Physicists at the Georgia Institute of Technology, US have introduced a novel way to generate entanglement between photons – an essential step in building scalable quantum computers that use photons as quantum bits (qubits). Their research, published in Physical Review Letters, leverages a mathematical concept called non-Abelian quantum holonomy to entangle photons in a deterministic way without relying on strong nonlinear interactions or irrevocably probabilistic quantum measurements.

Entanglement is fundamental to quantum information science, distinguishing quantum mechanics from classical theories and serving as a pivotal resource for quantum technologies. Existing methods for entangling photons often suffer from inefficiencies, however, requiring additional particles such as atoms or quantum dots and additional steps such as post-selection that eliminate all outcomes of a quantum measurement in which a desired event does not occur.

While post-selection is a common strategy for entangling non-interacting quantum particles, protocols for entangled state preparation that use post-selection are non-deterministic. This is because they rely upon making measurements, and the result of obtaining a certain state of the system after a measurement is associated with a probability, making it inevitably non-deterministic.

Non-Abelian holonomy

The new approach provides a direct and deterministic alternative. In it, the entangled photons occupy distinguishable spatial modes of optical waveguides, making entanglement more practical for real-world applications. To develop it, Georgia Tech’s Aniruddha Bhattacharya and Chandra Raman took inspiration from a 2023 experiment by physicists at Universität Rostock, Germany, that involved coupled photonic waveguides on a fused silica chip. Both works exploit a property known as non-Abelian holonomy, which is essentially a geometric effect that occurs when a quantum system evolves along a closed path in parameter space (more precisely, it is a matrix-valued generalization of a pure geometric phase).

In Bhattacharya and Raman’s approach, photons evolve in a waveguide system where their quantum states undergo a controlled transformation that leads to entanglement. The pair derive an analytical expression for the holonomic transformation matrix, showing that the entangling operation corresponds to a unitary rotation within an effective pseudo-angular momentum space. Because this process is fully unitary, it does not require measurement or external interventions, making it inherently robust.

Beyond the Hong-Ou-Mandel effect

A classic example of photon entanglement is the Hong–Ou–Mandel (HOM) effect, where two identical photons interfere at a beam splitter, leading to quantum correlations between them. The new method extends such interference effects beyond two photons, allowing deterministic entanglement of multiple photons and even higher-dimensional quantum states known as qudits (d-level systems) instead of qubits (two-level systems). This could significantly improve the efficiency of quantum information protocols.

Because state preparation and measurement are relatively straightforward in this approach, Bhattacharya and Raman say it is well-suited for quantum computing. Since the method relies on geometric principles, it naturally protects against certain types of noise, making it more robust than traditional approaches. They add that their technique could even be used to construct an almost universal set of near-deterministic entangling gates for quantum computation with light. “This innovative use of non-Abelian holonomy could shift the way we think about photonic quantum computing,” they say.

By providing a deterministic and scalable entanglement mechanism, Bhattacharya and Raman add that their method opens the door to more efficient and reliable photonic quantum technologies. The next steps will be to validate the approach experimentally and explore practical implementations in quantum communication and computation. Further in the future, it will be necessary to find ways of integrating this approach with other quantum systems, such as matter-based qubits, to enable large-scale quantum networks.

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Water molecules on the surface of an electrode flip just before they give up electrons to form oxygen – a feat of nanoscale gymnastics that explains why the reaction takes more energy than it theoretically should. After observing this flipping in individual water molecules for the first time, scientists at Northwestern University in the US say that the next step is to find ways of controlling it. Doing so could improve the efficiency of the reaction, making it easier to produce both oxygen and hydrogen fuel from water.

The water splitting process takes place in an electrochemical cell containing water and a metallic electrode. When a voltage is applied to the electrode, the water splits into oxygen and hydrogen via two separate half-reactions.

