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Space & Physics

The various avatars of the Hall effect

In this second article of Ed Publica’s series on the Hall effect, Dr. Saraubh Basu examines the physics of the Hall effect variants discovered over the course of the past century.

Dr. Saurabh Basu

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Wikimedia Xenon hall thruster
A xenon Hall thruster tested at a NASA facility. Credit: NASA/JPL-Caltech

This is the second article of Ed Publica’s series on the Hall effect, which covers the various manifestations of the Hall effect. You can read the first article here.

The ‘anomalous’ Hall effect

In 1881, just two years after Edwin Hall discovered the eponymous Hall effect, he spotted an anomaly when replicating the effect with ferromagnets.

He had observed a tenfold deflection of electric charges this time around, compared to non-magnetic conductors.

Suspecting the magnetic properties played a role, this avatar of the Hall effect is dubbed the anomalous Hall effect. The word ‘anomalous’ is used owing to the fact that external magnetic field no longer remains as a stringent requirement for the Hall effect; instead, the intrinsic magnetization (for instance, the ferromagnet in the above example) fulfils that criterion.

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The physicist Edwin Hall. Credit: Wikimedia

The Hall resistivity in ferromagnets increase steeply under the presence of very weak magnetic fields. However, in stronger magnetic fields, the Hall resistivity doesn’t increase further very much. This saturation is rather strange, for it is in contrast to the classical Hall effect where the Hall resistivity maintains its steady growth.

There are several other effects that play a crucial role in determining the anomalous Hall resistivity, thus making it a complicated phenomenon that physicists lack comprehensive understanding about, in comparison to the various other avatars of the Hall effect.

Quantum avatar(s)

The fact that a simple lab experiment showed how the Hall resistivity can be expressed as an equation that contains merely constants, opened up a a plethora of research to understand the cause of this ‘universality’. For it hinted to the involvement of a very fundamental phenomenon.

In 1980, Klaus von Klitzing discovered the quantum avatar of the Hall effect was detected. He was amidst research at a magnetic facility in Grenoble, France, working to improve electron mobility in metal oxide semiconductor field effect transistors (MOSFET). These are transistors that typically operate at extremely low temperatures and under intense magnetic fields.

von Klitzing observed his sample’s Hall resistivity assuming discretized values. This means the resistivity jumps in steps, by a fixed amount that can be scaled as multiples of an integer number (includes 0 along with whole numbers such as 1,2,3, and so on). This discretization reveals the underlying quantum mechanical behavior that has been unraveled at long last – thus bearing its name – the integer quantum Hall effect. von Klitzing later won the Nobel Prize in Physics for 1985 for this work.

Wikimedia quantum hall effect

The plot here depicts the transverse and longitudinal Hall resistivity (y-axis) increasing in integer steps as the magnetic field (x-axis) increases. This is due to the integer quantum Hall effect. Credit: Wikimedia

But the quantization isn’t limited to integer multiples. In fact, two years later, the fractional quantum Hall effect was observed in experiments. It was shown there were about 100 fractions, including those that aren’t whole numbers that were now in the formula.

Robert Laughlin, who would later win a share of the 1998 Nobel Prize in Physics, proposed a theory to explain the observations. It boils down to the interaction among electrons, either due to the Coulombic repulsion force or the Pauli exclusion principle.

These interacts would eventually split the degeneracy of these enormously degenerate Landau energy levels. These are quantum states occupied by electrons that complete circular revolutions under the influence of an external magnetic field. Splitting these degeneracies, lead to the opening of an energy gap, for the fractional quantum Hall effect to be observed. 

‘Spin’ avatar(s)

Just as there are electric charges in nature, so are there spin currents found in nature. ‘Spin’ is a key property found in quantum particles. Unlike what the name suggests, these quantum particles don’t spin or rotate about any axis passing through them. However, these particles carry an angular momentum as though it does spin.

In 1971, before von Klitzing observed the quantum Hall effect, Mikhail Dyakonov and Vladimir Perel hypothesized the spin Hall effect.

