Indefinite causal order: how quantum physics is challenging our understanding of cause and effect

09 Sep 2025 Hamish Johnston

Part of our International Year of Quantum Science and Technology coverage

In the fourth of our series of truly weird quantum effects, Hamish Johnston becomes a casual observer of the bizarre situation in which the causal order of events are in a quantum superposition

(Courtesy: Shutterstock/Hakim Graphy)

The concept of cause and effect plays an important role in both our everyday lives and in physics. If you set a ball down in front of a window and kick it hard, a split second later, the ball will hit the window and smash it. What we don’t observe is a world where the window smashes on its own, thereby causing the ball to be kicked – that would seem rather nonsensical. In other words, kick before smash, and smash before kick, are two different physical processes, each having a unique and definite causal order.

But, does definite causal order also reign supreme in the quantum world, where concepts like position and time can be fuzzy? Most physicists are happy to accept the paradox of Schrödinger’s cat – a thought experiment in which a cat hidden in a box is simultaneously dead and alive at the same time, until you open the box to check.

 

China’s Experimental Advanced Superconducting Tokamak smashes fusion confinement record

23 Jan 2025 Michael Banks

Record breaker: The Experimental Advanced Superconducting Tokamak based in Hefei, China, has sustained a high-pressure plasma for over 1000 seconds (Courtesy: Hefei Institute of Physical Science)

Update 20/02/2025: On 18 February, officials at France’s atomic energy commission (CEA) announced that its WEST Tokamak reactor, which is based in Cadarache, France, had maintained a steady-state high-confinement plasma for 1336 seconds.

A fusion tokamak in China has smashed its previous fusion record of maintaining a steady-state plasma. This week, scientists working on the Experimental Advanced Superconducting Tokamak (EAST) announced that they had produced a steady-state high-confinement plasma for 1066 seconds, breaking EAST’s previous 2023 record of 403 seconds.

 

Space ice reveals its secrets

07 Aug 2025 Isabelle Dumé

Structure amid disorder: Visual representation of the structure of low-density amorphous ice. Many tiny crystallites (white) are concealed in the amorphous material (blue). (Courtesy: Michael B Davies, UCL and University of Cambridge)

The most common form of water in the universe appears to be much more complex than was previously thought. While past measurements suggested that this “space ice” is amorphous, researchers in the UK have now discovered that it contains crystals. The result poses a challenge to current models of ice formation and could alter our understanding of ordinary liquid water.

Unlike most other materials, water is denser as a liquid than it is as a solid. It also expands rather than contracts when it cools; becomes less viscous when compressed; and exists in many physical states, including at least 20 polymorphs of ice.

 

How hot can you make a solid before it melts?

19 Aug 2025 Isabelle Dumé

Heating up: Researchers at SLAC's Matter in Extreme Conditions (MEC) instrument used a laser to superheat a sample of gold. They then sent a pulse of ultrabright X-rays from the Linac Coherent Light Source (LCLS) through the sample to measure the speed, and thus the temperature, of the atoms vibrating in the sample. (Courtesy: Greg Stewart/SLAC National Accelerator Laboratory)

Gold can remain solid at temperatures over 14 times its melting point, far beyond a long-accepted theoretical limit dubbed the “entropy catastrophe.” This finding is based on temperature measurements made using high-resolution inelastic X-ray scattering, and according to team leader Thomas White of the University of Nevada, US, it implies that the question “How hot can you make a solid before it melts?” has no good answer.

“Until now, we thought that solids could not exist above about three times their melting temperatures,” White says. “Our results show that if we heat a material rapidly – that is, before it has time to expand – it is possible to bypass this limit entirely.”

 

New mechanism explains the behaviour of materials exhibiting giant magnetoresistance

01 Jul 2025 Isabelle Dumé

Correspondence: Single-particle (left) dispersion of electrons is intrinsically tied to the dynamics of spins/magnons (right panel) in double-exchange ferromagnets. (Courtesy: J Herbrych)

Two distinctive features of materials known as quantum double-exchange ferromagnets are purely due to quantum spin effects and multiorbital physics, with no need for the lattice vibrations previously invoked to explain them. This theoretical result could lead to new insights into these technologically important materials, as it suggests that some of their properties may arise from interactions hitherto regarded as less important.

