When new assistant professor of physics Lee McCuller was young, he liked to build things. His uncle made him a power supply, which he integrated with electronic hobby kits from RadioShack to do simple things like use analog circuits to switch lights and motors on and off. Today, McCuller tinkers with what some would call the most advanced measurement device in the world: LIGO, or the Laser Interferometer Gravitational-wave Observatory.

McCuller is an expert on quantum squeezing, a method used at LIGO to make incredibly precise measurements of gravitational waves that travel millions and billions of light-years across space to reach us. When black holes and collapsed stars, called neutron stars, collide, they generate ripples in space-time, or gravitational waves. LIGO's detectorslocated in Washington and Louisianaspecialize in picking up these waves but are limited by quantum noise, an inherent property of quantum mechanics that results in photons popping in and out of existence in empty space. Quantum squeezing is a complex method for reducing this unwanted noise.

Research into quantum squeezing and related measurements ramped up as far back as the 1980s, with key theorical studies by Caltech's Kip Thorne (BS '62), Richard P. Feynman Professor of Theoretical Physics, Emeritus, along with physicist Carl Caves (PhD '79) and others worldwide. Those theories inspired the first experimental demonstration of squeezing in 1986 by Jeff Kimble, the William L. Valentine Professor of Physics, Emeritus. The next decades saw many other advances in squeezing research, and now McCuller is at the leading edge of this innovative field. For example, he has been busy developing "frequency-dependent" squeezing that will greatly enhance LIGO's sensitivity when it turns back on in May of this year.

After earning his bachelor's degree from the University of Texas at Austin in 2010, McCuller attended the University of Chicago, where he earned his PhD in physics in 2015. There he began work on an experiment called the Fermilab Holometer, which looked for a speculative type of noise that would link gravity with quantum mechanics. It was during this project that McCuller met LIGO scientists, including MIT's Rai Weisswho together with Thorne and Barry Barish, the Ronald and Maxine Linde Professor of Physics, Emeritus, won the Nobel Prize in Physics in 2017 for their groundbreaking work on LIGO. McCuller was inspired by Weiss and the LIGO project and decided to join MIT in 2016. He became an assistant professor at Caltech in 2022.

In the future, McCuller hopes to take the quantum measurement tools he has developed for LIGO and apply them to other problems. "If LIGO is the most precise ruler in the world, then we want to make those rulers available to everyone," he says.

We met with McCuller over Zoom to learn more about quantum squeezing and its future applications to other fields as well as what inspired McCuller to join Caltech.

After I graduated from University of Chicago in 2015, I went to work on LIGO at MIT. When I walked in the door, they were having a meeting about the first detection of gravitational waves! The public didn't know yet, but there had been rumors. It was exciting to learn the rumors were true, and it was nice to see everyone overjoyed that things were working.

There was a local experiment taking place at that time on using squeezed light in the frequency-dependent manner that will start up at LIGO later this year. My job was to help build the first full-scale demonstration of this. The group, before me, had previously demonstrated the concept but not at the full scale. I was there was to show exactly what would be needed to employ it in the LIGO observatories. This required a particularly challenging experimental setup.

At each of the observatory locations, LIGO uses laser beams to measure disturbances in space-timethe gravitational waves. The laser beams are shot out at 90-degrees from each other and travel down two 4-kilometer-long arms. They reflect off mirrors and travel back down the arms to meet back up. If a gravitational wave passes through space, it will stretch and squeeze LIGO arms such that the lasers will be pushed out of sync; when they meet back up, the combined laser will create an interference pattern.

At the quantum level, there are photons in the laser light that hit the mirrors at different times. We call this shot noise, or quantum noise. Imagine dumping out a can full of BBs. They all hit the ground and click and clack independently. The BBs are randomly hitting the ground, and that creates a noise. The photons are like the BBs and hit LIGO's mirrors at irregular times. Quantum squeezing, in essence, makes the photons arrive more regularly as if the photons are holding hands rather than traveling independently. And this means that you can more precisely measure the phase or frequency of the light inside LIGOand ultimately detect even fainter gravitational waves.

