Hello. I'm Matin Durrani, and welcome to a very special edition of the Physics World weekly podcast, where we're going to be revealing our top 10 breakthroughs in physics for 2024. And we've been running the Physics World breakthrough of the year since 2009, and there's been some amazing successes during that time, including the Higgs boson, gravitational waves, and the shadow around a black hole.
So taking you through our choices this year, I'm joined by Margaret Harris, Tammy Freeman, Michael Banks, and Hamish Johnson. And before we start, a quick message from our sponsors, IOP Publishing's flagship journal reports on progress in physics. Do visit ioppublishing.org/ropp to find out more. So let's get cracking on the top 10 breakthroughs in physics for 2024, and we always pick them based on the following three criteria.
So in addition to having been reported in Physics World in 2024, the selections have to represent a significant advance in knowledge or understanding, be important for scientific progress and or development of real world applications, and be of general interest to Physics World's readers.
So Hamish, we're gonna start with some, quantum physics because, it happens to be International Year of Quantum Science and Technology next year, and that this was some work we reported back in July, where physicists used entangled photons, which I think are pretty weird in their own right, and they used it for something really useful for imaging purposes, and particularly in biological samples. And have I got this right? They imaged a bee.
That's right. Yeah. Actually, I I should point out that that this, particular entry is 2. It's a superposition of 2, bits of research that were done last year. So you're right. Yes. 1 had to do with imaging bees, but the other was sort of slightly weirder.
And, yeah, quantum mechanics. Wow. I mean, I remember when I started studying it almost 40 years ago, entanglement was sort of a curiosity that you sort of looked at and shrugged your shoulders and went on to, I don't know, calculate the energy levels of a of a quantum harmonic oscillator. But today, entanglement forms the basis of a growing number of quantum technology. And, this top ten entry, as I said, involves 2 new imaging techniques that rely on entanglement.
And the first was developed by Chloe Vigner and Hugo Dauphine at Sorbonne University in Paris. And they've managed to use entangled photons to encode an image into a beam of light. And the magical thing is that the image is only revealed to an observer who can detect single photons. So you have to use a special single photon detector. And if you don't use that detector, you can't see the image. And, of course, no guesses for what image they in they
encoded. It was a cat, of course. So either the cat was there or not, depending on what sort of detector that you used. And and these 2 researchers, a little earlier this year, joined forces with Patrick Cameron at the universe, the UK's University of Glasgow and others to create another entanglement based, imaging system. And that's the one that you referred to, where they looked at sort of insect insects and and and things like that.
And this uses entangled photons to enhance adaptive optics imaging. And adaptive optics is something I think it was initially, developed by astronomers to deal with the turbulence in the atmosphere. And the idea is that you you sort of measure the turbulence, between you and what you want to look at. And then, you somehow try to correct for it.
And, it's been very successful in astronomy and other, imaging techniques, particularly for, you know, biological materials that are always sort of changing and and becoming turbulent. But here, they've used, entangled photons. And the technique gets rid of something called a guide star. You can see the, the astronomy connection here. So it simplifies the correction process quite a bit. And, so, that's that technique.
And both of these techniques, could have a range of uses in microscopy and other imaging applications as well as optical communications. So two clever uses of entanglement. That's our first entry. Brilliant, Hamish. So that was, some work well, 2 pieces of work on entanglement. So thanks, Hamish. Now Tammy, we can turn to you. Still on a sort of biological
theme. I really like this next choice, which we covered in September, and that involved researchers using the optical properties of a common yellow dye to image a mouse. I mean, this sounds really unbelievable. So, do you wanna talk us through this one? So this is all about imaging biological tissue. Now one of the difficulties when you're using optical imaging techniques is that tissue scatters light, which makes it opaque so you can't see through it.
And the problem arises because of different components in tissue, such as water and lipids, have different refractive indices, and that's what causes the scattering. So, the researchers from Stanford University in the US developed a way to make biological tissue transparent, and they did this using the common food dye, tartrazine, which they applied to the skin of live mice.
So this dye absorbs light in the near UV and blue parts of the spectrum, which alters the refractive index of the water at red wavelengths such that it more closely matches that of lipids in this part of the spectrum. Basically this reduces the refractive index contrast between the water and the lipids and makes the tissue appear more transparent.
