## Quantum interference in time

Bosons — especially photons — have a natural tendency to clump together. In 1987, three physicists conducted a remarkable experiment demonstrating this clustering property, known as the Hong-Ou-Mandel effect. Recently, researchers at ULB’s Centre for Quantum Information and Communication have identified another way in which photons manifest their propensity to stick together. This research has just been published in Proceedings of the National Academy of Sciences.

Since the very beginning of quantum physics, a hundred years ago, it has been known that all particles in the universe fall into two categories: fermions and bosons. For instance, the protons found in atomic nuclei are fermions, while bosons include photons — which are particles of light- as well as the Brout-Englert-Higgs boson, for which François Englert, a professor at ULB, was awarded a Nobel Prize in Physics in 2013.

Bosons — especially photons — have a natural tendency to clump together. One of the most remarkable experiments that demonstrated photons’ tendency to coalesce was conducted in 1987, when three physicists identified an effect that was since named after them: the Hong-Ou-Mandel effect. If two photons are sent simultaneously, each towards a different side of a beam splitter (a sort of semitransparent mirror), one could expect that each photon will be either reflected or transmitted.

Logically, photons should sometimes be detected on opposite sides of this mirror, which would happen if both are reflected or if both are transmitted. However, the experiment has shown that this never actually happens: the two photons always end up on the same side of the mirror, as though they ‘preferred’ sticking together! In an article published recently in US journal Proceedings of the National Academy of Sciences, Nicolas Cerf — a professor at the Centre for Quantum Information and Communication (École polytechnique de Bruxelles) — and his former PhD student Michael Jabbour — now a postdoctoral researcher at the University of Cambridge — describe how they identified another way in which photons manifest their tendency to stay together. Instead of a semi-transparent mirror, the researchers used an optical amplifier, called an active component because it produces new photons. They were able to demonstrate the existence of an effect similar to the Hong-Ou-Mandel effect, but which in this case captures a new form of quantum interference.

Quantum physics tells us that the Hong-Ou-Mandel effect is a consequence of the interference phenomenon, coupled with the fact that both photons are absolutely identical. This means it is impossible to distinguish the trajectory in which both photons were reflected off the mirror on the one hand, and the trajectory in which both were transmitted through the mirror on the other hand; it is fundamentally impossible to tell the photons apart. The remarkable consequence of this is that both trajectories cancel each other out! As a result, the two photons are never observed on the two opposite sides of the mirror. This property of photons is quite elusive: if they were tiny balls, identical in every way, both of these trajectories could very well be observed. As is often the case, quantum physics is at odds with our classical intuition.

The two researchers from ULB and the University of Cambridge have demonstrated that the impossibility to differentiate the photons emitted by an optical amplifier produces an effect that may be even more surprising. Fundamentally, the interference that occurs on a semi-transparent mirror stems from the fact that if we imagine switching the two photons on either sides of the mirror, the resulting configuration is exactly identical. With an optical amplifier, on the other hand, the effect identified by Cerf and Jabbour must be understood by looking at photon exchanges not through space, but through time.

When two photons are sent into an optical amplifier, they can simply pass through unaffected. However, an optical amplifier can also produce (or destroy) a pair of twin photons: so another possibility is that both photons are eliminated and a new pair is created. In principle, it should be possible to tell which scenario has occurred based on whether the two photons exiting the optical amplifier are identical to those that were sent in. If it were possible to tell the pairs of photons apart, then the trajectories would be different and there would be no quantum effect. However, the researchers have found that the fundamental impossibility of telling photons apart in time (in other words, it is impossible to know whether they have been replaced inside the optical amplifier) completely eliminates the possibility itself of observing a pair of photons exiting the amplifier. This means the researchers have indeed identified a quantum interference phenomenon that occurs through time. Hopefully, an experiment will eventually confirm this fascinating prediction.

Two-boson quantum interference in time. Proceedings of the National Academy of Sciences, 2020; 202010827 DOI: 10.1073/pnas.2010827117

## Sci-Advent – New type of atomic clock keeps time even more precisely

This is s a reblog of an article in ScienceDaily. See the original here.

A newly-designed atomic clock uses entangled atoms to keep time even more precisely than its state-of-the-art counterparts. The design could help scientists detect dark matter and study gravity’s effect on time.

Atomic clocks are the most precise timekeepers in the world. These exquisite instruments use lasers to measure the vibrations of atoms, which oscillate at a constant frequency, like many microscopic pendulums swinging in sync. The best atomic clocks in the world keep time with such precision that, if they had been running since the beginning of the universe, they would only be off by about half a second today.

Still, they could be even more precise. If atomic clocks could more accurately measure atomic vibrations, they would be sensitive enough to detect phenomena such as dark matter and gravitational waves. With better atomic clocks, scientists could also start to answer some mind-bending questions, such as what effect gravity might have on the passage of time and whether time itself changes as the universe ages.

