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Quantum entanglement gets a laser-like lift

Under certain circumstances, an ultraviolet photon can spontaneously split into two lower-energy infrared photons – this is known as down-conversion. The polarizations of these two photons are intimately related: a measurement of the polarization of one photon would reveal the polarization of the other, even if they were widely separated. This is an example of ‘entanglement’ – a correlation that can exist between quantum particles that is much stronger than those allowed in classical physics.

But entangled photon pairs of this kind arise rarely in ultraviolet beams. In order to create more pairs, Lamas-Linares and colleagues shone a pulsed ultraviolet laser through a crystal of barium borate. As expected, one of the millions of photons split into two infrared photons via the down-conversion process. These photons left the crystal at an angle to the direction of travel of the laser pulse, and mirrors then reflected them back into the crystal. Meanwhile, the laser pulse that passed through the crystal was also reflected back towards it. The mirrors were arranged so that the reflected laser pulse reached the crystal at exactly the same time as the entangled photons.

The quantum interaction of the entangled photons and the reflected pulse sparked the production of another pair of entangled photons. Classically, this would lead to two photon pairs, but because this is a quantum interference process, it can produce a maximum of four pairs of photons or a minimum of zero. This is analogous to the reinforcement or cancellation of light waves in a diffraction pattern, and can result in a four-fold increase in the number of entangled photons produced. The phenomenon can also multiply the number of entangled photons by sixteen if it is applied to an even rarer system composed of four entangled photons.

“Currently available sources of entangled photons are extremely weak, but the laser action for entangled photons can produce very bright sources of entangled photon pairs”, Lamas-Linares told PhysicsWeb. “Laser action will also create far more complicated entangled states that involve many photons and are likely to play an important role in the realization of several recent theoretical developments in quantum information.”

The amplification demonstrated by Lamas-Linares and co-workers is analogous to the light that bounces between the mirrors at the ends of a laser cavity. In practice, however, the light in a conventional laser cavity undergoes many reflections, whereas the initial entangled pair in the Oxford experiment is reflected only once.

“We are now working on a system in which the laser passes through the crystal many times”, says Lamas-Linares. The refined set-up could lead to fluxes containing up to 100 entangled photons.

Fragment of ancient asteroid falls to Earth

Asteroids are classified by their optical properties and their dynamics. Most meteorites are thought to be fragments of asteroids, so the spectrum and brightness of the light reflected from a meteorite helps astronomers to identify which type of asteroid it came from. The arrival time of the meteorite on Earth is also a crucial factor in matching it to the motion of a certain asteroid.

Hiroi and colleagues found that the Tagish Lake meteorite reflected just 3% of the light incident upon it, a similar value to D-type asteroids. The spectrum of this reflected light also very closely matched that of a particular D-type asteroid called 368 Haidea. But the team found that another candidate – asteroid 773 Irmintraud – was more likely to have lost fragments at the right time because it was more strongly distorted by Jupiter’s gravity.

Hiroi and colleagues hope that better observations of D-type asteroids and more detailed studies of their dynamics will allow them to identify the parent asteroid of the Tagish Lake meteorite with greater certainty. They warn, however, that it is difficult to match meteorites with asteroids: those meteorites that reach Earth are not a representative sample because many are too fragile to survive the journey through our atmosphere.

Pizzarello and co-workers believe that the Tagish Lake meteorite – preserved in freezing conditions – is one of the most pristine meteorites to be studied. Although rich in carbon, the carbon was present in a narrow range of rather light compounds – including carboxylic acids, aromatic hydrocarbons and fullerenes – compared with meteorites studied previously. This implies that many distinct chemical processes took place in the early solar system, according to the researchers.

The team believes this discovery is consistent with a parent asteroid that initially contained little carbon. The abundance of chain-like carboxylic acids would have formed as it acquired carbon and water from interstellar dust, before it ejected the meteorite.

