Category Archives: Quantum Physics

Scientists at USC Dornsife determine that electrons traveling through proteins between bacteria and solid surfaces outside the cell tend to adopt a particular quantum spin, a finding that could impact future electronic technologies, including spintronics. [3 min read]

Electrons that shuttle through protein wires between bacterial cells and surfaces outside those cells tend to have a particular quantum spin. (Illustration: Katya Kadyshevskaya.)

Electrons spin. Its a fundamental part of their existence. Some spin up while others spin down. Scientists have known this for about a century, thanks to quantum physics.

Theyve also known that magnetic fields can affect the direction of an electrons quantum spin, flipping it from up to down and vice versa. And it doesnt take much: Even a bacterial cell can do it.

Researchers at USC Dornsife College of Letters, Arts and Sciences and Israels Weizmann Institute of Science have found that protein wires connecting a bacterial cell to a solid surface tend to transmit electrons with a particular spin.

This ability to select an electrons quantum spin could have implications for the use of bacteria in the biotechnology industry and in burgeoning efforts to create bacteria-based energy cells, as well as future electronic technologies, they said.

Life on the rocks

Led by USC Dornsifes Moh El-Naggar, professor of physics and astronomy and chemistry, and Ron Naaman of the Weizmann Institute, the scientists have been studying certain bacteria that can use solid surfaces in the same way animals use oxygen to breathe. Instead of dumping electrons generated during metabolism onto inhaled oxygen molecules, the bacteria send the electrons down specialized proteins that plug into an external surface.

Sahand Pirbadian studies how proteins in rock-breathing bacteria select electrons quantum spin. (Photo: Tingting Yang.)

Unlike most organisms that are able to use oxygen as the electron acceptor, said USC Dornsife Senior Research Associate Sahand Pirbadian, these bacteria transfer the electrons to a solid mineral or, as they do in our lab, to electrodes that are outside the cell.

In terms of metabolism, they breathe the minerals or electrodes.

To reach the external surface, the electrons are shuttled through various protein molecules that form electrical conduits. These proteins have magnetic fields that can favor a particular spin as the electrons shuttle through.

Scientists found, says Pirbadian, that these magnetic fields are affected by a characteristic of the proteins called chirality.

A few words about chirality

Many molecules, especially biological molecules, appear in two versions, each a mirror image of the other. Scientists call this chirality. Its similar to human hands. Left and right hands have five fingers and a thumb, but theyre not exactly the same. Theyre both hands, but theyre mirror images of each other, oriented in opposite directions. Molecules can be the same way, and in fact, scientists refer to chiral molecules as being either left-handed or right-handed.

The left- or right-handedness of a protein may affect the polarity of the magnetic fields experienced by the electrons as they shuttle through the protein. Thats what happens to those electrons that travel along a protein wire to get to the outside of a rock-breathing bacterium, according to the researchers.

By the time the electrons traverse the molecule wire, the majority end up having the same quantum spin up or down depending on the chirality, said El-Naggar, who holds the Robert D. Beyer (81) Early Career Chair in Natural Sciences. This study is the first to confirm that the electrically conductive proteins in these cells are selecting the spin of electrons.

Putting the spin to use

El-Naggar and his colleagues have studied these rock-breathing bacteria, which one day might be used to produce sustainable energy, for years. Finding that the electron-conducting proteins in these bacteria can select for a particular electron spin based on their chirality could be useful in developing certain electronic devices called spintronics, El-Naggar says. Spintronics use not only the charge of electrons but also their quantum spin and may be especially useful in quantum computing.

There is an ongoing hunt for materials that can serve as the basis for new spintronic technologies, said El-Naggar. Our work shows that bacterial cytochromes may be interesting candidates for spintronics.

Understanding how proteins affect electrons quantum spin could also help the scientists understand how magnetic fields affect some biological processes.

About the study

Additional authors on the study include Suryakant Mishra, Amit Kumar Mondal and Ron Naaman of the Weizmann Institute of Science. The study appears as a cover story in the Dec. 11, 2019, issue of the Journal of the American Chemical Society. The study was supported by the U.S. Office of Naval Research Multidisciplinary University Research Initiative Grant No. N00014-18-1-2632 and by the Templeton Foundation and the Israel Science Foundation.

