The breakthrough, published in the prestigious international journal Science, was led by Prof. Jianhua Hao, Head of the Department of Physics and Materials, Chair Professor of Materials Physics and Devices and Associate Director of the PolyU-Wuhan Technology and Innovation Research Institute at PolyU. The collaboration included researchers from the National University of Singapore, The Hong Kong University of Science and Technology, Peking University, and the Singapore University of Technology and Design. Dr. Zehan Wu, Research Assistant Professor in the same department, is the first author of the research article. Their work addresses what the International Roadmap for Devices and Systems (IRDS) has identified as one of the most pressing bottlenecks in modern microelectronics, and it does so using a device architecture that the roadmap itself has flagged as the most promising successor to today’s dominant transistor design.

To appreciate why this matters, it helps to understand how the transistors inside every laptop, smartphone, and data centre actually work. Integrated circuits are built from complementary metal–oxide–semiconductor field-effect transistors, or MOSFETs, which switch electrical current by raising or lowering an energy barrier. In the OFF state, charges face a barrier they cannot cross; when a gating voltage is applied, charges gain enough thermal energy to spill over the top of that barrier in a process called thermionic emission. The steepness of this transition, measured by a figure known as the subthreshold swing (SS), determines how much voltage is needed to flip the transistor from OFF to ON. Here nature intervenes: because thermionic emission depends on the thermal distribution of electron energies, the subthreshold swing of any MOSFET cannot fall below 60 millivolts per decade of current at room temperature. That is the Boltzmann limit, and it is not an engineering shortfall but a consequence of statistical mechanics itself.

The consequences of this limit are profound. As chipmakers pack more transistors onto each processor, the 60 mV decade⁻¹ floor forces operating voltages to remain far higher than ideal, which means wasted power, wasted heat, and a hard ceiling on energy efficiency. Advanced MOSFETs typically require gate-voltage ranges of around 800 millivolts to operate, and every millivolt of that overhead is multiplied billions of times across a modern chip. For the artificial intelligence accelerators now driving enormous growth in global electricity consumption, the inability to switch transistors at lower voltages has become one of the defining engineering challenges of the decade. Breaking the 60 mV barrier is therefore not an incremental improvement but a potential inflection point for the entire industry.

The escape route identified by physicists is quantum tunnelling. Instead of forcing electrons to climb over an energy barrier, a tunnelling field-effect transistor (TFET) allows them to pass straight through it, exploiting the quantum-mechanical probability that particles can traverse barriers that would classically be impenetrable. Because tunnelling probability depends on the barrier’s width and height rather than on the thermal energy of the electrons, a TFET can in principle achieve subthreshold swings steeper than 60 mV decade⁻¹ even at room temperature. The IRDS has singled out TFETs as the most promising alternative to MOSFETs precisely for this reason. In practice, however, experimental TFETs have struggled for years with a frustrating trade-off: devices that achieved steep switching typically delivered painfully low output currents, undermining their usefulness in real circuits.

Prof. Hao’s team solved this problem with an elegantly engineered material system. Using pulsed laser deposition (PLD), a technique in which intense laser pulses vaporise a target material so that it recondenses as an ultra-thin film, the researchers fabricated an alternating heterostructure of two-dimensional bismuth (Bi) and indium selenide (InSe) layers. The choice of bismuth is the crucial insight. In its bulk form, bismuth is a semi-metal, a material that is neither a proper conductor nor a proper semiconductor. But when reduced to two-dimensional form with precise control over the layer structure at the nanoscale, bismuth transforms into a semiconductor. This transformation allowed the team to engineer an ideal energy band alignment between the bismuth and the indium selenide, creating exactly the conditions needed for charge carriers to tunnel efficiently from one material into the other through the quantum tunnelling mechanism.

The performance figures reported in the study are striking. The resulting Bi/InSe TFET achieved subthreshold swing values well below the 60 mV decade⁻¹ thermionic limit, and it maintained this steep switching behaviour across six orders of magnitude of current, a range that matters enormously for practical digital logic, where transistors must remain reliably OFF across many conditions rather than only at a single operating point. Remarkably, the device accomplished all of this at room temperature, on standard centimetre-scale silicon substrates, without exotic cryogenic cooling or specialised wafer treatments. The operating gate-voltage range required was only 160 millivolts, a fraction of the roughly 800 millivolts demanded by advanced MOSFETs, translating directly into the kind of power savings that next-generation computing demands.

