transmission spectra. We highlight the helium triplet as a potentially powerful probe of atmospheric composition, with complementary strengths and weaknesses to atmospheric retrievals. The main strength is its extreme sensitivity to metallicity in the scientifically significant range of 10--200x solar, and the main weakness is the enormous model uncertainties in outflow suppression and confinement mechanisms, such as magnetic fields and stellar winds, which can suppress the signal by at least a factor of ~several.

astro-ph.EP↗

arXiv2025

Joint Delay-Doppler Estimation using OFDMA Payloads for Integrated Sensing and Communications

The use of future communication systems for sensing offers the potential for a number of new applications. In this paper, we show that leveraging user data payloads in multi-node Orthogonal Frequency Division Multiple Access (OFDMA) networks for estimating target delay and Doppler-shift parameters can yield a significant advantage in SNR and addressable bandwidth. However, gaps in the frequency-time resources, reference signal boosting and amplitude modulation schemes introduce challenges for estimation at the sensing receiver. In this work, we propose a joint delay and Doppler-shift model-based estimator designed to address these challenges. Furthermore, we demonstrate that incorporating knowledge of the device model into the estimation procedure helps mitigate the effects of the non-ideal radar ambiguity function caused by amplitude-modulated user payloads and sparse reference signals. Simulation results demonstrate that the estimator achieves the theoretical lower bound on estimation variance.

eess.SP↗

arXiv2025

Bistatic Micro-Doppler Analysis of a Vertical Takeoff and Landing (VTOL) Drone in ICAS Framework

Integrated Communication and Sensing (ICAS) is a key technology that enables sensing functionalities within the next-generation mobile communication (6G). Joint design and optimization of both functionalities could allow coexistence, therefore it advances toward joint signal processing and using the same hardware platform and common spectrum. Contributing to ICAS sensing, this paper presents the measurement and analysis of the micro-Doppler signature of Vertical Takeoff and Landing (VTOL) drones. Measurement is performed with an OFDM-like communication signal and bistatic constellation, which is a typical case in ICAS scenarios. This work shows that micro-Doppler signatures can be used to precisely distinguish flight modes, such as take-off, landing, hovering, transition, and cruising.

eess.SP↗

arXiv2025

Smeared $R$-ratio in isospin symmetric QCD with Low Mode Averaging

Low Mode Average (LMA) is a technique to improve the quality of the signal-to-noise ratio in the long time separation of Euclidean correlation functions. We report on its beneficial impact in computing the vector-vector light connected two-point correlation functions and derived physical quantities in the mixed action lattice setup adopted by ETM collaboration. We focus on preliminary results of the computation within isospin symmetric QCD (isoQCD) of the $R$-ratio smeared with Gaussian kernels of widths down to $σ\sim250$ MeV, which is enough to appreciate the $ρ$ resonance around 770 MeV, using the Hansen-Lupo-Tantatlo (HLT) spectral-density reconstruction method.

hep-lat↗

arXiv2024

Newtonized Orthogonal Matching Pursuit for High-Resolution Target Detection in Sparse OFDM ISAC Systems

Integrated Sensing and Communication (ISAC) is a technology paradigm that combines sensing capabilities with communication functionalities in a single device or system. In vehicle-to-everything (V2X) sidelink, ISAC can provide enhanced safety by allowing vehicles to not only communicate with one another but also sense the surrounding environment by using sidelink signals. In ISAC-capable V2X sidelink, the random resource allocation results in an unstructured and sparse distribution of time and frequency resources in the received orthogonal frequency division multiplexing (OFDM) grid, leading to degraded radar detection performance when processed using the conventional 2D-FFT method. To address this challenge, this paper proposes a high-resolution off-grid radar target detection algorithm irrespective of the OFDM grid structure. The proposed method utilizes the Newtonized orthogonal matching pursuit (NOMP) algorithm to effectively detect weak targets masked by the sidelobes of stronger ones and accurately estimates off-grid range and velocity parameters with minimal resources through Newton refinements. Simulation results demonstrate the superior performance of the proposed NOMP-based target detection algorithm compared to existing compressed sensing (CS) methods in terms of detection probability, resolution, and accuracy. Additionally, experimental validation is performed using a bi-static radar setup in a semi-anechoic chamber. The measurement results validate the simulation findings, showing that the proposed algorithm significantly enhances target detection and parameter estimation accuracy in realistic scenarios.