To squeeze light, we are basically pushing the uncertainty inherent in light waves from one feature to another. We are making the light more certain in its phase, or frequency, and less certain in its amplitude, or power [the uncertainty principle says that both the exact frequency and amplitude of a light wave cannot be known at the same time]. To really explain the details of how squeezing actually works is very hard! I primarily know how to use math to describe it.

An interesting thing about squeezed light is that we aren't doing anything to the actual laser. We don't even touch it. When we operate LIGO, we offset the arms so that its wave interference is not perfectly darka small amount of light gets through. The little bit of light that remains has an electrical field that interferes with quantum fluctuations in the vacuum, or empty space, and this leads to the shot noise or the photons acting like BBs as we talked about earlier. When we squeeze light, we are actually squeezing the vacuum so that the photons have lower uncertainty in their frequency.

Up until now, we have been squeezing light in LIGO to reduce uncertainty in the frequency. This allows us to be more sensitive to the high-frequency gravitational waves within LIGO's range. But if we want to detect lower frequencieswhich occur earlier in, say, a black hole merger, before the bodies collidewe need to do the opposite: we want to make the light's amplitude, or power, more certain and the frequency less certain. At the lower frequencies, the shot noise, our BB-like photons, push the mirrors around in different ways. We want to reduce that. Our new frequency-dependent cavity at the LIGO detectors is designed to reduce the frequency uncertainty in the high frequencies and the amplitude uncertainties in the low frequencies. The goal is to win everywhere and reduce the unwanted mirror motions.

Part of the reason this technology is more important in the next run is because we are turning up the power on our lasers. With more power, you get more pressure on the mirrors. Our new squeezing technology will allow us to turn the power up without creating the unwanted mirror motions.

What this means is that we will be even more sensitive to the early phases of black hole and neutron star mergers, and that we can see even fainter mergers.

One project I'm working on involves Kathryn Zurek and Rana Adhikari. We are building a tabletop-size detector that will attempt to pick up signatures of quantum gravity, or pixels in space and time as some people say. The idea there is to make interferometers more like high-energy-physics detectors. The detectors would click when something passes through it, largely circumventing the impacts of shot noise. I love the motivation of the projectquantum gravity, which is the quest to merge theories of gravity with quantum physics. It is a very lofty goal.

In general, what I hope to do is grow from the LIGO work and apply quantum measurement techniques to not only enhance the gravitational wave detectors but also to see where other fundamental physics experiments or technologies can be improved. I want to use quantum optics not necessarily for computation or for information but for measurement. Squeezing light is one of the first demonstrations of these concepts in a real experiment. The hope is that we can keep using these quantum techniques in more and more experiments. We want to take the advantages of LIGO and find all the places where we can apply them.

Caltech has a lot of mission-oriented scientists. It's not just about learning or demonstrating or exploringit's the mix of all these things. I like a place where the goal is to integrate technologies and do new experiments. Take LIGO for instance. Few people know how the whole thing works and many of them are here. Caltech is a place where people understand that what we are doing is hard. Good projects require both narrow and broad expertise, and a combination of the right people. The students are similarly motivated by both the science goals and the process. We are not just trying to build something that reliably works, we are also trying to build something that's at the edge of what is possible.

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At the Edge of Physics - Caltech

Titans says DO cross the streams

The multiverse, every mega-franchises (and Oscar-winning non-franchise) favorite idea can often seem like homework. Why, it is reasonable to wonder, does every other blockbuster movie ticket come with a five-minute lecture on quantum physics for casuals? It doesnt have to be this way. Instead, the multiverse can just be like it is on the most recent episode of Titans: a goofy nod to a long-running franchises history that is great fun but ultimately not that vital to the experience.

With the all-timer episode title Dude, Wheres My Gar?, Titans which resumed its fourth and final season after a five-month hiatus this month sends Beast Boy, a.k.a. Gar Logan (Ryan Potter) on a metaphysical trip across the DC multiverse. Hes being coached by Dominic Mndawe/Freedom Beast on how to connect with The Red, the DC Universes cosmic force that ties together all animal life, and counterpart to The Green, which connects all plant life.

