Monthly Archives: February 2020

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

This research was published in Physical Review Letters.

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

As experimental proposals go, this one certainly doesnt lack ambition. First, take a black hole. Now make a second black hole that is quantum entangled with it, which means that anything that happens to one of the black holes will seem to have an effect on the other, regardless of how far apart they are.

The rest sounds a bit easier, but a lot weirder. Feed some information into the first black hole, encoded in a quantum particle. As it falls beyond the event horizon the point beyond which not even light can escape the information is rapidly smeared throughout the black hole and is scrambled seemingly beyond recall.

But have patience if youve linked the two black holes in the right way, after a short wait the quantum information will pop out of the second one, fully refocused into readable form. To get there, it will have traveled through a shortcut in space-time that links the two objects a wormhole.

That, at least, is what physicists have predicted. Now a group led by Sepehr Nezami of the California Institute of Technology has suggested how to actually perform this extraordinary experiment and they are beginning to work with collaborators to put the idea to the test.

If the predictions are borne out, the work may offer clues about where to look for the most elusive theory in physics: one that unites quantum mechanics with the theory of general relativity that describes gravity. And, for good measure, it would support the idea that space-time is not the fundamental backdrop against which the universe plays out but is itself woven from the interconnections between particles described by quantum entanglement.

This experiment, as you might have guessed,doesnt require black holes in the usual sense, meaning massive stars that have collapsed by their own gravity to an infinitesimally small volume. The researchers say that it could be done on a lab benchtop using just a few atoms or ions. All the same, the idea arises out of theoretical research on astrophysical black holes that has struggled to resolve a deep and unsettling question: Do these all-devouring monsters destroy information irreversibly?

Its widely thought that information, like energy, should obey a conservation rule: The total amount of information in the universe will always stay the same. Thats what quantum mechanics seems to imply: The wave functions that describe quantum entities always evolve smoothly in an information-conserving way and cant be suddenly snuffed out.

But black holes do seem to remove information from the universe. If, say, a quantum bit, or qubit, falls into a black hole, it can no longer be observed from outside the event horizon.

One possible resolution of this black hole information paradox can be found within the radiation that black holes emit from their event horizons. Hawking radiation, predicted by Stephen Hawking in the 1970s, will cause a black hole to lose gravitational energy and thus mass. In effect, black holes are not eternal. They slowly evaporate.

Hawking initially believed that even if a black hole fully evaporated, the information it had consumed would remain lost forever. But an idea known as the AdS/CFT correspondence shows how the photons of Hawking radiation might be able to encode information about the interior of the black hole, thereby carrying that information back out into the universe at large.

The AdS/CFT correspondence was postulated by the theoretical physicist Juan Maldacena in 1997, and its widely regarded as one of the most promising directions in which to pursue theories of quantum gravity. It suggests that the physical structure of space-time in, say, four dimensions is equivalent to the operation of a quantum theory at a three-dimensional boundary.

This connection is strange, deep and surprising. It says that if you construct a space-time with a particular kind of curvature (and thus gravity) known as an anti-de Sitter space thats the AdS part the mathematical description turns out to be equivalent to the description of a kind of quantum field theory called a conformal field theory thats the CFT part in one fewer dimension. In other words, the correspondence works like a hologram all the information in the higher-dimensional space-time projection is encoded within the lower-dimensional quantum interactions. This holographic principle was first proposed by the physics Nobel laureate Gerard t Hooft, and Maldacenas AdS/CFT correspondence provided the first concrete picture of how it might work for a particular form of space-time.

In this view, what looks like continuous space in the AdS universe manifests in the CFT quantum view as entanglement the interdependence of quantum bits. Here, said Maldacena, the emergence of space-time is supposed to happen in systems with a large number of qubits that are highly entangled and highly interacting. In other words, quantum entanglement can produce a space-time that seems to have gravity in it. Gravity, you might say, is spun from quantum effects.

