In this episode of Physics World Stories, theoretical physicist, cosmologist and author Paul Davies discusses his latest book, Quantum 2.0: the Past, Present and Future of Quantum Physics. A Regents Professor at Arizona State University, Davies reflects on how the first quantum revolution transformed our understanding of nature – and what the next one might bring.
He explores how emerging quantum technologies are beginning to merge with artificial intelligence, raising new ethical and philosophical questions. Could quantum AI help tackle climate change or tackle issues like hunger? And how far should we go in outsourcing planetary management to machines that may well prioritize their own survival?
Davies also turns his gaze to the arts, imagining a future where quantum ideas inspire music, theatre and performance. From jazz improvized by quantum algorithms to plays whose endings depend on quantum outcomes, creativity itself could enter a new superposition.
Hosted by Andrew Glester, this episode blends cutting-edge science and imagination in trademark Paul Davies style.
Photodetectors convert light into electrical signals and are essential in technologies ranging from consumer electronics and communications to healthcare. They also play a vital role in scientific research. Researchers are continually working to improve their sensitivity, response speed, spectral range, and design efficiency.
Since the discovery of graphene’s remarkable electrical properties, there has been growing interest in using graphene and other two-dimensional (2D) materials to advance photodetection technologies. When light interacts with these materials, it excites electrons that must travel to a nearby contact electrode to generate an electrical signal. The ease with which this occurs depends on the work functions of the materials involved, specifically, the difference between them, known as the Schottky barrier height. Selecting an optimal combination of 2D material and electrode can minimize this barrier, enhancing the photodetector’s sensitivity and speed. Unfortunately, traditional electrode materials have fixed work functions which are limiting 2D photodetector technology.
PEDOT:PSS is a widely used electrode material in photodetectors due to its low cost, flexibility, and transparency. In this study, the researchers have developed PEDOT:PSS electrodes with tunable work functions ranging from 5.1 to 3.2 eV, making them compatible with a variety of 2D materials and ideal for optimizing device performance in metal-semiconductor-metal architectures. In addition, their thorough investigation demonstrates that the produced photodetectors performed excellently, with a significant forward current flow (rectification ratio ~10⁵), a strong conversion of light to electrical output (responsivity up to 1.8 A/W), and an exceptionally high Ilight/Idark ratio of 10⁸. Furthermore, the detectors were highly sensitive with low noise, had very fast response times (as fast as 3.2 μs), and thanks to the transparency of PEDOT:PSS, showed extended sensitivity into the near-infrared region.
This study demonstrates a tunable, transparent polymer electrode that enhances the performance and versatility of 2D photodetectors, offering a promising path toward flexible, self-powered, and wearable optoelectronic systems, and paving the way for next-generation intelligent interactive technologies.
Quantum Conference Key Agreement (QCKA) is a cryptographic method that allows multiple parties to establish a shared secret key using quantum technology. This key can then be used for secure communication among the parties.
Unlike traditional methods that rely on classical cryptographic techniques, QCKA leverages the principles of quantum mechanics, particularly multipartite entanglement, to ensure security.
A key aspect of QCKA is creating and distributing entangled quantum states among the parties. These entangled states have unique properties that make it impossible for an eavesdropper to intercept the key without being detected.
Researchers measure the efficiency and performance of the key agreement protocol using a metric known as the key rate.
One problem with state-of-the-art QCKA schemes is that this key rate decreases exponentially with the number of users.
Previous solutions to this problem, based on single-photon interference, have come at the cost of requiring global phase locking. This makes them impractical to put in place experimentally.
However, the authors of this new study have been able to circumvent this requirement, by adopting an asynchronous pairing strategy. Put simply, this means that measurements taken by different parties in different places do not need to happen at exactly at the same time.
Their solution effectively removes the need for global phase locking while still maintaining the favourable scaling of the key rate as in other protocols based on single-photon interference.
The new scheme represents an important step towards realising QCKA at long distances by allowing for much more practical experimental configurations.
Schematic representation of quantum group network via circular asynchronous interference (Courtesy: Hua-Lei Yin)
Coherent crystalline interfaces Atomic-resolution image of a superconducting germanium:gallium (Ge:Ga) trilayer with alternating Ge:Ga and silicon layers demonstrating precise control of atomic interfaces. (Courtesy: Salva Salmani-Rezaie)
The ability to induce superconductivity in materials that are inherently semiconducting has been a longstanding research goal. Improving the conductivity of semiconductor materials could help develop quantum technologies with a high speed and energy efficiency, including superconducting quantum bits (qubits) and cryogenic CMOS control circuitry. However, this task has proved challenging in traditional semiconductors – such as silicon or germanium – as it is difficult to maintain the optimal superconductive atomic structure.
In a new study, published in Nature Nanotechnology, researchers have used molecular beam epitaxy (MBE) to grow gallium-hyperdoped germanium films that retain their superconductivity. When asked about the motivation for this latest work, Peter Jacobson from the University of Queensland tells Physics World about his collaboration with Javad Shabani from New York University.