This results in Gar getting briefly lost in the multiverse, where he glimpses mostly through archival footage and audio numerous DC film and television adaptations, including Cesar Romeros Joker, Grant Gustins Flash, Zachary Levis Shazam, and even animated Beast Boy from Teen Titans Go!. He also meets a few characters in the flesh, namely Stargirl (Brec Bassinger) and being reunited with Cyborg from Doom Patrol.

Gars trip through the multiverse isnt as expansive or ambitious as the Arrowverses truly bonkers Crisis on Infinite Earths crossover its more of an Easter egg than a big event but theres a sense of fun to it that makes it a counterweight to the grave stakes of something like the MCUs Multiverse Saga.

It also helps that DC has a multiverse to speak of since the company has never worked as hard (or as successfully) as its competitor to craft a continuous cinematic universe, it can just retroactively label every DC TV and film property as part of the multiverse. And, according to my copy of Quantum Physics for Dummies, that all makes perfect sense.

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Titans connects every DC show, movie in big Beast Boy multiverse scene - Polygon

String theorys unexpected and robust mathematical relationships may hold the key to understanding the universe, even if they don't neatly fit our current understanding of reality, writes Tasneem Zehra Husain.

Is string theory worth pursuing? This question, like the perennial Jack-in-the-Box, never stays down for long and - like said toy - creates a commotion every time it arises. Before calm is (temporarily) restored, it seems there are certain tunes that must be played. Typically, heres how it goes:

Critics dismiss string theory on the charge that its not science. A successful scientific theory must incorporate known physical phenomena and make verifiable predictions about the natural world. String theory hasnt, so its just a fantasy, they say; it should be abandoned. Proponents argue this assessment is superficial. They agree that concrete, testable, predictions are an essential feature of a mature theory - and remain an active goal of research - but string theory is still growing. It is not old enough to be oracular. More time is needed, they say; condemning it now would be premature.

SUGGESTED READINGEric Weinstein: The String Theory WarsBy Alexis Papazoglou

You have had decades, the critics object. String theorists respond with a list of the many times this has happened before. From atoms (postulated two and a half millennia before being observed), to gravitational waves (detected a hundred years after prediction), the Higgs boson (found after a half-century long search), quantum entanglement (an empirically falsifiable prediction took three decades to formulate; verification took two more), and countless others. String theory would not be the first theory to ask for a bit of patience, and none has ever had better reason! Lest it be drowned out by the noise, string theorists reiterate the magnitude of the problem at hand. They are attempting to obtain the fundamental equations that encompass everything which unfolds in the universe - from the Big Bang to now, from light years to the Planck length - and then, they need to figure out how to test the implications of these equations.

Todays state-of-the-art technology can probe length scales up to 10^-17cm; the Planck length is ten million billion times smaller. Direct experimental verification is obviously not an option, but the complications of string theory are not limited to technology. Mathematically, too, the theory is immensely complex - more intricate than anything else we have chanced upon. It is any wonder that progress is slow, string theorists ask.

From here on, it plays out much as you might expect. Arguments and counterarguments weave back and forth until the landscape makes an appearance and - at the mention of these 10^500 (or so) possible universes the theory describes - we reach the crescendo.

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They are attempting to obtain the fundamental equations that encompass everything which unfolds in the universe - from the Big Bang to now, from light years to the Planck length - and then, they need to figure out how to test the implications of these equations

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The discussion is so animated, so diverting, that its easy to lose track of the fact that when we debate whether or not string theory is true, we limit ourselves to only one kind of truth. Most evaluations of string theory are based on how well it describes, explains, and predicts observations; in short, on physical truth. This is - as it should be - the central criterion. Physics is, after all, the study of what manifests, not a catalog of what could have been. The importance of physical truth is obvious and uncontested. What is mentioned far less often, is mathematical truth; this too, plays a role.

Just to be clear, physical and mathematical truths cannot - and should not - be conflated. They are neither equivalent nor interchangeable. Every mathematical equation need not be physically realized, but the converse does not hold. Every observable phenomenon is, simply by virtue of existing, a logically consistent system and therefore, mathematically expressible in principle.

