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Category: quantum physics

How to Measure a Tiny Beam Shift

Measuring very small displacements of a laser beam is important in many areas of science and technology, such as in an atomic force microscope. A quantum trick called weak-value amplification (WVA) has previously led to extremely sensitive measurements of beam shifts within interferometers. Now Carlotta Versmold of the Ludwig Maximilian University of Munich and her colleagues have extended such measurements to beam displacements outside of an interferometer [1]. For example, a laser beam reflecting off of a distant window could encode vibrations resulting from conversations inside the building.

In the WVA version applicable to shifts within an interferometer, a light beam is split and routed along two slightly unequal paths that later merge and lead to two output ports—a “bright” port where the beams largely reinforce one another and a “dark” port where they mostly cancel each other out. Any slight displacement of either beam is amplified in the position of the dim spot at the dark port. However, shifts in the beam entering the interferometer lead to offsetting shifts of the internal beams and thus to no measurable signal.

To extend the method to shifts of the incoming beam, Versmold and her colleagues added a so-called Dove prism to one of the beam paths. This type of prism generates an additional reflection, which effectively leads to opposite shifts in the two paths, resulting in an amplified signal at the dark port.

Physicists bring unruly molecules to the quantum party

Scientists have made leaps and bounds in bending atoms to their will, making them into everything from ultraprecise clocks to bits of quantum data. Translating these quantum technologies from obedient atoms to unruly molecules could offer greater possibilities. Molecules can rotate and vibrate. That makes molecules more sensitive to certain changes in the environment, like temperature.

“If you’re sensitive to something, it can be a curse, because you would like to not be sensitive, or it can be a blessing,” said NIST physicist Dietrich Leibfried. “You can use that sensitivity to your advantage.”

But that same sensitivity has made molecules difficult to control. Recently, physicists at the National Institute of Standards and Technology (NIST) achieved new levels of control over molecules. In a study published in Physical Review Letters, they were able to manipulate a calcium hydride molecular ion—made up of one atom of hydrogen and one atom of calcium, with one electron removed to make it a charged molecule—with almost perfect success. And this control opens possibilities for quantum technology, chemical research and exploring new physics.

A simple spin swap reveals exotic anyons

Researchers from the University of Innsbruck, the Collège de France, and the Université Libre de Bruxelles have developed a simple yet powerful method to reveal anyons—exotic quantum particles that are neither bosons nor fermions—in one-dimensional systems. Their paper is published in Physical Review Letters.

In conventional three-dimensional space, particles belong to one of two categories: fermions or bosons. In low-dimensional settings, however, quantum mechanics allows for more exotic behavior. Here, anyons can emerge—quasi-particles whose exchange properties continuously interpolate between those of bosons and fermions, leading to fractional statistics. Detecting and engineering such particles in one dimension has long been a central challenge, typically requiring, as theory proposals suggest, intricate scattering schemes or density-dependent tunneling processes.

The new study by teams led by Hanns-Christoph Nägerl at the University of Innsbruck and Nathan Goldman at the Université Libre de Bruxelles and Collège de France (CNRS) now introduces a remarkably simple yet powerful approach. The researchers propose an effective “swap” model that leverages the spin degree of freedom of ultracold atoms. By assigning a complex phase to the exchange—or “swap”—of two spins, the system naturally acquires the fractional statistical behavior characteristic of anyons.

Silicon atom processor links 11 qubits with more than 99% fidelity

In order to scale quantum computers, more qubits must be added and interconnected. However, prior attempts to do this have resulted in a loss of connection quality, or fidelity. But, a new study published in Nature details the design of a new kind of processor that overcomes this problem. The processor, developed by the company Silicon Quantum Computing, uses silicon—the main material used in classical computers—along with phosphorus atoms to link 11 qubits.

The new design uses precision-placed phosphorus atoms in isotopically purified silicon-28, which are arranged into two multi-nuclear spin registers. One register contains four phosphorus atoms, while the other contains five, and each register shares an electron spin. The two registers are linked by electron exchange interaction, allowing for non-local connectivity across the registers and 11 linked qubits.

Because of the placement of silicon and phosphorus in the periodic table, the design is referred to as the “14|15 platform.” This 11-qubit atom processor in silicon is the largest of its kind to date, marking a major accomplishment for quantum computing.

Scientists build a quantum computer that can repair itself using recycled atoms

Like their conventional counterparts, quantum computers can also break down. They can sometimes lose the atoms they manipulate to function, which can stop calculations dead in their tracks. But scientists at the US-based firm Atom Computing have demonstrated a solution that allows a quantum computer to repair itself while it’s still running.

