At low temperatures, the electron gas of graphene is expected to show both very weak coupling to thermal baths and rapid thermalization, properties which are desirable for use as a sensitive bolometer. We demonstrate an ultrasensitive, wide-bandwidth measurement scheme based on Johnson noise to probe the thermal-transport and thermodynamic properties of the electron gas of graphene, with a resolution of $2\mathrm{mK}/\sqrt{\mathrm{Hz}}$ and a bandwidth of 80 MHz. We have measured the electron-phonon coupling directly through energy transport, from 2–30 K and at a charge density of $2\times {10}^{11}{\mathrm{cm}}^{-2}$. We demonstrate bolometric mixing and utilize this effect to sense temperature oscillations with a period of 430 ps and determine the heat capacity of the electron gas to be $2\times {10}^{-21}\mathrm{J}/(\mathrm{K}·\mu {\mathrm{m}}^2)$ at 5 K, which is consistent with that of a two-dimensional Dirac electron gas. These measurements suggest that graphene-based devices, together with wide-bandwidth noise thermometry, can generate substantial advances in the areas of ultrasensitive bolometry, calorimetry, microwave and terahertz photo-detection, and bolometric mixing for applications in fields such as observational astronomy and quantum information and measurement.

When an infectious disease strikes a population, the number of newly reported cases is often the only available information during the early stages of the outbreak. An important goal of early outbreak analysis is to obtain a reliable estimate for the basic reproduction number, ${\mathcal{R}}_0$. Over the past few years, infectious disease epidemic processes have gained attention from the physics community. Much of the work to date, however, has focused on the analysis of an epidemic process in which the disease has already spread widely within a population; conversely, very little attention has been paid, in the physics literature or elsewhere, to formulating the initial phase of an outbreak. Careful analysis of this phase is especially important as it could provide policymakers with insight on how to effectively control an epidemic in its initial stage. We present a novel method, based on the principles of network theory, that enables us to obtain a reliable real-time estimate of the basic reproduction number at an early stage of an outbreak. Our method takes into account the possibility that the infectious period has a wide distribution and that the degree distribution of the underlying contact network is heterogeneous. We validate our analytical framework with numerical simulations.

Quantum Hall (QH) and quantum spin Hall (QSH) phases have very different edge states and, when going from one phase to the other, the direction of one edge state must be reversed. We study this phenomenon in graphene in the presence of a strong perpendicular magnetic field on top of a spin-orbit (SO)-induced QSH phase. We show that, below the SO gap, the QSH phase is virtually unaffected by the presence of the magnetic field. Above the SO gap, the QH phase is restored. An electrostatic gate placed on top of the system allows the creation of a QSH-QH junction which is characterized by the existence of a spin-polarized chiral state, propagating along the topological interface. We find that such a setup naturally provides an extremely sensitive spin-polarized current switch which could pave the way to novel spin-based electronic devices.

Demonstrating nonclassical effects over longer and longer distances is essential for both quantum technology and fundamental science. The main challenge is the loss of photons during propagation, because considering only those cases where photons are detected opens a “detection loophole” in security whenever parties or devices are untrusted. Einstein-Podolsky-Rosen steering is equivalent to an entanglement-verification task in which one party (device) is untrusted. We derive arbitrarily loss-tolerant tests, enabling us to perform a detection-loophole-free demonstration of Einstein-Podolsky-Rosen steering with parties separated by a coiled 1-km-long optical fiber, with a total loss of 8.9 dB (87%).

This paper describes the production of thin, focused microjets with velocities of up to $850\mathrm{m}/\mathrm{s}$ by the rapid vaporization of a small mass of liquid in an open liquid-filled capillary. The vaporization is caused by the absorption of a low-energy laser pulse. A likely explanation of the observed phenomenon is based on the impingement of the shock wave caused by the nearly instantaneous vaporization on the free surface of the liquid. We conduct an experimental study of the dependence of the jet velocity on several parameters and develop a semiempirical relation for its prediction. The coherence of the jets and their high velocity, good reproducibility, and controllability are unique features of the system. A possible application is to development of needle-free drug-injection systems that would be of great importance for health care worldwide.

