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Category: quantum physics

Electrons in metals try to behave like obedient motorists, but they end up more like bumper cars. They may be reckless drivers, but a new Cornell-led study confirms this chaos has a limit established by the laws of quantum mechanics.

The team’s paper, “T-Linear Resistivity From an Isotropic Planckian Scattering Rate,” written in collaboration with researchers led by Louis Taillefer from the University of Sherbrooke in Canada, published July 28 in Nature. The paper’s lead author is Gael Grissonnanche, a postdoctoral fellow with the Kavli Institute at Cornell for Nanoscale Science.

Metals carry electric current when electrons all move together in tandem. In most metals, such as the copper and gold used for electrical wiring, the electrons try to avoid each other and flow in unison. However, in the case of certain “strange” metals, this harmony is broken and electrons dissipate energy by bouncing off each other at the fastest rate possible. The laws of quantum mechanics essentially play the role of an electron traffic cop, dictating an on how often these collisions can occur. Scientists previously observed this limit on the collision rate, also known as the “Planckian limit,” but there is no concrete theory that explains why the limit should exist, nor was it known how electrons reach this limit in . So Ramshaw and his collaborators set out to carefully measure it.

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Quarks are the fundamental building blocks from which matter is constructed. They combine to form hadrons, namely baryons, such as the proton and the neutron, which consist of three quarks, and mesons, which are formed as quark-antiquark pairs. In recent years a number of so-called exotic hadrons—particles with four or five quarks, instead of the conventional two or three—have been found. Today’s discovery is of a particularly unique exotic hadron, an exotic exotic hadron if you like.

The contains two and an up and a down antiquark. Several tetraquarks have been discovered in recent years (including one with two quarks and two charm antiquarks), but this is the first one that contains two charm quarks, without charm antiquarks to balance them. Physicists call this “open charm” (in this case, “double open charm”). Particles containing a charm quark and a charm antiquark have “hidden charm”—the charm quantum number for the whole particle adds up to zero, just like a positive and a negative electrical charge would do. Here the charm quantum number adds up to two, so it has twice the charm!

The content of Tcc+, has other interesting features besides being open charm. It is the first particle to be found that belongs to a class of tetraquarks with two heavy quarks and two light antiquarks. Such particles decay by transforming into a pair of mesons, each formed by one of the and one of the light antiquarks. According to some theoretical predictions, the mass of tetraquarks of this type should be very close to the sum of masses of the two mesons. Such proximity in mass makes the decay “difficult,” resulting in a longer lifetime of the particle, and indeed Tcc+, is the longest-lived exotic hadron found to date.

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Electron-positron pair generation from nonlinear quantum electrodynamics is predicted at high intensities that are, so far, beyond experimental capabilities. Here, simulations predict a high yield of positrons can be obtained from gamma-gamma photon collisions in the linear regime, using counter-propagating pulses and a microstructured target.

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Project offers new step toward study of emergence, ‘materials by design,’ and future nanomagnets.

Using a D-Wave quantum-annealing computer as a testbed, scientists at Los Alamos National Laboratory have shown that it is possible to isolate so-called emergent magnetic monopoles, a class of quasiparticles, creating a new approach to developing “materials by design.”

“We wanted to study emergent magnetic monopoles by exploiting the collective dynamics of qubits,” said Cristiano Nisoli, a lead Los Alamos author of the study. “Magnetic monopoles, as elementary particles with only one magnetic pole, have been hypothesized by many, and famously by Dirac, but have proved elusive so far.”

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A pair of studies in Nature show that a quasiparticle, known as a plasmon polariton, can be pulled with and against a flow of electrons, a finding that could lead to more efficient ways of manipulating light at the nanoscale.

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One of the most important open questions in science is how our consciousness is established. In the 1990s, long before winning the 2020 Nobel Prize in Physics for his prediction of black holes, physicist Roger Penrose teamed up with anaesthesiologist Stuart Hameroff to propose an ambitious answer.

They claimed that the brain’s neuronal system forms an intricate network and that the consciousness this produces should obey the rules of quantum mechanics —the theory that determines how tiny particles like electrons move around. This, they argue, could explain the mysterious complexity of human consciousness.

Penrose and Hameroff were met with incredulity. Quantum mechanical laws are usually only found to apply at very low temperatures. Quantum computers, for example, currently operate at around -272°C. At higher temperatures, classical mechanics takes over. Since our body works at room temperature, you would expect it to be governed by the classical laws of physics. For this reason, the quantum consciousness theory has been dismissed outright by many scientists—though others are persuaded supporters.

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