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'Rock-breathing' bacteria are electron spin doctors, study shows - USC Dornsife College of Letters, Arts and Sciences

Gauge invariance with cold atoms

There is considerable interest in developing quantum computational technologies that can simulate a series of physical phenomena inaccessible by classical computers. Mil et al. propose a modular scheme for quantum simulation of a U(1) lattice gauge theory based on heteronuclear spin-changing collisions in a mixture of two bosonic quantum gases isolated in single wells of a one-dimensional optical lattice. They engineered the elementary building block for a single well and demonstrate its reliable operation that preserves the gauge invariance. The potential for scalability of the proposed scheme opens up opportunities to address challenges in quantum simulating the continuum limit of the gauge theories.

Science, this issue p. 1128

In the fundamental laws of physics, gauge fields mediate the interaction between charged particles. An example is the quantum theory of electrons interacting with the electromagnetic field, based on U(1) gauge symmetry. Solving such gauge theories is in general a hard problem for classical computational techniques. Although quantum computers suggest a way forward, large-scale digital quantum devices for complex simulations are difficult to build. We propose a scalable analog quantum simulator of a U(1) gauge theory in one spatial dimension. Using interspecies spin-changing collisions in an atomic mixture, we achieve gauge-invariant interactions between matter and gauge fields with spin- and species-independent trapping potentials. We experimentally realize the elementary building block as a key step toward a platform for quantum simulations of continuous gauge theories.

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A scalable realization of local U(1) gauge invariance in cold atomic mixtures - Science Magazine

Decades after Einstein described quantum entanglement as spooky action at a distance, it was still seen as a quirkyif fascinatingfeature of quantum theory. But that changed in the early 1990s when physicists predicted that entanglement would allow the state of one particle to be transferredor, teleportedto another. Suddenly entanglement was not just something weird, but something useful, says Guifre Vidal, a graduate student at the time.

To know how useful a quantum state was, you had to know how much entanglement it contained, and this was exactly what Vidal and other theorists wanted to quantify. In a 2002 paper, Vidal, by then a postdoc at the University of Innsbruck, Austria, and his coauthor Reinhard Werner of the Technical University of Braunschweig, Germany, focused on a measure of entanglement that would apply to mixed states. Compared with the pure states that had been considered before, mixed states are more representative of those that can be created in experiments.

The measure, known as logarithmic negativity, was attractive to experimentalists because it allowed them to compare entanglement in different setups, says Vidal. Negativity is still used today to assess, for example, new ways of creating entangled photons for quantum information technologies. But Vidal, who now works at X (formerly, Google X), values the entanglement measures for a different reason. In his view, it led researchers to powerful mathematical tools for describing quantum systems, called tensor networks. This formalism has influenced the development of quantum gravity, quantum field theory, quantum simulation, and artificial intelligence.

Jan Sperling, a theorist who recently began leading a group at Paderborn University in Germany, says Vidal and Werners paper influenced the way physicists characterize a quantum systems unique characteristicsin essence, its quantumness. It also laid the groundwork for so-called quantum resource theories. Much like thermodynamics sets limits on the efficiency of an engine, a resource theory defines the capabilities of a quantum system.

Sperling says he was drawn to the fundamental and application-oriented sides of entanglement, which he sees as increasingly converging. It is interesting to observe how basic concepts of quantum physics have inspired and revolutionized quantum information technologies, potentially benefiting society as a whole.

G. Vidal and R. F. Werner, Computable measure of entanglement, Phys. Rev. A 65, 032314 (2002).

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50 Years of Physical Review A: The Legacy of Three Classics - Physics

The world of physics was turned on its head once quantum mechanics was discovered, and since that point, in time, the topic has been a big topic of debate in the scientific world.

