Electronically programmable photonic molecule


**Electronically programmable photonic molecule
Microwave-controlled photonic molecule. a) The photonic molecule is realized by a pair of identical coupled optical microring resonators (resonant frequency ω1=ω2). The system has two distinct energy levels

Physical systems with discrete energy levels are ubiquitous in nature and form fundamental building blocks of quantum technology. Artificial atom-like and molecule-like systems were previously demonstrated to regulate light for coherent and dynamic control of the frequency, amplitude and the phase of photons. In a recent study, Mian Zhang and colleagues engineered a photonic molecule with two distinct energy levels, using coupled lithium niobate micro-ring resonators that could be controlled via external microwave excitation. The frequency and phase of light could be precisely operated by programmed microwave signals using canonical two-level systems to include Autler-Townes splitting, Stark shift, Rabi oscillation and Ramsey interference phenomena in the study. Through such coherent control, the scientists showed on-demand optical storage and retrieval by reconfiguring the photonic molecule into a bright-dark mode pair. The dynamic control of light in a programmable and scalable electro-optic system will open doors for applications in microwave-signal processing, quantum photonic gates in the frequency domain and to explore concepts in optical computing as well as in topological physics.

The results are now published on Nature Photonics, where Zhang et al. overcame the existing performance trade-off, to realize a programmable photonic two-level that can be controlled dynamically via gigahertz . To accomplish this, the scientists created a microwave addressable photonic molecule using a pair of integrated lithium niobate micro-ring resonators patterned close to each other (radius 80 μm). The combined effects of low optical loss, efficient co-integration of optical waveguides and microwave electrodes allowed the simultaneous realization of a large electrical bandwidth (> 30 GHz), strong modulation efficiency and long photon lifetime (~2 ns).

A photonic analogue of a two-level system can typically facilitate the investigation of complex physical phenomena in materials, electronics and optics. Such systems convey important functions, including unique on-demand photon storage and retrieval, coherent optical frequency shift and optical quantum information processing at room temperature. For dynamic of photonic two-level systems, electro-optic methods are ideally suited due to their fast response, programmability and possibility for large-scale integration.

Device and experimental setup detail. a) Scanning electron microscope (SEM) image of the gap between the coupled microring resonators. b) Cross-section of the optical mode profile in the ring resonator. c) Microring image of the full device …more

For electro-optic control of a two-level system, the photon lifetime of each energy state must be longer than the time required for the system to be driven from one state to the other. Conventional integrated photonic platforms have not met the requirements of a simultaneously long photon life-time and fast modulation so far. Electrically active photonic platforms (based on silicon, graphene and other polymers), allow fast electro-optic modulation at gigahertz frequencies but suffer from shorter photon lifetimes. However, pure electrical tuning is still highly desirable, as narrowband microwave signals offer much better control with minimal noise and scalability.

In their work, Zhang et al. showed that optical transmission of the photonic molecule measured using a telecom-wavelength laser, supported a pair of well-defined optical energy levels. The evanescent coupling of light from one resonator to another was enabled through a 500 nm gap between the micro-ring resonators to form the two well-resolved optical energy levels. The scientists explored the analogy between an atomic and photonic two-level system to demonstrate control of the photonic molecule.

Extended experimental setup. The device is optically pumped by a tunable telecom laser centered around 1630 nm. The light is sent through an external electro-optic modulator and polarization controllers (PLC) before coupling into the chip …more

In the experiments, light from the tunable telecom wavelength laser was launched into the lithium niobate waveguides and collected from them via a pair of lensed optical fibres. The scientists used an arbitrary waveform generator to operate microwave control signals before sending them to electrical amplifiers. The efficient overlap between microwaves and optical fields observed in the system enabled higher tuning/modulation efficiency than those previously observed with bulk electro-optic systems. Such coherent microwave-to-optical conversion can link electronic quantum processes and memories via low-loss optical telecommunication, for applications in future quantum information networks.

Zhang et al. next used a continuous-wave coherent microwave field to control a photonic two-level system. In this system, the number of photons that could populate each of the two levels was not limited to one. The splitting frequency of the system was precisely controlled up to several gigahertz by controlling the amplitude of the microwave signals. The effect was used to control the effective coupling strength between the energy levels of the photonic molecule. Coherent spectral dynamics in the photonic molecule were investigated for a variety of microwave strengths applied to the photonic two-level system. The scientists also described the controlled amplitude and phase of the system using Rabi oscillation and Ramsey interference, while using Bloch spheres/geometric representations of the photonic two-level energy system to represent the phenomena.