“I had been working on superconducting circuits when I met Javad and discovered the new materials their team was making,” he explains. “We are all trying to understand how to control materials and tune interfaces in ways that could improve quantum devices.”
Germanium: from semiconductor to superconductor
Germanium is a group IV element, so its properties bridge those of both metals and insulators. Superconductivity can be induced in germanium by manipulating its atomic structure to introduce more electrons into the atomic lattice. These extra electrons interact with the germanium lattice to create electron pairs that move without resistance, or in other words, they become superconducting.
Hyperdoping germanium (at concentrations well above the solid solubility limit) with gallium induces a superconducting state. However, this material is traditionally unstable due to the presence of structural defects, dopant clustering and poor thickness control. There have also been many questions raised as to whether these materials are intrinsically superconducting, or whether it is actually gallium clusters and unintended phases that are solely responsible for the superconductivity of gallium-doped germanium.
Considering these issues and looking for a potential new approach, Jacobson notes that X-ray absorption measurements at the Australian Synchrotron were “the first real sign” that Shabani’s team had grown something special. “The gallium signal was exceptionally clean, and early modelling showed that the data lined up almost perfectly with a purely substitutional picture,” he explains. “That was a genuine surprise. Once we confirmed and extended those results, it became clear that we could probe the mechanism of superconductivity in these films without the usual complications from disorder or spurious phases.”
Epitaxial growth improves superconductivity control
In a new approach, Jacobson, Shabani and colleagues used MBE to grow the crystals instead of relying on ion implantation techniques, allowing the germanium to by hyperdoped with gallium. Using MBE forces the gallium atoms to replace germanium atoms within the crystal lattice at levels much higher than previously seen. The process also provided better control over parasitic heating during film growth, allowing the researchers to achieve the structural precision required to understand and control the superconductivity of these germanium:gallium (Ge:Ga) materials, which were found to become superconducting at 3.5 K with a carrier concentration of 4.15 × 1021 holes/cm3. The critical gallium dopant threshold to achieve this was 17.9%.
Using synchrotron-based X-ray absorption, the team found that the gallium dopants were substitutionally incorporated into the germanium lattice and induced a tetragonal distortion to the unit cell. Density functional theory calculations showed that this causes a shift in the Fermi level into the valence band and flattens electronic bands. This suggests that the structural order of gallium in the germanium lattice creates a narrow band that facilitates superconductivity in germanium, and that this superconductivity arises intrinsically in the germanium, rather than being governed by defects and gallium clusters.
The researchers tested trilayer heterostructures – Ge:Ga/Si/Ge:Ga and Ge:Ga/Ge/Ge:Ga – as proof-of-principle designs for vertical Josephson junction device architectures. In the future, they hope to develop these into fully fledged Josephson junction devices.
Commenting on the team’s future plans for this research, Jacobson concludes: “I’m very keen to examine this material with low-temperature scanning tunnelling microscopy (STM) to directly measure the superconducting gap, because STM adds atomic-scale insights that complement our other measurements and will help clarify what sets hyperdoped germanium apart”.
It is book week here at Physics World and over the course of three days we are presenting conversations with the authors of three fascinating and fun books about physics. First up is my Physics World colleague Michael Banks, whose book Physics Around the Clock: Adventures in the Science of Everyday Living starts with your morning coffee and ends with a formula for making your evening television viewing more satisfying.
As well as the rich physics of coffee, we chat about strategies for finding the best parking spot and the efficient boarding of aeroplanes. If you have ever wondered why a runner’s ponytail swings from side-to-side when they reach a certain speed – we have the answer for you.
Other daily mysteries that we explore include how a hard steel razor blade can be dulled by cutting relatively soft hairs and why quasiparticles called “jamitons” are helping physicists understand the spontaneous appearance of traffic jams. And a warning for squeamish listeners, we do talk about the amazing virus-spreading capabilities of a flushing toilet.
This episode is supported by the APS Global Physics Summit, which takes place on 15–20 March, 2026, in Denver, Colorado, and online.
“This is one of the big remaining frontiers in astronomy,” says Phil Bull, a cosmologist at the Jodrell Bank Centre for Astrophysics at the University of Manchester. “It’s quite a pivotal era of cosmic history that, it turns out, we don’t actually understand.”
Bull is referring to the vital but baffling period in the early universe – from 380,000 years to one billion years after the Big Bang – when its structure went from simple to complex. To lift the veil on this epoch, experiments around the world – from Australia to the Arctic – are racing to find a specific but elusive signal from the earliest hydrogen atoms. This signal could confirm or disprove scientists’ theories of how the universe evolved and the physics that governs it.
Hydrogen is the most abundant element in the universe. As neutral hydrogen atoms change states, they can emit or absorb photons. This spectral transition, which can be stimulated by radiation, produces an emission or absorption radio wave signal with a wavelength of 21 cm. To find out what happened during that early universe, astronomers are searching for these 21 cm photons that were emitted by primordial hydrogen atoms.
But despite more teams joining the hunt every year, no-one has yet had a confirmed detection of this radiation. So who will win the race to find this signal and how is the hunt being carried out?