Now a new kind of atomic clock designed by MIT physicists may enable scientists explore such questions and possibly reveal new physics.

The researchers report in the journal Nature that they have built an atomic clock that measures not a cloud of randomly oscillating atoms, as state-of-the-art designs measure now, but instead atoms that have been quantumly entangled. The atoms are correlated in a way that is impossible according to the laws of classical physics, and that allows the scientists to measure the atoms’ vibrations more accurately.

The new setup can achieve the same precision four times faster than clocks without entanglement.

“Entanglement-enhanced optical atomic clocks will have the potential to reach a better precision in one second than current state-of-the-art optical clocks,” says lead author Edwin Pedrozo-Peñafiel, a postdoc in MIT’s Research Laboratory of Electronics.advertisement

If state-of-the-art atomic clocks were adapted to measure entangled atoms the way the MIT team’s setup does, their timing would improve such that, over the entire age of the universe, the clocks would be less than 100 milliseconds off.

The paper’s other co-authors from MIT are Simone Colombo, Chi Shu, Albert Adiyatullin, Zeyang Li, Enrique Mendez, Boris Braverman, Akio Kawasaki, Saisuke Akamatsu, Yanhong Xiao, and Vladan Vuletic, the Lester Wolfe Professor of Physics.

Time limit

Since humans began tracking the passage of time, they have done so using periodic phenomena, such as the motion of the sun across the sky. Today, vibrations in atoms are the most stable periodic events that scientists can observe. Furthermore, one cesium atom will oscillate at exactly the same frequency as another cesium atom.

To keep perfect time, clocks would ideally track the oscillations of a single atom. But at that scale, an atom is so small that it behaves according to the mysterious rules of quantum mechanics: When measured, it behaves like a flipped coin that only when averaged over many flips gives the correct probabilities. This limitation is what physicists refer to as the Standard Quantum Limit.advertisement

“When you increase the number of atoms, the average given by all these atoms goes toward something that gives the correct value,” says Colombo.

This is why today’s atomic clocks are designed to measure a gas composed of thousands of the same type of atom, in order to get an estimate of their average oscillations. A typical atomic clock does this by first using a system of lasers to corral a gas of ultracooled atoms into a trap formed by a laser. A second, very stable laser, with a frequency close to that of the atoms’ vibrations, is sent to probe the atomic oscillation and thereby keep track of time.

And yet, the Standard Quantum Limit is still at work, meaning there is still some uncertainty, even among thousands of atoms, regarding their exact individual frequencies. This is where Vuletic and his group have shown that quantum entanglement may help. In general, quantum entanglement describes a nonclassical physical state, in which atoms in a group show correlated measurement results, even though each individual atom behaves like the random toss of a coin.

The team reasoned that if atoms are entangled, their individual oscillations would tighten up around a common frequency, with less deviation than if they were not entangled. The average oscillations that an atomic clock would measure, therefore, would have a precision beyond the Standard Quantum Limit.

Entangled clocks

In their new atomic clock, Vuletic and his colleagues entangle around 350 atoms of ytterbium, which oscillates at the same very high frequency as visible light, meaning any one atom vibrates 100,000 times more often in one second than cesium. If ytterbium’s oscillations can be tracked precisely, scientists can use the atoms to distinguish ever smaller intervals of time.

The group used standard techniques to cool the atoms and trap them in an optical cavity formed by two mirrors. They then sent a laser through the optical cavity, where it ping-ponged between the mirrors, interacting with the atoms thousands of times.

“It’s like the light serves as a communication link between atoms,” Shu explains. “The first atom that sees this light will modify the light slightly, and that light also modifies the second atom, and the third atom, and through many cycles, the atoms collectively know each other and start behaving similarly.”

In this way, the researchers quantumly entangle the atoms, and then use another laser, similar to existing atomic clocks, to measure their average frequency. When the team ran a similar experiment without entangling atoms, they found that the atomic clock with entangled atoms reached a desired precision four times faster.

“You can always make the clock more accurate by measuring longer,” Vuletic says. “The question is, how long do you need to reach a certain precision. Many phenomena need to be measured on fast timescales.”

He says if today’s state-of-the-art atomic clocks can be adapted to measure quantumly entangled atoms, they would not only keep better time, but they could help decipher signals in the universe such as dark matter and gravitational waves, and start to answer some age-old questions.

“As the universe ages, does the speed of light change? Does the charge of the electron change?” Vuletic says. “That’s what you can probe with more precise atomic clocks.”

Entanglement on an optical atomic-clock transition. Nature, 2020 DOI: 10.1038/s41586-020-3006-1

## Sci-Advent – Tiny quantum computer solves real optimization problem

This is a reblog of an article in ScienceDaily. See the original here.

