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Category: quantum physics

Corralling Interfering Anyons

Link.aps.org/doi/10.1103/Physics.19.

A delicate interference experiment elucidates the collective behavior of quasiparticles that are neither bosons nor fermions, but something in between.

When you live in theory-land, as I do, anyons in fractional quantum Hall (FQH) systems are an emblem of elegance. They address a fundamental question in quantum mechanics—the classification of indistinguishable particles—by breaking the long-rooted dichotomy between fermions and bosons and replacing it with a continuum of possibilities. Their implications are far reaching. Anyons account for the “hierarchy” of FQH states and they inspire visions of topologically protected quantum computation [1]. In experiment-land, the most direct manifestation of anyons is the phase that the system’s wave function acquires when two anyons are interchanged or when one winds around another. This phase is at the heart of a new experiment performed by Noah Samuelson and Andrea Young of the University of California, Santa Barbara, and their collaborators [2].

Electric current stabilizes spins at unstable points for new types of computing

A research team has discovered a new way to control tiny magnetic properties inside materials using electric current, which could possibly pave the way for new types of computing technologies. The work is based on spintronics, a field that uses not only the electric charge of electrons but also their “spin,” a quantum property that can be thought of as a tiny magnet.

Spintronics is already used in magnetic random access memory (MRAM), a type of memory that keeps data even when the power is turned off. This is different from conventional memory, which loses information without electricity.

In MRAM, data is stored depending on whether spins point “up” or “down.” These two stable states are separated by an energy barrier, which helps keep the data secure. However, this stability also makes it harder to switch between states, requiring strong electric currents.

First quantum oscillations observed in gallium nitride holes

Gallium nitride, a semiconductor that can operate at high voltages, temperatures, and frequencies, has enabled technologies from LED lighting to high-power electronics. Now Cornell researchers have observed a quantum property of the material for the first time, an advance that could expand its technological reach.

Much of gallium nitride’s value as a semiconductor lies in how quickly negatively charged electrons move through the material. But the material could become even more useful if scientists better understood its positively charged “holes,” which behave like mobile pockets of missing electrons but have been difficult to study.

Understanding how to control the flow of the holes—as engineers have achieved in silicon semiconductors—would allow gallium nitride to reach its full potential.

Physicists find electronic agents that govern flat band quantum materials

Physicists have directly visualized the fundamental electronic building blocks of flat-band quantum materials, a class of systems in which electron motion is effectively quenched and strong interactions give rise to emergent phases of matter. In a study published in Nature Physics, Qimiao Si’s group at Rice University, in collaboration with researchers at the Weizmann Institute of Science, identified compact molecular orbitals that act as the key electronic agents governing the exotic behavior of these materials.

“In flat band materials, electron motion experiences destructive interference,” said Si, the Harry C. and Olga K. Wiess Professor of Physics and Astronomy and director of Rice’s Extreme Quantum Materials Alliance.

These flat band materials are also topological with properties that are preserved as the material continuously bends or stretches in any symmetry-preserving way.

New controls can stretch, blur and even reverse quantum time flow

In new research published in Physical Review X, scientists have designed quantum control protocols that generate processes more consistent with time flowing backward than forward. The protocols—techniques to control quantum systems—modify a quantum system’s “arrow of time,” the concept of time as moving in one forward direction. The work opens up possibilities for energy extraction from quantum systems and for quantum state preparation.

A quantum system, such as a collection of qubits, is governed by the laws of quantum mechanics. The team’s control protocols can prevent the emergence of the arrow of time in a quantum system or even invert its direction—that is, cause quantum time to appear to flow in reverse.

As an application of their research, the team leveraged their control protocols to design a measurement engine that extracts energy from quantum measurements performed on the system.

Superconducting quantum processor performs well with significantly less wiring

Quantum computers, computing systems that process information using quantum mechanical effects, could outperform classical computers on some computational tasks. These computers rely on qubits, the basic units of quantum information, which can exist in multiple states (0, 1 or both simultaneously), due to quantum effects known as superposition and entanglement.

Many of the quantum computers developed in recent years are based on conventional superconductors, materials that exhibit an electrical resistance of zero at extremely low temperatures. To operate reliably and exhibit superconductivity, circuits based on these materials need to be cooled down to millikelvin temperatures.

In quantum computers, each qubit typically requires its own control line. This means that engineers need to introduce several wires that carry electrical pulses (i.e., signal lines), and the number of necessary wires increases with the number of qubits. As quantum computers grow larger, this can be problematic, as processors become harder to build and reliably operate.

Quantum computers could have a fundamental limit after all

The performance of quantum computers could cap out after around 1,000 qubits, according to a new analysis published in the Proceedings of the National Academy of Sciences. Through new calculations, Tim Palmer at the University of Oxford has reconsidered the mathematical foundations underlying the quantum principles behind the technology, concluding that restrictions on the information-carrying capacity of large quantum systems could make their computing power far more limited than many researchers predict.

For some time, quantum physicists have been growing increasingly excited—and concerned—about the seemingly limitless potential of quantum computers. In a classical computer, information content generally grows linearly as the number of bits increases. But in a quantum computer, each extra qubit doubles the number of quantum states the system can occupy.

Since these states can encode multiple possibilities at the same time, the overall system appears to become exponentially more powerful with each added qubit—at least according to our current understanding of quantum mechanics.

Sean M. Carroll

“I like to say that physics is hard because physics is easy, by which I mean we actually think about physics as students.”

Up next, The Multiverse is real. Just not in the way you think it is. ► • The Multiverse is real. Just not in the wa…

Physics seems complicated, until you realize why it works so well, says physicist Sean Carroll, revealing the basis of the field’s greatest successes: Radical simplicity.

Carroll takes us from Newton’s clockwork universe to Laplace’s demon, to Einstein’s spacetime revolution, exploring the historical shockwaves each breakthrough caused. If you’ve wondered how stripping the world down to its simplest parts can reveal deeper truths, this is where that story begins.

00:00:00 Radical simplicity in physics.
00:00:55 Chapter 1: The physics of free will.
00:04:55 Laplace’s Demon.
00:06:27 The clockwork universe paradigm.
00:07:41 Determinism and compatibilism.
00:08:45 Chapter 2: The invention of spacetime.
00:17:30: Einstein’s general theory of relativity.
00:24:27 Chapter 3: The quantum revolution.
00:28:05 The 2 biggest ideas in physics.
00:32:27 Visualizing physics.
00:38:17 Quantum field theory.
00:46:51 The Higgs boson particle.
00:47:28 The standard model of particle physics.
00:52:53 The core theory of physics.
01:02:03 The measurement problem.
01:13:47 Chapter 4: The power of collective genius.
01:16:19 A timeline of the theories of physics.

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