A blank spot
Let’s first return to about 380,000 years after the Big Bang, when the universe had expanded and cooled to below 3000 K. At this stage, neutral atoms, including atomic hydrogen, could form. Thanks to the absence of free electrons, ordinary matter particles could decouple from light, allowing it to travel freely across the universe. This ancient radiation that permeates the sky is known as the cosmic microwave background (CMB).
But after that we don’t know much about what happened for the next few hundred million years. Meanwhile, the oldest known galaxy MoM-z14 – which existed about 280 million years after the Big Bang – was observed in April 2025 by the James Webb Space Telescope. So there is currently a gap of just under 280 million years in our observations of the early universe. “It’s one of the last blank spots,” says Anastasia Fialkov, an astrophysicist at the Institute of Astronomy of the University of Cambridge.
This “blank spot” is a bridge between the early, simple universe and today’s complex structured cosmos. During this early epoch, the universe went from being filled with a thick cloud of neutral hydrogen, to being diversely populated with stars, black holes and everything in between. It covers the end of the cosmic dark ages, the cosmic dawn, and the epoch of reionization – and is arguably one of the most exciting periods in our universe’s evolution.
During the cosmic dark ages, after the CMB flooded the universe, the only “ordinary” matter (made up of protons, neutrons and electrons) was neutral hydrogen (75% by mass) and neutral helium (25%), and there were no stellar structures to provide light. It is thought that gravity then magnified any slight fluctuations in density, causing some of this primordial gas to clump and eventually form the first stars and galaxies – a time called the cosmic dawn. Next came the epoch of reionization, when ultraviolet and X-ray emissions from those first celestial objects heated and ionized the hydrogen atoms, turning the neutral gas into a charged plasma of electrons and protons.
Stellar imprint
The 21 cm signal astronomers are searching for was produced when the spectral transition was excited by collisions in the hydrogen gas during the dark ages and then by the first photons from the first stars during the cosmic dawn. However, the intensity of the 21 cm signal can only be measured against the CMB, which acts as a steady background source of 21 cm photons.
When the hydrogen was colder than the background radiation, there were few collisions, and the atoms would have absorbed slightly more 21 cm photons from the CMB than they emitted themselves. The 21 cm signal would appear as a deficit, or absorption signal, against the CMB. But when the neutral gas was hotter than the CMB, the atoms would emit more photons than they absorbed, causing the 21 cm signal to be seen as a brighter emission against the CMB. These absorption and emission rates depend on the density and temperature of the gas, and the timing and intensity of radiation from the first cosmic sources. Essentially, the 21 cm signal became imprinted with how those early sources transformed the young universe.
One way scientists are trying to observe this imprint is to measure the average – or “global” – signal across the sky, looking at how it shifts from absorption to emission compared to the CMB. Normally, a 21 cm radio wave signal has a frequency of about 1420 MHz. But this ancient signal, according to theory, has been emitted and absorbed at different intensities throughout this cosmic “blank spot”, depending on the universe’s evolutionary processes at the time. The expanding universe has also stretched and distorted the signal as it travelled to Earth. Theories predict that it would now be in the 1 to 200 MHz frequency range – with lower frequencies corresponding to older eras – and would have a wavelength of metres rather than centimetres.
Importantly, the shape of the global 21 cm signal over time could confirm the lambda-cold dark matter (ΛCDM) model, which is the most widely accepted theory of the cosmos; or it could upend it. Many astronomers have dedicated their careers to finding this radiation, but it is challenging for a number of reasons.
Unfortunately, the signal is incredibly faint. Its brightness temperature, which is measured as the change in the CMB’s black body temperature (2.7 K), will only be in the region of 0.1 K.
a A simulation of the sky-averaged (global) signal as a function of time (horizontal) and space (vertical). b A typical model of the global 21 cm line with the main cosmic events highlighted. Each experiment searching for the global 21 cm signal focuses on a particular frequency band. For example, the Radio Experiment for the Analysis of Cosmic Hydrogen (REACH) is looking at the 50–170 MHz range (blue).
There is also no single source of this emission, so, like the CMB, it permeates the universe. “If it was the only signal in the sky, we would have found it by now,” says Eloy de Lera Acedo, head of Cavendish Radio Astronomy and Cosmology at the University of Cambridge. But the universe is full of contamination, with the Milky Way being a major culprit. Scientists are searching for 0.1 K in an environment “that’s a million times brighter”, he explains.
And even before this signal reaches the radio-noisy Earth, it has to travel through the atmosphere, which further distorts and contaminates it. “It’s a very difficult measurement,” says Rigel Cappallo, a research scientist at the MIT Haystack Observatory. “It takes a really, really well calibrated instrument that you understand really well, plus really good modelling.”
The EDGES instrument is a dipole antenna, which resembles a ping-pong table with a gap in the middle (see photo at top of article for the 2024 set-up). It is mounted on a large metal groundsheet, which is about 30 × 30 m. Its ground-breaking observation was made at a remote site in western Australia, far from radio frequency interference.
But in the intervening seven years, no-one else has been able to replicate the EDGES results.