What does all this have to do with black holes? The black hole information paradox asks what happens to the information that gets tossed into a black hole. The AdS/CFT correspondence is a key component of one proposed solution, since it supplies the means by which quantum entanglement could imprint the information on Hawking radiation and prevent it from being irrevocably lost.In 2004, Hawking himself explained how, assuming the AdS/CFT conjecture is true, we could recover this information by capturing every single Hawking photon a black hole radiates over its entire lifetime before fully evaporating. As Norman Yao of the University of California, Berkeley described it, If you were God and you collected all these Hawking photons, there is in principle some ungodly calculation you can do to re-extract the information in [each swallowed] qubit.

Up until the halfway point of a black holes evaporation, the information inside it remains concealed. After that point, however, the black hole starts to reveal its information in its Hawking radiation. So you have a long wait before you can start to get at it. And according to an argument made in 1993 by the physicist Don Page of the University of Alberta, it will then seep out gradually, at a constant rate.

But in 2007, Patrick Hayden and John Preskill revised this picture by showing that, in fact, after the halfway point the information emerges more rapidly than that. Weirdly enough, once the black hole is half-evaporated, any further quantum bit of information tossed into it literally bounces right back, said Yao. This is because the black hole has by that stage become so quantum-entangled with the Hawking radiation it has already emitted that any more information it swallows is effectively registered at once in any further radiation it emits. The black hole, Hayden and Preskill said, then acts like an information mirror.

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Wormholes Reveal a Way to Manipulate Black Hole Information in the Lab - Quanta Magazine

February 28, 2020• Physics 13, 25

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

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

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

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

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

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

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

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

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

This research is published in Physical Review Letters.

Philip Ball

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

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

How did the universe begin? How does quantum mechanics, the study of the smallest things relate to gravity and the study of big things? These are some of the questions physicists have been working to solve ever since Einstein released his theory of relativity.

Formulas show that baby universes pops in and out of the main universe. However, we dont realize or experience this as humans. To calculate how this scales, theoretical physicists devised the so-called JT gravity, which turns the universe into a toy-like model with only one dimension of time or space. These restricted parameters allows for a model in which scientists can test their theories.

Building on the work of others, Professor Kazumi Okuyama of Shinshu University and Kazuhiro Sakai of Meiji Gakuin University set out to show how JT gravity, KdV equation and the macroscopic loop are related, thus pointing to the fact that gravity and quantum mechanics are unified. In the process, the duo succeeded in calculating the birth of baby universes 46 times which has never been done before, due to the fact the more times this is calculated the more things get increasingly complicated. Previously, Peter Zograf was able to calculate this 20 times.

The mathematical KdV equation formulated in the late 19th Century has been thought to be linked to the gravity since the 1990s. The KdV equation was first used to show how water waves behave, for example inside the canals in waterway laden Holland, solitons can be observed, or how a crest of a water wave continues unchanged for a long time when not disturbed. The macroscopic loop was also said to be related to the gravity in the 1990s.

Waves and gravity are thought to be comparable in how they manifest themselves. The holographic principle was introduced by Gerard t Hooft as a way to understand how gravity and quantum mechanics work. When these theories are combined, one can think of the 3D physical as the gravity and the information that it is sprung from; flat like how a hologram is on a credit card. This speaks to the dimensions in space. There is no formula yet for the holographic principle.

The bulk-boundary correspondence idea is similar to this in that the bulk is the three-dimensional manifestation of the boundary which is the information that gives rise to the hologram.

Professor Okuyama was able to show in this study that the JT gravity, KdV equation and macroscopic loop are intimately connected, pointing to the fact that quantum mechanics and gravity are indeed unified holographically in this model. He hopes to keep working to solve this problem in physics by devising a method to calculate the birth of baby universes not just in the toy model but for the existent universe.