Quantum double-exchange ferromagnets have interested scientists since the late 1980s, when physicists led by Albert Fert and Peter Grünberg found that their electrical resistance depends strongly on the magnitude of an external magnetic field. This phenomenon is known as giant magnetoresistance (GMR), and its discovery led to an enormous increase in the storage capacity of modern hard-disk drives, which incorporate GMR structures into their magnetic field sensors. It also led, in 2007, to a Nobel Prize for Fert and Grünberg.

 

Antiferromagnets could be better than ferromagnets for some ultrafast, high-density memories

30 Sep 2025 Isabelle Dumé

How it works On the left is a schematic illustration of memory device consisting of a chiral antiferromagnet Mn3Sn / nonmagnetic metal heterostructure. On the right is a scanning electron microscope image of the fabricated device with a Mn3Sn nanodot and a nonmagnetic metal channel. (Courtesy: Yutaro Takeuchi et al)

While antiferromagnets show great promise for spintronics applications, they have proven more challenging to control compared to ferromagnets. Researchers in Japan have now succeeded in switching an antiferromagnetic manganese–tin nanodot using electric current pulses as short as 0.1 ns. Their work demonstrates that these materials can be utilized to create efficient, high-speed, high-density memories that operate at gigahertz frequencies, thereby outperforming ferromagnets in this range.

In antiferromagnets, spins can flip quickly, potentially reaching frequencies well beyond the gigahertz. Such rapid spin flips are possible because neighbouring spins in antiferromagnets align antiparallel to each other thanks to strong interactions among the spins. This is different from ferromagnets, which have parallel electron spins.

 

‘Breathing’ crystal reversibly releases oxygen

09 Sep 2025 Isabelle Dumé

Schematic illustration of oxygen-breathing Scientists have developed a special type of crystal with oxygen-breathing abilities that could be used in clean energy technologies and next-generation electronics. (Courtesy: Hyoungjeen Jeen/Pusan National University)

A new transition-metal oxide crystal that reversibly and repeatedly absorbs and releases oxygen could be ideal for use in fuel cells and as the active medium in clean energy technologies such as thermal transistors, smart windows, and new types of batteries. The “breathing” crystal, discovered by scientists at Pusan National University in Korea and Hokkaido University in Japan, is made from strontium, cobalt, and iron and contains oxygen vacancies.

Transition-metal oxides exhibit a wide range of electrical properties that can be tuned from insulating to superconducting. This means they can find applications in areas as diverse as energy storage, catalysis, and electronic devices.

 

Electrochemical loading boosts deuterium fusion in a palladium target

25 Aug 2025

Thunderbird Reactor The benchtop particle accelerator has revealed how electrolysis can enhance deuterium fusion. (Courtesy: University of British Columbia/Berlinguette Lab)

Researchers in Canada have utilized electrochemistry to enhance the rate of nuclear fusion within a metal target bombarded with high-energy deuterium ions. While the process is unlikely to lead to a new source of energy – it consumes far more energy than it produces – further research could provide new insights into fusion and other areas of science.

Although modern fusion reactors are huge projects sometimes costing billions, the first evidence for an artificial fusion reaction – observed by Mark Oliphant and Ernest Rutherford in 1934 – was a simple experiment in which deuterium nuclei in a solid target were bombarded with deuterium ions.

 

General Fusion lays off staff due to ‘unexpected and urgent financing constraints’

06 May 2025 Michael Banks

Hot stuff: General Fusion’s Lawson Machine 26 employs magnetized target fusion technology. (Courtesy: General Fusion)

The Canadian firm General Fusion is set to lay off about 25% of its 140-strong workforce and scale back the operation of its fusion device, dubbed Lawson Machine 26 (LM26). The announcement was made in an open letter published on 5 May by the company’s chief executive Greg Twinney. The move follows what the firm says is an “unexpected and urgent financing constraint”.

