Light amplification in lasers usually relies on populating higher energy levels with more light emitters than lower energy levels. Scientists propose a new way of amplifying light without such population inversion, based on their discovery of resonant superradiant emission from an atomic ensemble interacting with a driving light field.

Visualizing in polymeric liquid how the microscopic polymer molecules react to, and back-act on, the macroscopic liquid flow requires observations of very different scales and is obviously challenging technically. A new experiment overcomes this challenge and shows directly how the extension of individual polymer molecules are tied to the flow properties in an engineered slender thinning filaments of a flowing polymer solution.

Coupling thermodynamics together with information processing is highly nontrivial conceptually, as demonstrated by the notion of Maxwell’s demon. Theorists present a fundamental framework that generalizes the second law of thermodynamics to include the physical effects of information processing.

Thermal radiation from conventional emitters, such as the warm glow of a light bulb, increases with temperature: the hotter the bulb, the more it glows. Thermal emitters that buck this trend could lead to many unconventional thermal devices. Researchers have engineered such a (meta)material by exploiting the unique structural and electronic phase changes of vanadium oxide at around 70${}^{\circ }$C.

Known metamaterial-based “invisibility cloaks” have been observed to work only for narrow ranges of electromagnetic waves, for example, making an object invisible to red light, but highly visible to blue light. With a comprehensive and quantitative theoretical analysis, researchers now provide a concrete understanding of the observations and also propose a design for broadband cloaks using diamagnetic or superconducting thin cloaking layers.

Single-photon sources required for quantum communication and computation are commonly based on semiconductor quantum dots; however, charge fluctuations in the semiconductor lead to unwanted variations in the “color” (wavelength) of the emitted photons. Scientists use a hybrid system of a quantum dot and a constant-wavelength laser to measure and then cancel out fluctuations in the wavelength, generating a perfect stream of single photons of one color.

Google’s search engine algorithmically determines the relative importance of the world’s webpages by exploiting the physics of a classical random walker on the complex network of nodes (pages) and links (hyperlinks). What happens if the classical random walker is replaced by a quantum one? Researchers develop and investigate a simple model of a quantum walker on a complex network, uncovering interesting quantum-classical correspondence as well as fundamentally intriguing differences.

At ambient temperatures, graphene conducts heat via lattice vibrations called phonons, but at very low temperatures, electron-phonon coupling becomes weak and thermal conductance is mostly due to electron diffusion. High-sensitivity measurements across a wide temperature range provide new, state-of-the-art data on electron-phonon coupling, and reveal intriguing deviations from theoretical predictions regarding electron-based thermal and electric transport.

When exposed to intense infrared laser pulses of femtosecond duration, molecules such as N${}_2$, CO${}_2$, and H${}_2$O not only become ionized but can also produce lasers as a result of simultaneous population inversion. Scientists look deep into this “air lasing” phenomenon in N${}_2$ gas and uncover both quantum coherence in the rotational wave packets of the lasing molecular ions and its footprint in the air-laser spectrum.

Inspired by the two-step circulation-diffusion distribution strategy of human circulatory system, scientists have developed “hydrogel membrane microwindows” in microfludic devices as a powerful and versatile means to establish and manipulate chemical and electric-field gradients with speed.

An “active” invisibility cloak achieves its goal by canceling the electromagnetic field scattered by the cloaked object, thus making it invisible. Scientists demonstrate the first experimental realization of such a cloak for microwaves using thin layers of antennas and phase shifters that can be tuned for field cancellation on demand.

Stromatolites are fossilized remains of microbial mats that stopped growing billions of years ago. A large class of them have similar conical shapes. Understanding their growth dynamics would seem like a daunting task. However, positing diffusion of calcium ions and inorganic carbon-based molecules in microbial mats and their diffusion-limited precipitation as the primary growth mechanisms, a mathematical model not only successfully predicts the conical shape but also concludes that the thickness of the ancient microbial mats was similar to that of modern-day mats.

Adiabatic quantum computing (AQC) has intrinsic robustness to noise, but whether it’s fault-tolerant—robust to all forms of noise—has not been explored. While fault tolerance through error correction is theoretically achievable in standard quantum computing, scientists show that it may not be achievable in AQC.