Quantum computers have already managed to surpass ordinary computers in solving certain tasks — unfortunately, totally useless ones. The next milestone is to get them to do useful things. Researchers at Chalmers University of Technology, Sweden, have now shown that they can solve a small part of a real logistics problem with their small, but well-functioning quantum computer.

Interest in building quantum computers has gained considerable momentum in recent years, and feverish work is underway in many parts of the world. In 2019, Google’s research team made a major breakthrough when their quantum computer managed to solve a task far more quickly than the world’s best supercomputer. The downside is that the solved task had no practical use whatsoever — it was chosen because it was judged to be easy to solve for a quantum computer, yet very difficult for a conventional computer.

Therefore, an important task is now to find useful, relevant problems that are beyond the reach of ordinary computers, but which a relatively small quantum computer could solve.

“We want to be sure that the quantum computer we are developing can help solve relevant problems early on. Therefore, we work in close collaboration with industrial companies,” says theoretical physicist Giulia Ferrini, one of the leaders of Chalmers University of Technology’s quantum computer project, which began in 2018.

Together with Göran Johansson, Giulia Ferrini led the theoretical work when a team of researchers at Chalmers, including an industrial doctoral student from the aviation logistics company Jeppesen, recently showed that a quantum computer can solve an instance of a real problem in the aviation industry.

The algorithm proven on two qubits All airlines are faced with scheduling problems. For example, assigning individual aircraft to different routes represents an optimisation problem, one that grows very rapidly in size and complexity as the number of routes and aircraft increases.

Researchers hope that quantum computers will eventually be better at handling such problems than today’s computers. The basic building block of the quantum computer — the qubit — is based on completely different principles than the building blocks of today’s computers, allowing them to handle enormous amounts of information with relatively few qubits.

However, due to their different structure and function, quantum computers must be programmed in other ways than conventional computers. One proposed algorithm that is believed to be useful on early quantum computers is the so-called Quantum Approximate Optimization Algorithm (QAOA).

The Chalmers research team has now successfully executed said algorithm on their quantum computer — a processor with two qubits — and they showed that it can successfully solve the problem of assigning aircraft to routes. In this first demonstration, the result could be easily verified as the scale was very small — it involved only two airplanes.

Potential to handle many aircraft With this feat, the researchers were first to show that the QAOA algorithm can solve the problem of assigning aircraft to routes in practice. They also managed to run the algorithm one level further than anyone before, an achievement that requires very good hardware and accurate control.

“We have shown that we have the ability to map relevant problems onto our quantum processor. We still have a small number of qubits, but they work well. Our plan has been to first make everything work very well on a small scale, before scaling up,” says Jonas Bylander, senior researcher responsible for the experimental design, and one of the leaders of the project of building a quantum computer at Chalmers.

The theorists in the research team also simulated solving the same optimisation problem for up to 278 aircraft, which would require a quantum computer with 25 qubits.

“The results remained good as we scaled up. This suggests that the QAOA algorithm has the potential to solve this type of problem at even larger scales,” says Giulia Ferrini.

Surpassing today’s best computers would, however, require much larger devices. The researchers at Chalmers have now begun scaling up and are currently working with five quantum bits. The plan is to reach at least 20 qubits by 2021 while maintaining the high quality.

Applying the Quantum Approximate Optimization Algorithm to the Tail-Assignment ProblemPhysical Review Applied, 2020; 14 (3) DOI: 10.1103/PhysRevApplied.14.034009

## Sci-Advent – Artificial Intelligence, High Performance Computing and Gravitational Waves

In a recent paper published in the ArXiV, researchers have highlighted the advantages that artificial intelligence techniques bring to the research of fields such as astrophysics. They are making their models available and that is always a great thing to see. They mention the use of these techniques to detect binary neutron stars, and to forecast the merger of multi-messenger sources, such as binary neutron stars and neutron star-black hole systems. Here are some highlights from the paper:

Finding new ways to use artificial intelligence (AI) to accelerate the analysis of gravitational wave data, and ensuring the developed models are easily reusable promises to unlock new opportunities in multi-messenger astrophysics (MMA), and to enable wider use, rigorous validation, and sharing of developed models by the community. In this work, we demonstrate how connecting recently deployed DOE and NSF-sponsored cyberinfrastructure allows for new ways to publish models, and to subsequently deploy these models into applications using computing platforms ranging from laptops to high performance computing clusters. We develop a workflow that connects the Data and Learning Hub for Science (DLHub), a repository for publishing machine learning models, with the Hardware Accelerated Learning (HAL) deep learning computing cluster, using funcX as a universal distributed computing service. We then use this workflow to search for binary black hole gravitational wave signals in open source advanced LIGO data. We find that using this workflow, an ensemble of four openly available deep learning models can be run on HAL and process the entire month of August 2017 of advanced LIGO data in just seven minutes, identifying all four binary black hole mergers previously identified in this dataset, and reporting no misclassifications. This approach, which combines advances in AI, distributed computing, and scientific data infrastructure opens new pathways to conduct reproducible, accelerated, data-driven gravitational wave detection.