So to test this idea, the researchers rubbed the dye on the abdomen, scalp, and hind limbs of the live mice and they saw that their skin became transparent in just a few minutes. They could then see the animal's internal organs, such as their liver, small intestine, and bladder through the skin. They could also observe fluorescent protein labeled nerve cells in the abdomen, visualize blood flow in their brains, and see the fine structure of muscle fibers in their legs.
And then when you don't need transparent mice anymore the effect can be reversed by simply rinsing off the dye. So so far this has only been used with animals, but the researchers say that if it was extended to humans there could be a lot of benefits. So for example, doctors might be able to diagnose deep tumors simply by imaging a person's tissue and that would remove the need for an invasive biopsy. It could also make blood draws less painful by helping to locate veins under the skin.
And the team is now working to find other dye molecules with greater efficiency in achieving this tissue transparency. So there's there's lots of potential ahead with this. So something amazing to think about next time you're doing some cooking with, food dye, that type. But, Tammy, we always keep a cut a close eye on medical physics here at Physics World.
And, the 3rd top ten breakthrough of the year was for some research that involved modeling cells in the lung in a way that could help us personalize radiotherapy treatments. And now that sounds really potentially good news for patients. Yeah. Indeed. So over half of all patients with lung cancer are treated using radiotherapy. And while this is an effective treatment, it leaves up to a third of recipients with radiation induced injuries.
And and this potential to damage healthy tissue also limits the dose that can be delivered to the tumor. So, to minimize the risk of radiation damage, researchers from the University of Surrey in the UK, GSI Helmholtz Centre For Heavy Iron Research in Germany, and Massachusetts General Hospital Harvard Medical School in the US have created a computational model that could improve radiotherapy outcomes for patients with lung cancer.
So, what they did is they developed an agent based model, which consists of separate interacting units or agents that mimic lung cells coupled with Monte Carlo simulations. And, the model simulates irradiation of lung tissue, in this case 18 alveoli, which are the tiny air tiny air sacs within the lungs, and it simulates these at microscopic and nanoscopic scales.
Then based on the radiation dose that it predicts will be delivered to each cell and its distribution, the model predicts whether each cell will live or die. It can also determine the severity of radiation damage, hours, days, months, or even years after treatment. So this is all really useful information.
Now, importantly, the researchers found that their model produced results that matched experimental observations from various labs and hospitals, and this suggests that it could, in principle, be used within clinical setting.
So, currently, radiotherapy fractionation, which is the scheme of small doses or fractions used to deliver the entire radiation course over time, so that this scheme isn't optimized for individual patients, but the researchers hope that one day their model could be used to create personalized treatment schemes for each patient. So basically, they'd use information from an individual's scans, biopsies, and other tests from feed this into the model.
And then it would output a tailored treatment plan that would improve the patient's chances of survival. So as you say, really potentially great news for patients. Thanks, Tammy. Now, Michael, we always love a bit of, astronomy and space science here at Physics World. And, one of the highlights for us this year was China's Changi 6 mission, which has returned samples from the dark side of the moon, for the first time, or should I say the far side of the
moon. And, that's made it onto our top 10 breakthroughs. Yeah. That's right. So while the moon's so called near side, so that's the side of the moon that always faces Earth, has been relatively well studied through, robotic missions, as well as crude ones. The so called far side, that's the side that doesn't face earth, hasn't. And that's partly because, due to its tricky terrain, it's got a lot not much flat surfaces and a lot of, giant craters and so
so forth. Yet scientists, of course, are interested in the relatively unexplored far side and why it happens to look so different from the near side. So in 2019, China launched the Changi 4 mission, which was the first to perform a soft landing on the moon's far side. It consisted of a lander and a rover, and together they made various measurements of the chemical composition of nearby rocks.
But, of course, to carry out more thorough measurements, scientists ideally need to bring samples back to Earth. And that happened actually for the first time this year, thanks to the Changi 6 mission, which launched on the 3rd May. It touched down in the Apollo basin and in June managed to bring back to Earth, around 2 kilograms of material.