**Electronically programmable photonic molecule
Microwave dressed photonic waveguides. a) When the applied microwave frequency is tuned to match the mode separation, dissipative coupling leads the two photonic levels to split into four levels. This effect is analogous to Autler–Townes …more

The work allowed controlled writing and reading of light into a resonator, from an external waveguide to achieve on-demand photon storage and retrieval, a critical task for optical signal processing. To facilitate this experimentally, Zhang et al. applied a large DC bias voltage (15 V) to reconfigure the double-ring system into a pair of bright and dark modes. In the setup, the mode localized in the first ring provided access to the optical waveguides and became optically bright (bright mode). The other mode was localized in the second ring that was geometrically decoupled from the input optical waveguide to become optically dark. Accordingly, the scientists demonstrated coherent and dynamic control of a two-level photonic molecule with fields and on-demand photon storage/retrieval through meticulous experiments in the study. The work opens a path to a new form of control on photons. The results are an initial step with potentially immediate applications in signal processing and quantum .

On-demand storage and retrieval of light using a photonic dark mode. a) The photonic molecule is programmed to result in localized bright and dark modes. As a result, the bright mode can be accessed from the optical waveguide, while the …more

The design parameters of the coupled resonators provide space to investigate the dynamic control of two-level and multi-level photonic systems, leading to a new class of photonic technologies. The scientists envision that these findings will lead to advances in topological photonics, advanced computation concepts and on-chip frequency-based optical quantum systems in the near future.

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Gravity is mathematically relatable to dynamics of subatomic particles


Gravity, the force that brings baseballs back to Earth and governs the growth of black holes, is mathematically relatable to the peculiar antics of the subatomic particles that make up all the matter around us.

Albert Einstein’s desk can still be found on the second floor of Princeton’s physics department. Positioned in front of a floor-to-ceiling blackboard covered with equations, the desk seems to embody the spirit of the frizzy-haired genius as he asks the department’s current occupants, “So, have you solved it yet?”

Einstein never achieved his goal of a unified theory to explain the natural world in a single, coherent framework. Over the last century, researchers have pieced together links between three of the four known physical forces in a “,” but the fourth force, gravity, has always stood alone.

No longer. Thanks to insights made by Princeton faculty members and others who trained here, gravity is being brought in from the cold—although in a manner not remotely close to how Einstein had imagined it.

Though not yet a “theory of everything,” this framework, laid down over 20 years ago and still being filled in, reveals surprising ways in which Einstein’s theory of gravity relates to other areas of physics, giving researchers new tools with which to tackle elusive questions.

The key insight is that gravity, the force that brings baseballs back to Earth and governs the growth of , is mathematically relatable to the peculiar antics of the that make up all the matter around us.

This revelation allows scientists to use one branch of physics to understand other seemingly unrelated areas of physics. So far, this concept has been applied to topics ranging from why black holes run a temperature to how a butterfly’s beating wings can cause a storm on the other side of the world.

This relatability between gravity and subatomic provides a sort of Rosetta stone for physics. Ask a question about gravity, and you’ll get an explanation couched in the terms of subatomic particles. And vice versa.

“This has turned out to be an incredibly rich area,” said Igor Klebanov, Princeton’s Eugene Higgins Professor of Physics, who generated some of the initial inklings in this field in the 1990s. “It lies at the intersection of many fields of physics.”

From tiny bits of string

The seeds of this correspondence were sprinkled in the 1970s, when researchers were exploring tiny subatomic particles called quarks. These entities nest like Russian dolls inside protons, which in turn occupy the atoms that make up all matter. At the time, physicists found it odd that no matter how hard you smash two protons together, you cannot release the quarks—they stay confined inside the protons.

One person working on quark confinement was Alexander Polyakov, Princeton’s Joseph Henry Professor of Physics. It turns out that quarks are “glued together” by other particles, called gluons. For a while, researchers thought gluons could assemble into strings that tie quarks to each other. Polyakov glimpsed a link between the theory of particles and the theory of strings, but the work was, in Polyakov’s words, “hand-wavy” and he didn’t have precise examples.

Meanwhile, the idea that fundamental particles are actually tiny bits of vibrating string was taking off, and by the mid-1980s, “string theory” had lassoed the imaginations of many leading physicists. The idea is simple: just as a vibrating violin string gives rise to different notes, each string’s vibration foretells a particle’s mass and behavior. The mathematical beauty was irresistible and led to a swell of enthusiasm for string theory as a way to explain not only particles but the universe itself.

One of Polyakov’s colleagues was Klebanov, who in 1996 was an associate professor at Princeton, having earned his Ph.D. at Princeton a decade earlier. That year, Klebanov, with graduate student Steven Gubser and postdoctoral research associate Amanda Peet, used string theory to make calculations about gluons, and then compared their findings to a string-theory approach to understanding a black hole. They were surprised to find that both approaches yielded a very similar answer. A year later, Klebanov studied absorption rates by black holes and found that this time they agreed exactly.