Research and development of AI models for gravitational wave astrophysics is evolving at a rapid pace. In less than four years, this area of research has evolved from disruptive prototypes into sophisticated AI algorithms that describe the same 4-D signal manifold as traditional gravitational wave detection pipelines for binary black hole mergers, namely, quasi-circular, spinning, non- precessing, binary systems; have the same sensitivity as template matching algorithms; and are orders of magnitude faster, at a fraction of the computational cost.

AI models have been proven to effectively identify real gravitational wave signals in advanced LIGO data, including binary black hole and neutron stars mergers. The current pace of progress makes it clear that the broader community will continue to advance the development of AI tools to realize the science goals of Multi-Messenger Astrophysics.

Furthermore, mirroring the successful approach of corporations leading AI innovation in industry and technology, we are releasing our AI models to enable the broader community to use and perfect them. This approach is also helpful to address healthy and constructive skepticism from members of the community who do not feel at ease using AI algorithms.

## Sci-Advent – Artificial intelligence improves control of powerful plasma accelerators

This is a reblog of the post by Hayley Dunning in the Imperial College website. See the original here.

Researchers have used AI to control beams for the next generation of smaller, cheaper accelerators for research, medical and industrial applications.

Experiments led by Imperial College London researchers, using the Science and Technology Facilities Council’s Central Laser Facility (CLF), showed that an algorithm was able to tune the complex parameters involved in controlling the next generation of plasma-based particle accelerators.

The techniques we have developed will be instrumental in getting the most out of a new generation of advanced plasma accelerator facilities under construction within the UK and worldwide.Dr Rob Shalloo

The algorithm was able to optimize the accelerator much more quickly than a human operator, and could even outperform experiments on similar laser systems.

These accelerators focus the energy of the world’s most powerful lasers down to a spot the size of a skin cell, producing electrons and x-rays with equipment a fraction of the size of conventional accelerators.

The electrons and x-rays can be used for scientific research, such as probing the atomic structure of materials; in industrial applications, such as for producing consumer electronics and vulcanised rubber for car tyres; and could also be used in medical applications, such as cancer treatments and medical imaging.

Several facilities using these new accelerators are in various stages of planning and construction around the world, including the CLF’s Extreme Photonics Applications Centre (EPAC) in the UK, and the new discovery could help them work at their best in the future. The results are published today in Nature Communications.

First author Dr Rob Shalloo, who completed the work at Imperial and is now at the accelerator centre DESY, said: “The techniques we have developed will be instrumental in getting the most out of a new generation of advanced plasma accelerator facilities under construction within the UK and worldwide.

“Plasma accelerator technology provides uniquely short bursts of electrons and x-rays, which are already finding uses in many areas of scientific study. With our developments, we hope to broaden accessibility to these compact accelerators, allowing scientists in other disciplines and those wishing to use these machines for applications, to benefit from the technology without being an expert in plasma accelerators.”

## First of its kind

The team worked with laser wakefield accelerators. These combine the world’s most powerful lasers with a source of plasma (ionised gas) to create concentrated beams of electrons and x-rays. Traditional accelerators need hundreds of metres to kilometres to accelerate electrons, but wakefield accelerators can manage the same acceleration within the space of millimetres, drastically reducing the size and cost of the equipment.

However, because wakefield accelerators operate in the extreme conditions created when lasers are combined with plasma, they can be difficult to control and optimise to get the best performance. In wakefield acceleration, an ultrashort laser pulse is driven into plasma, creating a wave that is used to accelerate electrons. Both the laser and plasma have several parameters that can be tweaked to control the interaction, such as the shape and intensity of the laser pulse, or the density and length of the plasma.

While a human operator can tweak these parameters, it is difficult to know how to optimise so many parameters at once. Instead, the team turned to artificial intelligence, creating a machine learning algorithm to optimise the performance of the accelerator.

The algorithm set up to six parameters controlling the laser and plasma, fired the laser, analysed the data, and re-set the parameters, performing this loop many times in succession until the optimal parameter configuration was reached.

Lead researcher Dr Matthew Streeter, who completed the work at Imperial and is now at Queen’s University Belfast, said: “Our work resulted in an autonomous plasma accelerator, the first of its kind. As well as allowing us to efficiently optimise the accelerator, it also simplifies their operation and allows us to spend more of our efforts on exploring the fundamental physics behind these extreme machines.”

