Highlights of the Year

Weird as it is, quantum mechanics says that measuring one particle in an entangled pair determines the state of its distant partner. Although this notion is widely accepted today, Einstein famously opposed it, proposing instead that each particle in the pair carries with it all the information needed to determine a measurement’s outcome. Three groups put to rest any doubts that this “local realism” hypothesis might be right (see Viewpoint: Closing the Door on Einstein and Bohr’s Quantum Debate). Performing a so-called Bell test that was free of questionable “loopholes,” scientists at NIST in Boulder, Colorado, and at the University of Vienna, Austria, showed that entangled photons have correlations that exceed the values local realism predicts. Researchers at the Delft University of Technology, Netherlands, found a similar violation by analyzing entangled electron spins. The experiments cap years of increasingly sophisticated Bell tests and lay the groundwork for secure quantum cryptography schemes.

The Large Hadron Collider Beauty Collaboration (LHCb) marked the 20th anniversary of its inception with an unexpected gift: the observation of two pentaquarks (see Viewpoint: Elusive Pentaquark Comes into View). These new particles contain four quarks and an antiquark and are the first five-quark composites to have been detected. LHCb was studying the decay of a bottom-quark particle known as the $\Lambda b$ baryon when the discovery was made. The long lifetime of this baryon, compared to others produced in LHC’s proton-proton collisions, made it possible to spot the pentaquark among the collision debris. The new pattern of quarks presents a unique opportunity to test models of the complex forces that bind quarks together.

X-ray crystallography is the main method for determining the structure of biomolecules. But it requires crystallized samples, which aren’t always available. Imaging single molecules without the need of a crystal is one of the top goals of x-ray free-electron lasers, the brightest x-ray sources currently available. Researchers used the intense pulses from the XFEL at SLAC National Laboratory to obtain the first image of the 3D structure of a single virus (see Viewpoint: X-Ray Imaging of a Single Virus in 3D). By combining hundreds of diffraction patterns from randomly oriented single viruses, the authors were able to reconstruct the electron density of the interior of the mimivirus, one of the largest known viruses. This proof of principle experiment opens the door to studying smaller pathogenic viruses like HIV, herpes, or influenza.

Our view of the Universe is partially obstructed by giant clumps of dark matter that distort the appearance of distant galaxies. But astronomers aren’t complaining. Instead they use this gravitational effect (lensing) to spot where dark matter is. The Dark Energy Survey (DES) in Chile is one such observational campaign. Analyzing a 139-square-degree region of the Southern sky, the DES team released the largest contiguous map of dark matter (see Viewpoint: Sky Survey Casts Light on the Dark Universe). They inferred the presence of dark matter clumps by measuring a slight elongation in the shapes of nearby galaxies. The DES is on its way to complete a much larger map (about 1/8 of the full sky), with which they will measure the pace of the cosmic acceleration—an effect caused by the mysterious dark energy.

In 1929, the German mathematician Hermann Weyl published a simple equation that predicted a new kind of fermionic particle with zero mass. No fundamental particles fitting the description of a “Weyl fermion” have ever been found. But analogues to the fermions have been theorized to exist as electronic excitations in a hypothetical solid—a Weyl semimetal. Two experimental groups, at Princeton and the Chinese Academy of Sciences, discovered that tantalum-arsenide is such a solid (see Viewpoint: Where the Weyl Things Are). Using photoemission, they showed that electron energy bands on the material’s surface have the characteristic arc shape expected for Weyl semimetals. A group at the Massachusetts Institute of Technology found Weyl-like states for microwaves in a photonic crystal. Because Weyl fermions behave like massless particles, researchers speculate they might be useful as information carriers in high-speed electronic devices.

Photons have been used to securely transmit quantum encryption keys over more than 300 kilometers of optical fiber. Ultimately, light attenuation limits how far a fiber can transmit a signal without degrading its quantum properties. But satellite-to-Earth links might soon open new frontiers for quantum communication. Researchers from the University of Padua and the Matera Laser Ranging Observatory, both in Italy, demonstrated that qubits encoded in photons can preserve their fragile quantum properties even after a round trip to satellites located more than one thousand kilometers away from Earth (see Viewpoint: Sending Quantum Messages Through Space). The authors encoded qubits in the photons’ polarization and sent them to five satellites that bounced the light back to Earth. After the long journey, different qubit states could be distinguished reliably enough for viable quantum protocols.

Physicists were able to make “electron-like” atoms sit still for a photo. Images of single atoms cooled and trapped by lasers had been taken before, but only using bosons. Fermionic atoms, which have the same spin as electrons and other fundamental particles, have proved harder to cool and assemble into optical traps. Three independent teams, one at the Massachusetts Institute of Technology, one at Harvard University, and one at the University of Strathclyde in the UK, got fermionic atoms to “say cheese” (see Synopsis: Quantum Microscope Images Fermionic Atoms). The basic idea is to use lasers that both cool and image at the same time. Having this imaging capability is important for quantum simulations, in which the interactions between the fermionic atoms can be tuned to simulate, for instance, strongly correlated electrons in superconductors and colossal magnetoresistance materials.

One of the most popular physics videos of 2015 showed that balloons burst in two distinct ways (see Focus: Two Modes of Balloon Bursting Revealed). The video was shot by physicists from the École Normale Supérieure (ENS) in France, who reported that balloons pop with a single crack when the internal gas pressure is below a certain threshold; above this threshold, they pop by forming many cracks radiating from the puncture point. Based on balloon-bursting videos and analysis of the rubber fragments, the researchers explained that at high pressures, single cracks can't propagate fast enough to release the high tension in the rubber. Instead, multiple cracks form to allow a more rapid tension release. The results may also apply to fast fragmentation of other materials, such as glass or rocks.