We study the spin and orbital dynamics of single nitrogen-vacancy (NV) centers in diamond between room temperature and 700 K. We find that the ability to optically address and coherently control single spins above room temperature is limited by nonradiative processes that quench the NV center’s fluorescence-based spin readout between 550 and 700 K. Combined with electronic structure calculations, our measurements indicate that the energy difference between the ${}^{3}E$ and ${}^{1}A_1$ electronic states is approximately 0.8 eV. We also demonstrate that the inhomogeneous spin lifetime ($T_2^{*}$) is temperature independent up to at least 625 K, suggesting that single NV centers could be applied as nanoscale thermometers over a broad temperature range.

Magnetic reconnection is a fundamental plasma process. Recent theoretical studies and numerical simulations have suggested that electrons can be efficiently accelerated in contracting magnetic islands and when magnetic islands coalesce. Using data from the STEREO spacecraft, we report in this article the first observation of macroscopic magnetic-island coalescence and a possible splitting, and the associated electron acceleration. On 24 May 2010, two magnetic islands were observed by both the STEREO-A and STEREO-B spacecraft to propagate out along the current sheet behind a coronal mass ejection (CME) and merge. Electron acceleration to above 10 keV is inferred through the observation of a type-III-like radio burst. The acceleration process occurred at a macroscopic scale, likely during the merging of the two magnetic islands. Our observation of the magnetic-island coalescence is supported by a 2.5-D axisymmetric magnetohydrodynamic simulation of CME in which the merging of post-CME magnetic islands is clearly identified.

Magnetotransport and superconducting properties are investigated for uniformly La-doped ${\mathrm{SrTiO}}_3$ films and ${\mathrm{GdTiO}}_3/{\mathrm{SrTiO}}_3$ heterostructures, respectively. ${\mathrm{GdTiO}}_3/{\mathrm{SrTiO}}_3$ interfaces exhibit a high-density 2D electron gas on the ${\mathrm{SrTiO}}_3$ side of the interface, while, for the ${\mathrm{SrTiO}}_3$ films, carriers are provided by the dopant atoms. Both types of samples exhibit ferromagnetism at low temperatures, as evidenced by a hysteresis in the magnetoresistance. For the uniformly doped ${\mathrm{SrTiO}}_3$ films, the Curie temperature is found to increase with doping and to coexist with superconductivity for carrier concentrations on the high-density side of the superconducting dome. The Curie temperature of the ${\mathrm{GdTiO}}_3/{\mathrm{SrTiO}}_3$ heterostructures scales with the thickness of the ${\mathrm{SrTiO}}_3$ quantum well. The results are used to construct a stability diagram for the ferromagnetic and superconducting phases of ${\mathrm{SrTiO}}_3$.

Random, spatially uncorrelated nuclear-hyperfine fields in organic materials dramatically affect electronic transport properties such as electrical conductivity, photoconductivity, and electroluminescence. The influence of these nuclear-hyperfine fields can be overwhelmed by a uniform externally applied magnetic field, even at room temperature where the thermodynamic influences of the resulting nuclear and electronic Zeeman splittings are negligible. As a result, even in applied magnetic fields as small as 10 mT, the kinetics of exciton formation, bipolaron formation, and single-carrier hopping are all modified at room temperature, leading to changes in transport properties in excess of 10% in many materials. Here, we demonstrate a new method of controlling the electrical conductivity of an organic film at room temperature, using the spatially varying magnetic fringe fields of a magnetically unsaturated ferromagnet. (The fringe field is the magnetic field emanating from a ferromagnet, associated with magnetic dipole interactions or, equivalently, the divergence of the magnetization within and at the surfaces of the ferromagnet.) The ferromagnet’s fringe fields might act as a substitute for either the applied magnetic field or the inhomogeneous hyperfine field. The size of the effect, the magnetic-field dependence, and hysteretic properties rule out a model where the fringe fields from the ferromagnet provide a local magnetic field that changes the electronic transport properties through the hyperfine field, and show that our effects originate from electrical transport through the inhomogeneous fringe fields coming from the ferromagnet. Surprisingly, these inhomogeneous fringe fields vary over length scales roughly 2 orders of magnitude larger than the hopping length in the organic materials, challenging the fundamental models of magnetoresistance in organic layers which require the correlation length of the inhomogeneous field to correspond roughly to the hopping length.