## Future designs and further improvements

The team demonstrated their technique using the Gemini laser systemat the CLF, and have already begun to use it in further experiments to probe the atomic structure of materials in extreme conditions and in studying antimatter and quantum physics.

The data gathered during the optimisation process also provided new insight into the dynamics of the laser-plasma interaction inside the accelerator, potentially informing future designs to further improve accelerator performance.

The experiment was led by Imperial College London researchers with a team of collaborators from the Science and Technology Facilities Council (STFC), the York Plasma Institute, the University of Michigan, the University of Oxford and the Deutsches Elektronen-Synchrotron (DESY). It was funded by the UK’s STFC, the EU Horizon 2020 research and innovation programme, the US National Science Foundation and the UK’s Engineering and Physical Sciences Research Council.

Automation and control of laser wakefield accelerators using Bayesian optimisation’ by R.J. Shalloo et al. is published in Nature Communications.

## Sci-Advent – New superhighway system discovered in the Solar System

Researchers have discovered a new superhighway network to travel through the Solar System much faster than was previously possible. Such routes can drive comets and asteroids near Jupiter to Neptune’s distance in under a decade and to 100 astronomical units in less than a century. They could be used to send spacecraft to the far reaches of our planetary system relatively fast, and to monitor and understand near-Earth objects that might collide with our planet.

The arches of chaos in the Solar System. Science Advances, 2020; 6 (48): eabd1313 DOI: 10.1126/sciadv.abd1313

In their paper, published in the Nov. 25 issue of Science Advances, the researchers observed the dynamical structure of these routes, forming a connected series of arches inside what’s known as space manifolds that extend from the asteroid belt to Uranus and beyond. This newly discovered “celestial autobahn” or “celestial highway” acts over several decades, as opposed to the hundreds of thousands or millions of years that usually characterize Solar System dynamics.

The most conspicuous arch structures are linked to Jupiter and the strong gravitational forces it exerts. The population of Jupiter-family comets (comets having orbital periods of 20 years) as well as small-size solar system bodies known as Centaurs, are controlled by such manifolds on unprecedented time scales. Some of these bodies will end up colliding with Jupiter or being ejected from the Solar System.

The structures were resolved by gathering numerical data about millions of orbits in our Solar System and computing how these orbits fit within already-known space manifolds. The results need to be studied further, both to determine how they could be used by spacecraft, or how such manifolds behave in the vicinity of the Earth, controlling the asteroid and meteorite encounters, as well as the growing population of artificial human-made objects in the Earth-Moon system.

## Sci-Advent – Mathematics and physics help protect the sight of patients with diabetics: Sabino Chávez-Cerda

This is a translation of the article by Antimio Cruz in Crónica. You can read the original in Spanish here. As a former student of Prof. Chávez-Cerda, I am very pleased to see that his research continues getting traction and the recognition it deserves.

Exotic Beam Theory was created by Sabino Chávez-Cerda, but for over four years it was rejected until it gained acceptance.

The Mexican scientist Sabino Chávez-Cerda has received numerous international awards over thirty years for his contributions to understanding one of the most complex phenomena in nature: light. This year, together with one of his graduates and other collaborators from Mexico and England, he presented a model that reproduces with great precision the operation of the flexible lens located behind the iris of the human eye: the crystalline lens. This work was recognised as one of the most important investigations in optics of the year 2020 by the magazine Optics and Photonics News.

Now, in conversation for the readers of Crónica, the researcher from the Instituto National de Asfrofísica, Óptica y Electrónica (INAOE) in Mexico says that we all should be made aware that science is much closer to our daily life than we imagine. For example, his studies are able to help care better for the eyes of patients with diabetes.

“My studies on how light travels have enabled me and my collaborators to make important contributions through the use of the physics and mathematics, the same that everyone is able to learn. I have made new interpretations that at first were rejected and with the availability of more evidence they have ended up being accepted”, says the man who created the Theory of Exotic Beams for which he was elected as Fellow member in 2013, one of the most important accolades in the field, by the Optical Society of America (OSA), one of the most prestigious organisations in the world.

“My recent work with the lenses of human eyes began in an interesting way. A few years ago some ophthalmologist surgeons from Puebla, Mexico, invited us to organise a seminar on how light propagates. This was because they had equipment to perform laser surgeries, but they had doubts on the subject aberrations. I then realised that what we were investigating about beams could be of great use in healthcare,” says Chávez-Cerda, who since childhood has lived in many cities in Mexico and abroad. He mentions that one of the things he most enjoys is watching the sunset on the shores of the Mexican Pacific.