The spectrum dip that EDGES detected was very different from what theorists had expected. “There is a whole family of models that are predicted by the different cosmological scenarios,” explains Ravi Subrahmanyan, a research scientist at Australia’s national science agency CSIRO. “When we take measurements, we compare them with the models, so that we can rule those models in or out.”
In general, the current models predict a very specific envelope of signal possibilities (see figure 1). First, they anticipate an absorption dip in brightness temperature of around 0.1 to 0.2 K, caused by the temperature difference between the cold hydrogen gas (in an expanding universe) and the warmer CMB. Then, a speedy rise and photon emission is predicted as the gas starts to warm when the first stars form, and the signal should spike dramatically when the first X-ray binary stars fire up and heat up the surrounding gas. The signal is then expected to fade as the epoch of reionization begins, because ionized particles cannot undergo the spectral transition. With models, scientists theorize when this happened, how many stars there were, and how the cosmos unfurled.
2 Weird signal
(Courtesy: SARAS Team)
The 21 cm signals predicted by current cosmology models (coloured lines) and the detection by the EDGES experiment (dashed black line).
“It’s just one line, but it packs in so many physical phenomena,” says Fialkov, referring to the shape of the 21 cm signal’s brightness temperature over time. The timing of the dip, its gradient and magnitude all represent different milestones in cosmic history, which affect how it evolved.
The EDGES team, however, reported a dip of more than double the predicted size, at about 78 MHz (see figure 2). While the frequency was consistent with predictions, the very wide and deep dip of the signal took the community by surprise.
“It would be a revolution in physics, because that signal will call for very, very exotic physics to explain it,” says de Lera Acedo. “Of course, the first thing we need to do is to make sure that that is actually the signal.”
A spanner in the works
The EDGES claim has galvanized the cosmology community. “It set a cat among the pigeons,” says Bull. “People realized that, actually, there’s some very exciting science to be done here.” Some groups are trying to replicate the EDGES observation, while others are trying new approaches to detect the signal that the models promise.
The Radio Experiment for the Analysis of Cosmic Hydrogen (REACH) – a collaboration between the University of Cambridge and Stellenbosch University in South Africa – focuses on the 50–170 MHz frequency range. Sitting on the dry and empty plains of South Africa’s Northern Cape, it is targeting the EDGES observation (Nature Astronomy 6 984).
The race to replicate REACH went online in the Karoo region of South Africa in December 2023. (Courtesy: Saurabh Pegwal, REACH collaboration)
In this radio-quiet environment, REACH has set up two antennas: one looks like EDGES’ dipole ping-pong table, while the other is a spiral cone. They sit on top of a giant metallic mesh – the ground plate – in the shape of a many-pointed star, which aims to minimize reflections from the ground.
Hunting for this signal “requires precision cosmology and engineering”, says de Lera Acedo, the principal investigator on REACH. Reflections from the ground or mesh, calibration errors, and signals from the soil, are the kryptonite of cosmic dawn measurements. “You need to reduce your systemic noise, do better analysis, better calibration, better cleaning [to remove other sources from observations],” he says.
Desert, water, snow
Another radio telescope, dubbed the Shaped Antenna measurement of the background Radio Spectrum (SARAS) – which was established in the late 2000s by the Raman Research Institute (RRI) in Bengaluru, India – has undergone a number of transformations to reduce noise and limit other sources of radiation. Over time, it has morphed from a dipole on the ground to a metallic cone floating on a raft. It is looking at 40 to 200 MHz (Exp. Astron.51 193).
After the EDGES claim, SARAS pivoted its attention to verifying the detection, explains Saurabh Singh, a research scientist at the RRI. “Initially, we were not able to get down to the required sensitivity to be able to say anything about their detection,” he explains. “That’s why we started floating our radiometer on water.” Buoying the experiment reduces ground contamination and creates a more predictable surface to include in calculations.
Floating telescope Evolution of the SARAS experiment and sites up to 2020. The third edition of the telescope, SARAS 3, was deployed on lakes to further reduce radio interference. (Courtesy: SARAS Team)
Using data from their floating radiometer, in 2022 Singh and colleagues disfavoured EDGES’ claim (Nature Astronomy 6 607), but for many groups the detection still remains a target for observations.
While SARAS has yet to detect a cosmic-dawn signal of its own, Singh says that non-detection is also an important element of finding the global 21 cm signal. “Non-detection gives us an opportunity to rule out a lot of these models, and that has helped us to reject a lot of properties of these stars and galaxies,” he says.
Raul Monsalve Jara – a cosmologist at the University of California, Berkeley – has been part of the EDGES collaboration since 2012, but decided to also explore other ways to detect the signal. “My view is that we need several experiments doing different things and taking different approaches,” he says.
The Mapper of the IGM Spin Temperature (MIST) experiment, of which Monsalve is co-principal investigator, is a collaboration between Chilean, Canadian, Australian and American researchers. These instruments are looking at 25 to 105 MHz (MNRAS 530 4125). “Our approach was to simplify the instrument, get rid of the metal ground plate, and to take small, portable instruments to remote locations,” he explains. These locations have to fulfil very specific requirements – everything around the instrument, from mountains to the soil, can impact the instrument’s performance. “If the soil itself is irregular, that will be very difficult to characterize and its impact will be difficult to remove [from observations],” Monsalve says.