Reference: JT gravity, KdV equations and macroscopic loop operators by Kazumi Okuyama and Kazuhiro Sakai, 24 January 2020, Journal of High Energy Physics.DOI: 10.1007/JHEP01(2020)156

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Witnessing the Birth of Baby Universes 46 Times: Space Is Information Encoded in a Lower-Dimensional Boundary? - SciTechDaily

Since the fall of 2004, Assistant Professor Christopher Search has been working with the Physics department. For five years, Search exclusively taught graduate-level courses until he was offered the opportunity to teach undergraduate-level courses. When asked about the nature of the transition from graduate to undergraduate classes, Search said, It is like what the Romans did to the Christians: they threw them to the lions. He later added, It is not because students are disruptive or disinterested, but, rather, there were too many students in a given lecture.

Students who have taken Electricity & Magnetism with Search, such as myself, are familiar with his distinct teaching style of drawing connections between physics theories and how they manifest in the real world. Search emphasized, I like the lower-level courses more than graduate courses because there is more attention to fundamental physics concepts that entail some proficiency in mathematics, but it does not involve fancy mathematics to provide an explanation of how the world works.

Search declared that many of his students have sought him out for career advice or general advice, which he is happy to share. Not unlike many undergraduate students trying to find their niche in engineering, business, or other fields, Search received two masters degrees: one in Biomedical Engineering and one in Electrical Engineering from the University of Michigan, after obtaining his bachelors degree in Physics from the University of Arizona. This was an effort to ascertain the direction of his physics career. For Search, the moment of clarity followed his elective course in Quantum Mechanics, which inspired him to pursue a Ph.D. in Applied Physics.

As of now, Search, in collaboration with other faculty, is working on developing the fairly new Optical Engineering degree program here at Stevens. As the Associate Chair for the Undergraduate Physics Program, he is responsible for class scheduling, degree requirements, and curriculum developments.

When asked about what the Physics department is currently lacking, he said, more funding is necessary in order to hire more lab TAs and upgrade lab equipment. Additionally, he maintained that the large lectures for the Mechanics and Electricity & Magnetism courses create an impersonal teaching environment. This makes it nearly impossible for a professor to connect with individual students unless they stop by office hours. In contrast, the scholars class section does not have a recitation or TA. This allows Professor Search to interact with students and witness first-hand which students need extra help. Search believes that the discrepancy between the regular and scholars sections for introductory physics courses significantly impacts the learning outcomes. Lastly, he asserted, All students should get what they paid for.

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A day in the life of a physics professor - The Stute

By Columbia University School of Engineering and Applied ScienceFebruary 29, 2020

Illustration of an integrated micro-ring resonator based low loss optical cavity with semiconductor 2D material on top of the waveguide. Credit: Ipshita Datta and Aseema Mohanty, Lipson Nanophotonics Group/Columbia Engineering

Researchers use 2D materials1/100,000 the size of a human hairto manipulate the phase of light without changing its amplitude, at extremely low power loss; could enable applications such as LIDAR, phased arrays, optical switching, and quantum and optical neural networks.

Optical manipulation on the nano-scale, or nanophotonics, has become a critical research area, as researchers seek ways to meet the ever-increasing demand for information processing and communications. The ability to control and manipulate light on the nanometer scale will lead to numerous applications including data communication, imaging, ranging, sensing, spectroscopy, and quantum and neural circuits (think LIDAR light detection and ranging for self-driving cars and faster video-on-demand, for example).

Today, silicon has become the preferred integrated photonics platform due to its transparency at telecommunication wavelengths, ability for electro-optic and thermo-optic modulation, and its compatibility with existing semiconductor fabrication techniques. But, while silicon nanophotonics has made great strides in the fields of optical data communications, phased arrays, LIDAR, and quantum and neural circuits, there are two major concerns for large-scale integration of photonics into these systems: their ever-expanding need for scaling optical bandwidth and their high electrical power consumption.

Existing bulk silicon phase modulators can change the phase of an optical signal, but this process comes at the expense of either high optical loss (electro-optic modulation) or high electrical power consumption (thermo-optic modulation). A Columbia University team, led by Michal Lipson, Eugene Higgins Professor of Electrical Engineering and professor of applied physics at Columbia Engineering, announced that they have discovered a new way to control the phase of light using 2D materials atomically thin materials, ~0.8 nanometer, or 1/100000 the size of a human hair without changing its amplitude, at extremely low electrical power dissipation.