Founded in 2002 by the Canadian plasma physicist Michel Laberge, General Fusion is based in Richmond, British Columbia. It is one of the first private fusion companies and has attracted more than $325 million in funding from both private investors, including Amazon founder Jeff Bezos, and the Canadian government.

 

Radioactive BEC could form a ‘superradiant neutrino laser’

04 Oct 2025

Cool and creative Building a superradiant neutrino laser would be a significant challenge. (Courtesy: iStock/Vitacops)

Radioactive atoms in a Bose–Einstein condensate (BEC) could form a “superradiant neutrino laser” in which the atomic nuclei undergo accelerated beta decay. A hypothetical laser has been proposed by two US researchers, who claim that it could be built and tested. While such a neutrino laser has no obvious immediate applications, further developments could potentially assist in the search for background neutrinos from the Big Bang – an important goal of neutrino physicists.

Neutrinos – the ghostly particles produced in beta decay – are notoriously difficult to detect or manipulate because of the weakness of their interaction with matter. They cannot be used to produce a conventional laser because they would pass straight through mirrors unimpeded. More fundamentally, neutrinos are fermions rather than bosons such as photons. This prevents neutrinos from forming a two-level system with a population inversion, as only one neutrino can occupy each quantum state in a system.

 

Is materials science the new alchemy for the 21st century?

06 Oct 2025 Honor Powrie

It can be challenging to define exactly what materials science is all about. But Honor Powrie believes the field is the most rewarding and challenging in all of science, with its practitioners striving to create “new gold”

Stuff that matters Whether it's metals, ceramics, bioplastics or the auxetic materials in this Nike running shoe, the world is being transformed thanks to materials science. (Courtesy: Ethan Jull, University of Leeds)

For many years, I’ve been a judge for awards and prizes linked to research and innovation in engineering and physics. It’s often said that it’s better to give than to receive, and it’s certainly true in this case. But another highlight of my involvement with awards is learning about cutting-edge innovations I either hadn’t heard of or didn’t know much about.

 

Deep-blue LEDs get a super-bright, non-toxic boost

21 Aug 2025 Isabelle Dumé

Deep blue: Scientists have developed hybrid copper iodide crystals that emit deep-blue light. (Courtesy: Kun Zhu/Jing Li Lab/Rutgers University)

A team led by researchers at Rutgers University in the US has discovered a new semiconductor that emits bright, deep-blue light. The hybrid copper iodide material is stable, non-toxic, and can be processed in solution, having already been integrated into a light-emitting diode (LED). According to its developers, it could find applications in solid-state lighting and display technologies.

Creating white light for solid-state lighting and full-colour displays requires bright, pure sources of red, green, and blue light. While stable materials that efficiently emit red or green light are relatively easy to produce, those that generate blue light (especially deep-blue light) are much more challenging. Existing blue-light emitters based on organic materials are unstable, meaning they lose their colour quality over time.

 

New metalaser is a laser researcher’s dream

11 Aug 2025 Isabelle Dumé

Tailored profiles: The working principle of the new metalaser. The designed geometric phase profile is introduced to the metasurface by rotating the hole in each silicon nitride nanodisk. Then the laser emission can be tailored to a focal spot, focal line, doughnut or even a holographic image. (Courtesy: Q Song)

A new type of nanostructured lasing system, called a metalaser, emits light with highly tunable wavefronts – a feat that had previously proved impossible to achieve with conventional semiconductor lasers. According to the researchers in China who developed it, the new metalaser can generate speckle-free laser holograms and could revolutionize the field of laser displays.

The first semiconductor lasers were invented in the 1960s, and many variants have since been developed. Their numerous advantages – including small size, long lifetimes, and low operating voltages – mean they are routinely employed in applications ranging from optical communications and interconnects to biomedical imaging and optical displays.

