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Category: quantum physics – Page 28

The World as a Neural Network

We discuss a possibility that the entire universe on its most fundamental level is a neural network. We identify two different types of dynamical degrees of freedom: “trainable” variables (e.g., bias vector or weight matrix) and “hidden” variables (e.g., state vector of neurons). We first consider stochastic evolution of the trainable variables to argue that near equilibrium their dynamics is well approximated by Madelung equations (with free energy representing the phase) and further away from the equilibrium by Hamilton–Jacobi equations (with free energy representing the Hamilton’s principal function). This shows that the trainable variables can indeed exhibit classical and quantum behaviors with the state vector of neurons representing the hidden variables.

Trembling Photons in Non-Abelian Electric Fields

A ring of optical fiber can be made to host phenomena that originated in the realm of high-energy physics.

In non-Abelian gauge theories, particles interact with each other; operation order matters; and a so-called gauge symmetry ensures the invariance of physical laws under local transformations via fields mediated by photons, gluons, and the W and Z bosons. Such theories describe the strong and weak forces; provide the mathematical backbone of the standard model; and underpin efforts to understand the early Universe, quantum gravity, and exotic topological phases of matter. For decades, subjecting these theories to direct experimental scrutiny often meant resorting to enormous particle accelerators. Recently, an experimental approach known as photonic synthetic dimensions has offered a tabletop alternative. Applications of the approach have already realized a non-Abelian magnetic field [1]. Now Shu Yang of the University of Hong Kong and his colleagues have synthesized the corresponding non-Abelian electric field [2].

Water’s local asymmetry drives proton hopping between molecules, simulations reveal

Complex simulations—the most intricate of their kind to date—carried out by an international research team led by scientists at Heidelberg University’s Institute for Physical Chemistry, have revealed how water governs the way protons move through it. Using their modeling, researchers from Cambridge (U.K.), Bochum, Dijon (France) and Heidelberg were able to trace, in full quantum detail, the movements of a proton shared among six water molecules. At its core, the work addresses how a proton moves through water: not as a single particle drifting along, but by “hopping” from one molecule to the next.

This “hopping” motion has been known since the 19th century. When an acid dissolves in water, the released proton—a positively charged hydrogen ion—does not remain bound to a single water molecule. It is highly mobile and constantly jumps from one molecule to the next.

Thin films ‘dance’ with substrates that are no longer inert, opening path toward 3D chips

Many of today’s electronic devices—from the semiconductors in your cell phone to the photovoltaic cells in your solar panels—are built on thin-film substrates. The thin film is an electrically conductive material, while the substrate is an inert material. Or is it?

Physicists and materials scientists have long assumed substrates do not react to electrical stimuli, but new research from the University of California San Diego and a team of collaborators has shown that substrates are not inert after all. The discovery has the potential to help engineers build the dense, three-dimensional, brain-inspired computer chips needed for more energy-efficient computing. This work appears in Science.

The research began four years ago in UC San Diego Associate Professor of Physics Alex Frañó’s lab. Frañó is a principal investigator and assistant director at the Quantum Materials for Energy-Efficient Neuromorphic Computing (Q-MEEN-C), one of the U.S. Department of Energy’s Energy Frontier Research Centers. One of the goals of Q-MEEN-C is to develop quantum materials that can be used in neuromorphic, or “brain-like,” computing.

Quantum neural networks get their first hardware test

Neural networks have transformed how machines find patterns in data, from recognizing faces in photos to predicting the shapes of proteins. So far, all of this progress has been made on ordinary classical computers, but with quantum computers now edging into practical use, there is a real possibility that neural networks could tap into distinctly quantum effects and operate in ways that classical machines never could. So far, however, neural networks have proven far more difficult to run on quantum hardware.

Through new research published in Physical Review Letters, Djamil Lakhdar-Hamina and colleagues at the University of Maryland, College Park, have built a neural network that runs on two different types of quantum computer, allowing them to test directly whether these systems can live up to their theoretical promise.

New quantum chip architecture could use built-in vibrations to link distant qubits

A new concept from Warwick researchers could help solve one of the biggest challenges to building large-scale quantum computers: enabling communication between vast numbers of quantum bits (qubits) over long distances across a single chip.

Published in APL Quantum, researchers from the University of Warwick and NRC Canada introduce the concept of Quantum Phononic Links (QPLs), a new approach to communication between qubits. It uses sound-like vibrations traveling through a specially engineered material to carry quantum information between qubits that are physically far apart.

Today’s leading quantum chips typically allow only neighboring qubits to communicate directly with each other. To build useful quantum computers, engineers expect to need to coordinate millions of qubits spread across an entire semiconductor chip, not just clusters of adjacent qubits. The team’s proposed approach uses sound vibrations, known as phonons, as an inherent communication system, allowing qubits to exchange quantum information over much greater distances than currently possible.

Simple semiconductor films break light’s front-back symmetry

Light typically interacts with a material the same way whether it enters through the front or the back—like polarized sunglasses that work the same from either side. Cornell researchers have demonstrated a simple route to breaking that symmetry, opening new possibilities for photonics and quantum information processing.

Optical reciprocity, the principle that a system responds identically regardless of which side faces the light, underlies most lenses, mirrors and other optical devices. A new study published in Nature Materials demonstrates that some materials can be engineered to exhibit nonreciprocal absorption and emission of linearly polarized light.

“Imagine window blinds with sunlight coming through their horizontal slats, but from the opposite side, the same blinds let light through as if the slats were vertical, completely inverted,” said corresponding author Richard Robinson, professor of materials science and engineering in the Cornell Duffield College of Engineering. “To get this type of behavior, you typically need complex metamaterials or external magnetic fields, but we show that it can be achieved in simple, solution-processed semiconductor nanoclusters.”

A quantum heat engine that simultaneously provides work and refrigeration

The laws of thermodynamics state that heat naturally flows from hotter systems or regions to colder systems or regions until a state of thermal equilibrium is reached. This simple principle underpins the operation of numerous technologies, ranging from refrigerators to power plants.

Yet quantum systems, which are governed by quantum mechanics, can exhibit unusual behaviors that cannot be explained by classical physics. These behaviors could be used to create innovative thermal devices.

Researchers at Qufu Normal University, the University of Hong Kong and the University of Palermo recently observed an anomalous thermal effect that allows a quantum system to absorb heat from colder thermal reservoirs. This unusual effect, outlined in a paper in Physical Review Letters, was leveraged to develop a new quantum heat engine that simultaneously produces work (i.e., mechanical energy created by converting heat into motion or power) and refrigeration.

Near a black hole, gravity changes a quantum circuit’s readings, not its rules

Imagine holding one of our most precise quantum devices at a fixed position outside a black hole. A Josephson junction—two superconductors separated by an ultrathin barrier—can turn a voltage into a quantum oscillation with extraordinary precision. Would intense gravity change that quantum rule, or only change how a faraway observer reads the device?

In our study published in the Journal of High Energy Physics, we found a clean answer. The local Josephson physics remains intact. Gravity instead changes the translation between measurements made beside the circuit and measurements assigned by an observer far from the black hole.

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