“I was born in Celaya, Guanajuato, Mexico. My father was an agronomist and we had to move many times. So during my childhood and youth I lived in Nayarit, Veracruz, Guanajuato and Mexico City”, says the physicist and PhD who has also carried out research in England, China, Brazil and the United States.

COMPLEX QUESTIONS – Light is the part of electromagnetic radiation that can be perceived by the human eye and that may have a complex behaviour. It is made up of photons, which have the duality of being a wave and a massless particle. The field of science that studies light, optics, has become so diverse that today it can be compared to a tree with diverse branches: including the study of fibre optics, the use of laser light, non-linear optics and many more.

“For example, physical optics tries to understand how light travels and how it changes when an obstruction or lens is put on its path. We have all seen when a CD generates a rainbow when placed in front of a light source. This is due to the physical phenomenon called diffraction, just like holograms. What happens there is that the light is ‘spread’ and that is one of the many phenomena that we study ”, details the INAOE researcher whose individual and team work averages around 4 thousand citations in four of the main databases of scientific articles: Web of Science (WoS), Scopus, Research Gate and Google Scholar.

His long academic career is based on his bachelor’s degree from the Escuela Superior de Física y Matemáticas (ESFM) of the Instituto Politécnico National (IPN) in Mexico. He later obtained an MSc at Centro the Investigaciones en Óptica (CIO) in León, Guanajuato, Mexico and his PhD in England, at the Imperial College London (IC).

The story of how he created the theory of exotic beams may require a larger, separate text. However, it is worth saying that it started from some reports made in the 1980s by the University of Rochester claiming that it was possible to create beams of light that were not ‘spread’ or did not show diffraction. That caused a stir because it violates the laws of physics and mathematics. When the Dr Chávez-Cerda showed interest in studying the subject his own English supervisors told him that they did not believe it was worth pursuing. He dedicated though several hours to study this and after performing many calculations and computer simulations he was able to find an answer that was not immediately understood by us all: the beams of light that did not ‘spread’ were not beams, but instead apparent beams, resulting from a phenomenon called interference.

“When I proposed this theory, it was rejected for four years. Over and over again they rejected my articles, but I improved and improved my ideas until there was no argument to reject them ”, says the professor who says that since he was young he has treasured two activities that he practiced for many years and that gave him love for discipline and freedom: martial arts and regional dance.

Now, he has received awards such as the annual award from the European Optical Society and the recognition of “Visiting Foreign Researcher of Excellence” by the Government of China. He is also able to boast his graduate students; today scientists who have membership in the National System of Researchers (SNI) in Mexico. [Translator note: and a few of us that are abroad too!!! — Thanks Sabino!]

“The human virtue that I value the most is honesty,” says the teacher, husband and father of two adult sons, and two 13-year-old twins. “Throughout my life and my professional experience I have met people who, due to lack of honesty, prevent me from moving in the right direction. That is why I know that when there is honesty, one can advance and everyone can grow a lot,” says the man who remembers the day his mother took him to a new elementary school in Tepic, Mexico where they were rude to both of them”. She told me, ‘Be the best you can,’ and that’s when I became good at maths,” he shared with Crónica’s readers.

## Sci-Advent – Physicists Nail Down the ‘Magic Number’ That Shapes the Universe

This is a reblog of the article in Nautilus by Natalie Wolchover. See the original here.

A team in Paris has made the most precise measurement yet of the fine-structure constant, killing hopes for a new force of nature.

As fundamental constants go, the speed of light, c, enjoys all the fame, yet c’s numerical value says nothing about nature; it differs depending on whether it’s measured in meters per second or miles per hour. The fine-structure constant, by contrast, has no dimensions or units. It’s a pure number that shapes the universe to an astonishing degree — “a magic number that comes to us with no understanding,” as Richard Feynman described it. Paul Dirac considered the origin of the number “the most fundamental unsolved problem of physics.”

Numerically, the fine-structure constant, denoted by the Greek letter α (alpha), comes very close to the ratio 1/137. It commonly appears in formulas governing light and matter. “It’s like in architecture, there’s the golden ratio,” said Eric Cornell, a Nobel Prize-winning physicist at the University of Colorado, Boulder and the National Institute of Standards and Technology. “In the physics of low-energy matter — atoms, molecules, chemistry, biology — there’s always a ratio” of bigger things to smaller things, he said. “Those ratios tend to be powers of the fine-structure constant.”

The constant is everywhere because it characterizes the strength of the electromagnetic force affecting charged particles such as electrons and protons. “In our everyday world, everything is either gravity or electromagnetism. And that’s why alpha is so important,” said Holger Müller, a physicist at the University of California, Berkeley. Because 1/137 is small, electromagnetism is weak; as a consequence, charged particles form airy atoms whose electrons orbit at a distance and easily hop away, enabling chemical bonds. On the other hand, the constant is also just big enough: Physicists have argued that if it were something like 1/138, stars would not be able to create carbon, and life as we know it wouldn’t exist.

