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Flash Physics: Self-gravitating midges, ALICE sees ‘enhanced strangeness’, single photon links quantum dots

26 Apr 2017

Flash Physics is our daily pick of the latest need-to-know developments from the global physics community selected by Physics World's team of editors and reporters

Swarm physics: high-speed videos reveal how midges stay together

Swarms of midges are self-gravitating

Swarms of midges may behave as self-gravitating systems held together by velocity-dependent forces – according to scientists in the UK and US. Many animals display collective group behaviour. In flocks of birds and schools of fish, local interactions between individuals appear to play a key role but swarms of flying insects, such as midges, behave differently. Although midges form cohesive groups and are tightly bound, the coupling inside the swarm is weak. Instead, Andrew Reynolds at Rothamsted Research in the UK and colleagues claim that the midges are bound to the centre of a swarm by forces that increase with flight speed. The researchers constructed models based on density profiles and velocity measurements taken from high-speed video recordings of midge swarms. These models demonstrated that attractive force increases with distance from the centre of the swarm – and that the force increases with an individual’s flight speed. The findings, presented in The European Physical Journal E, confirm a theory proposed last year by team member Nicholas Ouellette of Stanford University in the US. In the current study, the researchers also found that the speed-dependent forces may be governed by acoustic sensing – as previously proposed by Ouellette and team – and visual cues. Reynolds and colleagues add that the biophysical explanation for the swarm dynamics needs further examination.

ALICE spots “enhanced strangeness” in proton collisions

An abundance of particles containing strange quarks are produced when protons smash into each other in the Large Hadron Collider (LHC) at CERN. That is the surprising conclusion of physicists working on the ALICE experiment at the LHC, who have studied the quark–gluon plasma that is believed to form when protons collide at 7 TeV. The collision drives apart the quarks within the protons to create the quark–gluon plasma – a hot, dense “soup” of quarks, antiquarks and gluons that is thought to resemble the state of the universe just a few millionths of a second after the Big Bang. Over the past two decades, physicists have made quark–gluon plasmas by smashing heavy-nuclei together. More recently, evidence has emerged that quark–gluon plasmas can be created when protons collide at the LHC – something that was not expected. Now, ALICE physicists have detected kaon, lambda, xi and omega particles emerging from these collisions. All of these particles contain one strange quark, and this “enhanced strangeness” has been seen in quark–gluon plasmas created in heavy-nuclei collisions. The physicists believe that this enhanced strangeness is further evidence that proton–proton collisions can indeed produce quark–gluon plasmas. “We are very excited about this discovery,” says ALICE spokesperson Federico Antinori. “Being able to isolate the quark–gluon-plasma-like phenomena in a smaller and simpler system, such as the collision between two protons, opens up an entirely new dimension for the study of the properties of the fundamental state that our universe emerged from.” The measurements are described in Nature Physics.

Quantum dots linked with single photons

A single photon has been used to transfer quantum information a distance of 5 m between two quantum dots. The feat was performed by Atac Imamoğlu and colleagues at the Swiss Federal Institute of Technology in Zurich and could be an important step towards the creation of practical quantum computers in which quantum information stored in stationary quantum bits (qubits) is transferred via “flying” qubits. A quantum dot is a tiny piece of semiconductor that has well-defined electronic states much like an atom. These states can be used to store quantum information, making quantum dots potentially useful as stationary qubits. In this latest work, Imamoğlu’s team created a quantum dot that can emit a single photon that is in a coherent quantum superposition of two infrared wavelengths. This photon is then sent 5 m along an optical fibre to a second quantum dot that can absorb the photon – putting the second quantum dot into a superposition of two quantum states. As a result, quantum information held in the photon (the nature of the quantum superposition) is transferred to the second quantum dot – with the photon acting a flying qubit. The team is now working on extending its system so that different types of quantum information can be transferred. The research is described in Physical Review Letters.

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