Physics on the move MIST conducts measurements of the sky-averaged radio spectrum at frequencies below 200 MHz. Its monopole and dipole variants are highly portable and have been deployed in some of the most remote sites on Earth, including the Arctic (top) and the Nevada desert (bottom). (Courtesy: Raul Monsalve)
So far, the MIST instrument, which is also a dipole ping-pong table, has visited a desert in California, another in Nevada, and even the Arctic. Each time, the researchers spend a few weeks at the site collecting data, and it is portable and easy to set up, Monsalve explains. The team is planning more observations in Chile. “If you suspect that your environment could be doing something to your measurements, then you need to be able to move around,” continues Monsalve. “And we are contributing to the field by doing that.”
Aaron Parsons, also from the University of California, Berkeley, decided that the best way to detect this elusive signal would be to try and eliminate the ground entirely – by suspending a rotating antenna over a giant canyon with 100 m empty space in every direction.
His Electromagnetically Isolated Global Signal Estimation Platform (EIGSEP) includes an antenna hanging four storeys above the ground, attached to Kevlar cable strung across a canyon in Utah. It’s observing at 50 to 250 MHz. “It continuously rotates around and twists every which way,” Parsons explains. Hopefully, that will allow them to calibrate the instrument very accurately. Two antennas on the ground cross-correlate observations. EIGSEP began making observations last year.
More experiments are expected to come online in the next year. The Remote HI eNvironment Observer (RHINO), an initiative of the University of Manchester, will have a horn-shaped receiver made of a metal mesh that is usually used to construct skyscrapers. Horn shapes are particularly good for calibration, allowing for very precise measurements. The most famous horn-shaped antenna is Bell Laboratories’ Holmdel Horn Antenna in the US, with which two scientists accidentally discovered the CMB in 1965.
Initially, RHINO will be based at Jodrell Bank Observatory in the UK, but like other experiments, it could travel to other remote locations to hunt for the 21 cm signal.
Similarly, Subrahmanyan – who established the SARAS experiment in India and is now with CSIRO in Australia – is working to design a new radiometer from scratch. The instrument, which will focus on 40–160 MHz, is called Global Imprints from Nascent Atoms to Now (GINAN). He says that it will feature a recently patented self-calibrating antenna. “It gives a much more authentic measurement of the sky signal as measured by the antenna,” he explains.
In the meanwhile, the EDGES collaboration has not been idle. MIT Haystack Observatory’s Cappallo project manages EDGES, which is currently in its third iteration. It is still the size of a desk, but its top now looks like a box, with closed sides and its electronics tucked inside, and an even larger metal ground plate. The team has now made observations from islands in the Canadian archipelago and in Alaska’s Aleutian island chain (see photo at top of article).
“The 2018 EDGES result is not going to be accepted by the community until somebody completely independently verifies it,” Cappallo explains. “But just for our own sanity and also to try to improve on what we can do, we want to see it from as many places as possible and as many conditions as possible.” The EDGES team has replicated its results using the same data analysis pipeline, but no-one else has been able to reproduce the unusual signal.
All the astronomers interviewed welcomed the introduction of new experiments. “I think it’s good to have a rich field of people trying to do this experiment because nobody is going to trust any one measurement,” says Parsons. “We need to build consensus here.”
Taking off
Some astronomers have decided to avoid the struggles of trying to detect the global 21 cm signal from Earth – instead, they have their sights set on the Moon. Earth’s atmosphere is one of the reasons why the 21 cm signal is so difficult to measure. The ionosphere, a charged region of the atmosphere, distorts and contaminates this incredibly faint signal. On the far side of the Moon, any antenna would also be shielded from the cacophony of radio-frequency interference from Earth.
“This is why some experiments are going to the Moon,” says Parsons, adding that he is involved in NASA’s LuSEE-Night experiment. LuSEE-Night, or the Lunar Surface Electromagnetics Experiment, aims to land a low-frequency experiment on the Moon next year.
In July, at the National Astronomical Meeting in Durham, the University of Cambridge’s de Lera Acedo presented a proposal to put a miniature radiometer into lunar orbit. Dubbed “Cosmocube”, it will be a nanosatellite that will orbit the Moon searching for this 21 cm signal.
Taking the hunt to space Provisional illustration of the CosmoCube with its antenna deployed for the 21 cm signal detection, i.e. in operational mode in space. This nanosatellite would travel to the far side of the Moon to get away from the Earth’s ionosphere, which introduces substantial distortions and absorption effects to any radio signal detection. (CC BY 4.0 Artuc and de Lera Acedo 2024 RAS Techniques and Instruments4 rzae061)
“It is just in the making,” says de Lera Acedo, adding that it will not be in operation for at least a decade. “But it is the next step.”