Rocking ratchets are utilized in many different systems to create a directed current of energy or material and much of their behavior is universal across the systems. Scientists discover the time-shift invariance of the ratchet current to be the origin of the universality and based on this insight build a unified theory for rocking ratchets that can guide and interpret future research and applications.

Exciton polaritons are essentially photons “dressed” by and interacting via their interaction with excitons in a semiconductor. As bosons, they exhibit Bose-Einstein condensation, in which they self-organize into a coherent state. Scientists have now extended their lifetime from 10 to 100 picoseconds and have observed a number of dramatic new effects, including a new and dynamic coherent state of spatially localized polaritons at high densities.

The surface electronic structure of a three-dimensional topological superconductor is characterized by a Majorana cone—Majorana-fermion-like electronic excitations whose energies form a gapless continuum, rendering the surface conducting. It was believed that a gap in that electronic structure could only be created if time-reversal symmetry were broken. Theorists now show that, in the presence of strong electronic interaction, “gapping” and time-reversal symmetry can coexist if the surface has a special type of electronic excitations that are “anyons.”

Realization of Majorana fermions in solid-state systems offers a promising candidate for qubits in fault-tolerant quantum computing. How fast such qubits can be manipulated without loss in their quantum coherence motivates the need to understand the dynamics of solid-state Majorana fermions. Scientists discover that the dynamics is actually described by an effective variation of the original Dirac equation for the simpler relativistic free Majorana fermions and establish the speed limit for basic qubit manipulation.

Gauge theories such as quantum electrodynamics, in principle, give us a precise understanding of the interactions between subatomic particles, but often the calculations involved are beyond current computational capabilities. Quantum simulators based on experimental many-body systems could be a solution to this problem. Scientists exploit the current state-of-the-art experimental technology for manipulating cold trapped ions and propose a simulator of the one-dimensional version of quantum electrodynamics.

Properties of light propagating in some materials can be influenced by a static magnetic field. Such magneto-optical effects are useful, but their strength is limited traditionally by the materials used. Scientists demonstrate that a hybrid structure of a magneto-optical photonic waveguide with plasmonic (gold) nanowires significantly boosts an important effect, the so-called transverse magneto-optical Kerr effect, through resonant interaction between the waveguide photons and the plasmons.

Recent experiments on friction between a moving atomic-scale tip and a smooth surface have shown that the friction has a nonmonotonic dependence on temperature and decreases with the tip’s velocity. No single theoretical model provides consistent rationalizations of all these experimental data. A new theory based on the notion of “contact aging”—a thermally activated process of the tip-surface contact strengthening—correctly predicts all these recent results.

Why do humans cooperate with other unrelated humans? Correlated use of both a “reward” and a “punishment” strategy has been thought to be an evolutionary force underlying our predisposition for cooperation: if others are kind to us, we are kind to them; if they are exploitive, we may stop cooperating or tend to punish them. Recent human experiments show, however, that individuals mostly use one strategy or the other, rarely both. Now, statistical physical simulations of an evolutionary game model lend significant support to these findings.

A “monoplex” network, like a Facebook-based social network, can be represented by a set of nodes (people) linked by their Facebook connections (interactions). But real-world networks can be “multiplex,” with multiple types of interactions and where one type of interaction can influence another. A unifying framework for describing “multiplex” networks has been missing so far. Deftly employing the concept of tensors, theorists now present such a framework that will power studies of “multiplex” networks across many scientific disciplines.

Atomlike spin-based nitrogen-vacancy centers in diamond have emerged as a promising class of candidates for qubits in room-temperature quantum computing. Making them interact with each other in a controlled and scalable way even when they are separated over long distances is the next step. A new proposal shows that this goal can be achieved by coupling the spin qubits to a common ferromagnet and exploiting the fast traveling magnons in the ferromagnet as the agent mediating long-range qubit-qubit interactions.

Whether SmB${}_6$ is a true Kondo insulator has been a 40-year-old puzzle, as its unexpected finite low-temperature electric conductance defies that simple stereotyping. Using angle-resolved photoelectron spectroscopy, scientists find an important piece of the puzzle in the material’s electronic band structure, including the signature of topological surface conducting states.