eess.SP↗

arXiv2024

Optical properties and dynamics of direct and spatially and momentum indirect excitons in AlGaAs/AlAs quantum wells

We present an experimental study on optical properties and dynamics of direct and spatially and momentum indirect excitons in AlGaAs/AlAs quantum wells near the crossover between $\varGamma-$ and $X$-valley confined electron states. The time-integrated photoluminescence experiment at $T=$4.8 K revealed three simultaneously observed optical transitions resulting from (a) a direct exciton recombination, involving an electron and a hole states both located in the $\varGamma$-valley in the quantum well layer, and (b) two spatially and momentum indirect excitons, comprising of the confined electron states in the $X$-valley in the AlAs barrier with different effective masses and quantum well holes in the $\varGamma$-valley. This interpretation has been based on the optical pumping density-dependent, temperature-dependent and spatially-resolved photoluminescence measurements, which provided the characterization of the structure, crucial in potential system's applications. Additionally, the time-resolved photoluminescence experiments unveiled complex carrier relaxation dynamics in the investigated quantum well system, which is strongly governed by a non-radiative carrier recombination - the characteristics further critical in potential system's use. This solid state platform hosting both direct and indirect excitons in a highly tunable monolithic system can benefit and underline the operation principles of novel electronic and photonic devices.

cond-mat.mes-hall↗

arXiv2024

Anomalous dispersion via dissipative coupling in a quantum well exciton-polariton microcavity

Although energy level repulsion is typically observed in interacting quantum systems, non-Hermitian physics predicts the effect of level attraction, which occurs when significant energy dissipation is present. Here, we show a manifestation of dissipative coupling in a high-quality AlGaAs-based polariton microcavity, where two polariton branches attract, resulting in an anomalous, inverted dispersion of the lower branch in momentum dispersion. Using angle-resolved photoluminescence measurements we observe the evolution of the level attraction with exciton-photon detuning, leading to changes in anomalous dispersion shape within a single sample. The dissipative coupling is explained by the interaction with an indirect exciton, acting as a highly dissipative channel in our system, and the observed dispersions are well captured within a phenomenological model. Our results present a new mechanism of dissipative coupling in light-matter systems and offer a tunable and well-controlled AlGaAs-based platform for engineering the non-Hermitian and negative mass effects in polariton systems.

cond-mat.mes-hall↗

arXiv2024

Room-temperature polariton condensate in a two-dimensional hybrid perovskite

Layered 2D halide perovskites are chemically synthesized realizations of quantum well stacks with giant exciton oscillator strengths, tunable emission spectra and very large exciton binding energies. While these features render 2D halide perovskites a promising platform for room-temperature polaritonics, bosonic condensation and polariton lasing in 2D perovskites have so far remained elusive at ambient conditions. Here, we demonstrate room-temperature cavity exciton-polariton condensation in mechanically exfoliated crystals of the 2D Ruddlesden-Popper iodide perovskite $(BA)_{2}(MA)_{2}Pb_{3}I_{10}$ in an open optical microcavity. We observe a polariton condensation threshold of $P_{th}=6.76 fJ$ per pulse and detect a strong non-linear response. Interferometric measurements confirm the spontaneous emergence of spatial coherence across the condensate with an associated first-order autocorrelation reaching $g^{(1)}\approx 0.6$. Our results lay the foundation for a new class of room-temperature polariton lasers based on 2D halide perovskites with great potential for hetero-integration with other van-der-Waals materials and combination with photonic crystals or waveguides.

physics.optics↗

arXiv2024

Losses resistant verification of quantum non-Gaussian photon statistics

Quantum non-Gaussian states of light have fundamental properties that are essential for a multitude of applications in quantum technology. However, many of these features are difficult to detect using standard criteria due to optical losses and detector inefficiency. As the statistics of light are unknown, the loss correction on the data is unreliable, despite the fact that the losses can be precisely measured. To address this issue, we employ a loss-mitigated verification technique utilising quantum non-Gaussian witnesses, which incorporate the known optical losses and detector inefficiency into their derivation. This approach allows us to address the considerable challenge of experimentally demonstrating unheralded quantum non-Gaussian states of single photons and photon pairs.

quant-ph↗

arXiv2024

Accelerating Innovation in 6G Research: Real-Time Capable SDR System Architecture for Rapid Prototyping

