Broadband THz study of excitonic resonances in the high-density regime

We report the first terahertz study of the intra-excitonic 1s-2p transition at high excitation densities in GaAs/AlGaAs quantum wells. A strong shift, broadening, and ultimately the disappearance of this resonance occurs with increasing density, after ultrafast photoexcitation at the near-infrared exciton line. Densities of excitons and unbound electron-hole pairs are followed quantitatively using a model of the composite terahertz dielectric response. Comparison with near-infrared absorption changes reveals a significantly enhanced energy shift and broadening of the intra-excitonic resonance.Comment: 4 pages, 4 figure

Aggregation-fragmentation-diffusion model for trail dynamics

We investigate statistical properties of trails formed by a random process incorporating aggregation, fragmentation, and diffusion. In this stochastic process, which takes place in one spatial dimension, two neighboring trails may combine to form a larger one, and also one trail may split into two. In addition, trails move diffusively. The model is defined by two parameters which quantify the fragmentation rate and the fragment size. In the long-time limit, the system reaches a steady state, and our focus is the limiting distribution of trail weights. We find that the density of trail weight has power-law tail P(w)~w-γ for small weight w. We obtain the exponent γ analytically and find that it varies continuously with the two model parameters. The exponent γ can be positive or negative, so that in one range of parameters small-weight trails are abundant and in the complementary range they are rare

X-ray study of the liquid potassium surface: structure and capillary wave excitations

We present x-ray reflectivity and diffuse scattering measurements from the liquid surface of pure potassium. They strongly suggest the existence of atomic layering at the free surface of a pure liquid metal with low surface tension. Prior to this study, layering was observed only for metals like Ga, In and Hg, the surface tensions of which are 5-7 fold higher than that of potassium, and hence closer to inducing an ideal "hard wall" boundary condition. The experimental result requires quantitative analysis of the contribution to the surface scattering from thermally excited capillary waves. Our measurements confirm the predicted form for the differential cross section for diffuse scattering, $d\sigma /d\Omega \sim 1/q_{xy}^{2-\eta}$ where $\eta = k_BT q_z^2/2\pi \gamma$, over a range of $\eta$ and $q_{xy}$ that is larger than any previous measurement. The partial measure of the surface structure factor that we obtained agrees with computer simulations and theoretical predictions.Comment: 7 pages, 7 figures; published in Phys. Rev.

Learning to Grasp: from Somewhere to Anywhere

Robotic grasping is still a partially solved, multidisciplinary problem where data-driven techniques play an increasing role. The sparse nature of rewards make the automatic generation of grasping datasets challenging, especially for unconventional morphologies or highly actuated end-effectors. Most approaches for obtaining large-scale datasets rely on numerous human-provided demonstrations or heavily engineered solutions that do not scale well. Recent advances in Quality-Diversity (QD) methods have investigated how to learn object grasping at a specific pose with different robot morphologies. The present work introduces a pipeline for adapting QD-generated trajectories to new object poses. Using an RGB-D data stream, the vision pipeline first detects the targeted object, predicts its 6-DOF pose, and finally tracks it. An automatically generated reach-and-grasp trajectory can then be adapted by projecting it relatively to the object frame. Hundreds of trajectories have been deployed into the real world on several objects and with different robotic setups: a Franka Research 3 with a parallel gripper and a UR5 with a dexterous SIH Schunk hand. The transfer ratio obtained when applying transformation to the object pose matches the one obtained when the object pose matches the simulation, demonstrating the efficiency of the proposed approach

Observation of Entangled States of a Fully Controlled 20-Qubit System

We generate and characterise entangled states of a register of 20 individually controlled qubits, where each qubit is encoded into the electronic state of a trapped atomic ion. Entanglement is generated amongst the qubits during the out-of-equilibrium dynamics of an Ising-type Hamiltonian, engineered via laser fields. Since the qubit-qubit interactions decay with distance, entanglement is generated at early times predominantly between neighbouring groups of qubits. We characterise entanglement between these groups by designing and applying witnesses for genuine multipartite entanglement. Our results show that, during the dynamical evolution, all neighbouring qubit pairs, triplets, most quadruplets, and some quintuplets simultaneously develop genuine multipartite entanglement. Witnessing genuine multipartite entanglement in larger groups of qubits in our system remains an open challenge.Comment: 20 pages, 4 figure

Non-Inclusive Online Security: Older Adults\u27 Experience with Two-Factor Authentication

Older adults access critical resources online, including bank, retirement, and health insurance accounts. Thus, it is necessary to protect their accounts so they can confidently use these services that are increasingly being moved online. Two-factor authentication (2FA) protects online assets through efficient and robust authentication, but adoption and usability remain a challenge. Our in-depth qualitative research focuses on ten older adults’ (≥ 60 years) sustained (non)usage of 2FA for thirty days. Participants’ limited adoption of the security keys stemmed from its non-inclusive design, lack of tangible benefits, inconsistent instructions, and device dependencies. We propose design modifications, age-friendly instructions, effective risk communication, and appropriate assistance to encourage 2FA adoption among older adults and institutions entrusted with their data. We also introduce the concept of ‘Security Caregivers,’ who can ensure security and digital independence for the aging populatio