Lasers energise fusion research

Nuclear fusion is an attractive potential source of energy because the two isotopes of hydrogen used for fuel – deuterium and tritium – are readily available. It also produces no carbon dioxide or long-lived radioactive decay products. However, a temperature of about 50 million degrees is required to overcome the repulsion between the fusing deuterium and tritium nuclei.

The new ‘fast ignition’ technique has been demonstrated by Ryosuke Kodama and colleagues of Osaka University and researchers from the Rutherford Appleton Laboratory, Imperial College and the University of York in the UK. Like the conventional ‘inertial confinement’ method, fast ignition uses laser pulses to compress pellets of deuterium and tritium and the exploding plasma surrounding the pellets causes an equal and opposite implosion of the fuel. But unlike the old approach, in which the compressed fuel is heated by the collapse of accurately timed shock waves, fast ignition uses a second, shorter, laser pulse to start the fusion chain reaction.

The technique requires a less precise implosion but is more difficult because the short laser pulse is deflected by instabilities it creates in the plasma. The researchers resolved this problem by inserting a polymer cone inside the fuel pellet, which allows the beam to pass through unimpeded.

The use of a second laser halves the amount of energy needed to power the long-pulsed laser. The team now needs to establish if increasing the energy of the short-pulsed laser raises the fusion temperature. “At the moment the minimum energy conversion efficiency from laser to thermal energy is 20%,” says Kodama. “We want to see if this maintains itself as we go to higher energy levels when we will actually get ignition.”

Michael Key of the Lawrence Livermore National Laboratory in California, writing in a complementary article in Nature, says that if it is feasible at higher energies, fast ignition could lead to serious international efforts to produce fusion. But he cautions that the experiences of other fusion projects should temper such enthusiasm. “At such an early stage,” he says, “this new approach to fusion energy should be viewed as promising, but speculative, until much more work has been done.”

Silicon LED strides ahead

Light emission and absorption in semiconductors can be described by Planck’s theory of blackbody radiation. A semiconductor in thermal equilibrium with its surroundings absorbs the same amount of radiation as it emits. But if a voltage is applied across the semiconductor, the concentration of charge carriers grows exponentially and so does the light emission.

Green and colleagues realised that they could further amplify this growth if they could increase the ability of the semiconductor to absorb radiation. To do this, they etched an array of inverted pyramids onto the surface of the silicon LED – the pyramids reflect absorbed light back into the semiconductor. This ‘light trapping’ technique is commonly used in photovoltaic devices such as solar panels, and produced a factor of ten increase in the light emission of the silicon LED.

The team also reduced the number of ‘non-radiative recombination’ events in the device – these events occur when electrons and holes recombine but produce heat rather than light. The use of special surface treatment, small contacts and selective doping led to a further tenfold improvement in light emission.

Silicon LEDs currently have efficiencies in the range 0.01 to 0.1%, but the device developed by Green and co-workers exceeds a value of 1% at 300 kelvin. This is similar to the efficiencies of direct bandgap devices – based on materials like gallium arsenide – around a decade ago. “We believe we can reach an efficiency of 5% by improving the rear reflector in the device”, says Green. “We will also be developing ‘fast modulators’ that allow the diode brightness to be changed quickly”.

The development of an efficient silicon light emitter has long challenged physicists because silicon has an indirect bandgap, which makes it reluctant to emit light.

Sir Fred Hoyle 1915 – 2001

Hoyle’s 1953 prediction that an excited form of carbon-12 would be produced within stars was soon proved correct by the observations of William Fowler. Together with Geoffrey and Margaret Burbidge, Hoyle and Fowler published their theory of nucleosynthesis in 1957. The astronomical community was shocked when Fowler alone received the 1983 Nobel Prize for this groundbreaking work.

