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image:Fig. 1. left: the polarization induced by the vortical flow; right: the polarization induced by the shear flow. Red and yellow arrows represent the spin and momentum directions, respectively. view more

Credit: Shuai Liu

Chinese researchers recently discovered a new effect that can generate spin-polarization in fluid. The new effect, which is called "shear-induced polarization (SIP)," predicts that shear flow can induce polarization in the momentum space.

This research was conducted by scientists from the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS), together with their collaborators at Peking University and Central China Normal University, who studied polarization induced by shear flow for the first time. Their discoveries were published inPhysical Review LettersandJournal of High Energy Physics.

In flowing fluid, one may observe some special patterns of the flow field, such as those formed by the vortical flow, which rotates around a center and is related to the orbital angular momentum of the fluid. Due to spin-orbit coupling, the orbital angular momentum of the vortical flow can transfer to the spin of a particle. This vorticity-induced spin polarization hasbeen observed in a quantum fluid.

Besides vortical flow, shear flow is also quite common in fluids. However, it is far less intuitive how shear flow is related to angular momentum. Thus, how it affects spin polarization has never been investigated before.

In this research, using relativistic many-body quantum theory and linear response theory, the researchers systematically studied spin polarization in a hydrodynamic medium.They discovered that shear flow, although not intuitively related to orbital angular momentum, also generates spin polarization in the momentum space through spin-orbit coupling.

Employing a relativistic hydrodynamic model, the researchers then investigated how this new SIP effect manifests in relativistic heavy-ion collisions. Since previous studies do not include the SIP effect, their predictions always have the opposite sign compared to experimental observations. This discrepancy is sometimes called the "spin-sign puzzle"and has bothered the research community for several years.

However, once the SIP effect is included, the strange quark polarization predicted by the theory demonstrates a pattern similar to the measured Lambda polarization in experiments.

Considering the close relationship between strange quark polarization and Lambda polarization, thecurrent study is expected to be an essential step toward the final solution of the spin-sign puzzle.

This work was supported by the National Natural Science Foundation of China and the Strategic Priority Research Program of CAS.

Physical Review Letters

Experimental study

Not applicable

Shear-Induced Spin Polarization in Heavy-Ion Collisions

30-Sep-2021

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An engineer adjusts a laser to test chips with waveguides for quantum computing. (Photo by THOMAS KIENZLE / AFP)

Quantum computing applications may not be particularly mainstream now, although quantum computing as a field has been growing at an accelerated rate these past few years.

While the frequently-bandied about term may sound intimidating, quantum computing is essentially computing that can be performed at speeds and efficiencies far, far superior to what typical computers can do today. In short computing on steroids.

Aside from university labs, were already seeing it being used in a few sectors, such as cybersecurity, pharmaceuticals, and even logistics. Indeed, quantum computing has come a rather long way, in a short amount of time, mainly because of the immense benefit it can give to quickly compute and thus, analyze massive sets of data at breakneck speeds.

The rise of quantum computing has Big Tech to thank giants such as Microsoft, Amazon, Google, and IBM have been heavily investing in developing quantum computing and related technologies in recent years. The same has gone for governments such as China, South Korea, India, and Japan, all of whom have invested in or are planning to invest in developing this technology.

Just last month, UK-based Oxford Quantum Circuits launched the worlds first commercially available QCaaS (Quantum Computing as a Service), even. Prior to OQC, both Amazon and Honeywell had worked on developing and piloting commercial QCaaS.

Earlier this week, the National University of Singapores (NUS) Singapores Quantum Engineering Programme (QEP) announced that they would be working with Thales to develop and test quantum technologies for industry use.

The Memorandum of Understanding (MoU) signed on 29 September marks the start of a two-year partnership to jointly develop and test quantum technologies for commercial applications.

A Memorandum of Understanding was inked by (front row, from left) Professor Chen Tsuhan, Deputy President (Research and Technology), National University of Singapore, and Mr Kevin Chow, Country Director and Chief Executive, Thales in Singapore. The signing was witnessed by (back row, from left) Mr Ling Keok Tong, Director (Smart Nation and Digital Economy), National Research Foundation, Singapore, and Mr Chen Guan Yow, Vice President and Head (New Businesses), Economic Development Board. (IMG/Thales)

Under the MoU, Singapores Quantum Engineering Programme (QEP) and Thales aim to advance quantum technologies and prepare industry players for their arrival. The partnership will see industry and academic experts from Thales and QEP develop capabilities to test and evaluate interdisciplinary quantum security technologies.

They will also explore potential research collaboration opportunities in the fields of new materials and design for quantum sensing. Additionally, they will organise joint activities such as seminars and conferences to share their expertise and showcase their research outcomes.

The Quantum Engineering Programme (QEP) is an initiative launched in 2018 by the National Research Foundation, Singapore (NRF) and hosted at NUS. The projects under the collaboration span technologies for security and sensing, and involve QEP researchers across Singapores institutes of higher learning and research centres.

Professor Chen Tsuhan, NUS Deputy President (Research & Technology), said, Building on this momentum, QEPs partnership with Thales, a forerunner in the quantum revolution, will accelerate innovation and development of quantum solutions that are commercially attractive locally and globally.

With its track record in developing security and cybersecurity equipment, Thales will make available its SafeNet Luna Hardware Security Modules (HSMs) and high-speed network encryptors that support interfaces to quantum devices for research use.

The algorithms and quantum random number generation technology in these types of equipment provide the crypto-agility to easily implement quantum-safe crypto and combat the threats of quantum computing. This equipment would be deployed for proof-of-concept trials and testbeds in Singapore.