In this avatar of the Hall effect, quantum spins of opposite kinds accumulate at the edges of the sample, orthogonal to the direction in which the charge current passes.

The spin selection can be facilitated by the spin-orbit coupling. This refers to the modified energy levels in an atom when the electron’s motion is under the influence on the magnetic field generated by the nucleus. Strong coupling may be intrinsic to doped semiconductors. The proposal has triggered intense investigation of the phenomenon, with first experimental observations of the spin Hall effect seen in n-doped semiconductors and two-dimensional hole gases.

Karthik quantum spin

Quantum spins don’t really look like the depiction above, which is meant to showcase a fact that particles like electrons do have an intrinsic angular momentum nonetheless. Credit: Karthik / Ed Publica

For more than a decade, studies concerning the spin current and its application to novel spintronics (or spin electronics) have received plethora of attention. This is with regard to efficiently generating, manipulating and detecting spin accumulation in a sample material. Some progress has also occurred from the device fabrication perspective via techniques such as spin injection, among others.

A major advantage in dealing with the spin current lies in the non-dissipative (or very less dissipation) nature which arises owing to the time reversal invariance of the spin current. This presents a non-dissipative scenario (unlike the dissipative effects seen with charged currents), thus making it quite advantageous for spin transport phenomena.  

Furthermore, a quantized version of the spin Hall effect exists, with mercury telluride and cadmium telluride quantum well superlattices, showcasing this effect. In 2005, a quantum treatment was proposed by Charles Kane and Eugene Mele, in the form of a tight binding toy model of electrons operating in a two-dimensional honeycomb lattice.

In fact, the ‘wonder material’ graphene, which is a two-dimensional honeycomb lattice constituting carbon atoms, does satisfy some key requirements for the quantum spin Hall effect. However, it lacks a large spin-orbit coupling among other requirements.  

Nonetheless, graphene’s ability to entertain the quantum spin Hall effect, makes it a prospective candidate to find applications in next-generation spintronic devices.

Dr. Saurabh Basu is Professor at Department of Physics, Indian Institute of Technology (IIT) Guwahati. He works in the area of correlated electron systems with the main focus on bosonic superfluidity in (optical) lattices.

Space & Physics

Researchers Develop Ultra-Efficient Chip for Post-Quantum Security in Medical Devices

The breakthrough addresses a critical vulnerability in next-generation healthcare technology as quantum computing advances threaten current encryption standards.

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Credit: Courtesy of the researchers

Breakthrough Enables Strong Encryption on Tiny, Power-Constrained Devices

Researchers at the Massachusetts Institute of Technology have developed a highly energy-efficient microchip capable of running advanced post-quantum cryptography (PQC) on small, power-limited devices such as pacemakers, insulin pumps, and ingestible sensors. The breakthrough addresses a critical vulnerability in next-generation healthcare technology as quantum computing advances threaten current encryption standards.

The chip, roughly the size of a needle tip, integrates robust security features designed to protect sensitive patient data while maintaining extremely low power consumption. This makes it suitable for wireless biomedical devices that have historically lacked strong encryption due to energy constraints.

Why Post-Quantum Cryptography Matters

As quantum computers evolve, traditional encryption methods are expected to become obsolete. Governments and regulatory bodies, including the National Institute of Standards and Technology (NIST), are already preparing to transition toward PQC algorithms to safeguard digital infrastructure.

However, PQC techniques are computationally intensive, often increasing energy usage by up to 100–1000 times—making them impractical for small, battery-powered devices until now.

This new chip bridges that gap by enabling advanced encryption without significantly increasing energy demand.

Key Innovations Behind the Chip

Multi-Layered Security Design

The chip incorporates multiple PQC algorithms to ensure long-term resilience, even if one encryption method becomes vulnerable in the future.

Built-in Random Number Generator

A highly efficient on-chip random number generator strengthens encryption by producing secure cryptographic keys internally, eliminating reliance on external components.

Protection Against Physical Attacks

The design includes safeguards against “power side-channel attacks,” where hackers attempt to extract data by analyzing power consumption patterns.