But not yet in practice. Certain physical phenomena - black holes for instance - refuse to be rendered mathematically by the tools we currently have at our disposal. The equations evade us, but they exist. How do we know this? Because black holes do not cause the universe to implode. Had the laws of black holes been incompatible with the physics of the rest of the universe, one - or both - would collapse. But the universe exists, and black holes exist within it; so the equations governing black holes should exist also, as part of the mathematical framework that undergirds the universe. Neither quantum field theory nor general relativity rises to the challenge.

SUGGESTED VIEWINGThe code to the cosmosWith Marika Taylor, Peter Woit, Arif Ahmed, David Malone

We know exactly what the problem is: quantum field theory and general relativity - each unprecedentedly successful in their own domains (the former rules the very small, the latter the very heavy) - are fundamentally incompatible. As long as we restrict ourselves to systems that are either very small or very heavy - which, arguably, covers most of the universe - everything works out beautifully. There are, however, a few exotic places where the theories overlap; places where so many secrets are squeezed into tiny spaces that the very small becomes the very heavy. Here, where the deepest structure of the universe is revealed - at the big bang, or inside black holes - where both theories should hold sway, neither utters anything remotely coherent.

It is clear that we need a new paradigm. What isnt nearly as clear, is how we should go about obtaining it. The attitudes and assumptions of our two reigning theories are so different, it is difficult to imagine how they could be reconciled. Heres the crux of the issue: Quantum field theory describes the interactions of fundamental particles against a fixed space-time; should all the world be a stage, elementary particles would be the players and quantum field theory the script. But in general relativity, spacetime - the stage - is responsive and continually reacts to what unfolds upon it. In the vast majority of situations, we can get away with applying either quantum field theory, or general relativity; we pick either a play enacted upon a static stage, or a graceful dance in an ever-changing arena.

Should we try to implement both theories together, we create an untenable situation - a true postmodern nightmare! Each step the actors take, every gesture they make, causes the sets and stage to morph. With the ground moving under their feet, actors scramble to adapt to this transforming environment; their actions are modified constantly to keep pace with the shifting context, and the script is perpetually rewritten to somehow make sense. It doesnt. No matter how artfully we construct their lines, all the actors end up screaming infinities.

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The discussion is so animated, so diverting, that its easy to lose track of the fact that when we debate whether or not string theory is true, we limit ourselves to only one kind of truth

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A simple mashup just does not work. If we are ever to achieve a coherent formulation of quantum gravity, we need an entirely novel approach. But where will this come from? If we insist on taking our cue only from that which we know that which is physically realized - we are at a dead end. We have exhausted our intuition. In order to navigate the uncharted waters we now face, we will need to access an as yet untapped source of mathematical truths.

Or, we could just choose to stay on familiar land.

* * * * * * * * * *

One of the deep mysteries of mathematics is that, simply by requiring stability and consistency, we arrive at structures that seem imbued with an inner knowing. Time and again, we start to write, only to find our pens moving in invisible grooves, tracing figures we had not envisioned, uncovering consequences we could not have foreseen. Dirac set out to express the dynamics of an electron in a manner consistent with special relativity - but his equations held also the positron, paving the path for all of antimatter. The elegant equations of general relativity, so satisfyingly spare on the surface, hid black holes, left space for a cosmological constant, and prophesied gravitational waves - none of which was expected or even, initially, welcome. The annals of physics are full of such stories.

It feels like an incredible gift each time we are lifted to these foreign places we would never have reached on foot. Once we arrive, we chart the terrain, plot paths, add them to our maps, and work out ways to get there again. Future expeditions may be planned and purposeful, but that first time - when we have no idea where we will end up - there is a definite sense of being carried. In all our years of traveling by equation, string theory is by far the most extravagant structure we have encountered.

SUGGESTED READINGString theory is deadBy Peter Woit

For an arrangement this intricate to be stable is an incredible feat in itself. Had it been nothing but a house of cards, the very fact that it stands would be commendable - but string theory has proved to be robust, even load-bearing. The architecture is unfamiliar but the design is remarkably self-consistent. The unexpected flourishes, the peculiar shapes which curve in on, and around, themselves - they all fit together seamlessly. Moreover, string theory displays a tensile strength we could not have anticipated. We have thrown problems at it, and it has grown to accommodate them; most equations would have simply collapsed under the weight. Such stability is only possible in a structure that is mathematically true.

