Exciting an Avalanche Mohammad Maghrebi and Colleagues Have Nature Photon

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Shrink from tension closely related to networks characterized by be created from nothing. Hawking radiation Nature Mater. 11, 608–613 (2012) suboptimal equilibria that can be optimized via and the dynamical Casimir effect are structural constraints — like a traffic network examples of the macroscopic manifestation that profits from the removal of a road. In the of quantum vacuum fluctuations — near case of the proposed metamaterials, however, black-hole event horizons or accelerating the optimization occurs spontaneously, in boundaries, photons pop out of the vacuum. response to a change in applied force. AK Such spontaneous emission is also expected to occur in the presence of spinning bodies: Exciting an avalanche Mohammad Maghrebi and colleagues have Nature Photon. 6, 455–458 (2012) uncovered a new twist. Spontaneous emission from rotating An individual electron does not have a large objects was predicted in the 1970s, but influence on the world around it, which Maghrebi et al. have revisited the theory makes detection tricky. One successful and introduced an exact theoretical approach is to use avalanche amplification: treatment of vacuum fluctuations in one seed electron generates two ‘daughter’ the presence of a spinning body. Their electrons; this is repeated to create four, scattering-theory approach not only

© JUPITERIMAGES/GETTY IMAGES/THINKSTOCK © JUPITERIMAGES/GETTY and so on, until a measurable current is confirms that energy is emitted by the produced. Gabriele Bulgarini and colleagues spinning object even at zero temperature, Squeezing a material between your fingers have now applied this idea to the charge but also signals a previously unknown ordinarily results in compression. Imagine carriers trapped in single quantum dots. effect: the radiation from a rotating body then, if that same material were to expand Excitons — an electron and its positively exerts pressure on a nearby test object, in response to your touch. Counterintuitive charged counterpart, a hole — in a quantum which is dragged along and starts rotating though it may seem, Zachary Nicolaou and dot are one candidate for quantum bits. parallel to the rotation axis. A similar Adilson Motter have uncovered a design However, studies on single quantum dots effect is expected for the generalization to principle for metamaterials that exhibit often require that a measurement is repeated relativistic motion. IG such negative compressibility — expanding thousands of times: not ideal in a superfast with applied pressure and contracting quantum computer. Opposites form Cooper pairs under tension. Bulgarini et al. integrated their quantum Phys Rev. B. 85, 195206 (2012) Experience tells us such materials dot into a nanowire avalanche photodetector. shouldn’t exist, but that scepticism is also A photon generates a single exciton in the dot. In a superconductor, current is carried firmly grounded in thermodynamics: for The electron and hole tunnel out of the dot by bound pairs of like charges. For these a closed system in equilibrium, negative in opposite directions into a multiplication ‘Cooper pairs’ to form there must be some compressibility represents an unstable region, thus generating a useful photocurrent. mechanism to oppose their Coulombic configuration. This argument holds when The team show that 120 individual excitation repulsion: in conventional superconductors, the applied force increases infinitesimally, so events were required for electrical read-out of it’s mediated by phonons. Under certain that stability does not change as the force is the exciton — 10,000 times fewer than other conditions, however, Cooper pairs of varied. In reality, however, changes in applied approaches and a significant step towards the unlike charges — an electron and a force can be appreciable, changing stability ultimate goal of single-shot read-out. DG hole — might also form, provided their and causing the system to shift to a different Coulomb attraction is counteracted to stable configuration. Out of the vacuum impede recombination. Nicolaou and Motter exploit this concept Phys. Rev. Lett. 108, 230403 (2012) Marijn Versteegh and colleagues report to design materials for which the switch to evidence that electron–hole Cooper-pair a new configuration induces an expansion A surprising consequence of Heisenberg’s formation could occur in a photoexcited on application of pressure. The idea is uncertainty principle is that something can electron–hole gas in a semiconductor. At low density, Coulomb attraction binds electron–hole pairs tightly. But as the density Branching out Phys. Rev. Lett. 108, 231801 (2012) is increased, this attraction is screened and becomes gradually weaker. And at high According to the standard model of particle physics, flavour-changing neutral currents — densities it could become weak enough to interactions that can change the flavour of a particle but without changing its charge — are be balanced by Pauli repulsion, enabling suppressed. This physics emerged in the late 1960s from the study of K0 mesons, which Cooper pairs to form. didn’t seem to decay to pairs of muons as expected (and links to the postulation and Versteegh et al. subjected a ZnO subsequent discovery in 1974 of the charm quark). The effect is even more pronounced crystal to an intense infrared laser light. in heavier B0 mesons, and so the LHCb collaboration (R. Aaji et al.) has searched its data As they increased the laser intensity and for evidence of these particles decaying to pairs of muons: any measurable effect would decreased the temperature, they observed indicate some physics not yet included in the standard model. the emergence of a peak in the resulting LHCb finds none, and has instead set limits on the ‘branching fractions’ (a measure of the emission spectra consistent with the 0 probability of the decay) into a muon pair of two types of B meson — the Bs (a bound bottom recombination of preformed electron–hole and strange quark) and the B0 (a bound bottom and down quark). The limits are the tightest yet, Cooper pairs. EG constraining the branching fractions to be less than a few parts in a billion — and coming in close now to the tiny, almost negligible, branching fractions predicted in the standard model. AW Written by Iulia Georgescu, Ed Gerstner, David Gevaux, Abigail Klopper and Alison Wright

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