 

Laser-driven implosion could produce megatesla magnetic fields

20 Aug 2025 Isabelle Dumé

High field: Sawtooth-like inner blades on the cylindrical target induce off-axis charged flows under ultra-intense laser irradiation, driving strong loop currents and generating sub-megatesla magnetic fields. (Courtesy: Masakatsu Murakami)

Magnetic fields so strong that they are typically only observed in astrophysical jets and highly magnetized neutron stars could be created in the laboratory, say physicists at the University of Osaka, Japan. Their proposed approach relies on directing extremely short, intense laser pulses into a hollow tube housing sawtooth-like inner blades. The fields created in this improved version of the established “microtube implosion” technique can be used to simulate effects that occur in various high-energy-density processes, including nonlinear quantum phenomena and laser fusion, as well as astrophysical systems.

 

High-speed 3D microscope improves live imaging of fast biological processes

12 Sep 2025

Multifocus microscope A new microscope combines diffractive optics with 25 tiny cameras to simultaneously image at multiple depths. (Courtesy: Eduardo Hirata Miyasaki)

A new high-speed, multifocus microscope could facilitate discoveries in developmental biology and neuroscience, thanks to its ability to image rapid biological processes over the entire volume of tiny living organisms in real-time.

The pictures from many 3D microscopes are obtained sequentially by scanning through different depths, making them too slow for accurate live imaging of fast-moving natural functions in individual cells and microscopic animals. Even current multifocus microscopes that capture 3D images simultaneously have either relatively poor image resolution or can only image to shallow depths.

 

Harnessing quantum duality for object imaging

23 Sep 2025 Sophia Walls 

Duality ellipse The amount of coherence is linked to the amount of waveness and particleness of a quantum system through an duality ellipse relation. The coherence, modulated by the object shape, can be found by measuring the waveness and particleness of the photon. The more coherence is present at a specified location in the object, the more circular the duality ellipse relation. Finding the eccentricity of the duality ellipse relation at different points in the object then generates an object image. (Courtesy: Xiaofeng Qian)

The theory of quantum mechanics emerged from the need to explain how an object can behave as both a particle and a wave. Since then, researchers have been attempting to understand and quantify the degree of “waveness” and “particleness” of quantum systems.

Now, Pawan Khatiwada and Xiaofeng Qian, both based at the Stevens Institute of Technology in the US, have published a paper in Physical Review Research unveiling the missing piece of the puzzle in the unique relationship between the wave and particle nature of a quantum object. The key piece is coherence, which describes the statistical phase relationship between the possible states a quantum system can adopt. If the phase relationship is well-defined, that is, stable and consistent, the system is coherent and therefore has the potential to exhibit interference, a characteristic of waves.

 

Researchers perform the first real-time visualization of human embryo implantation

18 Aug 2025 Tami Fre

Videos of human embryo behaviour

The process of cell compaction in the embryo; the embryo invading the implantation platform. Courtesy: Institute for Bioengineering of Catalonia (IBEC)

Human reproduction is an inefficient process, with less than one-third of conceptions leading to live births. Failure of the embryo to implant in the uterus is one of the main causes of miscarriage. Recording this implantation process in vivo in real-time is not yet possible, but a team headed by the Institute for Bioengineering of Catalonia (IBEC) has designed a platform that enables the visualization of human embryo implantation in the laboratory. The researchers hope that quantifying the dynamics of implantation could impact fertility rates and help improve assisted reproductive technologies.