Hoyle also believed that the now-accepted ‘big bang’ theory – which describes how the universe was created in a cataclysmic explosion and has been expanding ever since – was flawed. In his opinion, the big bang could not have taken place unless space and time already existed. This led him to propose a ‘steady-state’ universe in which matter is continually generated by some as yet unknown mechanism. Hoyle could not explain the subsequent discovery of the microwave background radiation – widely believed to be a remnant of the big bang – but his challenge to the accepted theory prompted the research that ultimately proved it right.

In the early 1960s, Hoyle established the Institute of Theoretical Astronomy at Cambridge University, but his relationship with the university broke down following administrative quarrels. Hoyle was also persuaded by the sponsors of the Mullard Radioastronomy Observatory at Cambridge to attend a meeting at which his steady-state theory was publicly ridiculed.

Born in Yorkshire in 1915, Hoyle attended Bingley grammar school before gaining entrance to Cambridge University, where he obtained his PhD under the supervision of Paul Dirac. Ever the maverick, Hoyle’s recent investigations with Chandra Wickramasinghe explored the controversial idea that life on Earth may have originated from extra-terrestrial microbes.

First hints of novel nuclei

Physicists from RIKEN, the JINR and Kurchatov laboratories in Russia, and the GANIL laboratory in France detected the hydrogen-5 in collisions between a beam of helium-6 nuclei and a cryogenic hydrogen target. When a helium-6 nucleus collides with a hydrogen nucleus, it can donate all four of its neutrons to the hydrogen. This leaves two spare protons that are ejected from the target as a helium-2 nucleus, which quickly decays into two protons. The energies and angles of the protons were measured by the RIKEN detector, and revealed a peak in the energy spectrum consistent with a hydrogen-5 nucleus.

In the Brookhaven experiment, a team of 50 physicists from six countries created the strange nuclei by firing an intense beam of protons at a tungsten target to create a beam of ‘kaons’ – particles that contain one strange quark. The kaons impinged on a beryllium target to create both strange quarks and antiquarks. These quarks and antiquarks can then form a nucleus consisting of a proton, a neutron and two ‘lambda’ particles. Each lambda particle contains one up, one down and one strange quark, making the nucleus ‘doubly strange’.

Around 40 such nuclei were produced in the Alternating Gradient Synchrotron at Brookhaven. “That’s enough events to begin a study using statistical techniques”, says team member Adam Rusek. The team is optimistic that their method could pave the way for accurate measurements of the forces between lambda particles. The super-dense interiors of neutron stars are probably the only places in the universe where such strange matter exists in a stable form.

Mars-sized object probably created the Moon

The large angular momentum of the Earth-Moon system suggests that the Earth rotated once every five hours before the Moon formed. The motion of the Moon also shows that it is relatively light, which means that it probably contains a smaller proportion of iron than the Earth. These widely accepted ideas limit the kind of impact that could have given rise to the Moon.

Earlier studies identified a class of bodies with a large angular momentum that could have produced a Moon with the mass we observe today. But such an object must have collided with a well-developed Earth, and would not have increased its rotation period to 24 hours. This theory requires a second impact – for which there is little evidence – to have removed angular momentum from the system.

A collision with a second type of object could have resulted in the current rotational period of the Earth, but would have taken place before the Earth was fully formed. This means the Earth and Moon must have continued to gather similar matter after the collision. But the matter must have been rich in iron to explain the composition of the Earth, and this cannot account for the low density of the Moon today.

The new simulations – carried out at the Southwest Research Institute in Colorado and the University of California at Santa Cruz – show that a collision with a body about the size of Mars could lead to an iron-poor Moon and the current dynamics of the Earth and the Moon. The 36 calculations were based on ‘smooth particle hydrodynamics’, which modelled the dynamics of different impacts and the tens of thousands of ejected fragments.

“Our model requires a smaller impactor than previous models, making it more likely that the Earth should have a Moon as large as ours”, says Asphaug.

“The type of impact we now find can best produce the Earth-Moon system was erroneously ruled out a decade ago”, Canup told PhysicsWeb, “but our studies have a resolution ten times better than those early works”.