So scientists are now feverishly analyzing these samples, and some first results were published in November in which they found that the lunar far side was volcanically active at least 2,800,000,000 years ago. So thanks to this monumental achievement, we can now expect many more discoveries from those 2 kilograms of soil and rock fragments, over the coming months years ahead. So thanks, Michael. That's, 4 of the top 10 breakthroughs of the year done. So back to you,
Tammy. It wouldn't be a Physics World top 10 without something going on at that hotbed of physics, CERN. But, our next choice isn't particle physics, but something else going on at what is probably the world's biggest physics lab, and it involves cooling atoms. Okay. So, well, the atom in question is actually positronium, which is an atom like system made of an electron and a positron.
Now positronium is currently created in warmish clouds, in which the atoms have a large distribution of velocities, so this makes it difficult to do precision spectroscopy because the motion of all the atoms will broaden the measured spectral lines. So if you could cool the positronium to low temperatures you could open up new more accurate ways to study the properties of antimatter.
Now, this breakthrough nomination is given to 2 teams: the AEGIS collaboration at CERN, and a research team at the University of Tokyo, who simultaneously demonstrated laser cooling of clouds of positronium. Now, this laser cooling of positronium is actually really difficult to achieve, as positronium is really unstable and annihilates itself after 140 nanoseconds. So this means that the laser pulses must interact with the positronium cloud faster than it decays.
So to do this, the CERN team start the cooling process by containing a cloud of positrons in a penning trap and then shooting the positrons through a nano channel silicon converter, where they bind to electrons on its surface and create the positronium. So, this stage acts as a pre cooling step.
Next, the positronium atoms are collected in a vacuum chamber where their laser cooled, and this involves the atoms absorbing and re emitting photons from a laser and losing kinetic energy in the process. And using this method the CERN team was able to reduce the temperature of the positronium cloud from 380 to 170 ks.
But but why is this important? So, for 1, reducing the velocity of positronium enables researchers to produce 1 to 2 orders of magnitude more anti hydrogen, and this is an anti atom comprising a positron and an antiproton, and this is of really great interest to physicists for their experiments. Also, this research paves the way to using positronium to test aspects of the standard model, such as quantum electrodynamics, for example, or to probe the effects of gravity on antimatter.
So there's lots of really exciting studies ahead here. So thanks, Tammy. So that was all about cooling a gas of positronium, but going down to an even smaller scale to the nuclear physics level, we've got a cool bit of research, that involves measuring the decay of helium nuclei. But was it done in a rather unusual way? Marg Margaret, what's what's this one about?
Yeah, Matine. So this works actually on a bigger scale than positronium because it involves helium nuclei, so 2 protons and 2 neutrons. Helium nuclei sometimes called alpha particles. And you may remember stories about them from your history of physics classes, about people studying radioactivity by watching radioactive atoms spit out these alpha particles as they decay.
And that's kind of what the physicists in this next piece of research did, except they actually measured the recoil due to this ejection of the alpha particle. And they did this by embedding a big, fluffy, unstable radioactive atom, lead 212, in a microscopic sphere made of silica. And then levitating this microscopic sphere using optical tweezers, which is basically a focus laser beam with
a microsphere trapped at the focus. And once they had this microsphere suspended in space, they used the principle of conservation of momentum to determine when the radioactive atom in the microsphere decayed and kicked out a helium nucleus. This is really cool because, like, one of the researchers involved, Zhangxing Wang at Yale University, compared to to a person throwing a ball while they're standing on a skateboard. As soon as the ball gets thrown, the skateboard rolls backwards.
It's really pretty cool to be able to measure that back reaction. It involves physics everyone's heard about, and it might have applications for detecting neutrinos in the future. So that's why it's in our top ten. So thanks, Margaret. And, still on the nuclear physics theme, Hamish, we've picked a piece of research that we reported back in October. And I know you're always passionate about making
sure we, don't ignore theoretical physics. And this one combines those two areas, theory and nuclear physics. So this one's quite subtle. So, I'm waiting for you to explain it to us. Well, I'll do my best. So, we have a category on the Physics World website called particle particles and nuclei, or nuclear and particle. And that sort of makes the suggestion that nuclear physics and particle physics are somehow different. But, you know, the reality is that, a nucleus is just a big particle.