It is an oft-told story, how strings were first uncovered by accident, on a dig for a theory of the strong interaction. They never quite belonged because no matter how you rubbed them, you just could not remove the peculiar glint from their surface It took a while for people to recognize this glint as gravity, but once that happened, there was massive jubilation. String theory, it was thought, heralded the ultimate answer. It would show us a way to not only bridge quantum field theory and gravity, but unify them. It would lead us to a place from where all the matter and forces in the universe would appear to be notes played by a string; all we know, all we can know, arising from a vibrating strand of energy. At least, that was the idea.

Things didnt quite turn out that neatly.

Equations can be quite chatty if you let them, and to those who listened, string theory had plenty to say. Its true, what the critics allege - strings did not do what we asked of them; they did more. They enlarged the space of what we thought possible. Nestled in the nooks and crannies of string theory were problems we had not prepared for, elements we could not have expected - an entire menagerie of issues was unleased; the process of taming them is an ongoing education. But, amid all the creative chaos, there have also been answers - whispered answers to questions we didnt even know to ask.

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For an arrangement this intricate to be stable is an incredible feat in itself. Had it been nothing but a house of cards, the very fact that it stands would be commendable - but string theory has proved to be robust, even load-bearing

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No one expected the theory to be correct in its initial formulation. The name is merely historic. We always knew strings could not take us to a full-blown theory of quantum gravity on their own - but in a fascinating twist, they led us to the things that might. Until string theory came along, we had only a point-particle view of the universe, which - it turns out - was very restrictive. Strings had a whole new umwelt. They saw and showed us - so much more than particles could. Open strings, probing space with mathematical tentacles, found their endpoints getting stuck in certain places; through their motion, they sketched the shapes of these invisible traps, thus revealing a whole family of higher dimensional membranes that until then, lay hidden.

String theory has offered up many revolutionary ideas - extra dimensions, large and small, and a possible explanation for black hole entropy among them - but one particular triumph is the discovery of duality. A duality is when two apparently unrelated theories - potentially containing different particles, different dynamics, operating in space-times of different shapes and dimensions - describe physically equivalent content. Or, to flip this around: duality says that the same physical situation can be modeled equally well in two unrecognizably different ways. Whats more, the descriptions work in concert with each other. Questions asked of one theory may be answered by appealing to the other. Its as if the script of a play was equivalent to the score of a symphony, and by listening to the music, actors could learn their lines and take stage directions.

The insights obtained from string theory may have emerged in the unrealistic contexts of AdS space (which we dont inhabit) or supersymmetric black holes (which cannot exist in our present universe), but the relationships they describe are not necessarily limited to those situations. At the very least, they tell us that certain arrangements and interdependencies are possible; ideally, they will turn out to be preliminary sketches - or even blueprints - for the physical world.

SUGGESTED VIEWINGFrom one universe to the nextWith Roger Penrose

This may sound like wishful thinking, but in fact the history of physics is replete with such examples. Heres one of my favorites. Imaginary numbers do not, by definition, exist in the world around us. If the purpose of numbers is solely to quantify objects in the physical world, imaginary numbers make no sense at all - and yet, they are absolutely essential in formulating the equations of both quantum field theory and general relativity. Where i on its own may be dismissed as a flight of fancy, the abstract relationship it represents is enacted over and over again in the physical world.

A mathematically consistent framework is larger than, and independent of, the context from which it emerges. It is proof that a certain set of relationships, a particular pattern of evolution, is possible - that it is logically sound and structurally stable. The identities of those who enact these relationships is irrelevant. Symbols are roles anyone can step into, as long as the choreography of the equations is obeyed. It is, thus, entirely possible to extract mathematical truths from situations that are physically unrealized - even unrealistic - and store them as models, building up a library to consult when you encounter previously unmapped phenomena.

We stand today upon a cliff edge, facing what appears to be complete chaos. We are unable to identify any forms - not because they do not exist, but because we see only that which we have trained ourselves to see. What we behold now is completely new, and so we must learn to see again. If we are to articulate what lies ahead, we need a new lexicon of words to hold these amorphous realities. We need new patterns to connect what we find here, new templates to measure against, a new list of possible interactions.

