Letters in 2012

Shor’s quantum algorithm factorizes integers, and implementing this is a benchmark test in the early development of quantum processors. Researchers now demonstrate this important test in a solid-state system: a circuit made up of four superconducting qubits factorizes the number 15.

It is known that graphene exhibits natural ripples with characteristic lengths of around 10 nm. But when it is stretched across nanometre-scale trenches that form in a reconstructed copper surface, it develops even tighter corrugations that cannot be explained by continuum theory.

Doping a topological insulator with manganese makes it magnetic. Moreover, decreasing the concentration of Dirac fermions in a Mn-doped topological insulator with an electric field increases the strength of its magnetic characteristics—a trait that could be valuable to the use of topological insulators in the development of spintronics.

When a low-viscosity fluid penetrates a fluid of higher viscosity confined by parallel plates, finger-like patterns propagate at the interface between the two fluids. Experiments now show that tapering the fluid cell can suppress this instability - providing interfacial control via a simple change in geometry.

In metals, the Coulomb potential of charged impurities is strongly screened, but in graphene, the potential charge of a few-atom cluster of cobalt can extend up to 10 nm. By measuring differences in the way electron-like and hole-like Dirac fermions are scattered from this potential, the intrinsic dielectric constant of graphene can be determined.

Two-dimensional Bose fluids—such as liquid-helium films, or confined ultracold atoms—cannot form a condensate, but become superfluid instead. Frictionless flow, proving superfluid behaviour, has now been observed in an ultracold two-dimensional Bose gas that is stirred with a laser beam.

Chemical reactions between a single trapped ion and a condensate of ultracold neutral atoms are investigated by controlling the quantum states of both ion and atoms—revealing the effect of the hyperfine interaction on the reaction dynamics.

In systems of oscillators, phase-locking behaviour can, in theory, coexist with incoherent dynamics—invoking the fabled chimera state. Now, experimental realization of a coupled-map lattice reveals dynamical states displaying coexisting spatial domains of coherence and incoherence.

Chimera states describing the stable coexistence of synchronous and incoherent dynamics have so far only been realized numerically. An experimental demonstration of these states in a network of discrete chemical oscillators reveals behaviour that differs from that predicted by existing phase-oscillator models.

How and why Fermi arcs—disconnected segments of the Fermi surface—emerge in the pseudogap phase of cuprate superconductors is a mystery. A technique for analysing angle-resolved photoemission spectroscopy data that removes momentum broadening effects suggests these arcs do not reflect true Fermi surface states, which would explain why they do not form continuous loops.

Nuclear spin is seen as a robust qubit. Electrons can be used to ‘read’ to the nuclear state, but their presence causes decoherence. Researchers now show that this problem can be circumvented using a temporary spin state, thus enabling entanglement of the nuclear state at unprecedented speeds.

A technique capable of detecting the electric field associated with individual atoms is now demonstrated. Atomic-resolution differential phase-contrast imaging using aberration-corrected scanning transmission electron microscopy provides a sensitive probe of the gradient of the electrostatic potential in a crystal lattice.

Density functional theory provides a powerful framework for probing electronic structure in many-body systems. A new functional for particles interacting via short-range potentials extends its applicability to ultracold atoms in optical lattices.

Hilbert space is made up of a potentially infinite number of dimensions that correspond to all the parameters needed to fully define a system. The idea is seen as an important resource for quantum information processing. A technique for estimating the number of dimensions in an unknown system based on the results of measurements performed on it—a so-called dimension witness—is now experimentally demonstrated.

Is it possible to deduce the number of dimensions of a completely unknown system only from the results of measurements performed on it? So-called dimension witnesses allow such an estimation, and are now experimentally demonstrated using pairs of entangled photons.

You influence a system by measuring it. This back-action is an important consideration when studying tiny structures in which quantum effects play a crucial role. Researchers now show that quantum interference could provide a way to negate back-action in quantum-dot-qubit circuits.

Uranium ruthenium silicide exhibits a discontinuity in its specific heat at 17.5 K. The underlying cause of this anomaly is hotly debated. A first-principles study of high-order correlations in its electronic structure suggests this behaviour is the result of the emergence of rank-5 nematic order.

Scanning tunnelling microscopy images of the evolution of the pseudogap phase of a hole-doped cuprate superconductor suggest that it emerges in localized clusters that grow with increasing doping. Moreover, the eventual coalescence of these clusters coincides with the emergence of superconductivity.

A quantum memory that combines high-efficiency and long lifetime is now demonstrated. Employing a collective excitation, or spin wave, in an ensemble of atoms in a trap improves memory lifetime, while incorporating the trap into an optical ring cavity simultaneously aids higher retrieval efficiency.

The penetration of a superconducting current from a superconductor into a half-metallic ferromagnet is usually forbidden. Resonances in the conductance spectra of superconductor/half-metal heterostructures suggest this restriction is lifted by the occurrence of unconventional equal-spin Andreev reflection.