Physicists have more or less given up on a century-old obsession over where alpha’s particular value comes from; they now acknowledge that the fundamental constants could be random, decided in cosmic dice rolls during the universe’s birth. But a new goal has taken over.

Physicists want to measure the fine-structure constant as precisely as possible. Because it’s so ubiquitous, measuring it precisely allows them to test their theory of the interrelationships between elementary particles — the majestic set of equations known as the Standard Model of particle physics. Any discrepancy between ultra-precise measurements of related quantities could point to novel particles or effects not accounted for by the standard equations. Cornell calls these kinds of precision measurements a third way of experimentally discovering the fundamental workings of the universe, along with particle colliders and telescopes.

Today, in a new paper in the journal Nature, a team of four physicists led by Saïda Guellati-Khélifa at the Kastler Brossel Laboratory in Paris reported the most precise measurement yet of the fine-structure constant. The team measured the constant’s value to the 11th decimal place, reporting that α = 1/137.03599920611. (The last two digits are uncertain.)

With a margin of error of just 81 parts per trillion, the new measurement is nearly three times more precise than the previous best measurement in 2018 by Müller’s group at Berkeley, the main competition. (Guellati-Khélifa made the most precise measurement before Müller’s in 2011.) Müller said of his rival’s new measurement of alpha, “A factor of three is a big deal. Let’s not be shy about calling this a big accomplishment.”

Guellati-Khélifa has been improving her experiment for the past 22 years. She gauges the fine-structure constant by measuring how strongly rubidium atoms recoil when they absorb a photon. (Müller does the same with cesium atoms.) The recoil velocity reveals how heavy rubidium atoms are — the hardest factor to gauge in a simple formula for the fine-structure constant. “It’s always the least accurate measurement that’s the bottleneck, so any improvement in that leads to an improvement in the fine-structure constant,” Müller explained.

The Paris experimenters begin by cooling the rubidium atoms almost to absolute zero, then dropping them in a vacuum chamber. As the cloud of atoms falls, the researchers use laser pulses to put the atoms in a quantum superposition of two states — kicked by a photon and not kicked. The two possible versions of each atom travel on separate trajectories until more laser pulses bring the halves of the superposition back together. The more an atom recoils when kicked by light, the more out of phase it is with the unkicked version of itself. The researchers measure this difference to reveal the atoms’ recoil velocity. “From the recoil velocity, we extract the mass of the atom, and the mass of the atom is directly involved in the determination of the fine-structure constant,” Guellati-Khélifa said.

In such precise experiments, every detail matters. Table 1 of the new paper is an “error budget” listing 16 sources of error and uncertainty that affect the final measurement. These include gravity and the Coriolis force created by Earth’s rotation — both painstakingly quantified and compensated for. Much of the error budget comes from foibles of the laser, which the researchers have spent years perfecting.

For Guellati-Khélifa, the hardest part is knowing when to stop and publish. She and her team stopped the week of February 17, 2020, just as the coronavirus was gaining a foothold in France. Asked whether deciding to publish is like an artist deciding that a painting is finished, Guellati-Khélifa said, “Exactly. Exactly. Exactly.”

Surprisingly, her new measurement differs from Müller’s 2018 result in the seventh digit, a bigger discrepancy than the margin of error of either measurement. This means — barring some fundamental difference between rubidium and cesium — that one or both of the measurements has an unaccounted-for error. The Paris group’s measurement is the more precise, so it takes precedence for now, but both groups will improve their setups and try again.

Though the two measurements differ, they closely match the value of alpha inferred from precise measurements of the electron’s g-factor, a constant related to its magnetic moment, or the torque that the electron experiences in a magnetic field. “You can connect the fine-structure constant to the g-factor with a hell of a lot of math,” said Cornell. “If there are any physical effects missing from the equations , we would be getting the answer wrong.”

Instead, the measurements match beautifully, largely ruling out some proposals for new particles. The agreement between the best g-factor measurements and Müller’s 2018 measurement was hailed as the Standard Model’s greatest triumph. Guellati-Khélifa’s new result is an even better match. “It’s the most precise agreement between theory and experiment,” she said.

And yet she and Müller have both set about making further improvements. The Berkeley team has switched to a new laser with a broader beam (allowing it to strike their cloud of cesium atoms more evenly), while the Paris team plans to replace their vacuum chamber, among other things.

What kind of person puts such a vast effort into such scant improvements? Guellati-Khélifa named three traits: “You have to be rigorous, passionate and honest with yourself.” Müller said in response to the same question, “I think it’s exciting because I love building shiny nice machines. And I love applying them to something important.” He noted that no one can single-handedly build a high-energy collider like Europe’s Large Hadron Collider. But by constructing an ultra-precise instrument rather than a super-energetic one, Müller said, “you can do measurements relevant to fundamental physics, but with three or four people.”

