Volume 10 Issue 4, April 2014

A tiny drum converts between infrared and microwave signals with record efficiency by keeping the beat of both.

Surface states are the salient feature of topological insulators. Now, experiments demonstrate that surface states can be enhanced or destroyed by applying compressive or tensile strain.

Spin pumping and spin-to-charge conversion in hybrid metal–organic devices reveal the physical mechanisms at work in semiconducting polymers.

Gravity and quantum mechanics tend to stay out of each other's way, but this might change as we devise new experiments to test the applicability of quantum theory to macroscopic systems and larger length scales.

Recent advances in quantum information theory reveal the deep connections between entanglement and thermodynamics, many-body theory, quantum computing and its link to macroscopicity.

Revisiting the notion of causality in quantum mechanics may lead to new directions in quantum information theory and quantum gravity research.

There are good reasons to consider nonlocality to be the defining feature of quantum mechanics, but stronger nonlocal correlations than those predicted by quantum theory could exist, which raises the intriguing question of what lies beyond.

Testing the limits of the quantum mechanical description of nature has become a subject of intense experimental interest. Recent advances in investigating macroscopic quantum superpositions are pushing these limits.

Starting with wave-particle duality, experiments with light have played a major role in the development of quantum theory. Advances in photonic technologies allow for improved tests of quantum complementarity, delayed-choice and nonlocality.

A quantum spin liquid is a spin state with no magnetic order even at the lowest temperatures. To explain neutron scattering data on a ‘kagome lattice’ antiferromagnet, visons (elementary excitations of vortices) must be included, in addition to the usual fractionalized spinons.

Bismuth selenide is a prototypical 3D topological insulator; its electronic spectrum features a Dirac cone populated by surface states. Now, it is experimentally and numerically shown that a bandgap forms beyond a certain critical compressive strain, destroying the surface states.

Superconductivity in iron pnictide compounds occurs near a magnetic phase and magnetic spin fluctuations are prime candidates for the superconducting pairing mechanism. What does this mean now that a second magnetic phase, next to another superconducting phase, is found at higher doping levels?

Bad metals, such as the copper oxide superconductors, do conduct electricity but the origin of their poor conductivity is unclear. A study of disordered rare-earth nickelates now provides microscopic insights into bad-metal behaviour

The transport and relaxation mechanisms in organic semiconductors are still insufficiently understood, but measurements now show that in these materials polarons carry pure spin currents over extended distances with long relaxation times, and uncover the role of spin-orbit coupling in this process.

Spin–orbit coupling in Bose gases is expected to lead to new phenomena, but the thermodynamic properties are not yet fully understood. An ultracold atom experiment using artificial spin–orbit coupling uncovers the finite-temperature phase diagram and a transition between a stripe-ordered and a magnetized phase.

An optomechanical system that converts microwaves to optical frequency light and vice versa is demonstrated. The technique achieves a conversion efficiency of approximately 10%. The results indicate that the device could work at the quantum level, up- and down-converting individual photons, if it were cooled to millikelvin temperatures. It could, therefore, form an integral part of quantum-processor networks.

It's time for a life lesson.

Over the past two decades, the fields of quantum information theory and quantum technology have emerged and matured. The theoretical and experimental tools developed in this context are now making it possible to revisit the very foundations of quantum theory, and to explore the terra incognitathat may lie beyond. In this Insight, we survey recent trends in the study of the foundations of quantum mechanics: from the expansion or even rethinking of quantum theory, to ambitious new experiments that will seek the elusive effects of quantum gravity.