That work was limited to the example of gluons and black holes. It took an insight by Juan Maldacena in 1997 to pull the pieces into a more general relationship. At that time, Maldacena, who had earned his Ph.D. at Princeton one year earlier, was an assistant professor at Harvard. He detected a correspondence between a special form of gravity and the theory that describes particles. Seeing the importance of Maldacena’s conjecture, a Princeton team consisting of Gubser, Klebanov and Polyakov followed up with a related paper formulating the idea in more precise terms.

Another physicist who was immediately taken with the idea was Edward Witten of the Institute for Advanced Study (IAS), an independent research center located about a mile from the University campus. He wrote a paper that further formulated the idea, and the combination of the three papers in late 1997 and early 1998 opened the floodgates.

“It was a fundamentally new kind of connection,” said Witten, a leader in the field of string theory who had earned his Ph.D. at Princeton in 1976 and is a visiting lecturer with the rank of professor in physics at Princeton. “Twenty years later, we haven’t fully come to grips with it.”

Two sides of the same coin

This relationship means that gravity and subatomic particle interactions are like two sides of the same coin. On one side is an extended version of gravity derived from Einstein’s 1915 theory of general relativity. On the other side is the theory that roughly describes the behavior of subatomic particles and their interactions.

The latter theory includes the catalogue of particles and forces in the “standard model” (see sidebar), a framework to explain matter and its interactions that has survived rigorous testing in numerous experiments, including at the Large Hadron Collider.

In the standard model, quantum behaviors are baked in. Our world, when we get down to the level of particles, is a quantum world.

Notably absent from the standard model is gravity. Yet quantum behavior is at the basis of the other three forces, so why should gravity be immune?

The new framework brings gravity into the discussion. It is not exactly the gravity we know, but a slightly warped version that includes an extra dimension. The universe we know has four dimensions, the three that pinpoint an object in space—the height, width and depth of Einstein’s desk, for example—plus the fourth dimension of time. The gravitational description adds a fifth dimension that causes spacetime to curve into a universe that includes copies of familiar four-dimensional flat space rescaled according to where they are found in the fifth dimension. This strange, curved spacetime is called anti-de Sitter (AdS) space after Einstein’s collaborator, Dutch
astronomer Willem de Sitter.

The breakthrough in the late 1990s was that mathematical calculations of the edge, or boundary, of this anti-de Sitter space can be applied to problems involving quantum behaviors of subatomic particles described by a mathematical relationship called conformal field theory (CFT). This relationship provides the link, which Polyakov had glimpsed earlier, between the theory of particles in four space-time dimensions and string theory in five dimensions. The relationship now goes by several names that relate gravity to particles, but most researchers call it the AdS/CFT (pronounced A-D-S-C-F-T) correspondence.

Tackling the big questionsThis correspondence, it turns out, has many practical uses. Take black holes, for example. The late physicist Stephen Hawking startled the physics community by discovering that black holes have a temperature that arises because each particle that falls into a black hole has an entangled particle that can escape as heat.

Using AdS/CFT, Tadashi Takayanagi and Shinsei Ryu, then at the University of California-Santa Barbara, discovered a new way to study

entanglement in terms of geometry, extending Hawking’s insights in a fashion that experts consider quite remarkable.

In another example, researchers are using AdS/CFT to pin down chaos theory, which says that a random and insignificant event such as the flapping of a butterfly’s wings could result in massive changes to a large-scale system such as a faraway hurricane. It is difficult to calculate chaos, but black holes—which are some of the most chaotic quantum systems possible—could help. Work by Stephen Shenker and Douglas Stanford at Stanford University, along with Maldacena, demonstrates how, through AdS/CFT, black holes can model quantum chaos.

One open question Maldacena hopes the AdS/CFT correspondence will answer is the question of what it is like inside a black hole, where an infinitely dense region called a singularity resides. So far, the relationship gives us a picture of the black hole as seen from the outside, said Maldacena, who is now the Carl P. Feinberg Professor at IAS.

“We hope to understand the singularity inside the black hole,” Maldacena said. “Understanding this would probably lead to interesting lessons for the Big Bang.”

The relationship between gravity and strings has also shed new light on quark confinement, initially through work by Polyakov and Witten, and later by Klebanov and Matt Strassler, who was then at IAS.

Those are just a few examples of how the relationship can be used. “It is a tremendously successful idea,” said Gubser, who today is a professor of physics at Princeton. “It compels one’s attention. It ropes you in, it ropes in other fields, and it gives you a vantage point on theoretical physics that is very compelling.”

The relationship may even unlock the quantum nature of gravity. “It is among our best clues to understand gravity from a quantum perspective,” said Witten. “Since we don’t know what is still missing, I cannot tell you how big a piece of the picture it ultimately will be.”