And but traditionally, there have been 2 different ways of looking at nuclei. The particle physics view is that nuclei are a bunch of quarks, lots of quarks, in the case of a lead nucleus, for example, that are all held together by gluons, which mediate the strong force. And then, there's the nuclear physics point of view. And that sort of assumes that nuclei comp comprise protons and neutrons, so individual particles, distinct particles, that again, interact with each other via gluons,
and the strong force. But, of course, the real picture is somewhere in between. And it's very complicated, which is, of course, the reason why we tend to have the two views of the nucleus. But now, 2 physicists, Andrew Denniston at MIT and Thomas Gesso at the University of nucleus. And that's when the distinction is to each other in a nucleus. And that's when the distinction is blurred. You know, are they individual protons or neutrons, or is it just a bunch of quarks,
getting very close together? So they've tested and refined their model with experimental data, that from nuclei that range from tiny helium, all the way up to hefty lead. And they've found very good agreement. So we see this as an important step forward in understanding nuclear structure and also the strong force itself. So very fundamental. And that's why it's, it's in our top ten this year. Sounds a worthy contender. And, Hamish, back down to earth and the real world, if you
like. And now back in July, we reported on researchers who've made a new kind of titanium sapphire laser. Now those aren't lasers aren't new, but this one seems to be a lot cheaper and smaller and kind of better in lots of different ways. Yeah. The titanium sapphire laser is is a very, very useful device, in science and technology. They produce powerful beams of both narrow band continuous light, and they can also be used to create broadband pulses of light.
And the latter have been crucial in the development of frequency cones. And and these are lasers that have played important well, they play important roles in metrology and spectroscopy, including atomic clocks. So this is a central laser technology. But the problem so far is that they tend to be very big and very expensive. So their use is is fairly limited.
But now, Yelena Vukovich and colleagues at Stanford University in the US have created a titanium sapphire laser that is a 1000 times smaller, and possibly even more important, a 1000 times cheaper than existing systems. So, I I think in the in the news story, that we wrote about this, we described it as a potential democratization of the laser, in that there's gonna be a lot more, scientific and and also technological applications of of this device.
Of this device. You know, for for example, these lasers can be used, in spectroscopy, but, you know, you can't really take it out of the lab, or if you try to take it out of the lab, it's it's can't really take it out of the lab, or if you try to take it out of the lab, it's going to be very
difficult. But if you've got this tiny sort of titanium sapphire laser on a chip, you can imagine lots of, you know, for example, environmental monitoring applications that you could use, and industrial applications as well. So who knows? Maybe there'll be one coming to your mobile phone soon, you know, allow you to do spectroscopy on, on on, I don't know, the food you buy in the shop to make sure it's still fresh. I'm not sure I
wanna tie Saflids in my pocket. I'm not sure in the shop to make sure it's still fresh. I'm not sure I wanna tie saffles in my pocket. There's powerful stuff there. Well, we'll see. Yeah. So yeah. That I mean, we thought that that was a huge, scientific and technological breakthrough. So, yep, that's why it's on our list this year. So thanks, Hamish. So we're down to the final 2 top 10 breakthroughs of the year for 2024.
And, well, 20 years on from the creation of Graphene for the first time, Margaret, we'll be recognizing some work in that area. So what's this, pick all about? Yeah. So just to remind our listeners, Graphene is this two dimensional sheet of carbon atoms arranged in a hexagonal lattice like honeycomb. So back to the bees again. And when it was first isolated 20 years ago, there was a lot of excitement about its properties. It's a great conductor of electricity,
for example. It's better than a metal in that respect. And there was almost an expectation that it would revolutionize electronics. That revolution hasn't quite happened yet for a variety of reasons. It took time to work out how to make graphene well and reliably and in the right quantities with the right parameters and so on. But this year has seen some exciting developments. We've made 2 of them a joint entry in our top ten because they both involve electronic switching.
The first piece of research we're covering here was led by Marcelo Lozada Hidalgo of the University of Manchester. And his team used the fact that graphene is good at conducting electrons and protons to create a switch that can perform both logic and memory operations in a single device. So roughly speaking, there are some more complicated details, of course, they use the proton current in graphene to do the logic operation
and the electron current from memory storage. And they were able to control those two things independently. Now putting those two functions together is important because today's computers definitely don't do that. If you look inside a standard silicon based computer chip, you'll find that the circuit elements that do that do the logic operations are physically separated from the ones that do the memory operations. And that separation increases data transfer times and raises power consumption.