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If the purpose of numbers is solely to quantify objects in the physical world, imaginary numbers make no sense at all - and yet, they are absolutely essential in formulating the equations of both quantum field theory and general relativity

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We need an entirely new set of models to contain this strangeness, but where, and how, do we look for something when we have no idea what it looks like? There is no prescribed path, no plan to follow, no way to mount a targeted search. Instead, we must wander, leaving ourselves open to serendipitous encounters, paying attention to whatever it is we find. If the romance of this quest does not appeal, it may help to remember that, while this may not be the most efficient way forward, we dont know any other.

String theory has proved to be a veritable trove of mathematical treasure. Odd objects to be sure, but fascinating - and in the spirit of discovery, we study them. We may not know what purpose, if any, they will serve but we are in no rush to use them; they are stable, they will keep. Our mathematical cabinet of curiosities expands as, one by one, we place this motley assortment on our shelves, in the hope that one day in the future, sitting at our desk, turning over some unidentifiable phenomenon in our hands, we will hold it up to the light and find its contours eerily familiar. Perhaps there will be a moment of recognition. Perhaps we will spring to the cabinet, pull down a model from the shelves, dust it off - and see that we already have exactly what we need.

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It's too soon to dump string theory | Tasneem Zehra Husain - IAI

It is one of the most exciting races in modern physics: How can we produce the best superconductors that remain superconducting even at the highest possible temperatures and ambient pressure? In recent years, a new era of superconductivity has begun with the discovery of nickelates. These superconductors are based on nickel, which is why many scientists speak of the "nickel age of superconductivity research." In many respects, nickelates are similar to cuprates, which are based on copper and were discovered in the 1980s.

But now a new class of materials is coming into play: In a cooperation between TU Wien and universities in Japan, it was possible to simulate the behaviour of various materials more precisely on the computer than before. There is a "Goldilocks zone" in which superconductivity works particularly well. And this zone is reached neither with nickel nor with copper, but with palladium. This could usher in a new "age of palladates" in superconductivity research. The results have now been published in the scientific journal Physical Review Letters.

The search for higher transition temperatures

At high temperatures, superconductors behave very similar to other conducting materials. But when they are cooled below a certain "critical temperature," they change dramatically: their electrical resistance disappears completely and suddenly they can conduct electricity without any loss. This limit, at which a material changes between a superconducting and a normally conducting state, is called the "critical temperature."

"We have now been able to calculate this "critical temperature" for a whole range of materials. With our modelling on high-performance computers, we were able to predict the phase diagram of nickelate superconductivity with a high degree of accuracy, as the experiments then showed later," says Prof. Karsten Held from the Institute of Solid State Physics at TU Wien.

Many materials become superconducting only just above absolute zero (-273.15C), while others retain their superconducting properties even at much higher temperatures. A superconductor that still remains superconducting at normal room temperature and normal atmospheric pressure would fundamentally revolutionise the way we generate, transport and use electricity. However, such a material has not yet been discovered. Nevertheless, high-temperature superconductors, including those from the cuprate class, play an important role in technology -- for example, in the transmission of large currents or in the production of extremely strong magnetic fields.

Copper? Nickel? Or Palladium?

The search for the best possible superconducting materials is difficult: there are many different chemical elements that come into question. You can put them together in different structures, you can add tiny traces of other elements to optimise superconductivity. "To find suitable candidates, you have to understand on a quantum-physical level how the electrons interact with each other in the material," says Prof. Karsten Held.

This showed that there is an optimum for the interaction strength of the electrons. The interaction must be strong, but also not too strong. There is a "golden zone" in between that makes it possible to achieve the highest transition temperatures.

Palladates as the optimal solution

This golden zone of medium interaction can be reached neither with cuprates nor with nickelates -- but one can hit the bull's eye with a new type of material: so-called palladates. "Palladium is directly one line below nickel in the periodic table. The properties are similar, but the electrons there are on average somewhat further away from the atomic nucleus and each other, so the electronic interaction is weaker," says Karsten Held.

The model calculations show how to achieve optimal transition temperatures for palladium data. "The computational results are very promising," says Karsten Held. "We hope that we can now use them to initiate experimental research. If we have a whole new, additional class of materials available with palladates to better understand superconductivity and to create even better superconductors, this could bring the entire research field forward."