One of the leading physicists who are pursuing an understanding of quantum mechanics is theoretical physicist Sean Carroll. In Carroll's new book, 'Something Deeply Hidden: Quantum Worlds And The Emergence Of Spacetime' he pushes forth the Many World's Theory, or parallel universe understanding quantum mechanics. Now, this is where things get tricky to understand, and rather than attempting to recite such a complex topic, I'd advise you to watch the above video.

Carroll is taking his theory to Australian audiences in "Our Preposterous Universe" tour, and has also recently spoken to News.com.au about his theory. According to News.com.au, Carroll's base theory on observing an electron in the quantum state is that the electron is actually in all of its possible positions at once, but just different parallel universes. Here's what is stated by the publication, "Out of the known mechanics of quantum states must emerge multiple, parallel worlds."

Carroll says, "But there's a lot more going on, not every world you imagine actually comes true. There are still equations, physical rules, patterns that must be obeyed. Some possible alternate worlds can come true. But not all of them."

If you are after anymore information regarding what Carroll has to say about his theory, check out this link here.

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Parallel Universes do exist and we will find them, says physicist - TweakTown

A conservationist in France haas uncovered a highly valuable first edition copy of the great physicist Isaac Newton's pioneering text Philosophi Naturalis Principiaone of the most important works in the history of science and mathematics.

The textwhose title translates as Mathematical Principles of Natural Philosophy from the original Latinwas first published in 1687 and consists of three books.

In the manuscript, the English scientist famously outlined his three laws of motion, which formed the bedrock of classical mechanicsa branch of physics which deals with the movement (or equilibrium) of bodies under the influence of forces.

The first edition text in question was found by Vannina Schirinsky-Schikhmatoff, a director of conservation at the Fesch public heritage librarylocated in Ajaccio on the French island of Corsica the AFP reported.

The library, which contains around 50,000 books, was founded by Lucien Bonaparte, a brother of the famous statesman and military leader Napolon Bonaparte who was Emperor of France at the beginning of the 19th century.

Schirinsky-Schikhmatoff came across the workoften referred to simply as the "Principia"while examining an index created by Lucien Bonaparte. The conservationist noted the text appeared to be surprisingly well preserved.

"I found the Holy Grail in the main room, hidden in the upper shelves," Schirinsky-Schikhmatoff told the AFP. "The cover has a little damage but inside it's in excellent conditionthis is the cornerstone of modern mathematics."

After initial publication of the work, English translations were printed. However, first edition Latin copies of the Principiawhich were intended for distribution on the European mainlandare highly prized, given that only around 400 were produced, The Guardian reported. Only about half of these are thought to remain in existence today.

One such first edition copy became the most expensive printed science book ever sold in December 2016, after an anonymous buyer paid around $3.7 million for it during an auction at Christie's in New York.

The Principia helped to shape the evolution of modern physics. In fact, the work was famously described by Albert Einstein as "perhaps the greatest intellectual stride that it has ever been granted to any man to make."

In the work, Newton outlines his universal physical laws of gravitation and motion, helping to explain phenomena which were previously described by renowned scientists such as Copernicus, Galileo, and Kepler, according to Christie's.

"For the first time a single mathematical law could explain the motion of objects on earth as well as the phenomena of the heavens," a Christie's description of the book read. "It was this grand conception that produced a general revolution in human thought, equaled perhaps only by that following Darwin's Origin of Species."

The famous three laws of motion can be described as follows, according Stanford University:

While the advent of Einstein's theory of relativity and the rise of quantum mechanics posed a challenge to Newton's ideas, the laws outlined in Principia are still key to our understanding of the universe, centuries after publication.

The masterwork may never have been published, however, if it wasn't for the efforts of the renowned English astronomer Edmond Halleythe man who Halley's comet was named after. Halley encouraged Newton to produce a text outlining his ideas and edited the work once written. The astronomer also covered most of the printing costs because the Royal Societythe world's oldest independent scientific academyhad run out of money at the time and was no longer able to finance the project.

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Rare Copy of Isaac Newton's Masterwork Discovered on French Island - Newsweek

This month in Insights & Outcomes, Yale scientists take a deep dive into thermodynamics, discover a new drug to treat seizures, and unify the algebra of everything.