Still, the AdS/CFT correspondence, while powerful, relies on a simplified version of spacetime that is not exactly like the real universe. Researchers are working to find ways to make the theory more broadly applicable to the everyday world, including Gubser’s research on modeling the collisions of heavy ions, as well as high-temperature superconductors.

Also on the to-do list is developing a proof of this correspondence that draws on underlying physical principles. It is unlikely that Einstein would be satisfied without a proof, said Herman Verlinde, Princeton’s Class of 1909 Professor of Physics, the chair of the Department of Physics and an expert in string , who shares office space with Einstein’s desk.

“Sometimes I imagine he is still sitting there,” Verlinde said, “and I wonder what he would think of our progress.”

Physicists Just Smashed Another Record For High-Temperature Superconductivity


Scientists in Germany say they have hit a new superconductivity milestone. According to their paper, they achieved resistance-free electrical current at the highest temperature yet: just 250 Kelvin, or -23 degrees Celsius (-9.4 degrees Fahrenheit).

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Although the team’s superconducting material has yet to be verified, the claim has merit – the work was led by Mikhail Eremets, a physicist at the Max Planck Institute for Chemistry, who set the previous high temperature record for superconductivity in 2014, at 203 Kelvin (-70 degrees Celsius).

Superconductivity, first discovered in 1911, is a curious phenomenon. Usually, the flow of an electrical current encounters some degree of resistance – a bit like how air resistance pushes back on a moving object, for example.

The higher the conductivity of a material, the less electrical resistance it has, and the current can flow more freely.

But at low temperatures in some materials, something strange happens. Resistance lowers to zero, and the current flows unimpeded. When accompanied by something called the Meissner effect – the expulsion of the material’s magnetic fields as it transitions below that critical temperature – this is called superconductivity.

So-called room-temperature superconductivity, above 0 degrees Celsius, is something of a white whale for scientists. If it could be achieved, it would revolutionise electrical efficiency, vastly improving power grids, high-speed data transfer, and electrical motors, to name a few potential applications.

So it’s something that many laboratories around the world have been working on, with new claims of high-temperature superconductivity rearing from time to time that then fail reproducibility tests.

Eremets and his team achieved the previous high-temperature superconductivity record using hydrogen sulfide – yep, the compound that makes rotten eggs and human flatulence stinky – under 150 gigapascals of pressure (Earth’s core is between 330 and 360 gigapascals).

Scientists who rushed to understand hydrogen sulfide superconductivity believe that this result is possible because hydrogen sulfide is so light a material that it can vibrate at high speeds, which means higher temperatures – but the pressure is needed to keep it from vibrating itself apart.

This new research used a different material, called lanthanum hydride, under about 170 gigapascals of pressure. Earlier this year, the team reported they had achieved superconductivity using this material at 215 Kelvin (-58.15 C°, -72 F°) – and now, just a few months later, they have improved on that result.

The new temperature is nearly half the average winter temperature at the North Pole.

“This leap, by 50 Kelvin, from the previous critical temperature record of 203 Kelvin,” the researchers wrote in their paper, “indicates the real possibility of achieving room-temperature superconductivity (that is at 273 Kelvin) in the near future at high pressures, and the perspective of conventional superconductivity at ambient pressure.”

The result is yet to be verified by the scientific community, and the paper is awaiting peer-review.

There are three tests, reports MIT Technology Review, that are considered the gold standard for superconductivity, and the team has only achieved two: the drop in resistance below a critical temperature threshold, and replacing elements in the material with heavier isotopes to observe a corresponding drop in superconductivity temperature.

The third is the Meissner effect, which is the name given to one of the signatures of superconductivity. As the material passes below the critical temperature and transitions into superconductivity, it ejects its magnetic field.

The team has yet to observe this phenomenon because their sample is so small – well below the detection capabilities of their magnetometer. However, the transition into superconductivity has an effect on the external magnetic field, too. It’s not a direct detection, but the team was able to observe this effect.

It’s not the Meissner effect, but it does look promising. And you can bet that physicists with the ability to do so will be falling over each other to verify and attempt to replicate the team’s result.

Physics Explains How to Build a Time Machine Using This Simple Construction


The concept of time travel has always captured the imagination of physicists and laypersons alike. But is it really possible? Of course it is. We’re doing it right now, aren’t we? We are all traveling into the future one second at a time.

But that was not what you were thinking. Can we travel much further into the future? Absolutely. If we could travel close to the speed of light, or in the proximity of a black hole, time would slow down enabling us to travel arbitrarily far into the future. The really interesting question is whether we can travel back into the past.

See also: ‘Future Man’ Makes ‘Doctor Who’ Time Travel Look Weak AF

I am a physics professor at the University of Massachusetts, Dartmouth, and first heard about the notion of time travel when I was 7, from a 1980 episode of Carl Sagan’s classic TV series, Cosmos. I decided right then that someday I was going to pursue a deep study of the theory that underlies such creative and remarkable ideas: Einstein’s relativity. Twenty years later, I emerged with a Ph.D. in the field and have been an active researcher in the theory ever since.

