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Category: quantum physics

A new study illuminates surprising choreography among spinning atoms. In a paper appearing in the journal Nature, researchers from MIT and Harvard University reveal how magnetic forces at the quantum, atomic scale affect how atoms orient their spins.

In experiments with ultracold lithium , the researchers observed different ways in which the spins of the atoms evolve. Like tippy ballerinas pirouetting back to upright positions, the spinning atoms return to an equilibrium orientation in a way that depends on the between individual atoms. For example, the atoms can spin into equilibrium in an extremely fast, “ballistic” fashion or in a slower, more diffuse pattern.

The researchers found that these behaviors, which had not been observed until now, could be described mathematically by the Heisenberg model, a set of equations commonly used to predict magnetic behavior. Their results address the fundamental nature of magnetism, revealing a diversity of behavior in one of the simplest magnetic materials.

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More than two-thirds of the energy used worldwide is ultimately ejected as “waste heat.” Within that reservoir of discarded energy lies a great and largely untapped opportunity, claim scientists in MIT’s Department of Nuclear Science and Engineering (NSE). As reported in a recent issue of Nature Communications, the MIT team—led by Assistant Professor Mingda Li, who heads NSE’s Quantum Matter Group—has achieved a breakthrough in thermoelectric generation, which offers a direct means of converting thermal energy, including waste heat, into electricity.

A , or difference, within a material such as a metal or semiconductor can, through a phenomenon known as the Seebeck effect, give rise to an that drives a current. “For many materials, the is too low to be useful,” explains NSE Research Scientist Fei Han. “Our goal is to find materials with conversion efficiencies high enough to make thermoelectric generation more practical.”

The efficiency of thermoelectric energy conversion is proportional to a material’s , electrical , and something called the “” squared; it is inversely proportional to the . Because efficiency goes up with temperature, most thermoelectric materials used today operate in the range of hundreds of degrees centigrade. “But in our lives, most of the stuff around us is at room temperature,” Han says. “That’s why we’re trying to discover new materials that work effectively at or below room temperature.”

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Since the very beginning of quantum physics, a hundred years ago, it has been known that all particles in the universe fall into two categories: fermions and bosons. For instance, the protons found in atomic nuclei are fermions, while bosons include photons — which are particles of light — as well as the BroutEnglert-Higgs boson, for which Francois Englert, a professor at ULB, was awarded a Nobel Prize in Physics in 2013.

Bosons — especially photons — have a natural tendency to clump together. One of the most remarkable experiments that demonstrated photons’ tendency to coalesce was conducted in 1987, when three physicists identified an effect that was since named after them: the Hong-Ou-Mandel effect. If two photons are sent simultaneously, each towards a different side of a beam splitter — a sort of semitransparent mirror — one could expect that each photon will be either reflected or transmitted.

Logically, photons should sometimes be detected on opposite sides of this mirror, which would happen if both are reflected or if both are transmitted. However, the experiment has shown that this never actually happens: the two photons always end up on the same side of the mirror, as though they ‘preferred’ sticking together! In an article published recently in US journal Proceedings of the National Academy of Sciences, Nicolas Cerf — a professor at the Centre for Quantum Information and Communication (École polytechnique de Bruxelles) — and his former PhD student Michael Jabbour — now a postdoctoral researcher at the University of Cambridge — describe how they identified another way in which photons manifest their tendency to stay together. Instead of a semi-transparent mirror, the researchers used an optical amplifier, called an active component because it produces new photons.

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New technique could be useful for quantum information processing.

A new technique to cool reactive molecules to temperatures low enough to achieve quantum degeneracy – something not generally possible before – has been created by researchers in the US. In this temperature regime, the dominance of quantum effects over thermal fluctuations should allow researchers to study new quantum properties of molecules. As a first example, the researchers demonstrated how a slight change in applied electric field can alter the reaction rate between molecules by three orders of magnitude. The researchers hope their platform will enable further exploration of molecular quantum degeneracy, with potential applications ranging from quantum many body physics to quantum information processing.

When atoms are cooled close to absolute zero, the blur created by thermal effects that govern their behaviour in the classical world around us is removed, making their quantum nature clear. This has led to some fascinating discoveries. In ultracold quantum bosonic or fermion-pair quantum gases, for example, all the atoms in a trap can simultaneously occupy the quantum ground state, resulting in a wavefunction that is macroscopic.

Cooling and trapping molecules is much trickier because they are inherently more complex than atoms. Whereas atoms can only contain quanta of energy in electronic excitations, the chemical bonds in molecules can stretch, rotate and bend – and cooling molecules involves removing energy from all of these degrees of freedom. Moreover, the complexity of molecules increases the complexity of their collisions. Although elastic collisions are necessary to knock the fastest-moving molecules out of a trap and cool it, inelastic collisions dissipate heat in the trap.

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Fast spinning black holes could have features different from those predicted by general relativity.

General relativity is a profoundly complex mathematical theory, but its description of black holes is amazingly simple. A stable black hole can be described by just three properties: its mass, its electric charge, and its rotation or spin. Since black holes aren’t likely to have much charge, it really takes just two properties. If you know a black hole’s mass and spin, you know all there is to know about the black hole.

This property is often summarized by the no-hair theorem. Specifically, the theorem asserts that once matter falls into a black hole, the only characteristic that remains is mass. You could make a black hole out of a Sun’s worth of hydrogen, chairs, or those old copies of National Geographic from Grandma’s attic, and there would be no difference. Mass is mass as far as general relativity is concerned. In every case the event horizon of a black hole is perfectly smooth, with no extra features. As Jacob Bekenstein said, black holes have no hair.

But with all its predictive power, general relativity has a problem with quantum theory. This is particularly true with black holes. If the no-hair theorem is correct, the information held within an object is destroyed when it crosses the event horizon. Quantum theory says that information can never be destroyed. So the valid theory of gravity is contradicted by the valid theory of the quanta. This leads to problems such as the firewall paradox, which can’t decide whether an event horizon should be hot or cold.

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Be it with smartphones, laptops, or mainframes: The transmission, processing, and storage of information is currently based on a single class of material—as it was in the early days of computer science about 60 years ago. A new class of magnetic materials, however, could raise information technology to a new level. Antiferromagnetic insulators enable computing speeds that are a thousand times faster than conventional electronics, with significantly less heating. Components could be packed closer together and logic modules could thus become smaller, which has so far been limited due to the increased heating of current components.

Information transfer at room temperature

So far, the problem has been that the information transfer in antiferromagnetic insulators only worked at low temperatures. But who wants to put their smartphones in the freezer to be able to use it? Physicists at Johannes Gutenberg University Mainz (JGU) have now been able to eliminate this shortcoming, together with experimentalists from the CNRS/Thales lab, the CEA Grenoble, and the National High Field Laboratory in France as well as theorists from the Center for Quantum Spintronics (QuSpin) at the Norwegian University of Science and Technology. “We were able to transmit and process information in a standard antiferromagnetic insulator at room temperature—and to do so over long enough distances to enable information processing to occur”, said JGU scientist Andrew Ross. The researchers used iron oxide (α-Fe2O3), the main component of rust, as an antiferromagnetic , because iron oxide is widespread and easy to manufacture.

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Circa 2014

We propose the new dark matter particle candidate—the “black hole atom,” which is an atom with the charged black hole as an atomic nucleus and electrons in the bound internal quantum states. As a simplified model we consider the central Reissner-Nordström black hole with the electric charge neutralized by the internal electrons in bound quantum states. For the external observers these objects would look like the electrically neutral Schwarzschild black holes. We suppose the prolific production of black hole atoms under specific conditions in the early universe.

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