The upcoming 3GPP global mobile communication standard 6G strives to push the technological limits of radio frequency (RF) communication even further than its predecessors: Sum data rates beyond 100 Gbit/s, RF bandwidths above 1 GHz per link, and sub-millisecond latency necessitate very high performance development tools. We propose a new SDR firmware and software architecture designed explicitly to meet these challenging requirements. It relies on Ethernet and commercial off-the-shelf network and server components to maximize flexibility and to reduce costs. We analyze state-of-the-art solutions (USRP X440 and other RFSoC-based systems), derive architectural design goals, explain resulting design decision in detail, and exemplify our architecture's implementation on the XCZU48DR RFSoC. Finally, we validate its performance via measurements and outline how the architecture surpasses the state-of-the-art with respect to sustained RF recording, while maintaining high Ethernet bandwidth efficiency. Building a 6G integrated sensing and communication (ISAC) example, we demonstrate its real-time and rapid application development capabilities.

eess.SP↗

arXiv2024

Infrared magneto-polaritons in MoTe$_2$ mono- and bilayers

MoTe$_2$ monolayers and bilayers are unique within the family of van-der-Waals materials since they pave the way towards atomically thin infrared light-matter quantum interfaces, potentially reaching the important telecommunication windows. Here, we report emergent exciton-polaritons based on MoTe$_2$ monolayer and bilayer in a low-temperature open micro-cavity in a joint experiment-theory study. Our experiments clearly evidence both the enhanced oscillator strength and enhanced luminescence of MoTe$_2$ bilayers, signified by a 38 \% increase of the Rabi-splitting and a strongly enhanced relaxation of polaritons to low-energy states. The latter is distinct from polaritons in MoTe$_2$ monolayers, which feature a bottleneck-like relaxation inhibition. Both the polaritonic spin-valley locking in monolayers and the spin-layer locking in bilayers are revealed via the Zeeman effect, which we map and control via the light-matter composition of our polaritonic resonances.

cond-mat.mes-hall↗

arXiv2024

Topologically tunable polaritons based on two-dimensional crystals in a photonic lattice

Topological photonics is an emergent research discipline which interlinks fundamental aspects of photonics, information processing and solid-state physics. Exciton-polaritons are a specifically interesting platform to study topological phenomena, since the coherent light matter coupling enables new degrees of freedom such as tunable non-linearities, chiralities and dissipation. Room-temperature operation of such exciton-polaritons relies on materials comprising both, large exciton binding energies and oscillator strength. We harness widely spectrally tunable, room temperature exciton-polaritons based on a WS2 monolayer in an open optical cavity to realize a polariton potential landscape which emulates the Su-Schrieffer-Heeger (SSH) Hamiltonian. It comprises a domain boundary hosting a topological, exponentially localized mode at the interface between two lattices characterized by different Zak-phases which features a spectral tunability over a range as large as 80 meV. Moreover, we utilize the unique tilt-tunability of our implementation, to transform the SSH-lattice into a Stark-ladder. This transformation couples the topologically protected defect mode to propagating lattice modes, and effectively changes the symmetry of the system. Furthermore, it allows us to directly quantify the Zak-phase difference $Δ_{Zak}=(1.13\pm 0.11)π$ between the two topological phases. Our work comprises an important step towards in-situ tuning topological lattices to control and guide light on non-linear chips.

physics.optics↗

arXiv2024

Correlated magnetism of moiré exciton-polaritons on a triangular electron-spin lattice

We demonstrate evidence of correlated magnetism for exciton-polaritons in a MoSe$_{2}$/WS$_{2}$ moiré heterostructure with near-parallel alignment subject to electron doping. In our experiments, interactions between electrons and moiré excitons are controlled electrostatically by field-effect doping, and the polaritonic regime of strong light-matter coupling is established in an open cryogenic microcavity. Remarkably, at filling fractions around one electron per moiré cell, we observe drastic and nonlinear enhancement of the effective polariton Landé factor as a hallmark of correlated magnetism, which is cavity-controlled via resonance tuning of light and matter polariton constituents. Our work establishes moiré van der Waals heterostructures as an outstanding platform for studies of correlated phenomena in the presence of strong light-matter coupling and many-body phases of lattice-ordered excitons, charges and spins.

cond-mat.mes-hall↗

arXiv2024

A source of entangled photons based on a cavity-enhanced and strain-tuned GaAs quantum dot