Early Fault Detection

The chip can detect voltage irregularities and abort compromised operations early, preventing energy waste and potential security breaches.

Major Gains in Energy Efficiency

The researchers report that the chip achieves 20 to 60 times greater energy efficiency compared to existing PQC implementations, while also occupying a smaller physical footprint.

This efficiency breakthrough is crucial for expanding secure computing to edge devices—systems that operate outside traditional data centers, often with strict power limitations.

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Space & Physics

The Universe Is Ringing

How gravitational waves from colliding black holes are opening an entirely new way of exploring the cosmos

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The Gravitational-Wave Transient Catalog 4.0, pictured, is a record of cosmic mergers detected between 2015 and 2024 by the LIGO, Virgo, and KAGRA gravitational wave observatories. Each panel is a time and frequency signature of an individual event — the merger of two black holes, two neutron stars, or one of each, somewhere out in the cosmos. Credit: Ryan Nowicki / Bill Smith / Karan Jani

More than a century after Albert Einstein predicted them, gravitational waves are transforming astronomy. Ripples in space-time produced by colliding black holes and neutron stars are now being detected routinely, revealing a universe filled with violent mergers and cosmic echoes that have travelled billions of years to reach Earth.

A Ripple Across the Cosmos

When the densest objects in the universe collide, the impact does not simply end with the destruction or merger of stars. It sends ripples through the very fabric of space and time.

These ripples—known as gravitational waves—spread outward at the speed of light, crossing galaxies and cosmic voids for millions or even billions of years. By the time they reach Earth, they are unimaginably faint distortions of space itself.

Yet scientists have learned how to detect them.

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Image by Gerd Altmann from Pixabay

A global network of observatories now monitors these tiny disturbances: the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States, the Virgo detector in Italy, and the Kamioka Gravitational Wave Detector (KAGRA) in Japan. Together, these instruments form one of the most sensitive scientific experiments ever constructed, capable of detecting distortions smaller than the width of a proton.

Through them, astronomers have begun to “listen” to the universe.

And what they are hearing is astonishing.

A Universe Filled with Collisions

The LIGO–Virgo–KAGRA (LVK) Collaboration has now released the latest compilation of gravitational-wave detections, to appear in a special issue of Astrophysical Journal Letters. The findings suggest that the cosmos is reverberating with collisions far more frequently than scientists once imagined.

The newly released Gravitational-Wave Transient Catalog-4.0 (GWTC-4) includes detections from part of the observatories’ fourth observing run, conducted between May 2023 and January 2024.

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In just nine months, the detectors recorded 128 new gravitational-wave candidates—signals that likely originated from extreme astrophysical events occurring hundreds of millions or billions of light-years away.

This newest batch more than doubles the size of the gravitational-wave catalog, which previously contained 90 candidates from earlier observing runs.

“The beautiful science that we are able to do with this catalog is enabled by significant improvements in the sensitivity of the gravitational-wave detectors as well as more powerful analysis techniques,” says Nergis Mavalvala, a member of the LVK collaboration and dean of the MIT School of Science.

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Albert Einstein /Credit: Wikipedia

What began in 2015 with the first historic detection has now become a steady stream of discoveries.

“In the past decade, gravitational wave astronomy has progressed from the first detection to the observation of hundreds of black hole mergers,” says Stephen Fairhurst, professor at Cardiff University and spokesperson for the LIGO Scientific Collaboration. “These observations enable us to better understand how black holes form from the collapse of massive stars, probe the cosmological evolution of the universe and provide increasingly rigorous confirmations of the theory of general relativity.”

When Black Holes Dance

Most gravitational waves detected so far originate from binary black holes—pairs of black holes locked in orbit around each other.

Over time, gravity draws them closer together. As they spiral inward, they release enormous amounts of energy in the form of gravitational waves. In the final fraction of a second, the two objects merge in a titanic collision, forming a single, larger black hole.

These cosmic dances are among the most energetic events in the universe.

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Black holes themselves are born when massive stars collapse at the end of their lives, compressing enormous amounts of matter into regions so dense that not even light can escape.

Many form in pairs. When they eventually collide, the event sends gravitational waves surging through space.