Now, one of my doctoral students has just published a paper in the journal Classical and Quantum Gravity that describes how to build a time machine using a very simple construction.

Closed Time-Like Curves

Einstein’s general theory of relativity allows for the possibility of warping time to such a high degree that it actually folds upon itself, resulting in a time loop. Imagine you’re traveling along this loop; that means that at some point, you’d end up at a moment in the past and begin experiencing the same moments since, all over again — a bit like deja vu, except you wouldn’t realize it. Such constructs are often referred to as “closed time-like curves” or CTCs in the research literature, and popularly referred to as “time machines.” Time machines are a byproduct of effective, faster-than-light travel schemes, and understanding them can improve our understanding of how the universe works.

time loop
Here we see a time loop. Green shows the short way through wormhole. Red shows the long way through normal space. Since the travel time on the green path could be very small compared to the red, a wormhole can allow for the possibility of time travel.

Over the past few decades, well-known physicists like Kip Thorne and Stephen Hawking produced seminal work on models related to time machines.

The general conclusion that has emerged from previous research, including Thorne’s and Hawking’s, is that nature forbids time loops. This is perhaps best explained in Hawking’s “Chronology Protection Conjecture,” which essentially says that nature doesn’t allow for changes to its past history, thus sparing us from the paradoxes that can emerge if time travel were possible.

Perhaps the most well-known amongst these paradoxes that emerge due to time travel into the past is the so-called “grandfather paradox” in which a traveler goes back into the past and murders his own grandfather. This alters the course of history in a way that a contradiction emerges: The traveler was never born and therefore cannot exist. There have been many movie and novel plots based on the paradoxes that result from time travel — perhaps some of the most popular ones being the Back to the Future movies and Groundhog Day.

Exotic Matter

Depending on the details, different physical phenomena may intervene to prevent closed time-like curves from developing in physical systems. The most common is the requirement for a particular type of “exotic” matter that must be present in order for a time loop to exist. Loosely speaking, exotic matter is matter that has negative mass. The problem is negative mass is not known to exist in nature.

Caroline Mallary, a doctoral student at the University of Massachusetts Dartmouth has published a new model for a time machine in the journal Classical and Quantum Gravity. This new model does not require any negative mass exotic material and offers a very simple design.

Mallary’s model consists of two super long cars — built of material that is not exotic, and have positive mass — parked in parallel. One car moves forward rapidly, leaving the other parked. Mallary was able to show that in such a setup, a time loop can be found in the space between the cars.

An animation shows how Mallary’s time loop works. As the spacecraft enters the time loop, its future self appears as well, and one can trace back the positions of both at every moment afterwards. This animation is from the perspective of an external observer, who is watching the spacecraft enter and emerge from the time loop.

So Can You build This in Your Backyard?

If you suspect there is a catch, you are correct. Mallary’s model requires that the center of each car has infinite density. That means they contain objects — called singularities — with an infinite density, temperature, and pressure. Moreover, unlike singularities that are present in the interior of black holes, which makes them totally inaccessible from the outside, the singularities in Mallary’s model are completely bare and observable and, therefore, have true physical effects.

Physicists don’t expect such peculiar objects to exist in nature, either. So, unfortunately, a time machine is not going to be available anytime soon. However, this work shows that physicists may have to refine their ideas about why closed time-like curves are forbidden.

Prophylactic cranial irradiation fails to improve overall survival in NSCLC


Dr Alex Sun.

An updated 10-year analysis of the RTOG 0214 trial showed that the use of prophylactic cranial irradiation (PCI) improved disease-free survival (DFS) and reduced brain metastases, but failed to improve overall survival (OS) in patients with locally advanced non-small-cell lung carcinoma (LA-NSCLC).

The results of the 10-year follow-up of this phase III study, done from September 2001 to August 2007 in 356 patients with stage III A/B LA-NSCLC (median age, 60), showed that PCI did not improve OS rate vs observation alone (17.6 percent vs 13.3 percent; hazard ratio [HR], 1.23; 95 percent confidence interval [CI], 0.95 to 1.59; p=0.124)  [Sun A, et al, WCLC 2018 abstract OA01.01]

Patients who underwent PCI, however, experienced better DFS (12.6 percent vs 7.5 percent; HR, 1.32; 95 percent CI, 1.03 to 1.69; p=0.0298) and less central nervous system (CNS) metastasis (16.7 percent vs 28.3 percent; HR, 2.33; 95 percent CI, 1.31 to 4.15; p=0.0298) vs those who were just observed.

“There was only 45 percent power to detect the hypothesized difference [HR=1.25], and if we were able to accrue the targeted number, there may have been a benefit in OS,” said study investigator Dr Alex Sun from the University of Toronto, Toronto, Ontario, Canada.