A quantum-light source that delivers photons with a high brightness and a high degree of entanglement is fundamental for the development of efficient entanglement-based quantum-key distribution systems. Among all possible candidates, epitaxial quantum dots are currently emerging as one of the brightest sources of highly entangled photons. However, the optimization of both brightness and entanglement currently requires different technologies that are difficult to combine in a scalable manner. In this work, we overcome this challenge by developing a novel device consisting of a quantum dot embedded in a circular Bragg resonator, in turn, integrated onto a micromachined piezoelectric actuator. The resonator engineers the light-matter interaction to empower extraction efficiencies up to 0.69(4). Simultaneously, the actuator manipulates strain fields that tune the quantum dot for the generation of entangled photons with corrected fidelities to a maximally entangled state up to 0.96(1). This hybrid technology has the potential to overcome the limitations of the key rates that plague QD-based entangled sources for entanglement-based quantum key distribution and entanglement-based quantum networks.

quant-ph↗

arXiv2024

Laser excitation of the $^{229}$Th nuclear isomeric transition in a solid-state host

LiSrAlF$_6$ crystals doped with $^{229}$Th are used in a laser-based search for the nuclear isomeric transition. Two spectroscopic features near the nuclear transition energy are observed. The first is a broad excitation feature that produces red-shifted fluorescence that decays with a timescale of a few seconds. The second is a narrow, laser-linewidth-limited spectral feature at $148.38219(4)_{\textrm{stat}}(20)_{\textrm{sys}}$ nm ($2020407.3(5)_{\textrm{stat}}(30)_{\textrm{sys}}$ GHz) that decays with a lifetime of $568(13)_{\textrm{stat}}(20)_{\textrm{sys}}$ s. This feature is assigned to the excitation of the $^{229}$Th nuclear isomeric state, whose energy is found to be $8.355733(2)_{\textrm{stat}}(10)_{\textrm{sys}}$ eV in $^{229}$Th:\thor:LiSrAlF$_6$.

physics.atom-ph↗

arXiv2024

Engineering the impact of phonon dephasing on the coherence of a WSe$_{2}$ single-photon source via cavity quantum electrodynamics

Emitter dephasing is one of the key issues in the performance of solid-state single photon sources. Among the various sources of dephasing, acoustic phonons play a central role in adding decoherence to the single photon emission. Here, we demonstrate, that it is possible to tune and engineer the coherence of photons emitted from a single WSe$_2$ monolayer quantum dot via selectively coupling it to a spectral cavity resonance. We utilize an open cavity to demonstrate spectral enhancement, leveling, and suppression of the highly asymmetric phonon sideband, finding excellent agreement with a microscopic description of the exciton-phonon dephasing in a truly two-dimensional system. Moreover, the impact of cavity tuning on the dephasing is directly assessed via optical interferometry, which points out the capability to utilize light-matter coupling to steer and design dephasing and coherence of quantum emitters in atomically thin crystals.

quant-ph↗

arXiv2024

Bistatic Reflectivity and Micro-Doppler Signatures of Drones for Integrated Communication and Sensing

The integration of wireless communication and radar sensing is gaining the interest of researchers from wireless communication and radar societies. Sensing in Integrated Communication and Sensing (ICAS) systems differs from the traditional radar system in the configuration of transmitter-target-receiver, the operating frequency bands, and the transmitting waveform. It is necessary to understand how target electromagnetic signatures behave in this context. Therefore, this paper presents measurements and analysis of two important target signatures, reflectivity and micro-Doppler, for sensing in ICAS. These target signatures are measured in the state-of-the-art measurement system, Bistatische-Radar-Messeinrichtung (BiRa).

eess.SP↗

arXiv2024

Second-order temporal coherence of polariton lasers based on an atomically thin crystal in a microcavity

Bosonic condensation and lasing of exciton-polaritons in microcavities is a fascinating solid-state phenomenon. It provides a versatile platform to study out-of-equilibrium many-body physics and has recently appeared at the forefront of quantum technologies. Here, we study the photon statistics via the second-order temporal correlation function of polariton lasing emerging from an optical microcavity integrated with an atomically thin MoSe2 crystal. Furthermore, we investigate the macroscopic polariton phase transition for varying excitation powers and temperatures. The lower-polariton exhibits photon bunching below the threshold, implying a dominant thermal distribution of the emission, while above the threshold, the second-order correlation transits towards unity, which evidences the formation of a coherent state. Our findings are in agreement with a microscopic numerical model, which explicitly includes scattering with phonons on the quantum level.

cond-mat.mes-hall↗