The problem is that the half-reaction that produces oxygen, known as the oxygen evolution reaction (OER), is difficult and inefficient and takes more energy than predicted by theory. “It should require 1.23 V,” says Franz Geiger, the Northwestern physical chemist who led the new study, “but in reality, it requires more like 1.5 or 1.8 V.” This extra energy cost is one of the reasons why water splitting has not been implemented on a large scale, he explains.

Determining how water molecules arrange themselves

In the new work, Geiger and colleagues wanted to test whether the orientation of the water’s oxygen atoms affects the kinetics of the OER. To do this, they directed an 80-femtosecond pulse of infrared (1034 nm) laser light onto the surface of the electrode, which was in this case made of nickel. They then measured the intensity of the reflected light at half the incident wavelength.

This method, which is known as second harmonic and vibrational sum-frequency generation spectroscopy, revealed that the water molecules’ alignment on the surface of the electrode depended on the applied voltage. By analysing the amplitude and phase of the signal photons as this voltage was cycled, the researchers were able to pin down how the water molecules arranged themselves.

They found that before the voltage was applied, the water molecules were randomly oriented. At a specific applied voltage, however, they began to reorient. “We also detected water dipole flipping just before cleavage and electron transfer,” Geiger adds. “This allowed us to distinguish flipping from subsequent reaction steps.”

An unexplored idea

The researchers’ explanation for this flipping is that at high pH levels, the surface of the electrode is negatively charged due to the presence of nickel hydroxide groups that have lost their protons. The water molecules therefore align with their most positively charged ends facing the electrode. However, this means that the ends containing the electrons needed for the OER (which reside in the oxygen atoms) are pointing away from the electrode. “We hypothesized that water molecules must flip to align their oxygen atoms with electrochemically active nickel oxo species at high applied potential,” Geiger says.

This idea had not been explored until now, he says, because water absorbs strongly in the infrared range, making it appear opaque at the relevant frequencies. The electrodes typically employed are also too thick for infrared light to pass through. “We overcame these challenges by making the electrode thin enough for near-infrared transmission and by using wavelengths where water’s absorbance is low (the so-called ‘water window’),” he says.

Other challenges for the team included designing a spectrometer that could measure the second harmonic generation amplitude and phase and developing an optical model to extract the number of net-aligned water molecules and their flipping energy. “The full process – from concept to publication – took three years,” Geiger tells Physics World.

The team’s findings, which are detailed in Science Advances, suggest that controlling the orientation of water at the interface with the electrode could improve OER catalyst performance. For example, surfaces engineered to pre-align water molecules might lower the kinetic barriers to water splitting. “The results could also refine electrochemical models by incorporating structural water energetics,” Geiger says. “And beyond the OER, water alignment may also influence other reactions such as the hydrogen evolution reaction and CO₂ reduction to liquid fuels, potentially impacting multiple energy-related technologies.”

The researchers are now exploring alternative electrode materials, including NiFe and multi-element catalysts. Some of the latter can outperform iridium, which has traditionally been the best-performing electrocatalyst, but is very rare (it comes from meteorites) and therefore expensive. “We have also shown in a related publication (in press) that water flipping occurs on an earth-abundant semiconductor, suggesting broader applicability beyond metals,” Geiger reveals.

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Complex hydrogel structures created using 3D printing are increasingly employed in fields including flexible electronics, soft robotics and regenerative medicine. Currently, however, such hydrogels are often soft and fragile, limiting their practical utility. Researchers at Zhejiang University in China have now fabricated 3D-printed hydrogels that can be easily, and repeatably, switched between soft and hard states, enabling novel applications such as smart medical bandages or information encryption.

“Our primary motivation was to overcome the inherent limitations of 3D-printed hydrogels, particularly their soft, weak and fragile mechanical properties, to broaden their application potential,” says co-senior author Yong He.

The research team created the hard/soft switchable composite by infusing supersaturated salt solution (sodium acetate, NAAC) into 3D-printed polyacrylamide (PAAM)-based hydrogel structures. The hardness switching is enabled by the liquid/solid transition of the salt solution within the hydrogel.