“As compared with previous trials, PCI employing delivery of 30 Gy in 15 fractions as used in the RTOG 0214 study might also be too low a dose to exert its desirable effects,” commented discussant Dr John Armstrong of St Luke’s Radiation Oncology Network, Dublin, Ireland. [Radiat Oncol 2016;11:67]

“However, a subgroup analysis among 225 patients who did not have surgery as primary treatment showed that patients who underwent PCI had better OS [p=0.026] and DFS [p=0.014] and a lower incidence of brain metastases [p=0.003],” said Sun.

Most patients in the study experienced grade 1 (14.6 percent) or grade 2 (35 percent) acute toxicities or grade 1 (12.7 percent) or grade 2 (8.7 percent) late toxicities, with neurocognitive-associated toxicities being the most commonly reported.

“The most probable reason why we do not do much PCI is due to concerns about its reported toxicities such as somnolence, cognitive disturbances, neuropathy, memory impairment and dizziness. It is difficult to convince patients to undergo a procedure which will unlikely alter their survival,” said Armstrong. [J Clin Oncol 2018;36:2366-2377]

A previous study in 113 cancer patients with brain metastases showed that hippocampus-sparing intensity-modulated radiation therapy (IMRT) is associated with a significantly lower decline in Hopkins Verbal Learning Test-Revised Delayed Recall scores vs historical controls (p<0.001). [J Clin Oncol 2014;32:3810-3816]

In the future, PCI in NSCLC is foreseen to involve identification of ultra-high-risk individuals and performance of research which can be used for profiling patients who will experience brain metastases. For these patients, aggressive surveillance with volumetric MRI and early intervention with stereotactic surgery should be performed.

Nobel Prize in Physics won by scientists using lasers to solve the universe’s smallest mysteries


Winners Arthur Ashkin, Gérard Mourou and Donna Strickland helped develop technology dreamed up in science fiction that led to breakthroughs such as eye surgery.

The 2018 Nobel Prize in Physics has been given to scientists who used lasers to solve some of the universe’s smallest mysteries.

The award was given to Arthur Ashkin and the other half jointly to Gérard Mourou and Donna Strickland.

Ashkin was given the prize for “optical tweezers and their application to biological systems”, the committee wrote. Those optical tweezers use lasers to grab particles, atoms, viruses and other living cells.

That in turn allowed for something from science fiction’s dreams: using light to move physical objects around. He found that he could push small particles towards the centre of the beam and hold them there.

As well as being a stunning breakthrough in itself, the discovery led to further work as scientists could use the tweezers to investigate the tiny processes that power the universe. They can grab bacteria without harming them, for instance, allowing Ashkin and other scientists to investigate what the committee called the “machinery of life”.

Mourou and Strickland allowed mankind to create the shortest and most intense laser pulses ever seen. With a technique called chirped pulse amplification, or CPA, they allowed for high-intensity lasers, of the kind that are used today millions of times to carry out corrective eye surgeries.

Their discoveries also laid the foundation for the work done by Ashkin. And the full implications of their work have still not yet been found.

“The innumerable areas of application have not yet been completely explored,” the committee wrote. “However, even now these celebrated inventions allow us to rummage around in the microworld in the best spirit of Alfred Nobel – for the greatest benefit to humankind.”

Strickland is the first woman to be named a Nobel laureate since 2015. She is also only the third to have won the physics prize, with the first being Marie Curie in 1903.

CERN Scientists Say The LHC Has Confirmed Two New Particles, And Possibly Discovered a Third


They are known as bottom baryons.

The Large Hadron Collider is at it again, showing us new wonders in the world of particle physics. Scientists working on the Large Hadron Collider beauty (LHCb) collaboration have observed two new particles that have never been seen before – and seen evidence of a third.

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The two new particles, predicted by the standard quark model, are baryons – the same family of particles as the protons used in LHC particle acceleration experiments.

Baryons are what most of the Universe is made up of, including protons and neutrons – composite particles consisting of three fundamental particles called quarks, which have different ‘flavours’, or types: up, down, top, bottom, charm, and strange.

Protons consist of two up quarks and one down quark, while neutrons consist of one up quark and two down quarks, for instance. But the two new particles discovered have a slightly different composition.

Named Σb(6097)+ and Σb(6097), they consist of two up quarks and one bottom quark; and two down quarks and one bottom quark, respectively.

These particles are known as bottom baryons, and they are related to four particles previously observed at Fermilab. However, the new observations mark the first time scientists have detected these higher-mass counterparts; they are about six times more massive than a proton.

So what’s the third particle candidate we mentioned earlier?

The researchers think it might be a strange type of composite particle called a tetraquark. These are an exotic kind of meson, which normally have two quarks. But a tetraquark is composed of four quarks – well, two quarks and two antiquarks, to be more accurate.