We report the ab-initio study of rare-earth adatoms (Gd) on an insulating surface. This surface is of interest because of previous studies by scanning tunneling microscopy showing spin excitations of transition-metal adatoms. The present work is the first study of rare-earth spin-coupled adatoms, as well as the geometry effect of spin coupling and the underlying mechanism of ferromagnetic coupling. The exchange coupling between Gd atoms on the surface is calculated to be antiferromagnetic in a linear geometry and ferromagnetic in a diagonal geometry. We also find that the Gd dimers in these two geometries are similar to the nearest-neighbor and the next-nearest-neighbor Gd atoms in GdN bulk. We analyze how much direct exchange, superexchange, and Ruderman-Kittel-Kasuya-Yosida interactions contribute to the exchange coupling for both geometries by additional first-principles calculations of related model systems.

The ability to coherently spectrally manipulate quantum information has the potential to improve qubit rates across quantum channels and find applications in optical quantum computing. In this paper, we present experiments that use a multielement solenoid combined with the three-level gradient echo memory scheme to perform precision spectral manipulation of optical pulses. These operations include separate bandwidth and frequency manipulation with precision down to tens of kHz, spectral filtering of up to three separate frequency components, as well as time-delayed interference between pulses with both the same, and different, frequencies. If applied in a quantum information network, these operations would enable frequency-based multiplexing of qubits.

A loophole-free violation of Bell inequalities is of fundamental importance for demonstrating quantum nonlocality and long-distance device-independent secure communication. However, transmission losses represent a fundamental limitation for photonic loophole-free Bell tests. A local precertification of the presence of the photons immediately before the local measurements may solve this problem. We show that local precertification is feasible by integrating three current technologies: (i) enhanced single-photon down-conversion to locally create a flag photon, (ii) nanowire-based superconducting single-photon detectors for a fast flag detection, and (iii) superconducting transition-edge sensors to close the detection loophole. We carry out a precise space-time analysis of the proposed scheme, showing its viability and feasibility.

Although iron-based superconductors are multiorbital systems with complicated band structures, we demonstrate that the low-energy physics which is responsible for their high-$T_c$ superconductivity is essentially governed by an effective two-orbital Hamiltonian near half filling. This underlying electronic structure is protected by the $S_4$ symmetry. With repulsive or strong next-nearest-neighbor antiferromagnetic exchange interactions, the model results in a robust $A_{1g}$ $s$-wave pairing which can be mapped exactly to the $d$-wave pairing observed in cuprates. The classification of the superconducting (SC) states according to the $S_4$ symmetry leads to a natural prediction of the existence of two different phases, named the $A$ and $B$ phases. In the $B$ phase, the superconducting order has an overall sign change along the $c$ axis between the top and bottom As (or Se) planes in a single Fe-As (or Fe-Se) trilayer structure, the common building block of iron-based superconductors. The sign change is analogous to the sign change in the $d$-wave superconducting state of cuprates upon 90° rotation. Our derivation provides a unified understanding of iron pnictides and iron chalcogenides, and suggests that cuprates and iron-based superconductors share an identical high-$T_c$ superconducting mechanism.