## Quantum magic squares

Quantum magic squares

In a new paper in the Journal of Mathematical Physics, Tim Netzer and Tom Drescher from the Department of Mathematics and Gemma De las Cuevas from the Department of Theoretical Physics have introduced the notion of the quantum magic square, which is a magic square but instead of numbers one puts in matrices.

This is a non-commutative, and thus quantum, generalization of a magic square. The authors show that quantum magic squares cannot be as easily characterized as their “classical” cousins. More precisely, quantum magic squares are not convex combinations of quantum permutation matrices. “They are richer and more complicated to understand,” explains Tom Drescher. “This is the general theme when generalizations to the non-commutative case are studied. Check out the paper!

Quantum magic squares: Dilations and their limitations: Journal of Mathematical Physics: Vol 61, No 11

## 2020 Nobel Prize in Physics – Black holes

I had intended to post this much ealier on, and certainly closer to the actual announcement of the Nobel Prizes in early October. It has however been a very busy period. Better late than never, right?

I was very pleased to see that the winners of the 2020 Nobel Prize in Physics were a group that combined the observational with the theoretical. Sir Roger Penrose, Reinhard Genzel, and Andrea Ghez are the recipients of the 2020 Nobel Prize in Physics. Penrose receives half the 10 million Swedish krona while Ghez and Genzel will share the other half.

Penrose’s work has taken the concept of black holes from the realm of speculation to a sound theoretical idea underpinning modern astrophysics. With the use of topology and general relativity, Penrose has provided us with an explanation to the collapse of matter due to gravity leading to the singularity at the centre of a black hole.

A few decades after the 1960’s work from Penrose we have Genzel and Ghez whose independent work using adaptive optics and speckle imaging enabled them to analyse the motion of stars tightly orbiting Sagittarius A*. Their work led to the conclusion that the only explanation for the radio source at the centre of the Milky Way’s was a black hole.

Ghez is the fourth woman to be named a Nobel physics laureate, after Donna Strickland (2018), Maria Goeppert Mayer (1963), and Marie Curie (1903).

#### From an Oddity to an Observation

In 1916 Karl Schwarzwild described a solution to Einstein’s field equation for the curved spacetime around a mass of radius $r$. Some terms in the solution either diverged or vanished for $r=\frac{2GM}{c}$ or $r=0$. A couple of decades later, Oppenheimer and his student Hartland Snyder realised that the former value corresponded to the radius within which light, under the influence of gravity, would no longer be able to reach outside observers – the so called event horizon. Their work would need more than mathematical assumptions to be accepted.

By 1964 Penrose came up with topological picture of the gravitational collapse described and crucially doing so without the assumptions made by Oppenheimer and Snyder. His work required instead the idea of a trapped surface. In other words a 2D surface in which all light orthogonal to it converges. Penrose’s work showed that inside the event horizon, the radial direction becomes time-like. It is impossible to reverse out of the black hole and the implication is that all matter ends up at the singularity. Penrose’s research established black holes as plausible explanation for objets such s quasars and other active galactic nuclei.

#### Closer to Home

Although our own galaxy is by no means spewing energy like your average quasar, it still emits X-rays and other radio signals. Could it be that there is a black hole-like object at the heart of the Milky Way? This was a question that Genzel and Ghez would come to answer in time.

With the use of infrared (IR) spectroscopy, studies of gas clouds near the galactic centre showed rising velocities with decreasing distances to the centre, suggesting the presence of a massive, compact source of gravitation. These studies in the 1980s were not definitive but provided a tantalising possibility.

In the mid 1990s, both Genzel and Ghez set out to obtain better evidence with the help of large telescopes operating in the near-IR to detect photons escaping the galactic center. Genzel and colleagues began observing from Chile, whereas Ghez and her team from Hawaii.

Their independent development of speckle imaging, a technique that corrects for the distortions caused by Earth’s atmosphere enabled them to make the crucial observations. The technique improves the images by stacking a series of exposures, bringing the smeared light of individual stars into alignment. In 1997, both groups published their measurements stars movements strongly favouring the black hole explanation.

Further to that work, the use of adaptive optics by both laureates not only improved the resolutions obtained, but also provided the possibility of carrying out spectroscopic analyses which enabled them to get velocities in 3D and therefore obtain precise orbits.

The “star” object in this saga is the so-called S0-2 (Ghez’s group) or S2 (Genzel’s group) star. It approaches within about 17 light-hours of Sagittarius A* every 16 years in a highly elliptical orbit.

Congratulations to Ghez and Genzel, and Penrose.