The team zeroed in on quantum computers that use neutral atoms (atoms with equal numbers of protons and electrons). These individual atoms are the qubits, or the basic building blocks of a quantum computer’s memory. They are held in place by laser beams called optical tweezers, but the setup is not foolproof.

Occasionally, an atom slips out of its trap and disappears. When this happens mid-calculation, the whole process can grind to a halt because the computer can’t function with a missing part.

Flat Fermi surface in altermagnets enables quantum limit spin currents

The key feature of spintronic devices is their ability to use spin currents to transfer momentum, enabling low-energy, high-speed storage and logical signal control. These devices are usually manipulated by electric currents and fields. The charge-to-spin conversion efficiency (CSE) is a key metric for evaluating their performance.

Now, scientists from the Institute of Metal Research (IMR) of the Chinese Academy of Sciences have proposed a new deep correlation between the spin splitting torque (SST) and the Fermi surface geometry, achieving a quantum limit of 100% in a system with a flat Fermi surface. These results were published in Physical Review Letters on December 16.

Physicists Propose First-Ever Experiment To Manipulate Gravitational Waves

When massive cosmic objects such as black holes merge or neutron stars crash into one another, they can produce gravitational waves. These ripples move through the universe at the speed of light and create extremely small changes in the structure of space-time. Their existence was first predicted by Albert Einstein, and scientists confirmed them experimentally for the first time in 2015.

Building on this discovery, Prof. Ralf Schützhold, a theoretical physicist at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), is proposing a bold new step.

Schützhold has developed a concept for an experiment that would go beyond detecting gravitational waves and instead allow researchers to influence them. The proposal, published in the journal Physical Review Letters, could also help clarify whether gravity follows quantum rules, a question that remains unresolved in modern physics.

Physicists push superconducting diodes to high temperatures

For the first time, researchers in China have demonstrated a high-temperature superconducting diode effect, which allows a supercurrent to flow in both directions. Published in Nature Physics, the team’s result could help address the noisy signals that pose a fundamental challenge in quantum computing.

A diode is a device that shows an asymmetric electrical response, allowing current to flow more easily in one direction than the other. Until recently, diode behavior had only been observed in conventional, non-superconducting electrical systems—but in 2020, a team of researchers in Japan became the first to demonstrate the diode effect in a superconductor. Ever since, this effect has gained increasing attention for its potential in practical quantum computing.

“However, most of the reported superconducting diodes work at low temperatures around 10 Kelvin, and often require an external magnetic field,” explains Ding Zhang at Tsinghua University and the Beijing Academy of Quantum Information Sciences, who led the research. “The diode efficiency is also low for many superconducting diodes.”

Shortest light pulse ever created captures ultrafast electron dynamics

Electrons determine everything: how chemical reactions unfold, how materials conduct electricity, how biological molecules transfer energy, and how quantum technologies operate. But electron dynamics happens on attosecond timescales—far too fast for conventional measurement tools.

Researchers have now generated a 19.2-attosecond soft X-ray pulse, which effectively creates a camera capable of capturing these elusive dynamics in real time with unprecedented detail, enabling the observation of processes never observed before. Dr. Fernando Ardana-Lamas, Dr. Seth L. Cousin, Juliette Lignieres, and ICREA Prof. Jens Biegert, at ICFO, has published this new record in Ultrafast Science. At just 19.2 attoseconds long, it is the shortest and brightest soft X-ray pulse ever produced, giving rise to the fastest “camera” in existence.

Flashes of light in the soft X-ray spectral range provide fingerprinting identification, allowing scientists to track how electrons reorganize around specific atoms during reactions or phase transitions. Generating an isolated pulse this short, required innovations in high-harmonic generation, advanced laser engineering, and attosecond metrology. Together, these developments allow researchers to observe electron dynamics, which define material properties, at their natural timescales.

Conventional entanglement can have thousands of hidden topologies in high dimensions

Researchers from the University of the Witwatersrand in South Africa, in collaboration with Huzhou University, discovered that the entanglement workhorse of most quantum optics laboratories can have hidden topologies, reporting the highest ever observed in any system: 48 dimensions with over 17,000 topological signatures, an enormous alphabet for encoding robust quantum information.

Most quantum optics laboratories produce entangled photons by a process of spontaneous parametric downconversion (SPDC), which naturally produces entanglement in “space,” the spatial degrees of freedom of light. Now the team have found that hidden in this space is a world of high-dimensional topologies, offering new paradigms for encoding information and making quantum information immune to noise. The topology was shown using the orbital angular momentum (OAM) of light, from two dimensional to very high dimensions.

Reporting in Nature Communications, the team showed that if one measures the OAM of two entangled photons it can be shown to have a topology: an underlying feature of the entanglement itself. Since OAM can take on an infinite number of possibilities, so too can the topology.

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