New nano-technique hits the bull’s-eye

The Osaka team used a technique known as photopolymerization – in which the absorption of light causes a polymer to solidify – to create the micro-bull in a block of commercially available resin. By using two-photon photopolymerization, the team was able to beat the diffraction limit and create structures with a spatial resolution of about 120 nanometres, even though the laser used had a wavelength that was more than six times longer. This feat is possible thanks to the nonlinear relationship between the polymerization reaction and the light intensity.

Kawata and co-workers chose to make a micro-bull purely for demonstration purposes: ‘People can easily compare our fabrication with the actual image in their mind,’ said Kawata. ‘The micro-bull also clearly shows the unique power of two-photon photopolymerization: three-dimensional capability and high spatial resolution.’

The team also used the technique to make the smallest ever functional micromechanical system – a spring with a diameter of only 300 nanometres. Two-photon polymerization was developed by Kawata’s team in 1997.

Ion strings make brilliant beams

Collisions in high-energy ion beams reduce the beam intensity and can be remedied by extra focusing devices or the use of low-density beams. However, physicists predicted 20 years ago that in a sufficiently cool beam, the ions would not collide because their Coulomb repulsion would outweigh their kinetic energy.

Such ‘crystallization’ has been achieved before in ion traps – in which the ions are stationary – but it is more difficult in a circulating beam because of the motion of the ions and interactions between the beam and the storage ring. These problems affect both large storage rings – such as the Relativistic Heavy Ion Collider at Brookhaven – and smaller ones.

Schramm and co-workers injected magnesium ions into their 0.36-metre circumference storage ring, PALLAS – the Paul laser cooling acceleration system. The beam was laser-cooled and its fluorescence monitored. The team found that, at a certain laser wavelength, the diameter of the beam fell and the fluorescence peaked sharply. This pinpoints the transition to the crystalline state, during which the range of ion velocities drops by 75%.

The fluorescence measurements showed that the ring contained around 18 000 ions, and the temperature of the beam fell from 30 to 0.4 kelvin as the crystalline state emerged. In this new phase, the ions reach a speed of 2800 metres per second – corresponding to a beam energy of 1 electron volt – and resemble a one-dimensional thread. The beam can perform over 3000 revolutions of their storage ring without further cooling

According to Schramm, the technique could be used for a wide range of experiments. “Crystalline ion beams could aid inertial confinement fusion – which mimics stellar nuclear reactions – while precise experiments with relativistic beams could test special relativity”, he says.

True tritium tales, toxic and non-toxic

Tritium is one of the most dangerous substances known to modern science.

This is not something that one would suspect at first appearance. Tritium is an ordinary part of the modern environment and is continually produced in the Earth’s atmosphere by cosmic rays. Common consumer products contain it: some wristwatches use tritium in illuminated dials, as do emergency-exit signs that are now standard in office buildings. Tritium is also used in many vital medical-research procedures.

Nor would one suspect tritium’s dangers from reading science textbooks. A soft-beta-ray emitter, tritium is not an external hazard to the body and must be inhaled or ingested to be dangerous. Even then, it passes through the body like water and is excreted quickly. Atom for atom, tritium has a low radiological toxicity, and the biological damage per disintegration is small. While one disintegration of an alpha emitter may deposit 6 MeV, a disintegration of a tritium nucleus deposits 5 keV, making it more than a thousand times less harmful. Environmentally, tritium does not hang around long, having a short half-life of 12.5 years.

Ingested in high quantities, of course, tritium can be lethal, like many chemicals (aspirin, say). But for a radioisotope, tritium’s physical and biological properties are relatively benign.

Toxic tritium tales

Appearances deceive. Consider the following true stories about the damage that tritium has inflicted, or threatens to inflict, on modern science.