In the meanwhile, groups here on Earth are in a race to detect this elusive signal. The instruments are getting more sensitive, the modelling is improving, and the unknowns are reducing. “If we do the experiments right, we will find the signal,” Monsalve believes. The big question is who, of the many groups with their hat in the ring, is doing the experiment “right”.
Team effort Based at the University of Innsbruck, Ben Lanyon’s group has created a novel qubit register by trapping ten ions. (Courtesy: Victor Krutyanskiy/University of Innsbruck)
Researchers in Austria have entangled matter-based qubits with photonic qubits in a ten-ion system. The technique is scalable to larger ion-qubit registers, paving the way for the creation of larger and more complex quantum networks.
Ions in motion Each ion (large object) is moved one at a time into the “sweet spot” of the optical cavity. Once there, a laser beam drives the emission of a single photon (small object), entangled with the ion. The colours indicate ion–photon entanglement. (Courtesy: Universität Innsbruck/Harald Ritsch)
Quantum networks consist of matter-based nodes that store and process quantum information and are linked through photons (quanta of light). Already, Ben Lanyon’s group at the University of Innsbruck has made advances in this direction by entangling two ions in different systems. Now, in a new paper published in Physical Review Letters , they describe how they have developed and demonstrated a new method to entangle a string of ten ions with photons. In the future, this approach could enable the entanglement of sets of ions in different locations through light, rather than one ion at a time.
To achieve this, Lanyon and colleagues trapped a chain of 10 calcium ions in a linear trap in an optical cavity. By changing the trapping voltages in the trap, each ion was moved, one-by-one, into the cavity. Once inside, the ion was placed in the “sweet spot”, where the ion’s interaction with the cavity is the strongest. There, the ion emitted a single photon when exposed to a 393 nm Raman laser beam. This beam was tightly focused on one ion, guaranteeing that the emitted photon – collected in a single-mode optical fibre – comes out from one ion at a time. This process was carried out ten times, one per ion, to obtain a train of ten photons.
By using quantum state tomography, the researchers reconstructed the density matrix, which describes the correlation between the states of ions (i) and photons (j). To do so, they measure every ion and photon state in three different basis, resulting in nine Pauli-basis configurations of quantum measurements. From the density matrix, the concurrence (a measure of entanglement) between the ion (i) and photon (j) was found to be positive only when i = j, and equal to zero otherwise. This implies that the ion is uniquely entangled with the photon it produced, and unentangled with the photon produced by other ions.
From the density matrix, they also calculate the fidelity with the Bell state (a state of maximum entanglement), yielding an average 92%. As Marco Canteri points out, “this fidelity characterizes the quality of entanglement between the ion-photon pair for i=j”.
This work developed and demonstrated a technique whereby matter-based qubits and photonic qubits can be entangled, one at a time, in ion strings. Now, the group aims to “demonstrate universal quantum logic within the photon-interfaced 10-ion register and, building up towards entangling two remote 10-ion processors through the exchange of photons between them,” explains team member Victor Krutyanskiy. If this method effectively scales to larger systems, more complex quantum networks could be built. This would lead to applications in quantum communication and quantum sensing.
Measuring blood flow to the brain is essential for diagnosing and developing treatments for neurological disorders such as stroke, vascular dementia or traumatic brain injury. Performing this measurement non-invasively is challenging, however, and achieved predominantly using costly MRI and nuclear medicine imaging techniques.
Emerging as an alternative, modalities based on optical transcranial measurement are cost-effective and easy to use. In particular, speckle contrast optical spectroscopy (SCOS) – an offshoot of laser speckle contrast imaging, which uses laser light speckles to visualize blood vessels – can measure cerebral blood flow (CBF) with high temporal resolution, typically above 30 Hz, and cerebral blood volume (CBV) through optical signal attenuation.
Researchers at the California Institute of Technology (Caltech) and the Keck School of Medicine’s USC Neurorestoration Center have designed a lightweight SCOS system that accurately measures blood flow to the brain, distinguishing it from blood flow to the scalp. Co-senior author Charles Liu of the Keck School of Medicine and team describe the system and their initial experimentation with it in APL Bioengineering.
Seven simultaneous measurements Detection channels with differing source-to-detector distances monitor blood dynamics in the scalp, skull and brain layers. (Courtesy: CC BY 4.0/APL Bioeng. 10.1063/5.0263953)
The SCOS system consists of a 3D-printed head mount designed for secure placement over the temple region. It holds a single 830 nm laser illumination fibre and seven detector fibres positioned at seven different source-to-detector (S–D) distances (between 0.6 and 2.6 cm) to simultaneously capture blood flow dynamics across layers of the scalp, skull and brain. Fibres with shorter S–D distances acquire shallower optical data from the scalp, while those with greater distances obtain deeper and broader data. The seven channels are synchronized to exhibit identical oscillation frequencies corresponding to the heart rate and cardiac cycle.
When the SCOS system directs the laser light onto a sample, multiple random scattering events occur before the light exits the sample, creating speckles. These speckles, which materialize on rapid timescales, are the result of interference of light travelling along different trajectories. Movement within the sample (of red blood cells, for instance) causes dynamic changes in the speckle field. These changes are captured by a multi-million-pixel camera with a frame rate above 30 frames/s and quantified by calculating the speckle contrast value for each image.