Observational evidence of tetraquarks has been pretty elusive to date, and that is also the case here. Evidence of the candidate particle, called Zc(4100) and including two heavy charm quarks, was detected in the decay of heavier B mesons.

But the detection only had a significance of over 3 standard deviations. The usual threshold to claim the discovery of a new particle is 5 standard deviations. It will take future observations to either confirm or disprove the existence of Zc(4100).

The new bottom baryons, you’ll be pleased to know, blew that threshold out of the water: Σb(6097)+ and Σb(6097) had significances of 12.7 and 12.6 standard deviations respectively.

MIT Physicists Have Constructed a Bizarre Form of ‘Molecular’ Light With 3 Photons


Photons shouldn’t do this.

Five years ago, physicists from Harvard and MIT achieved a world first by forcing a pair of photons to interact with one another in ways that shouldn’t seem possible.

What do you do when you’ve achieved such a lofty goal? You try to add a third photon, of course.

With all eyes on light as the future of computing, researchers are keen to discover new ways to manipulate photons.

By most accounts, the massless particles that make up the electromagnetic spectrum don’t have a whole lot to do with one another.

We often smash atoms together in giant accelerators and search for new physics in the resulting carnage.

The same can’t be said for photons. You can cross even the strongest of laser beams without risking so much as a gentle bump between two light particles.

For years physicists theorised there were conditions where this rule could be bent, and in 2013 they finally saw it in action.

“What we have done is create a special type of medium in which photons interact with each other so strongly that they begin to act as though they have mass, and they bind together to form molecules,” Harvard physicist Mikhail Lukin said at the time.

To do this, they passed a weak laser consisting of a few photons through a cloud of rubidium atoms chilled to near standstill.

Moving from atom to atom the light hands over some of its energy. Yet a strange thing happens when a nearby photon tries to do the same thing.

It’s called a Rydberg blockade – effectively, neighbouring particles can’t be excited to the same degree.

So as one photon buzzes an atom, a nearby photon with the same properties can’t cause another atom to share the same level of excitement. So it sticks around, briefly forming an atom-light hybrid called a polariton.

As a result, there’s a pushing and pulling of polaritons as the photons slowly make their way through the rubidium cloud. On exiting out the other side, they end up stuck together.

The same team of physicists has now used the same setup to determine if this special partnership could also be a triad, by throwing a third photon into the mix.

“For example, you can combine oxygen molecules to form O2 and O3 (ozone), but not O4, and for some molecules you can’t form even a three-particle molecule,” says the study’s senior author, Vladan Vuletic from MIT.

“So it was an open question: Can you add more photons to a molecule to make bigger and bigger things?”

Sure enough, out popped clusters of photons in twos and threes, showing it was indeed possible. And sticking together these pairs and triplets of photons into a kind of “molecule” could have many useful applications.

Scientists have been busy in recent years controlling light’s speed in a vacuum, twisting it into new configurations, and contorting it to have strange properties.

All of this lays the groundwork for technologies that no longer use clunky old electrons to do the grunt work of computers, but photons that can be entangled, encoded, and sent long distances at high speeds packed with more information.

So what’s next for the team? Will we be seeing photon quads? Vuletic is open minded.

“With repulsion of photons, can they be such that they form a regular pattern, like a crystal of light? Or will something else happen? It’s very uncharted territory.”

Scary Physics of ‘Curve 9’ Leads to Terrifying Olympic Luge Crash


On Tuesday, Americans woke up to the news that Emily Sweeney of Team USA crashed during her luge run at the Olympics. Sweeney refused a stretcher and was able to walk away from the frightening accident — a tough-as-nails moment after careening down what’s essentially a roller coaster made of ice. Sweeney crashed after losing control during her final heat at Curve 9 — already notorious among Olympic sliders before the games even began.

Emily Sweeney, luge

The insane physics of the Winter Olympic’s “fastest sport on ice” means that coming out of a curve during the luge feels like, in the words of 2014 Olympian Chris Mazder, “launching into space on a rocket.” Sliders can reach speeds of 90 miles per hour after launching onto the ice with a 50-pound sled, propelling forward with spike-equipped gloves, and steering with their calves.

All that speed means that luge races are timed to one-thousandth of a second, and any time lost on a curve can ruin an athlete’s chances of placement. Defending Gold medalist Felix Loch of Team USA bumped into Curve 9 on Sunday, losing hundredths of a second and his chance at a 2018 medal.

Luge races take place on a track built with a length of 1,000 to 1,500 meters with a difference in elevation between 110 to 130 meters and an average slope of nine to 11 percent. Curve 9 is just one of 16 obstacles on Pyeongchang’s Alpensia Sliding Center track. As the athletes shoot down the U-shaped groove of the course they have to maneuver through left curves, right curves, hairpin curves, S-shaped curves, and a three-turn combination called a labyrinth.