Just as importantly, the device resolved the long-standing weakness of previous experimental TFETs. The PolyU-led team demonstrated a high output current of up to several microamps per micrometre alongside an exceptionally high ON/OFF current ratio. Output current is not a vanity metric: it determines whether a single transistor can drive multiple downstream logic gates, a capability engineers call fan-out, and whether signals propagate through a circuit with minimal delay. A steep-switching transistor that cannot deliver adequate current would force designers into cumbersome workarounds and erode the very efficiency gains it promises. By combining steep slopes with robust current drive, the Bi/InSe device demonstrates compatibility with existing integrated-circuit design practices and even the potential for a generational upgrade of current chips rather than a disruptive replacement.

The manufacturing dimension of the work may prove as consequential as the physics. Pulsed laser deposition has long been valued in research laboratories for its precision, but questions have lingered about whether it can serve high-precision, wafer-scale production of two-dimensional materials. This study demonstrates the practical viability of PLD for exactly that role, producing uniform ultra-thin heterostructures on silicon substrates that are the backbone of the existing semiconductor industry. Because the device integrates seamlessly with traditional silicon-based manufacturing processes, the researchers argue that their approach provides a scalable roadmap toward energy-efficient microchips, rather than a laboratory curiosity that would require an entirely new industrial ecosystem to deploy. That compatibility with ultra-short channel lengths, which two-dimensional materials are particularly well suited to support, positions the technology for the transistor geometries that future process nodes will demand.

The implications extend well beyond the laboratory bench. Ultra-low-power, high-performance integrated circuits built on tunnelling transistors could reshape the energy economics of artificial intelligence, where specialised hardware already consumes power on the scale of entire cities, and where every reduction in switching voltage compounds across billions of operations per second. Prof. Hao has emphasised that the technology paves the way for the ICs essential to emerging AI chips and advanced semiconductor applications, and the publication of the findings in Science signals that the broader research community regards the result as a landmark. For decades, the Boltzmann limit has stood as an immovable fact of life for chip designers, a boundary drawn by thermodynamics itself. By replacing thermionic emission with quantum tunnelling in a carefully crafted two-dimensional heterostructure, the PolyU-led team has shown that the boundary can be crossed, and in doing so has brought a long-awaited experimental technology a decisive step closer to commercial reality.

Subject of Research: A quantum-tunnelling field-effect transistor based on two-dimensional bismuth and indium selenide heterostructures that overcomes the Boltzmann switching limit in integrated circuits

Article Title: PolyU develops quantum-tunnelling field-effect transistor to overcome barriers to integrated-circuit chip development

Article References: PolyU develops quantum-tunnelling field-effect transistor to overcome barriers to integrated-circuit chip development. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: quantum tunnelling, tunnelling field-effect transistor, TFET, two-dimensional materials, bismuth, indium selenide, Boltzmann limit, subthreshold swing, pulsed laser deposition, integrated circuits, low-power electronics, AI chips

The achievement rests on a technique that has become one of the most powerful control knobs in attosecond science: two-colour waveform synthesis. Instead of hitting the tunnel junction with a single laser beam, the researchers combined two coherent light fields of different colours, a fundamental frequency and its second harmonic. When these two fields overlap, their sum is no longer a simple sinusoidal oscillation. Within each optical cycle, the combined electric field becomes an asymmetric waveform, with a strong half-cycle in one direction and a weaker, differently shaped excursion in the other. For an electron contemplating a quantum jump across the tunnel barrier, that asymmetry matters enormously. The instantaneous field tilts the barrier, lowers it on one side, and determines the precise instant, within the optical cycle, when tunnelling is most probable. By adjusting the relative phase between the two colours, the team could shift that favoured instant back and forth, and with it, flip the direction of the resulting current pulse.

What makes the result remarkable is the combination of timescale and spatial precision. The junction in a scanning tunnelling microscope is a few nanometres or less across, and the tunnelling process itself is confined to a region far smaller than that. The researchers demonstrated sub-ångström topographic sensitivity, meaning the laser-driven current responded to height changes on the surface at the level of individual atoms, while achieving a lateral spatial resolution of two nanometres. In other words, the attosecond current they generated and steered remained a genuinely microscopic probe, not a diffuse photoresponse smeared across the sample. And they did it under ambient conditions, at room temperature and in air, rather than in the cryogenic vacuum that most ultrafast microscopy experiments demand.