Metallic silicides interface with silicon in most microelectronic devices. But when the miniaturization of the devices takes their sizes down to the nanoscale, is the chemical and structural integrity of the interfacing materials maintained, and if not, how are the electronic properties of the device affected? Scientists investigate nanoscale transistors based on PtSi/Si/PtSi heterostructures and find that Pt diffuses into the silicon channel to form PtSi clusters that behave as metallic quantum dots in a semiconductor matrix.

The ability of liquid-crystal materials to self-assemble into different structures extends even to the defects in them. Ordered arrays of defect-induced structures are used in optical applications, such as microlenses. By inserting colloid particles—which behave as defects—into layered (smectic) liquid crystals to control molecular alignment, scientists develop a new way of generating and manipulating specific optical “flower textures” that may see applications.

Many commonly used electrical resistive switching devices are based on films of metal-oxide-metal “sandwiches.” Now scientists have observed ultrafast and reversible resistive switching in a thin-film device composed of a metallic platinum layer and two ultrathin layers of insulating oxides, LaAlO${}_3$ and SrTiO${}_3$, and revealed its origin in the oxides’ charge-carrying oxygen vacancies.

Nonclassical quantum states—those with no correspondence to classical states—can be characterized by a negative Wigner function and are vital to quantum-information processing. A new all-optical scheme achieves, for the first time, creation, storage, and on-demand release of highly nonclassical photonic states by using two coupled optical cavities, one for creation and storage, and the other for dynamical tuning of the release.

A radio receiver that can tune to and digitize millions of frequencies per second, even if the signals are very weak, requires isolating the desired signal from stronger, unwanted noise. Scientists demonstrate a new kind of metamaterial, built with individual radio-frequency superconducting quantum-interference devices (rf SQUIDs), that allows such fast and long-range tuning by exploiting the large tunability of the nonlinear effective inductance of the Josephson junction in each SQUID.

Coupling a light-amplifying gain material with a light-absorbing lossy material should, if their strengths are carefully balanced, result in a system with no net amplification or absorption. However, this overall symmetry can spontaneously break down at a specific value of gain-loss strength. Surprisingly, scientists find that this “symmetry-breaking” point is extremely robust to modifications of the system by its optical boundary conditions and explain why.

Light bullets, solitarily propagating spatiotemporal waves recently observed in a two-dimensional waveguide array, are complex solitons. For the first time, scientists observe and investigate vortex light bullets—stably bound and propagating triplets of light bullets with an energy vortex at their centers.

Recently nonlinear fiber optics has revealed the existence of “breathers,” a new form of solitons with periodic oscillations on a finite background. But, how do such breathers, when they appear at the same time, interact with each other? A new experiment demonstrates that two such breathers, when their initial shapes and propagations are properly controlled, can collide to make a new giant “rogue” wave.

Pump-probe spectroscopy based on ultrashort laser pulses is gaining a surging interest as a method for probing electronic dynamics in solid-state materials. But how to make sense of the spectroscopic measurements remains a fundamental challenge. Theorists now report a timely development of a concrete and general understanding of pump-probe spectroscopy studies of electron-phonon coupling.

When two superconductors are connected through a weak link, discrete quasiparticle states localized at the link, called Andreev levels, are known to appear and leave their signatures in a “supercurrent” that flows through the link. Using microwave excitation and supercurrent measurements, scientists reveal the fundamental nature of the Andreev levels associated with a single-atom link.

Binary metallic Platinum Group Metal (PGM) systems are a class of materials important for chemical, petroleum, and automotive industries as well as for aeronautics and electronics. A state-of-the-art high-throughput computational materials modeling yields a large number of predictions of stable new PGM systems that will keep experimental materials scientists busy for years to come.

The proximity of an antiferromagnetic phase to the superconducting phase in iron pnictides raises the tantalizing possibility of a fundamental connection between magnetism and superconductivity. With an experiment of unprecedented precision, scientists find unequivocal evidence that puts that possibility on a firmer footing.

Heavily doped transparent conducting oxides are believed to be promising alternatives to noble metals in low-loss plasmonic applications in the technologically important near-infrared range of light. Scientists now report a timely study of the optical properties of doped zinc oxide, assessing its performance in plasmonic devices and establishing a hitherto unrealized connection from doping to crystal structure and optical properties.