But it was Curve 9 that everyone had been talking about before the Olympics kicked off. Before her own run, Sweeney described the curve as like “driving on a slanted road, but having your call getting pulled in a direction away from the way you’re steering.” It’s the angle of the curve that’s so rough — the turn sends the lugers to the right, but the track is actually designed to go 45 degrees to the left.

And when lugers hit the angle of the curve, their force against the ice can be as high as eight times that of gravity. The aerodynamic position of their body combined with the tiny amount of contact the steel (the name for the sled’s blades) makes with the ice minimizes the force of the drag, adding incredible speed to the centrifugal force that emerges as a reaction between the ice, the athlete, and the inertia. All of that means when a curve shoots you to the right, but the track goes to the left, retaining control is going to be incredibly difficult.

It’s ultimately what got Sweeney, who began sliding at severe and alternating angles after losing control after Curve 9. She was ultimately thrown from her sled into a tumble — a scary finale for a first-time Olympic run.

Blade Dynamics Explains How Olympic Figure Skaters Can Stand on Each Other


On Sunday night, Team Canada’s figure skating power pair, Tessa Virtue and Scott Moir, skated their way to an Olympic gold with a rapturous performance to Moulin Rouge’s “El Tango De Roxanne.” In their heart-stopping routine, they dazzled audiences with an insane lift in which Virtue literally steps onto Moir’s quads with her blades, then just stands there, arms outstretched, like some glorious figurehead of a Spandex-clad ship. It’s just mind-boggling.

scott moir tessa virtue olympics

If you, like me, spent at least three hours watching the clip of their routine on repeat today, a serious question may have crossed your mind: How are Moir’s thighs not gushing blood onto the ice?

It’s a legitimate question. The blades on figure skates are sharp enough to cut the skin on a person’s face and can even slice deeper, resulting in some serious injuries. But a closer look at the blade — and its positioning on the lifter’s leg — shows how it can be done safely.

Here’s a version of the lift from a previous performance of the same routine at the Canadian National Skating Championships in Vancouver this January, in which the pair got a perfect score. Even with proper technique, this can’t be comfortable, but look at them beam! How do they do it?

tessa virtue scott moir lift
Unreal!

Part of the reason this can be done safely is because the blade of a figure skate is not like the blade of a knife or sword. If you look at a figure skate’s blade up close, you’ll notice that its cross-section is concave like the letter C, with its two arms making up the outer and inner edge that actually touch the ice. In the middle is the groove. Regular knife or sword blades have a thin, singular edge.

Both edges of a concave figure skate blade are very sharp, but having two edges helps diffuse the weight of the wearer over a greater surface area, reducing the force on any one point on the leg. If skate blades were like knives, all of the wearer’s weight would be concentrated on a single, localized edge.

cross section figure skating blade
When it comes to distributing force, two edges are better than one.

When a force (in this case, Virtue’s weight) gets exerted perpendicularly on a surface (the blade’s twin edges, pressing into Moir’s thigh), the pressure is calculated by dividing the force by the surface area on which it rests. With the greater surface area supplied by two edges, the pressure is smaller than it would be if there was only a single blade. Whatever that pressure is in this situation, it’s not enough to cut through Moir’s uniform and flesh.

That’s not the only factor skaters have to consider to do this safely. Another reason Moir is not bleeding out beneath Virtue’s blades is because they’re not moving on his thighs. Once she’s in place, she stays perfectly still.

 Doing so prevents the edges of the blades from sawing into him, the way you might use a knife to slice a tomato. Chefs and physicists alike know that using a saw-like slicing action with a blade seems to cut more efficiently, and in 2012 researchers at Harvard University discovered why.

Pressing a wire against a gel and looking closely at the point of contact, they saw that cuts didn’t appear in the gel until the wire pushed down far enough to form the beginnings of a crack. But as the BBC put it, “the key difference between ‘chopping’ and slicing is that, in the former case the wire mostly just compresses the gel, whereas in the latter case the sideways movement of the wire also stretches it.” That stretching weakened the gel further, making slicing easier.

In the paper, they explained that the extra stretching is caused by friction between the wire and the surface. Applying this principle to Moir and Virtue, any friction between her blade edges and his leg would initiate a cut faster than a motionless blade simply pressing downward. Again, their impeccable technique saves the day.

The crazy lift starts around the 3:50 mark.

Of course, just because physics can determine a safe way to do this move doesn’t mean most people should try it. Even serious skaters rarely do it. The Goose lift that has become virtually synonymous with Virtue and Moir has rarely been replicated by any other pair, so for the love of Roxanne, leave this power move to these two perfect, prolific professionals.

The incredible “Goose lift.”
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