A persistent obstacle in laser-driven STM has been heat. When laser light strikes the tip or the sample, some of its energy is inevitably absorbed, raising the temperature of the junction and producing thermal expansion of the tip, thermally assisted currents and other artefacts that can masquerade as genuine ultrafast dynamics. Earlier femtosecond and picosecond experiments in laser-assisted STM were often haunted by exactly this problem, and distinguishing a true sub-cycle electronic response from a slow thermal signal became a central methodological challenge for the field. The Technion team addressed this directly. Because their current control depends on the sub-cycle waveform of the two-colour field, flipping the carrier-envelope phase flips the current direction, whereas a thermal effect would be completely indifferent to the sign of the electric field. The observed phase-dependent, direction-reversing current is therefore an unambiguous signature of coherent, lightwave-driven electron transport, and the experiment was explicitly designed to be free of thermal artefacts.

To understand the mechanism, the researchers turned to theory at two levels of description. On the one-electron side, they employed a strong-field model of tunnelling, in the spirit of the Keldysh framework that has underpinned strong-field physics since 1965, treating the electron as a wavepacket launched through a field-tilted barrier. On the many-body side, collaborators performed time-dependent density functional theory calculations of the full metal-insulator-metal junction, capturing the collective response of the electrons in the electrodes. Projecting the experimental data onto both theoretical descriptions revealed a consistent physical picture: a three-step transport process operating in the non-adiabatic tunnelling regime. In this regime, the optical field changes so quickly that the electron cannot follow it adiabatically; instead, the field launches the electron into the barrier, the wavepacket propagates across the gap while the field continues to evolve, and the electron is finally collected on the far side, with the whole sequence compressed into a fraction of an optical cycle.

From their theoretical analysis, the team extracted a concrete number for the duration of the current burst: 860 attoseconds. That figure, less than a single femtosecond, quantifies how briefly the electrons stream through the junction during each pulse. It also places the experiment firmly in the lightwave electronics regime, where the oscillating carrier wave of light, rather than a sequence of electronic switches, dictates the timing of current flow. Lightwave electronics has been advancing rapidly in recent years, with petahertz-scale field control demonstrated in nanotips, nanoantennas and solid-state devices, but the STM junction offers something none of those platforms can: the ability to position the driven current with atomic-scale precision on a surface and, in principle, to image ultrafast charge dynamics as they unfold at specific sites, such as a single molecule or a defect.

The lineage of this work runs through several landmark experiments. Attosecond science itself was born from the recognition that strong laser fields can ionize atoms within a fraction of an optical cycle, and that the emitted electrons carry timing information about that process. In 2011, researchers at the Max Planck Institute of Quantum Optics showed attosecond control of electrons emitted from a nanoscale metal tip, extending strong-field physics to nanostructures. Later experiments traced attosecond electron emission from tungsten nanotips and demonstrated attosecond field emission from nanotips driven by terahertz-scale waveforms. In parallel, ultrafast STM developed along its own track, first with pulsed lasers that suffered from thermal complications, then with single-cycle terahertz pulses that enabled femtosecond imaging of molecular motion, and more recently with all-optical sub-cycle microscopy on atomic length scales. The new work synthesizes these threads, bringing waveform-level attosecond control, the hallmark of gas-phase strong-field physics, into the spatially resolved world of the STM.

The two-colour approach itself has a distinguished history. Phase-controlled ionization with two colours was explored in atoms in the 1990s, and coherent control of above-threshold photoemission from tungsten nanotips followed in 2016. What the Technion team added was the junction geometry and the diagnostic power of the STM platform. In a tunnel junction, the current direction is a measurable, meaningful quantity: electrons can flow from tip to sample or from sample to tip, and the net laser-induced current reflects the asymmetry of the driving waveform. By scanning the two-colour phase and recording how the current reverses, the researchers effectively performed attosecond streaking inside the junction, using the optical waveform itself as the clock that timestamps the tunnelling event.

The practical implications reach toward a long-sought goal: triggering and imaging ultrafast charge dynamics at specific locations on a surface. Many of the most important processes in chemistry, materials science and nanotechnology, from charge transfer in molecular junctions to carrier dynamics in photovoltaic materials, begin with electron motion on attosecond to femtosecond timescales localized to nanometre-scale regions. A microscope that can launch a directional attosecond current pulse at a chosen point and then read out the response could watch those processes unfold in real space and real time simultaneously. The two-nanometre lateral resolution and sub-ångström vertical sensitivity reported here, achieved under ambient conditions, suggest that such spatiotemporal microscopy need not be confined to specialized cryogenic laboratories.