Human testing
The researchers used the SCOS system to perform CBF and CBV measurements in 20 healthy volunteers. To isolate and obtain surface blood dynamics from brain signals, the researchers gently pressed on the superficial temporal artery (a terminal branch of the external carotid artery that supplies blood to the face and scalp) to block blood flow to the scalp.
In tests on the volunteers, when temporal artery blood flow was occluded for 8 s, scalp-sensitive channels exhibited significant decreases in blood flow while brain-sensitive channels showed minimal change, enabling signals from the internal carotid artery that supplies blood to the brain to be clearly distinguished. Additionally, the team found that positioning the detector 2.3 cm or more away from the source allowed for optimal brain blood flow measurement while minimizing interference from the scalp.
“Combined with the simultaneous measurements at seven S–D separations, this approach enables the first quantitative experimental assessment of how scalp and brain signal contributions vary with depth in SCOS-based CBF measurements and, more broadly, in optical measurements,” they write. “This work also provides crucial insights into the optimal device S–D distance configuration for preferentially probing brain signal over scalp signal, with a practical and subject-friendly alternative for evaluating depth sensitivity, and complements more advanced, hardware-intensive strategies such as time-domain gating.”
The researchers are now working to improve the signal-to-noise ratio of the system. They plan to introduce a compact, portable laser and develop a custom-designed extended camera that spans over 3 cm in one dimension, enabling simultaneous and continuous measurement of blood dynamics across S–D distances from 0.5 to 3.5 cm. These design advancements will enhance spatial resolution and enable deeper brain measurements.
“This crucial step will help transition the system into a compact, wearable form suitable for clinical use,” comments Liu. “Importantly, the measurements described in this publication were achieved in human subjects in a very similar manner to how the final device will be used, greatly reducing barriers to clinical application.”
“I believe this study will advance the engineering of SCOS systems and bring us closer to a wearable, clinically practical device for monitoring brain blood flow,” adds co-author Simon Mahler, now at Stevens Institute of Technology. “I am particularly excited about the next stage of this project: developing a wearable SCOS system that can simultaneously measure both scalp and brain blood flow, which will unlock many fascinating new experiments.”
As we enter the final stretch of the International Year of Quantum Science and Technology (IYQ), I hope you’ve enjoyed our extensive quantum coverage over the last 12 months. We’ve tackled the history of the subject, explored some of the unexplained mysteries that still make quantum physics so exciting, and examined many of the commercial applications of quantum technology. You can find most of our coverage collected into two free-to-read digital Quantum Briefings, available here and here on the Physics World website.
Over the last 100 years since Werner Heisenberg first developed quantum mechanics on the island of Helgoland in June 1925, quantum mechanics has proved to be an incredibly powerful, successful and logically consistent theory. Our understanding of the subatomic world is no longer the “lamentable hodgepodge of hypotheses, principles, theorems and computational recipes”, as the Israeli physicist and philosopher Max Jammer memorably once described it.
In fact, quantum mechanics has not just transformed our understanding of the natural world; it has immense practical ramifications too, with so-called “quantum 1.0” technologies – lasers, semiconductors and electronics – underpinning our modern world. But as was clear from the UK National Quantum Technologies Showcase in London last week, organized by Innovate UK, the “quantum 2.0” revolution is now in full swing.
The day-long event, which is now in its 10th year, featured over 100 exhibitors, including many companies that are already using fundamental quantum concepts such as entanglement and superposition to support the burgeoning fields of quantum computing, quantum sensing and quantum communication. The show was attended by more than 3000 delegates, some of whom almost had to be ushered out of the door at closing time, so keen were they to keep talking.
The first day, focusing on the foundations of quantum mechanics, ended with a panel discussion – chaired by my colleague Tushna Commissariat and Daisy Shearer from the UK’s National Quantum Computing Centre – with physicists Fay Dowker (Imperial College), Jim Al-Khalili (University of Surrey) and Peter Knight. They talked about whether the quantum wavefunction provides a complete description of physical reality, prompting much discussion with the audience. As Al-Khalili wryly noted, if entanglement has emerged as the fundamental feature of quantum reality, then “decoherence is her annoying and ever-present little brother”.
Knight, meanwhile, who is a powerful figure in quantum-policy circles, went as far as to say that the limit of decoherence – and indeed the boundary between the classical and quantum worlds – is not a fixed and yet-to-be revealed point. Instead, he mused, it will be determined by how much money and ingenuity and time physicists have at their disposal.
On the second day of the IOP conference at the RI, I chaired a discussion that brought together four future leaders of the subject: Mehul Malik (Heriot-Watt University) and Sarah Malik (University College London) along with industry insiders Nicole Gillett (Riverlane) and Muhammad Hamza Waseem (Quantinuum).
As well as outlining the technical challenges in their fields, the speakers all stressed the importance of developing a “skills pipeline” so that the quantum sector has enough talented people to meet its needs. Also vital will be the need to communicate the mysteries and potential of quantum technology – not just to the public but to industrialists, government officials and venture capitalists. By many measures, the UK is at the forefront of quantum tech – and it is a lead it should not let slip.