In 1997 at the Brookhaven National Laboratory in the US, a small leak of water that contained tritium was found at the spent-fuel pool of the High Flux Beam Reactor (HFBR), one of the most important tools for neutron scientists worldwide. A total of 5 curies of tritium had seeped out – less than a quarter of that inside a single typical emergency-exit sign. The tritium was not in the drinking-water supply. It decayed near the pool and was not a health hazard. Yet the discovery triggered a media and political outcry, in the wake of which the Department of Energy (DOE) fired the lab’s contractor and ordered the reactor to be shut down.

Activists are trying to terminate the National Tritium Labeling Facility (NTLF), funded by the National Institutes of Health, at the Lawrence Berkeley National Laboratory in California. The facility, which labels tritium for research into curing diseases and other purposes, releases a minute amount of tritium in its operations. However, Berkeley’s city council has voted resolutions to shut down the facility because of tritium fears. Activists cited a consultant’s view that a catastrophic fire storm, consuming the entire tritium inventory of the NTLF, could expose a damaging amount to a jogger passing a few metres away. (This possibility was found to be exaggerated by several orders of magnitude.)

Anti-NTLF activists have also targeted a famous museum for closure – the Lawrence Hall of Science at the University of California at Berkeley. The tritium exposure there, due to the nearby NTLF, is about 0.02 millirems a year (one-thirtieth of the level permitted by US regulations). A single round-trip plane flight from Berkeley to New York exposes passengers to more radiation than a year spent living at the museum. Yet the Alameda county school board voted in favour of a moratorium on student field trips to the museum, although the resolution was later modified merely to note “differences of opinion” about the danger.

The National Ignition Facility at the Lawrence Livermore National Laboratory, also in California, has been targeted by activists claiming that its tritium releases (equal to that from an average hospital that conducts in vitro research) will be life-threatening.

All of these stories reveal that tritium poses a potential threat to any modern scientific and healthcare institutions that use tritium or occasionally produce it, in however minute quantities.

Non-toxic tritium tales

Yet tritium is not always harmful. Consider the following stories.

Last year the High Flux Isotope Reactor at the Oak Ridge National Laboratory in Tennessee was discovered to be leaking tritium-containing water. The amount of tritium in the resulting plume appears to be greater than that at the HFBR at Brookhaven, yet the consequences for the Oak Ridge contractor were not severe. Indeed, the Department of Energy wants to upgrade the reactor.

The DOE’s Savannah River site, which has produced tritium and plutonium for the US weapons-production programme, released hundreds of thousands of curies of tritium a year into the environment up until the 1990s. Now it releases less than 50,000 curies a year.

In June 1997 a Brookhaven radiation-protection team received a call from the New Jersey State Health Department about a 16-year-old New Jersey boy who, as a prank, had removed several tritium-powered exit signs from a dump, broken them open and ingested some tritium. His urine samples showed a concentration of 28.6 million picocuries per litre – almost 20 times the peak concentration in the tritium that had seeped from the spent-fuel pool at Brookhaven.

Yet the New Jersey health authorities determined that the incident did not create a health hazard. They also concluded that there was no possibility that the boy could have received enough of a radiation dose to cause an immediate health effect, such as radiation sickness.

The critical point

I have heard scientists speak of public scares involving minute amounts of tritium as cases of irrationality – “modern Salem stories”. This is wrong: there are underlying reasons. Fear of tritium can be generated and exploited in the presence of “social prereceptors” – specific sensitivities to external stimuli. Ideologues can then treat minute amounts of tritium not as a health issue but as a tool to further political agendas or Utopian crusades.

Many scientific instruments – from accelerators to reactors – create tritium in minute amounts. Given this, isn’t an in-depth study of these social prereceptors important to the future of science? Considering that so many public-health institutions – from hospitals to clinical research programmes – use tritium in indispensable ways, isn’t such an analysis of interest to any socially progressive individual?

Copyright © 2026 by IOP Publishing Ltd and individual contributors