So a device that can avoid those two things, those two problems, is pretty cool, actually. The other piece of research that we've squeezed into this entry for our top ten comes from Walter Daher and Lei Ma at the Georgia Institute of Technology in the US and Tianjin University in China. And they kind of did the opposite. They worked out how to make a form of graphene that doesn't conduct electricity quite so well.
This new form of graphene is called epigraphene, and it's a semiconductor, like silicon. And that means it has a band gap that electrons have to overcome if the current is going to flow. That might sound like a disadvantage, but it's that band gap that means you can make transistors, which are really just a fancy kind of switch, out of silicon. Except in Graphene, the electron mobility is 10 times higher than in silicon.
DeHir had a nice analogy. He compared electron mobility in silicon to driving on a gravel road. Whereas in epigraphene, you're cruising on electron freeway. It's more efficient. It doesn't heat up as much, and allows for higher speeds so that electrons can move faster. And again, you have the potential to create much more efficient electronic devices. And that's why it's in our top ten. Excellent, Margaret. So that was, graphene. And finally, we've got some work. Well, again, it's 2
pieces of work that we've, squeezed in. And one was quite late breaking, and this has to do with quantum computing. I mean, there's so much going on in that area, Margaret. But, these two bits of work really stood out, didn't they? Yes. As you say, this is another 2 for 1 entry. And it's kind of nice because one of the results, as you say, came out just a couple days ago, actually. And the other came out, funnily enough, just after we announced our list of last year's top 10 breakthroughs.
So the works in this breakthrough kind of bookend the year and they're both about error correction in quantum computing. The first of them chronologically comes from Misha Lukin and colleagues at Harvard, MIT, Queer Computing, and the NIST University of Maryland Joint Center For Quantum Information and Computer Science, all in the US. At the very tail end of last year, they surprised the quantum community really by announcing that they created a quantum processor with 48 logical cubits.
Now, if you follow quantum computing a little bit but not super closely, that might not sound like a big deal. There are other devices out there that have more than a 1000 cubits. So what's the big deal about 48? Well, you may have noticed, if you're listening carefully, that I said that they were logical cubits. And what that means is that there are additional physical cubits in the system that are keeping these logical cubits in line and correcting their errors.
That's really important because errors are the Achilles heel of quantum computers. That's because the quantum superposition states they run on are really fragile and very prone to lose their quantum nature due to noise from their environment. So being able to correct those errors while manipulating multiple cubits at once is huge. And interestingly, Lukin and colleagues did it with a device that uses neutral atoms as cubits.
This is a cubit platform, as they call it, that until very recently had kinda lagged behind superconducting circuits and trapped ions in terms of its Like I said, this result was surprising. The second more recent work, really hot off the presses, comes from a team at Google Quantum AI. Their result was maybe less surprising since it happened not only after Lukin's work, but also after a previous Google Quantum AI result that fell just short of the goal.
But what this team did is just as impressive because they demonstrated a quantum processor with an error rate that goes down as you add more physical cubits to it. In technical terms, they showed that it's possible to do error correction below what's called the surface code threshold, which is the point where an error correction code called the surface code, lets you start to, in effect, win the game against noise in your machine. And the implications of that are huge.
It opens the door to making quantum computers that are much more powerful and capable of running much more complex algorithms than anything we've seen so far. Now there's still a long way to go before we get a quantum computer that's gonna solve important real world problems. But this result gives us more confidence that that's going to be possible eventually. And that's why it's in our list of this year's top 10 breakthroughs.
That sounds incredible, Margaret. So that was, the final entry in the physics world top 10 breakthroughs for the year. So as you've heard, there's been quantum physics, nuclear physics, biological, medical, atomic, space science, condensed matter, and lasers. But to find out which one of those bagged the overall top prize for the physics world breakthrough of the year, We'll be announcing on Thursday, 19th December, in a news story and in the next episode of the Physics
World weekly podcast. So do stay tuned for that. So finally, big thanks to Tammy, Margaret, Hamish, and Michael for joining me. Thanks to our sponsors, Reports on Probes in Physics, and to our producer, Fred Iles. And thanks for listening, and do join us again next week for the big reveal.