Lifeboat Foundation

The famous cat-in-a-box thought experiment by Austrian physicist Erwin Schrödinger is an illustration of one of the defining characteristics of quantum mechanics — the unpredictable behaviour of particles at the quantum level.

It makes working with quantum systems incredibly difficult; but what if we could make quantum predictions? A team of physicists believes it’s possible.

In a study published last year, they demonstrated their ability to predict something called a quantum jump, and even reverse the process after it’s started.

Physicists have measured the flight times of electrons emitted from a specific atom in a molecule upon excitation with laser light. This has enabled them to measure the influence of the molecule itself on the kinetics of emission.

Photoemission — the release of electrons in response to excitation by light — is one of the most fundamental processes in the microcosm. The kinetic energy of the emitted electron is characteristic for the atom concerned, and depends on the wavelength of the light employed. But how long does the process take? And does it always take the same amount of time, irrespective of whether the electron is emitted from an individual atom or from an atom that is part of a molecule? An international team of researchers led by laser physicists in the Laboratory for Attosecond Physics (LAP) at LMU Munich and the Max Planck Institute of Quantum Optics (MPQ) in Garching has now probed the influence of the molecule on photoemission time.

The theoretical description of photoemission in 1905 by Albert Einstein marked a breakthrough in quantum physics, and the details of the process are of continuing interest in the world of science and beyond. How the motions of an elementary quantum particle such as the electron are affected within a molecular environment has a significant bearing on our understanding of the process of photoemission and the forces that hold molecules together.

Asked by: Adam King, Huddersfield.

The ALICE collaboration has presented new results on the production rates of antideuterons based on data collected at the highest collision energy delivered so far at the Large Hadron Collider. The antideuteron is composed of an antiproton and an antineutron. The new measurements are important because the presence of antideuterons in space is a promising indirect signature of dark matter candidates. The results mark a step forward in the search for dark matter.

Understanding of these mechanisms is critical to the development of ultracompact particle accelerators and light sources.

Gail Overton

A team of researchers at California Institute of Technology has found that arrays of strontium Rydberg atoms show promise for use in a quantum computer. In their paper published in the journal Nature Physics, the researchers describe their study of quantum entangled alkaline-earth Rydberg atoms arranged in arrays and what they learned about them. In the same issue, Wenhui Li, with the National University of Singapore, has published a News & Views piece exploring the state of quantum computing research, and outlines the work done by the team at CIT.

Quantum computers capable of conducting real computing work have still not been realized, but work continues as scientists are confident that the goal will be reached. And as Li notes, most of the early-stage demo quantum computers are based on superconducting qubits or trapped ion platforms, though other systems are being studied, as well. One such system is based on neutral atoms in which the charges of the protons and electrons balance. In this new effort, the researchers looked at a type of neutral atom system based on Rydberg atoms (excited atoms with one or more electrons that also have a high quantum number). To use such atoms in a quantum computer, they must, of course, be entangled—and there needs to be a lot of them, generally arranged in an array.

In their work, the team at CIT developed a way to demonstrate entanglement of Rydberg atoms in arrays—and as part of the system, they were able to detect and control Rydberg qubits with unprecedented fidelities. To achieve this feat, they began with realizing photon coupling between different levels of Rydberg ground-state qubits, thus avoiding scattering. Doing so also allowed for efficient detection of Rydberg states, greatly improving detection fidelity. The researchers also demonstrated two-qubit entanglement using tweezer potentials, also with high fidelity.

Researchers from the Faculty of Physics at the University of Warsaw, ETH in Zurich and the University of Cambridge have synthesized and analysed active microparticles self-propelling in a fluid and reversing their propulsion direction depending on the wavelength of illuminating light. A research article summarising their work has recently been published in Nature Communications.

Active matter encompasses systems with self-propelling elements that draw energy from the environment and convert it into kinetic energy. This is currently a lively discipline in physics, spanning across many time and length scales, concerning, e.g., the behaviour of birds in flocks (such as murmurations of starlings), schools of fish (as a form of protection against predators), and also bacteria in biofilms and other aquatic microswimmers. It focuses both on the behaviour of individual elements and understanding their mechanisms of energy conversion, interaction and coupling with the environment so important for the survival, and on the collective effects and emergence of new phenomena in large populations. Both can be successfully described on different levels of precision, starting from simplistic minimal coarse-grained models, and up to refined numerical simulations.

Bacteria, algae, spermatozoa, ciliates and other unicellular organisms are an important group of active swimmers. Exploring the physical basis of their dynamics is often complicated by their immense diversity, biological complexity, and high sensitivity to external conditions. The aquatic microworld is, however, governed by the universal laws of fluid dynamics, which put limitations on all organisms.

Solving a difficult physics problem can be surprisingly similar to assembling an interlocking mechanical puzzle. In both cases, the particles or pieces look alike, but can be arranged into a beautiful structure that relies on the precise position of each component (Fig. 1). In 1983, the physicist Robert Laughlin made a puzzle-solving breakthrough by explaining the structure formed by interacting electrons in a device known as a Hall bar¹. Although the strange behaviour of these electrons still fascinates physicists, it is not possible to simulate such a system or accurately measure the particles’ ultrashort time and length scales. Writing in Nature, Clark et al.² report the creation of a non-electronic Laughlin state made of composite matter–light particles called polaritons, which are easier to track and manipulate than are electrons.

To picture a Laughlin state, consider a Hall bar, in which such states are usually observed (Fig. 2a). In these devices, electrons that are free to move in a two-dimensional plane are subjected to a strong magnetic field perpendicular to the plane. In classical physics, an electron at any position will start moving along a circular trajectory known as a cyclotron orbit, the radius of which depends on the particle’s kinetic energy. In quantum mechanics, the electron’s position will still be free, but its orbital radius — and, therefore, its kinetic energy — can be increased or decreased only in discrete steps. This feature leads to large sets of equal-energy (energy-degenerate) states called Landau levels. Non-interacting electrons added to the lowest-energy Landau level can be distributed between the level’s energy-degenerate states in many different ways.

Adding repulsive interactions between the electrons constrains the particles’ distribution over the states of the lowest Landau level, favouring configurations in which any two electrons have zero probability of being at the same spot. The states described by Laughlin have exactly this property and explain the main features of the fractional quantum Hall effect, whereby electrons in a strong magnetic field act together to behave like particles that have fractional electric charge. This work earned Laughlin a share of the 1998 Nobel Prize in Physics. Laughlin states are truly many-body states that cannot be described by typical approximations, such as the mean-field approximation. Instead, the state of each particle depends on the precise state of all the others, just as in an interlocking puzzle.

Graphene, a two-dimensional honeycomb structure made of carbon atoms with a thickness of only one atom, has numerous outstanding properties. These include enormous mechanical resistance and extraordinary electronic and optical properties. Last year a team led by the Empa researcher Roman Fasel was able to show that it can even be magnetic: they succeeded in synthesizing a molecule in the shape of a bowtie, which has special magnetic properties.

Now, researchers report another breakthrough. Theoretical work from 2007 predicted that graphene could exhibit magnetic behaviour if it were cut into tiny triangles. Over the last three years, several teams, including the Empa team, have succeeded in producing the so-called triangulenes, consisting of only a few dozen carbon atoms, by chemical synthesis under ultra-high vacuum.