There remain challenges to overcome before attosecond STM becomes a routine tool. The current bursts are generated by intense optical fields, and extending the technique to more delicate samples, to insulating substrates and to single-molecule junctions will require careful control of the near-field enhancement at the tip. The theoretical framework, spanning one-electron strong-field models and full many-body simulations, will need to keep pace as experiments move to more complex materials. But the conceptual barrier has now been crossed. Electrons tunnelling through the gap of a scanning tunnelling microscope have been caught in the act, timed to within a few hundred attoseconds, and steered at will by the waveform of light. The fastest event in electronics and the sharpest eye in microscopy are, at last, looking at the same thing at the same time.

Subject of Research: Attosecond control of laser-driven electron tunnelling currents in a scanning tunnelling microscope junction

Article Title: Attosecond current control and timing in a scanning tunnelling microscope

Article References: Davidovich, D., Ma, B., Goldner, A., Cohen, S., Chen, Z., Borisov, A. G., & Krüger, M. (2026). Attosecond current control and timing in a scanning tunnelling microscope. Nature Photonics. https://doi.org/10.1038/s41566-026-02021-y

Image Credits: AI Generated

DOI: 10.1038/s41566-026-02021-y

Keywords: attosecond science, scanning tunnelling microscopy, lightwave electronics, two-colour laser pulses, quantum tunnelling, ultrafast microscopy, strong-field physics, TDDFT, nanoscale junctions, carrier-envelope phase, electron transport, Nature Photonics

A new study published in the Journal of Cosmology and Astroparticle Physics by Robson Christie, Jaewoo Joo, Greg Kaplanek, Vincent Vennin and David Wands used a simplified cosmological model to investigate what determines which vacuum a field may end up in within an expanding Universe. The stakes of this question are illustrated by the most famous example the authors themselves invoke: the Higgs field. The vacuum value of the Higgs field contributes to giving mass to the particles of the Standard Model, the theory that describes the known elementary particles and three of the four fundamental forces, and it helps determine the structure of low-energy physics. According to some calculations based on the Standard Model, it is possible that the Higgs field does not sit in the lowest possible energy state but in a false vacuum, while at very large field values a second, deeper minimum may exist. The study is not directly about the Higgs field, but it uses the Higgs as a concrete example of what can happen when a field becomes trapped in a local minimum even though a lower-energy state is available.

“In principle, a transition to that deeper minimum would take the Universe into a radically different state, in which the structure of matter and the forces that govern it would be altered,” explains Robson Christie, a researcher at the School of Mathematics and Physics at the University of Portsmouth and first author of the study. Classically, such a transition would require the field to climb over the energy barrier separating the two minima, like a ball rolling out of a shallow valley and over a mountain before it can descend into a deeper one. Quantum mechanics changes this picture fundamentally. The state of a quantum system can extend beyond the barrier, leaving a small but nonzero probability that the system will simply appear on the other side. This phenomenon, quantum tunnelling, is the mechanism by which a field in a false vacuum could, in principle, escape to the true vacuum, and it has long been a central concern in calculations of vacuum stability in the early and late Universe.

What distinguishes the new work is its treatment of isolation. Many standard calculations of tunnelling treat the field as completely cut off from its surroundings, an idealisation that the authors argue is physically misleading. “We know, however, that perfect isolation is an idealisation,” says Greg Kaplanek, a researcher at Syracuse University in New York. In reality, fields continuously interact with other fields and with whatever surrounds them, in other words with their environment. Kaplanek draws an analogy from laboratory physics: “Think, for example, of quantum computers: we go to enormous lengths to protect the quantum information stored in these machines from the environment, because even weak interactions with it can quickly alter the quantum state. Something similar happens in cosmology: a field is never really alone.” These interactions produce decoherence, the process by which a quantum system that can exist in a superposition of different possibilities gradually loses the ability to maintain that superposition and begins to behave more and more like an ordinary classical system. Using the analogy of a coin, a quantum state is not simply heads or tails but includes both possibilities at once, and interaction with the environment makes it increasingly difficult to preserve such a combination.