Initially, the salt molecules are arranged randomly within the hydrogel and the PAAM/NAAC composite is soft and flexible. The energy barrier separating the soft and hard states prevents spontaneous crystallization, but can be overcome by artificially seeding a crystal nucleus (via exposure to a salt crystal or contact with a sharp object). This seed promotes a phase transition to a hard state, with numerous rigid, rod-like nanoscale crystals forming within the hydrogel matrix.

Superior mechanical parameters

The researchers created a series of PAAM/NAAC structures, using projection-based 3D printing to print hydrogel shapes and then soaking them in NAAC solution. Upon seeding, the structures rapidly transformed from transparent to opaque as the crystallization spread through the sample at speeds of up to 4.5 mm/s.

The crystallization dramatically changed the material’s mechanical performance. For example, a soft cylinder of PAAM/1.5NAAC (containing 150 wt% salt) could be easily compressed by hand, returning to its original shape after release. After crystallization, four 9x9x12 mm cylinders could support an adult’s weight without deforming.

For this composite, just 1 min of crystallization dramatically increased the compression Young’s modulus compared with the soft state. And after 24 h, the Young’s modulus grew from 110 kPa to 871.88 MPa. Importantly, the hydrogel could be easily returned to its soft state by heating and then cooling, a process that could be repeated many times.

The team also performed Shore hardness testing on various composites, observing that hardness values increased with increasing NAAC concentration. In PAAM/1.7NAAC composites (170 wt% salt), the Shore D value reached 86.5, comparable to that of hard plastic materials.

The hydrogel’s crosslinking density also impacted its mechanical performance. For PAAM/1.5NAAC composites, increasing the mass percentage of polymer crosslinker from 0.02 to 0.16 wt% increased the compression Young’s modulus to 1.2 GPa and the compression strength to 81.7 MPa. The team note that these parameters far exceed those of any existing 3D-printed hydrogels.

Smart plaster cast

He and colleagues demonstrated how the hard/soft switching and robust mechanical properties of PAAM/NAAC can create medical fixation devices, such as a smart plaster cast. The idea here is that the soft hydrogel can be moulded around the injured bone, and then rapidly frozen in shape by crystallization to support the injury and promote healing.

The researchers tested the smart plaster cast on an injured forearm. After applying a layer of soft cotton padding, they carefully wrapped around layers of the smart plaster bandage (packed within a polyethylene film to prevent accidental seeding). The flexible hydrogel could be conformed to the curved surface of limbs and then induced to crystallize.

After just 10 min of crystallization, the smart plaster cast reached a yield strength of 8.7 MPa, rapidly providing support for the injured arm. In comparison, a traditional plaster cast (as currently used to treat bone fractures) took about 24 h to fully harden, reaching a maximum yield strength of 3.9 MPa

To determine the safety of the exothermic crystallization process, the team monitored temperature changes in the plaster cast nearest to the skin. The temperature peaked at 41.5 °C after 25 min of crystallization, below the ISO-recommended maximum safe temperature of 50 °C.

The researchers suggest that the ease of use, portability and fast response of the smart plaster cast could provide a simple and effective solution for emergency and first aid situations. Another benefit is that, in contrast to traditional plaster casts that obstruct X-rays and hinder imaging, X-rays easily penetrate through the smart plaster cast to enable high-quality imaging during the healing process.

While the composites exhibit high strength and Young’s modulus, they are not as tough as ideally desired. “For example, the elongation at break was less than 10% in tensile testing for the PAAM/1.5NAAC and PAAM/1.7NAAC samples, highlighting the challenge of balancing toughness with strength and modulus,” He tells Physics World. “Therefore, our current research focuses on enhancing the toughness of these composite materials without compromising their modulus, with the goal of developing strong, tough and mechanically switchable materials.”

The hydrogel is described in the International Journal of Extreme Manufacturing.

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