As always, be sure to keep tabs on the latest research news in the Science & Technology and Health & Medicine pages on YaleNews. For now, grab your lab coat and lets go:

Its long been understood that you can eliminate wasted energy in a thermodynamic change from a cars speed to the biological processes in a cell as long as the change occurs slowly enough. But theres a limit to this, according to assistant professor of physics Benjamin Machta and graduate student Samuel Bryant.

In a new study, they report that all controlled thermodynamic changes must consume a minimum amount of energy, regardless of speed. Previous analyses have missed an important, but easily overlooked point, Bryant said. The control mechanism responsible for changing the object under consideration must necessarily waste some energy. Machta and Bryant said their work is particularly relevant for biology, where processes such as the release of calcium in muscles, do seem to be paying substantial costs in energy. The study appears in Proceedings of the National Academy of Sciences.

The prognosis for people suffering from chronic epilepsy is often poor, even after surgery and with anti-seizure medication. Now a Yale team has found that an experimental drug which targets a protein linked to two genetic disorders associated with intractable epilepsy reduces seizures and cell abnormalities in mouse models of the conditions. This is a completely new treatment with unexpected benefits, said Yales Angelique Bordey, professor of neurosurgery and of cellular and molecular physiology and senior author of the research.

Bordey and first author Longbo Zhang said they hope the drug that targets the protein FLNA might help reduce seizures in those with epilepsy as well as two genetic disorders, tuberous sclerosis complex and focal cortical dysplasia type II, which is found in a subset of patients. The study appears in Science Translational Medicine.

A study led by Yale emergency medicine assistant professor Edouard Coupet II, M.D., found that the Affordable Care Act (ACA) is not associated with a change in opioid overdoses (ODs). The researchers found no link between expansion of insurance for young adults under the ACA (via extended dependent coverage) and any fatal prescription, non-prescription opioids such as heroin, or methadone overdoses, nor emergency department encounters. Such a link had been suggested by advocates of ACA repeal. Researchers compared a group that was eligible before the ACA passed to a group that became eligible after ACA passed.

Coupet, a National Institute on Drug Abuse-sponsored Yale Drug Use, Addiction, and HIV Scholar, said the findings underscore the importance of increased insurance coverage in providing mental health and addiction services to vulnerable populations. Around the time when young adults are seeing a lot of behavioral health issues manifest, including substance use disorders, expanding access to providers allows them to pursue addiction treatment, he said. The study appears in the Journal of General Internal Medicine.

For more than a century, classical mechanics and quantum mechanics have been thought to be very different. Peter Morgan, a laboratory associate in physics, now argues that they can be unified. In a new study, Morgan takes an algebraic approach to demonstrating that classical and quantum mechanics are equally capable of modeling measurements and analyzing measurement results. A more precise understanding of their relationship, Morgan said, enables a unification of collapse and no-collapse interpretations of quantum mechanics. This recognition provides for new approaches to the unification of general relativity, which is essentially classical, and the standard model of particle physics, which is essentially quantum, he said. The study appears in Annals of Physics.

Researchers at Yale and the University of Michigan found, to their surprise, that people do not report bias toward emergency room physicians based on gender or race. The study enrolled 3,592 participants from across the U.S. and asked them to imagine that they had been admitted to the ER for stomach pain. Participants were presented with a physicians image a white man, white woman, black man, or black woman and two conflicting diagnoses, a conservative one established by the physician, and a more aggressive one (appendicitis) that the participant self-diagnosed using a web-based source. Researchers asked a series of questions related to confidence in the diagnosis and treatment plan. They found no loss of confidence or satisfaction in physicians based on race or gender.

These results have different implications for different groups. The results are positive from an organizational and policy standpoint, and make a strong case for continuing to build a diverse and inclusive physician workforce, said Basmah Safdar, M.D., associate professor of emergency medicine, who specializes in sex and gender-specific research. Safdar said the findings do not invalidate the many negative personal experiences women and black physicians experience due to patient bias. The study appears in JAMA Network Open.