The first such detection, announced in 2016, confirmed a century-old prediction of Einstein’s theory of general relativity. Since then, dozens—and now hundreds—of similar events have been observed.

But the latest catalog shows that the universe is far more diverse than scientists once believed.

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Pushing the Edges of Black Hole Physics

The newly detected signals reveal a remarkable variety of cosmic systems.

Among them are the heaviest black hole binaries ever detected, systems where the masses of the two black holes are strikingly unequal, and pairs spinning at astonishing speeds.

“The message from this catalog is: We are expanding into new parts of what we call ‘parameter space’ and a whole new variety of black holes,” says Daniel Williams, a research fellow at the University of Glasgow. “We are really pushing the edges, and are seeing things that are more massive, spinning faster, and are more astrophysically interesting and unusual.”

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Image by Iris,Helen,silvy from Pixabay

One particularly dramatic signal—GW231123_135430—appears to have originated from two enormous black holes, each roughly 130 times the mass of the Sun. Most previously observed mergers involved black holes closer to 30 solar masses.

The extraordinary size of these objects suggests they may themselves have formed from earlier black hole mergers—a kind of cosmic generational chain.

Another remarkable event, GW231028_153006, revealed a binary in which both black holes are spinning at around 40 percent of the speed of light.

And in GW231118_005626, scientists detected an unusually uneven pair where one black hole is roughly twice as massive as the other.

“One of the striking things about our collection of black holes is their broad range of properties,” says Jack Heinzel, an MIT graduate student who contributed to the catalog’s analysis. “Some of them are over 100 times the mass of our sun, others are as small as only a few times the mass of the sun. Some black holes are rapidly spinning, others have no measurable spin.”

“We still don’t completely understand how black holes form in the universe,” he adds, “but our observations offer a crucial insight into these questions.”

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Catching a Whisper in Space-Time

Detecting gravitational waves requires extraordinary precision.

The observatories use L-shaped interferometers with arms several kilometers long. Laser beams travel down these tunnels and reflect back to their source.

If a gravitational wave passes through the detector, it slightly stretches one arm while compressing the other, changing the distance the light travels by an incredibly tiny amount.

These changes can be smaller than one-thousandth the diameter of a proton.

Even with such advanced technology, detections remain unpredictable.

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Image by Stefan Keller from Pixabay

“You can’t ever predict when a gravitational wave is going to come into your detector,” says Amanda Baylor, a graduate student at the University of Wisconsin–Milwaukee who worked on the signal search. “We could have five detections in one day, or one detection every 20 days. The universe is just so random.”

Recent upgrades have dramatically improved the detectors’ reach. LIGO can now detect signals from neutron star collisions up to one billion light-years away, and black hole mergers far beyond that.

Testing Einstein’s Ultimate Theory

Gravitational waves are not only revealing spectacular cosmic events. They are also providing some of the most extreme tests ever conducted of Einstein’s theory of general relativity.

Black holes themselves are one of the most extraordinary predictions of the theory.

“Black holes are one of the most iconic and mind-bending predictions of general relativity,” says Aaron Zimmerman, associate professor of physics at the University of Texas at Austin.

When two black holes collide, he explains, they “shake up space and time more intensely than almost any other process we can imagine observing.”

One particularly powerful signal—GW230814_230901—allowed scientists to analyze the structure of the gravitational wave in exceptional detail.

“So far, the theory is passing all our tests,” Zimmerman says. “But we’re also learning that we have to make even more accurate predictions to keep up with all the data the universe is giving us.”

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Measuring the Expansion of the Universe

Gravitational waves are also becoming powerful tools for answering one of cosmology’s biggest questions: how fast the universe is expanding.

Astronomers measure this expansion using the Hubble constant, but different methods have produced conflicting results.

Gravitational waves offer an independent approach.

“Merging black holes have a really unique property: We can tell how far away they are from Earth just from analyzing their signals,” says Rachel Gray, a lecturer at the University of Glasgow.

“So, every merging black hole gives us a measurement of the Hubble constant, and by combining all of the gravitational wave sources together, we can vastly improve how accurate this measurement is.”