 

Delft Circuits: cryogenic RF cable innovations offer a flexible path to quantum scalability

22 Sep 2025 Sponsored by Delft Circuits

The Dutch manufacturer Delft Circuits is rewriting the rules of I/O cabling technology for full-stack quantum computing systems

Flexible thinking, scalable innovation Delft Circuits has established itself as a one-stop shop for scalable cryogenic I/O assemblies in quantum computing. The company’s Cri/oFlex® cabling platform combines fully integrated filtering with a compact footprint and low heatload. (Courtesy: Delft Circuits)

As manufacturers in the nascent quantum supply chain turn their gaze towards at-scale commercial opportunities in quantum computing, the scenic city of Delft in the Netherlands is emerging as a heavyweight player in quantum science, technology, and innovation. At the heart of this regional quantum ecosystem is Delft Circuits, a Dutch manufacturer of specialist I/O cabling solutions, which is aligning its product development roadmap to deliver a core enabling technology for the scale-up and industrial deployment of next-generation quantum computing, communications, and sensing systems.

 

NASA criticized over its management of $3.3bn Dragonfly mission to Titan

30 Sep 2025

Up in the air Cost overruns and delay have hit the drone-like rotorcraft Dragonfly mission to Titan. (Courtesy: NASA/Johns Hopkins APL/Steve Gribben)

An internal audit has slammed NASA over its handling of the Dragonfly mission to Saturn’s largest moon, Titan. The drone-like rotorcraft, designed to land on and gather samples from Titan, has faced a two-year delay, with costs surging by $1 billion to $3.3 billion. NASA now envisions a launch date of July 2028 with Dragonfly arriving at Titan in 2034.

NASA chose Dragonfly in June 2019 as the next mission under its New Frontiers programme. Managed by the Johns Hopkins University Applied Physics Laboratory, it is a nuclear-powered, car-sized craft with eight rotors.

 

Large-scale commercial applications of quantum computing remain a distant promise, claims report

28 Jun 2025

Growing demand: the Massachusetts Institute of Technology (MIT) Quantum Index Report 2025 finds that jobs in the quantum sector have tripled in the US since 2018. (Courtesy: istock/olemedia)

Quantum technology is rapidly growing, with job demand tripling in the US, alongside venture capital investments bringing in billions of dollars to the field. That is according to the inaugural Massachusetts Institute of Technology (MIT) Quantum Index Report 2025, which finds, however, that large-scale commercial applications for quantum computing still remain “far off”.

Conducted by the Initiative on the Digital Economy (IDE) at MIT, the report is based on data collected from academia, industry, and policy sources. It aims to track, measure, and visualize trends across various areas, including education, funding, research, and development.

 

Quantum gas keeps its cool

19 Sep 2025 Isabelle Dumé

A frozen state Despite being continually kicked and strongly interacting, the atoms no longer absorb energy. The system localizes in momentum space, the momentum distribution literally freezes, a remarkable phenomenon termed many-body dynamical localization. (Courtesy: Universität Innsbruck)

Adding energy to a system usually heats it up, but physicists at the University of Innsbruck in Austria have now discovered a scenario in which this is not the case. Their new platform – a one-dimensional fluid of strongly interacting atoms cooled to just a few nanokelvin above absolute zero and periodically “kicked” using an external force – could be used to study how objects transition from being quantum and ordered to classical and chaotic.

Our everyday world is chaotic, and chaos plays a crucial and often useful role in many areas of science – from nonlinear complex systems in mathematics, physics, and biology to ecology, meteorology, and economics.

 

Physicists extend the wave nature of large objects

19 Sep 2025 Candice Chua 

Quantum delocalization In an artistic rendering, a silica nanoparticle is held by laser light. The uncertainty in the particle’s position spreads and recompresses (blue trajectories), illustrating extended wave behaviour known as quantum delocalization. (Courtesy: Nicola Carlon Zambon)

Can quantum mechanics fully describe macroscopic reality? Everyday objects are typically well-described by classical mechanics, whereas atomic-scale objects are governed by quantum mechanics. Exploring the boundary between the two domains could enable fundamental tests of quantum mechanics and the development of new sensing technologies for gravitational measurements.

Now, a team of researchers at Switzerland’s ETH Zürich and Spain’s Institute of Photonic Sciences in Barcelona has taken an important step towards bridging the two regimes by extending the quantum wave nature of nanoparticles — objects a thousand times larger than atoms.