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Insights & Outcomes: Thermodynamics and the algebra of everything - Yale News

Wormhole traveling is mainly known for being a very fictional concept, and the general assumption when it comes to wormhole traveling seems to be that it has always stayed that way. However, one physicist believes that this may actually be proven to be true.

Express reports that Harvard physicist Daniel Jafferis weighed in on the concept of traveling through a wormhole and wormholes in general. Dr. Jafferis explained that it would take a long time to go through a wormhole, much longer compared to direct travel, and thus it would not be useful when it comes to navigating space. In his work, Dr. Jafferis hopes to make a quantum theory of gravity, which is something that scientists have always wanted to achieve for years.

Dr. Jafferis revealed that the key import of his work is its link to the black hole information problem as well as the link between gravity and quantum mechanics. I think it will teach us deep things about the gauge/gravity correspondence, quantum gravity, and even perhaps a new way to formulate quantum mechanics, He added.

The reason why wormholes would take longer to travel through is that the black holes, which essentially serve as openings to these wormholes, are not in a straight line. Dr. Jafferis likened wormhole traveling to quantum teleportation via black holes. However, just getting to a black hole is a challenge in itself as the nearest black hole to Earth is trillions of miles away.

Speaking of black holes, it was previously reported that this celestial enigma tends to reject the basic laws of quantum mechanics, including Albert Einsteins theory of relativity simply by consuming physical information and making it disappear. This is a concept that many scientists have debated and failed to agree on one definitive explanation to this object.

According to Italian astrophysicist, Fabio Panucci during a Ted Talk back in November of 2019, Throughout history, paradoxes have threatened everything we know, and just as often, they reshape our understanding of the world. Panucci continued to explain that one of the biggest paradoxes in the universe, referred to as the black hole information paradox, threatens to rewrite the theory of relativity as well as quantum mechanics.

The questions it raises for physics are far more urgent, that destruction of information would force us to rewrite some of our most fundamental scientific theories.

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Physicist believes that wormhole travel is possible - EconoTimes

What is the fastest way to heat a system which is coupled to a temperature-controlled oven?

The obvious answer is to utilize just the hottest temperature accessible.

According to a new theoretical study, cooling, as the first step before heating, maybe the fastest way to warm up certain materials. Such precooling could lead in some cases to exponentially quicker heating.

The concept is similar to the Mpemba effect, a process in which hot water can freeze faster than cold water.

Scientists, despite everything, dont concede to why the Mpemba impact occurs, and it isnt very easy to repeat the effect reliably.

Physicist Andrs Santos of Universidad de Extremadura in Badajoz, Spain, said, The new study is a way of thinking of effects like the Mpemba effect from a different perspective.

This potential for faster heating doesnt apply to pizza slices, yet to specific simplified theoretical models of materials, which scientists use to make calculations that assist them in understanding real materials.

Physicists Amit Gal and Oren Raz of the Weizmann Institute of Science in Rehovot, Israel, considered a theoretical framework called the Ising model, a 2-D grid of atoms which have magnetic poles that point either up or down. Specifically, they considered a version of the Ising model in which neighboring atoms tended to point their poles in opposite directions, behavior which is called antiferromagnetic. In that framework, heating could occur faster after a pre-cooling phase.

Gal said, For the new effect to occur, there must be some relevant property of the system other than a uniform temperature that is affected by the precooling. Otherwise, thered be no difference between a system that had been precooled and rewarmed, and one that hadnt. The temperature cannot tell the whole story.

When it comes to the antiferromagnetic Ising model, Scientists considered the overall magnetization produced from all the atoms and also the number of magnets pointed in the opposite direction of their neighbors. Cooling the material could change the ratio between those two properties in a way that would allow heating to proceed more quickly.

Physicist Adolfo del Campo of the Donostia International Physics Center in Spain said, The prospects are exciting. Scientists have been searching for ways to speed up the heating in tiny machines that follow the rules of quantum mechanics and can bypass some of the limits of standard machines (SN: 4/1/19). If the effect can be exploited in such minute machines, it would [be] quite handy.

The study is published in the Physical Review Letters.