Using the current gravitational-wave catalog, scientists estimate that the universe is expanding at roughly 76 kilometers per second per megaparsec.

For now, the uncertainty remains large—but future detections could sharpen the measurement significantly.

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Image by Johnson Martin from Pixabay

Listening to the Future

Only a decade ago, gravitational waves were purely theoretical signals.

Today, they are transforming astronomy.

With every new detection, scientists gain another glimpse into the hidden life of the universe: the birth of black holes, the evolution of galaxies, and the behavior of gravity under the most extreme conditions imaginable.

“Each new gravitational-wave detection allows us to unlock another piece of the universe’s puzzle in ways we couldn’t just a decade ago,” says Lucy Thomas, a postdoctoral researcher at the Caltech LIGO Lab.

“It’s incredibly exciting to think about what astrophysical mysteries and surprises we can uncover with future observing runs.”

The instruments on Earth are quiet, their lasers moving silently down vacuum tunnels. But far beyond our galaxy, black holes continue to collide.

And with each collision, the universe sends out another ripple—another echo across the cosmos—waiting for us to hear it.

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Space & Physics

NASA’s Artemis II Captures Stunning ‘Earthset’ Over the Moon

NASA’s Artemis II crew captures a rare Earthset over the Moon, revealing lunar basins, craters, and Earth’s night-day divide.

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Artemis II Captures Rare ‘Earthset’ Over Moon
Earth sets beyond the Moon’s horizon as seen by the Artemis II crew on April 6, 2026, revealing the lunar surface’s cratered terrain alongside Earth’s day–night divide over the Australia–Oceania region. Image credit: NASA

NASA’s Artemis II mission has captured a striking new perspective of the Moon, showing Earth setting beyond the lunar horizon in a rare and visually dramatic moment from deep space.

The image, taken on April 6, 2026, at 6:41 p.m. EDT by the Artemis II crew during their journey around the far side of the Moon, reveals Earth partially dipping behind the Moon’s curved limb—an event often described as an “Earthset.”

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Captured through Orion’s window during Artemis II’s lunar flyby on April 6, 2026, this image shows Earth setting behind the Moon’s cratered surface, with clouds visible over Australia and Oceania and the terraced Ohm crater in the foreground.Image Credit: NASA

A Geological Snapshot of the Moon

Beyond its visual impact, the image offers a detailed look at the Moon’s complex surface.

The Orientale basin, one of the Moon’s most prominent impact structures, is visible along the edge of the lunar surface. Nearby, the Hertzsprung Basin appears as faint concentric rings, partially disrupted by the younger Vavilov crater, which sits atop the older geological formation.

Also visible are chains of secondary craters—linear indentations formed by debris ejected during the massive impact that created the Orientale basin.

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Captured by the Artemis II crew on April 6, 2026, this image shows the Moon’s terminator—the boundary between day and night—where low-angle sunlight casts long shadows, revealing craters and rugged terrain in striking detail during the spacecraft’s far-side flyby. Image credit: NASA

Artemis II: Earth in Shadow and Light

The photograph also captures Earth in a moment of contrast.

The darkened portion of the planet is in nighttime, while the illuminated side reveals swirling cloud formations over Australia and the Oceania region, offering a reminder of Earth’s dynamic atmosphere even from hundreds of thousands of kilometres away.

Artemis II: A New Era of Lunar Exploration

The Artemis II mission marks a major step in NASA’s return to the Moon, carrying astronauts on a crewed journey around the lunar surface for the first time in over five decades.

Images like this not only provide scientific insights into lunar geology but also offer a powerful visual connection between Earth and its nearest celestial neighbour—highlighting both the scale of space exploration and the fragility of our home planet.

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Captured by the Artemis II crew on April 6, 2026, this image shows the Moon completely blocking the Sun during a rare 54-minute totality. The Sun’s corona forms a glowing halo around the lunar disk, while faint stars and Earth-reflected light illuminate the Moon’s surface—offering a unique deep-space perspective. Image Credit: NASA

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