Illustration of an integrated optical interferometer with semiconductor monolayers such as TMDs on both the arms of the silicon nitride (SiN) interferometer. One can probe the electro-optic properties of the monolayer with high precision using these on-chip optical interferometers. Credit: Ipshita Datta and Aseema Mohanty, Lipson Nanophotonics Group/Columbia Engineering

In this new study, published on February 24, 2020, by Nature Photonics, the researchers demonstrated that by simply placing the thin material on top of passive silicon waveguides, they could change the phase of light as strongly as existing silicon phase modulators, but with much lower optical loss and power consumption.

Phase modulation in optical coherent communication has remained a challenge to scale, due to the high optical loss that was associated with phase change, says Lipson. Now weve found a material that can change the phase only, providing us another avenue to expand the bandwidth of optical technologies.

The optical properties of semiconductor 2D materials such as transition metal dichalcogenides (TMDs) are known to change dramatically with free-carrier injection (doping) near their excitonic resonances (absorption peaks). However, very little is known about the effect of doping on the optical properties of TMDs at telecom wavelengths, far away from these excitonic resonances, where the material is transparent and therefore can be leveraged in photonic circuits.

The Columbia team, which included James Hone, Wang Fong-Jen Professor of Mechanical Engineering at Columbia Engineering, and Dimitri Basov, professor of physics at the University, probed the electro-optic response of the TMD by integrating the semiconductor monolayer on top of a low-loss silicon nitride optical cavity and doping the monolayer using an ionic liquid. They observed a large phase change with doping, while the optical loss changed minimally in the transmission response of the ring cavity. They showed that the doping-induced phase change relative to change in absorption for monolayer TMDs is approximately 125, which is significantly higher than that observed in materials commonly employed for silicon photonic modulators including Si and III-V on Si, while being simultaneously accompanied by negligible insertion loss.

We are the first to observe strong electro-refractive change in these thin monolayers, says the papers lead author Ipshita Datta, a PhD student with Lipson. We showed pure optical phase modulation by utilizing a low loss silicon nitride (SiN)-TMD composite waveguide platform in which the optical mode of the waveguide interacts with the monolayer. So now, by simply placing these monolayers on silicon waveguides, we can change the phase by the same order of magnitude, but at 10000 times lower electrical power dissipation. This is extremely encouraging for the scaling of photonic circuits and for low-power LIDAR.

The researchers are continuing to probe and better understand the underlying physical mechanism for the strong electrorefractive effect. They are currently leveraging their low-loss and low-power phase modulators to replace traditional phase shifters, and therefore reduce the electrical power consumption in large-scale applications such as optical phased arrays, and neural and quantum circuits.

Reference: Low-loss composite photonic platform based on 2D semiconductor monolayers by Ipshita Datta, Sang Hoon Chae, Gaurang R. Bhatt, Mohammad Amin Tadayon, Baichang Li, Yiling Yu, Chibeom Park, Jiwoong Park, Linyou Cao, D. N. Basov, James Hone and Michal Lipson, 24 February 2020, Nature Photonics.DOI: 10.1038/s41566-020-0590-4

The study was supported by Department of Energy, Office of Science, Basic Energy Sciences (EFRC Pro-QM #De-SC0019443), Defense Advanced Research Projects Agency (#HR001110720034 and #FA8650-16-7643), Air Force Office of Scientific Research MURI (#FA9550-18-1-0379), Office of Naval Research (#N00014-16-1-2219), and National Aeronautics and Space Administration (#NNX16AD16G). Competing interests: M.L., J.H., I.D., S.C., G.R.B. and D.N.B are named inventors on US provisional patent application 16/282,013 regarding the technology reported in this article.

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New Way to Control the Phase of Light Using Atomically Thin Materials Enables Quantum and Neural Circuits - SciTechDaily