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Cooling certain materials is the fastest way to heat them - Tech Explorist

Before it's observed, an electron is a hot mess of possibility. Just like the metaphorical Schrdinger's cat, it's only once we lift the lid from its metaphorical box and take a good, close look that an electron settles on a clear position around an atom.

We've now had a closer look at exactly how this settling happens. By taking a series of snapshots of a strontium ion held in an electric field, a team of physicists from Sweden, Germany and Spain have found an electron's transition from 'maybe' to 'reality' isn't quite an all or nothing affair.

For the better part of a century it's been fairly clear that the Universe we experience in our daily lives isn't quite like the one we see when we try to look at it up close.

One extraordinary consequence of the strangeness at the heart of physics is objects can only be described using sets of probabilities called superpositions - up until we poke them with probes and bombard them with light to determine for certain their size and nature.

In our classical world of absolutes, this is hard to picture. Even the famous physicist Erwin Schrdinger mocked the idea when he first heard it, posing a thought experiment involving an imagined cat that was at once alive and dead until we looked.

Only by opening the box and observing is the cat's potential life either sustained or extinguished, at least in the eyes of the observer.

Schrdinger found it silly, as did Einstein, but since then it's been shown time and time again that this metaphoricalcat is indeed an accurate description of the way physics works.

One question that remains is whether there's such a thing as an ideal quantum measurement, one that can measure aspects of a system without causing its entire superposition to collapse into a final answer.

In the 1940s, the American-Hungarian mathematician John von Neumann figured that measuring one part of a quantum system such as the position of an electron in an orbit would create sufficient quantum noise to it all to give up its probabilistic nature.

Years later, a German theoretical physicist named Gerhart Lders contested von Neumann's assumptions, pointing out that some undecided qualities of a particle's possibilities could stick around even while others become clear.

While physicists have agreed with Lders in theory, it's not the easiest thing to demonstrate experimentally, relying on measuring certain actions that occur naturally in a way that they don't interfere with one another.

The researchers settled on an atom of strontium with missing electrons, trapping the ion in a way that makes it unclear which of two orbits the remaining electrons are in, leaving them in a smear of both.

It's more or less the same set-up used in many quantum computers. A laser then forces the superposition of electrons in the ion to move, with the potential shift in orbit confirmed by detecting the light that's emitted as the electron falls back into place.

Only on detection of the light can we consider the absolute position of the electron as locked in place.

"Every time when we measure the orbit of the electron, the answer of the measurement will be that the electron was either in a lower or higher orbit, never something in between," says Stockholm University physicist Fabian Pokorny.

"The measurement in a sense forces the electron to decide in which of the two states it is."

Capturing numerous photons as the strontium ion is rotated into different states with separate laser provided the team with a picture of the process's evolution as it took place over a span of a millionth of a second.

They found that the transition of the quantum system from maybe to actually isn't an absolute affair. Aspects of it can be measured, such as the final resting place of the electron, while leaving some features of its superposition untouched and undecided. Just as Lders had argued.

"These findings shed new light onto the inner workings of nature and are consistent with the predictions of modern quantum physics", says lead researcher Markus Hennrich, also a physicist from Stockholm University.

What's more, this shift isn't instantaneous. By taking snapshots of the atom as one of its electrons adopts a clear orbit, the team showed the change is an unfolding one, as if the transition from complete uncertainty into a specific orbit is a matter of increasing probability, rather than a sudden decision.

This isn't the first experiment to show how quantum jumps in an electron's possibility is an unfolding process like "the eruption of a volcano", rather than a switch. But it does add some interesting details to the way this change occurs that allows for such ideal measurements.

Sadly none of this tells us what a transition of quantum possibilities into a clear measurement means in the grand scheme of things, let alone how to think of Schrdinger's poor cat as it waits patiently in the darkness.

All we know is lifting the lid on the poor animal doesn't rob it completely of its mystery. Even if it risks a slower death than von Neumann might have imagined.

This research was published in Physical Review Letters.

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Physicists Have Filmed The Moment an Atom Undergoes a Quantum Measurement - ScienceAlert

February 28, 2020• Physics 13, 25

A new attempt to detect the neutrons electric dipole moment tightens the constraints on theories of symmetry breaking in the early Universe.