We propose a compact single-plasmon source operating at near-infrared wavelengths on an integrated III-V semiconductor platform, with a thin ridge waveguide serving as the plasmon channel. By attaching an ultrasmall cavity to the channel, it is shown that both the plasmon-generation efficiency ($\beta$) and the spontaneous decay rate into the channel can be significantly enhanced. An analytical model derived with the Lorentz reciprocity theorem captures the main physics involved in the design of the source and yields results in good agreement with fully vectorial simulations of the device. At resonance, it is predicted that the ultrasmall cavity increases the $\beta$ factor by 70% and boosts the spontaneous decay rate by a factor of 20. The proposed design could pave the way toward integrated and scalable plasmonic quantum networks. Comparison of the present design with other fully dielectric competing approaches is addressed.

We propose a method to get experimental access to the physics of the ultrastrong- and deep-strong-coupling regimes of light-matter interaction through the quantum simulation of their dynamics in standard circuit QED. The method makes use of a two-tone driving scheme, using state-of-the-art circuit-QED technology, and can be easily extended to general cavity-QED setups. We provide examples of ultrastrong- and deep-strong-coupling quantum effects that would be otherwise inaccessible.

The microscopic mechanisms responsible for the ultrafast loss of magnetic order triggered in ferromagnetic metals by optical excitation are still under debate. One of the ongoing controversies is about the thermal origin of ultrafast demagnetization. Although different theoretical investigations support a main driving mechanism of thermal origin, alternative descriptions in terms of coherent interaction between the laser and the spin system or superdiffusive spin transport have been proposed. Another important matter of debate originates from the experimental observation of two time scales in the demagnetization dynamics of the $4f$ ferromagnet gadolinium. Here, it is still unclear whether it is necessary to invoke two distinct microscopic mechanisms to explain such behavior, or if one single mechanism is indeed sufficient. To uncover the physics behind these two unsolved issues, we explore the dependence of ultrafast-demagnetization dynamics in nickel through a survey of different laser intensities and ambient temperatures. Measurements in a large range of these external parameters are performed by means of the time-resolved magneto-optical Kerr effect and display a pronounced change in the maximum loss of magnetization and in the temporal profile of the demagnetization traces. The most striking observation is that the same material system (nickel) can show a transition from a one-step (one time scale) to a two-step (two time scales) demagnetization, occurring on increasing the ambient temperature. We find that the fluence and the temperature dependence of ultrafast demagnetization—including the transition from one-step to two-step dynamics—are reproduced theoretically assuming only a single scattering mechanism coupling the spin system to the temperature of the electronic system. This finding means that the origin of ultrafast demagnetization is thermal and that only a single microscopic channel is sufficient to describe magnetization dynamics in the $3d$ ferromagnets on all time scales.

Compressed sensing has triggered a major evolution in signal acquisition. It consists of sampling a sparse signal at low rate and later using computational power for the exact reconstruction of the signal, so that only the necessary information is measured. Current reconstruction techniques are limited, however, to acquisition rates larger than the true density of the signal. We design a new procedure that is able to reconstruct the signal exactly with a number of measurements that approaches the theoretical limit, i.e., the number of nonzero components of the signal, in the limit of large systems. The design is based on the joint use of three essential ingredients: a probabilistic approach to signal reconstruction, a message-passing algorithm adapted from belief propagation, and a careful design of the measurement matrix inspired by the theory of crystal nucleation. The performance of this new algorithm is analyzed by statistical-physics methods. The obtained improvement is confirmed by numerical studies of several cases.

The inevitable presence of decoherence effects in systems suitable for quantum computation necessitates effective error-correction schemes to protect information from noise. We compute the stability of the toric code to depolarization by mapping the quantum problem onto a classical disordered eight-vertex Ising model. By studying the stability of the related ferromagnetic phase via both large-scale Monte Carlo simulations and the duality method, we are able to demonstrate an increased error threshold of 18.9(3)% when noise correlations are taken into account. Remarkably, this result agrees within error bars with the result for a different class of codes—topological color codes—where the mapping yields interesting new types of interacting eight-vertex models.