Clear talker Jim Al-Khalili giving his Friday night discourse at the Royal Institution on 7 November 2025. (Courtesy: Matin Durrani)
The week ended with Al-Khalili giving a public lecture, also at the Royal Institution, entitled “A new quantum world: ‘spooky’ physics to tech revolution”. It formed part of the RI’s famous Friday night “discourses”, which this year celebrate their 200th anniversary. Al-Khalili, who also presents A Life Scientific on BBC Radio 4, is now the only person ever to have given three RI discourses.
After the lecture, which was sold out, he took part in a panel discussion with Knight and Elizabeth Cunningham, a former vice-president for membership at the IOP. Al-Khalili was later presented with a special bottle of “Glentanglement” whisky made by Glasgow-based Fraunhofer UK for the Scottish Quantum Technology cluster.
Significant progress towards answering one of the Clay Mathematics Institute’s seven Millennium Prize Problems has been achieved using deep learning. The challenge is to establish whether or not the Navier-Stokes equation of fluid dynamics develops singularities. The work was done by researchers in the US and UK – including some at Google Deepmind. Some team members had already shown that simplified versions of the equation could develop stable singularities, which reliably form. In the new work, the researchers found unstable singularities, which form only under very specific conditions.
The Navier–Stokes partial differential equation was developed in the early 19th century by Claude-Louis Navier and George Stokes. It has proved its worth for modelling incompressible fluids in scenarios including water flow in pipes; airflow around aeroplanes; blood moving in veins; and magnetohydrodynamics in plasmas.
No-one has yet proved, however, whether smooth, non-singular solutions to the equation always exist in three dimensions. “In the real world, there is no singularity…there is no energy going to infinity,” says fluid dynamics expert Pedram Hassanzadeh of the University of Chicago. “So if you have an equation that has a singularity, it tells you that there is some physics that is missing.” In 2000, the Clay Mathematics Institute in Denver, Colorado listed this proof as one of seven key unsolved problems in mathematics, offering a reward of $1,000,000 for an answer.
Computational approaches
Researchers have traditionally tackled the problem analytically, but in recent decades high-level computational simulations have been used to assist in the search. In a 2023 paper, mathematician Tristan Buckmaster of New York University and colleagues used a special type of machine learning algorithm called a physics-informed neural network to address the question.
“The main difference is…you represent [the solution] in a highly non-linear way in terms of a neural network,” explains Buckmaster. This allows it to occupy a lower-dimensional space with fewer free parameters, and therefore to be optimized more efficiently. Using this approach, the researchers successfully obtained the first stable singularity in the Euler equation. This is an analogy to the Navier-Stokes equation that does not include viscosity.
A stable singularity will still occur if the initial conditions of the fluid are changed slightly – although the time taken for them to form may be altered. An unstable singularity, however, may never occur if the initial conditions are perturbed even infinitesimally. Some researchers have hypothesized that any singularities in the Navier-Stokes equation must be unstable, but finding unstable singularities in a computer model is extraordinarily difficult.
“Before our result there hadn’t been an unstable singularity for an incompressible fluid equation found numerically,” says geophysicist Ching-Yao Lai of California’s Stanford University.
Physics-informed neural network
In the new work the authors of the original paper and others teamed up with researchers at Google Deepmind to search for unstable singularities in a bounded 3D version of the Euler equation using a physics-informed neural network. “Unlike conventional neural networks that learn from vast datasets, we trained our models to match equations that model the laws of physics,” writes Yongji Wang of New York University and Stanford on Deepmind’s blog. “The network’s output is constantly checked against what the physical equations expect, and it learns by minimizing its ‘residual’, the amount by which its solution fails to satisfy the equations.”
After an exhaustive search at a precision that is orders of magnitude higher than a normal deep learning protocol, the researchers discovered new families of singularities in the 3D Euler equation. They also found singularities in the related incompressible porous media equation used to model fluid flows in soil or rock; and in the Boussinesq equation that models atmospheric flows.
The researchers also gleaned insights into the strength of the singularities. This could be important as stronger singularities might be less readily smoothed out by viscosity when moving from the Euler equation to the Navier-Stokes equation. The researchers are now seeking to model more open systems to study the problem in a more realistic space.
Hassanzadeh, who was not involved in the work, believes that it is significant – although the results are not unexpected. “If the Euler equation tells you that ‘Hey, there is a singularity,’ it just tells you that there is physics that is missing and that physics becomes very important around that singularity,” he explains. “In the case of Euler we know that you get the singularity because, at the very smallest scales, the effects of viscosity become important…Finding a singularity in the Euler equation is a big achievement, but it doesn’t answer the big question of whether Navier-Stokes is a representation of the real world, because for us Navier-Stokes represents everything.”
He says the extension to studying the full Navier-Stokes equation will be challenging but that “they are working with the best AI people in the world at Deepmind,” and concludes “I’m sure it’s something they’re thinking about”.
The work is available on the arXiv pre-print server.