Link:

Better superconductors with palladium - Science Daily

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Even if you are not a quantum physicist, you will most likely have heard of Schrdinger's famous cat. Erwin Schrdinger came up with the feline that can be alive and dead at the same time in a thought experiment in 1935. The obvious contradictionafter all, in everyday life we only ever see cats that are either alive or deadhas prompted scientists to try to realize analogous situations in the laboratory. So far, they have managed to do so using, for instance, atoms or molecules in quantum mechanical superposition states of being in two places at the same time.

At ETH, a team of researchers led by Yiwen Chu, professor at the Laboratory for Solid State Physics, has now created a substantially heavier Schrdinger cat by putting a small crystal into a superposition of two oscillation states. Their results, which have been published this week in the journal Science, could lead to more robust quantum bits and shed light on the mystery of why quantum superpositions are not observed in the macroscopic world.

In Schrdinger's original thought experiment, a cat is locked up inside a metal box together with a radioactive substance, a Geiger counter and a flask of poison. In a certain time-framean hour, sayan atom in the substance may or may not decay through a quantum mechanical process with a certain probability, and the decay products might cause the Geiger counter to go off and trigger a mechanism that smashes the flask containing the poison, which would eventually kill the cat.

Since an outside observer cannot know whether an atom has actually decayed, he or she also doesn't know whether the cat is alive or deadaccording to quantum mechanics, which governs the decay of the atom, it should be in an alive/dead superposition state. (Schrdinger's idea is commemorated by a life-size cat figure outside his former home at Huttenstrasse 9 in Zurich).

"Of course, in the lab we can't realize such an experiment with an actual cat weighing several kilograms," says Chu. Instead, she and her co-workers managed to create a so-called cat state using an oscillating crystal, which represents the cat, with a superconducting circuit representing the original atom. That circuit is essentially a quantum bit or qubit that can take on the logical states "0" or "1" or a superposition of both states, "0+1."

The link between the qubit and the crystal "cat" is not a Geiger counter and poison, but rather a layer of piezoelectric material that creates an electric field when the crystal changes shape while oscillating. That electric field can be coupled to the electric field of the qubit, and hence the superposition state of the qubit can be transferred to the crystal. In the ETH Zurich experiment, the cat is represented by oscillations in a crystal (top and blow-up on the left), whereas the decaying atom is emulated by a superconducting circuit (bottom) coupled to the crystal. Credit: ETH Zurich

As a result, the crystal can now oscillate in two directions at the same timeup/down and down/up, for instance. Those two directions represent the "alive" or "dead" states of the cat. "By putting the two oscillation states of the crystal in a superposition, we have effectively created a Schrdinger cat weighing 16 micrograms," explains Chu. That is roughly the mass of a fine grain of sand and nowhere near that of a cat, but still several billion times heavier than an atom or molecule, making it the fattest quantum cat to date.

In order for the oscillation states to be true cat states, it is important that they be macroscopically distinguishable. This means that the separation of the "up" and "down" states should be larger than any thermal or quantum fluctuations of the positions of the atoms inside the crystal. Chu and her colleagues checked this by measuring the spatial separation of the two states using the superconducting qubit. Even though the measured separation was only a billionth of a billionth of a metersmaller than an atom, in factit was large enough to clearly distinguish the states.

In the future, Chu would like to push the mass limits of her crystal cats even further. "This is interesting because it will allow us to better understand the reason behind the disappearance of quantum effects in the macroscopic world of real cats," she says.

Beyond this rather academic interest, there are also potential applications in quantum technologies. For instance, quantum information stored in qubits could be made more robust by using cat states made up of a huge number of atoms in a crystal rather than relying on single atoms or ions, as is currently done. Also, the extreme sensitivity of massive objects in superposition states to external noise could be exploited for precise measurements of tiny disturbances such as gravitational waves or for detecting dark matter.

More information: Marius Bild et al, Schrdinger cat states of a 16-microgram mechanical oscillator, Science (2023). DOI: 10.1126/science.adf7553

Journal information: Science

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Heaviest Schrdinger cat achieved by putting a small crystal into a superposition of two oscillation states - Phys.org