The neutron is electrically neutral, but it might still have an asymmetric internal distribution of charge, a property quantified by the so-called electric dipole moment (EDM). A nonzero neutron EDM is predicted by several theories that seek to extend the standard model of particle physics, and an international collaboration has now placed a new upper limit on how big it could be. To determine the new limit, the team measured the interaction of a cloud of neutrons with electric and magnetic fields. The finding could help theorists narrow the options for explanations of the imbalance between matter and antimatter in the Universe.

For a full understanding of the physical laws of nature, the standard model is not enoughit cant, for example, reconcile quantum mechanics with general relativity. Researchers believe that a more complete theory is likely to involve breaking of the standard models so-called CP (charge-parity) symmetry of the strong force, which binds quarks into baryons (such as neutrons and protons). If such a symmetry violation did indeed occur in the early Universe during the formation of baryons, it would explain why there are many more baryons than antibaryons.

CP violation is predicted to create an asymmetric distribution of quarks inside the neutron, which would result in a nonzero EDM. So a neutron EDM would be a kind of fossil remnant of the symmetry breaking that occurred in the early Universe. And the size of the EDM, if it exists, might offer clues to when the event happened.

Attempts to measure the neutron EDM have been made since the 1950s [1]. More recently, researchers have also looked for the EDM of the electron [2] and of the nuclei of mercury atoms [3]. An attempt to detect a neutron EDM in 2006 [4] by an international team using the neutron source at the Laue-Langevin Institute (ILL) in France found no sign of it within the experimental uncertainty (an improved data analysis was published in 2015 [5]). A new effort led by Philipp Schmidt-Wellenburg of the Paul Scherrer Institute (PSI) in Switzerland and Guillaume Pignol of the Laboratory of Subatomic Physics and Cosmology (LPSC) in France has now searched with more sensitivity than any previous experiment.

A neutron does have a magnetic moment, so it rotates (precesses) in an applied magnetic field. The frequency of precession can be deduced by exciting it to a higher energy state with microwaves. If it has an EDM, an applied electric field should alter this precession frequency, but if not, the electric field would have no effect. Schmidt-Wellenburg and his colleagues looked for such a change in the precession frequency as they held ultracold neutrons from the PSI source in electric and magnetic fields within a cylindrical polystyrene chamber half a meter across. The main innovations of this experiment were in techniques and sensors for detecting variations in the magnetic field, allowing the team to make this field extremely uniform in space and to subtract the effects of the fields temporal fluctuations from the results.

The well-controlled magnetic field allowed the neutrons to precess in a coherent way for over 30 minutes. The researchers could only collect data for about three minutes because of the rate at which the neutrons were lost by collisions with the chambers walls, but that was still 50% longer than in the 2006 experiment [4]. The longer measuring time led to a more sensitive measurement. The researchers placed an upper limit of 1.81026e-cm on the magnitude of the neutron EDM (where e is the charge on the electron), which is almost half the previous best value [4, 5].

Timothy Chupp, an atomic physicist at the University of Michigan, Ann Arbor, who has made previous measurements of the EDM of neutrons and xenon atoms, calls the experiment beautiful work that provides the first new data in two decades. Molecular physicist David DeMille of Yale University in New Haven points out that the 2016 study of the EDM of the mercury nucleus [3] also estimated a limit on the neutron EDM based on some assumptions, and the result was close to the new one. But he says that the new experiment is cleaner theoretically and can be interpreted with much greater confidence.

At this stage, the tighter constraint on the neutron EDM doesnt rule out any candidate theories, says Schmidt-Wellenburg. But he adds that the result, taken together with those for other particles such as the electron, suggests that strong CP violation may have occurred earlier than the separation of the electromagnetic force from the weak nuclear force in the early Universe.

This research is published in Physical Review Letters.

Philip Ball

Philip Ball is a freelance science writer in London. His latest book isHow To Grow a Human (University of Chicago Press, 2019).

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Focus: New Limit on the Neutron's Internal Charge Asymmetry - Physics