Introduction
Cover story
To see a diamond, don't go to a jewellery shop — head for a spintronics or quantum-computation laboratory instead. The spin associated with a nitrogen vacancy centre — an impurity sitting at a vacancy site in the diamond lattice — has a long lifetime and is therefore promising for applications in quantum information processing. Ryan Epstein and colleagues have constructed a room-temperature microscope that is sensitive to the light emitted by a single nitrogen vacancy centre. Moreover, by precisely controlling the magnetic field, they can detect the presence of nearby non-luminescent nitrogen atoms that couple to the nitrogen vacancy centres. These 'dark' spins have an even longer lifetime than the bright spins. [Letter page 94; News and Views page 79]
What's shape got to do with it?
The dimensionality of two thin films joined by a point contact — is it one or two? Does it matter? In superconducting devices: certainly. A dissipationless current will flow between two superconductors separated by an insulator. This is the Josephson effect, which is the basis of the SQUID, our most sensitive device for measuring magnetic fields. But shrink that insulating layer to nothing, so that the two films meet at a point — will the structure remain superconducting? Michael Hermele and co-workers have taken a theoretical approach to answering this question. For this particular geometry, they find that a supercurrent can be sustained only at absolute zero. This means even at the lowest attainable temperature, such thin films will have finite resistance. Now it remains to test their theory. [Article page 117; News and Views page 83]
Spinor dynamics in ultracold gases
In a Bose–Einstein condensate, the constituent atoms all share the same wavefunction and phase. Now Ming-Shien Chang and colleagues show that this coherence extends to an internal degree of freedom. In a spin-1 condensate held in an optical trap, they watched how collisions between the atoms coherently and reversibly mix the spin components — unlike in a classical gas, in which such collisions seem to be random. But Chang et al. have gone beyond just observation: they actively controlled the evolution. Using magnetic-field pulses, they engineered the sample into a desired final spin state. These experiments underline how atomic Bose–Einstein condensates provide a clean and controllable macroscopic quantum system that can serve as a versatile test bed for studies that will lead to a better understanding of quantum coherence. [Article page 111; News and Views page 89]
Ultrasound-driven microexplosions
The role of ultrasound in medical imaging is familiar. But technology that uses acoustic microbubbles to destroy biological cells or deliver drugs has not yet left the laboratory. In particular, it's not clear how encapsulated microbubbles blow up individual cells. Paul Prentice and collaborators have used high-speed imaging to see what happens. They optically trapped single microbubbles next to a cell surface and then acoustically drove the bubbles to expand and collapse. Together with detailed atomic force microscopy of the cell surface, their observations provide fresh insight into sonoporation. The proximity of a rigid wall causes asymmetric collapse, sometimes resulting in a microjet that punctures a hole in the surface. It seems that for their high-pressure bubbles, the actual mechanism is a combination of several different ones. [Letter page 107]
Field-effect spin control
The development of a nanotube spin transistor could point the way to spintronic logic.
An aim of spintronics is to develop devices that use a charge-carrier's spin in the same way that conventional electronic devices use their charge. The archetypal spintronic device is the spin-valve, comprising two ferromagnetic layers separated by a thin non-magnetic tunnelling barrier. Spin-dependent scattering effects make the electrical resistance of a spin-valve dependent on the relative magnetization direction of its ferromagnetic layers. This discovery has led to profound improvements in the performance of computer hard-disks. Sangeeta Sahoo and colleagues take this idea a step further to develop a spintronic device analogous to a field-effect transistor, whose magnetoresistance can be tuned by the application of a voltage to its gate. [Letter page 99, News and Views page 85]
Atoms at a standstill
There is no way out for the rubidium atoms caught by Stefan Nu
mann and colleagues in their optical microcavity. Using laser cooling along all directions of motion, they can catch single atoms as they fly through a resonator and slow them down to the point that they nearly stand still. In this way, a well-controlled number of atoms can be trapped in the high-finesse cavity for, on average, 17 seconds. During this time they form a strongly coupled atom–resonator system, a setting that allows fine control of light–matter interactions. For practical purposes, the long trapping times mean that single atoms can be permanently confined to the cavity, which is an attractive property with regard to exploiting few-atom systems for both fundamental studies and applications. [Article page 122]