In the model constructed by Christie and colleagues, the environment is represented by other fields interacting with the main field under study. The main field can initially be in a quantum superposition involving both vacua, so the framework allows the researchers to track how the competition between the two minima evolves when the field is coupled to its surroundings. One of the surprises of the study is that the environment does not appear to play a decisive role in the initial choice of vacuum. Instead, what matters far more is whether the field is light or heavy relative to the Hubble scale, the characteristic rate at which the Universe is expanding. This comparison between the intrinsic dynamics of the field and the speed of cosmic expansion turns out to govern which minimum the field is likely to settle in during the expansion of space.

“A field that is heavy compared with the Hubble scale can quickly adjust to the changes as the Universe expands,” Christie explains, “and in this case it is highly likely to move towards the true vacuum, the deepest energy minimum.” Heavy fields, in this sense, have enough time during each doubling of cosmic scale to relax toward their lowest-energy configuration, so the expansion does not outpace their internal dynamics. Something quite different happens for lighter fields. “If the expansion is too rapid compared with the dynamics of the field, the system cannot keep up with the changes,” Christie continues. “In this case there can remain a significant probability that the field will also end up in the false vacuum.” In other words, the initial choice between the true and false vacuum is influenced mainly by the relationship between the field’s own dynamics and the rate of cosmic expansion, rather than by the details of the environmental coupling. A rapidly expanding Universe can effectively freeze a light field before it has had the opportunity to find the deepest valley in its energy landscape.

The second half of the story concerns what happens after a light field has landed in a false vacuum. In a perfectly isolated quantum system, tunnelling towards the deeper minimum would still be possible, however slowly. But in the authors’ model, interaction with the environment produces decoherence and destroys the quantum properties needed to maintain a coherent superposition between the two vacua. “The interesting thing is that it is not primarily the environment that decides where the field will end up,” Kaplanek explains. “Once the field has localised in one of the two minima, however, decoherence tends to keep it there. Tunnelling towards the other vacuum is strongly suppressed.” The authors call this phenomenon cosmic lockdown: a kind of lock that stabilises the field in whichever vacuum it has reached, preventing the slow quantum leakage that would otherwise, over vast timescales, allow transitions between minima.

The physical interpretation of cosmic lockdown connects it to a well-established quantum effect. The authors understand it as a manifestation of the quantum Zeno effect, the counterintuitive result that a quantum system which is continuously monitored can have much more difficulty moving from one state to another, because each measurement effectively resets its evolution. Of course, no one is literally observing the cosmological field in the model, and no conscious observer is required. “You do not need a conscious observer,” Kaplanek explains. “The environment continuously gathers information about the state of the system. This process destroys the coherence between the two possible vacua and makes tunnelling from one to the other much more difficult.” In this sense, the rest of the Universe acts as an ever-present measuring apparatus, constantly extracting information about the field’s state and, as a byproduct of that information gathering, freezing it in place. The effect is a striking example of how openness to the environment, usually seen as a nuisance in quantum technology, can instead act as a stabilising force on cosmological scales.

The work remains a simplified model, and the authors are careful not to overclaim its reach. The study does not show that our current Higgs vacuum is protected by cosmic lockdown, and translating the result into a statement about the actual stability of the Higgs field would require more realistic treatments of the many interactions present in the early and late Universe. The result does, however, establish an interesting principle: interactions with the environment can make a false vacuum more stable by suppressing tunnelling towards another state, reversing the usual intuition that openness always undermines stability. “If a mechanism of this kind were relevant in more realistic cosmological situations, then it could help stabilise a field that is already sitting in a false vacuum,” Wands concludes. “But understanding how far this can be applied to the Higgs field will require more realistic models.” If our Higgs field really is sitting in a false vacuum, as some Standard Model calculations suggest may be the case, then cosmic lockdown points to at least one reassuring possibility: the ceaseless interaction of the field with its surroundings could help make a transition to a radically different state, one in which the structure of matter and the forces governing it would be altered, even more difficult than existing tunnelling estimates imply.

Subject of Research: Decoherence-induced suppression of quantum tunnelling between false and true vacua in an expanding Universe

Article Title: Cosmic lockdown: how the environment can isolate quantum fields

Article References: Cosmic lockdown: how the environment can isolate quantum fields. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: cosmology, quantum fields, decoherence, false vacuum, quantum tunnelling, Higgs field, quantum Zeno effect, vacuum stability, Hubble scale, Standard Model, early Universe, JCAP