In May 2021, Thales launched a network encryption solution capable of protecting enterprise data from future quantum cyber-attacks. It supplements standard encryption with a scheme resistant to quantum computing that is under consideration for international standards.

Quantum technologies open almost infinite possibilities for the future and our researchers see real potential in three types of quantum applications, namely in sensors, communications and post-quantum cryptology, shared Mr Kevin Chow, Country Director and Chief Executive, Thales in Singapore.

Thales, which has 33,000 engineers across the world, also aims to be a key player in what is often called the second quantum revolution, which exploits subtle properties of quantum physics and requires mastery of the associated technologies.

Thales collaboration with QEP is a strong testament to the companys approach towards using quantum technologies to solve real-world, end-user challenges.

According to Chow, while this initial partnership will involve their network encryption technology to provide crypto-agility and cybersecurity, Thales will continue to work with the R&T ecosystem in Singapore to explore new topics, including using novel materials for quantum sensing or in secured communications in quantum technologies.

Additionally, the joint team of scientists and engineers will also develop devices that tap on quantum physics for higher performance. According to QEP, this is an area of focus under Singapores Research, Innovation and Enterprise 2025 Plan (RIE2025).

Mr Ling Keok Tong, Director (Smart Nation and Digital Economy) at NRF shared that quantum communications and security, as well as quantum devices and instrumentation, are two significant focus areas under the QEP.

Jamilah Lim| @TechieKitteh

Jam (she/they) is the editor of Tech Wire Asia. They are a humanist and feminist with a love for science and technology. They are also cognizant of the intersectionality of the above with ethics, morality, and its economic/social impact on people, especially marginalized/underdeveloped communities.

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Rice physicists teamed with colleagues at Europes Large Hadron Collider to study matter-generating collisions of light. Researchers showed the departure angle of debris from the smashups is subtly distorted by quantum interference patterns in the light prior to impact. Credit: Illustration by 123rf.com

Understanding photon collisions could aid search for physics beyond the Standard Model.

Hot on the heels of proving an 87-year-old prediction that matter can be generated directly from light, Rice University physicists and their colleagues have detailed how that process may impact future studies of primordial plasma and physics beyond the Standard Model.

We are essentially looking at collisions of light, said Wei Li, an associate professor of physics and astronomy at Rice and co-author of the study published in Physical Review Letters.

We know from Einstein that energy can be converted into mass, said Li, a particle physicist who collaborates with hundreds of colleagues on experiments at high-energy particle accelerators like the European Organization for Nuclear Researchs Large Hadron Collider (LHC) and Brookhaven National Laboratorys Relativistic Heavy Ion Collider (RHIC).

Accelerators like RHIC and LHC routinely turn energy into matter by accelerating pieces of atoms near the speed of light and smashing them into one another. The 2012 discovery of the Higgs particle at the LHC is a notable example. At the time, the Higgs was the final unobserved particle in the Standard Model, a theory that describes the fundamental forces and building blocks of atoms.

Rice physics professor Wei Li (left) and postdoctoral research associate Shuai Yang teamed with colleagues at the Large Hadron Colliders (LHC) Compact Muon Solenoid experiment to study matter-generating collisions of light that occurred in heavy ion experiments at LHC. Yang lead-authored a newly published study that detailed how the departure angle of debris from the smashups is subtly distorted by quantum interference patterns prior to impact. Credit: Photo by Jeff Fitlow

Impressive as it is, physicists know the Standard Model explains only about 4% of the matter and energy in the universe. Li said this weeks study, which was lead-authored by Rice postdoctoral researcher Shuai Yang, has implications for the search for physics beyond the Standard Model.

There are papers predicting that you can create new particles from these ion collisions, that we have such a high density of photons in these collisions that these photon-photon interactions can create new physics beyond in the Standard Model, Li said.

Yang said, To look for new physics, one must understand Standard Model processes very precisely. The effect that weve seen here has not been previously considered when people have suggested using photon-photon interactions to look for new physics. And its extremely important to take that into account.

The effect Yang and colleagues detailed occurs when physicists accelerate opposing beams of heavy ions in opposite directions and point the beams at one another. The ions are nuclei of massive elements like gold or lead, and ion accelerators are particularly useful for studying the strong force, which binds fundamental building blocks called quarks in the neutrons and protons of atomic nuclei. Physicists have used heavy ion collisions to overcome those interactions and observe both quarks and gluons, the particles quarks exchange when they interact via the strong force.

But nuclei arent the only things that collide in heavy ion accelerators. Ion beams also produce electric and magnetic fields that shroud each nuclei in the beam with its own cloud of light. These clouds move with the nuclei, and when clouds from opposing beams meet, individual particles of light called photons can meet head-on.

In a PRL study published in July, Yang and colleagues used data from RHIC to show photon-photon collisions produce matter from pure energy. In the experiments, the light smashups occurred along with nuclei collisions that created a primordial soup called quark-gluon plasma, or QGP.

At RHIC, you can have the photon-photon collision create its mass at the same time as the formation of quark-gluon plasma, Yang said. So, youre creating this new mass inside the quark-gluon plasma.

Yangs Ph.D. thesis work on the RHIC data published in PRL in 2018 suggested photon collisions might be affecting the plasma in a slight but measurable way. Li said this was both intriguing and surprising, because the photon collisions are an electromagnetic phenomena, and quark-gluon plasmas are dominated by the strong force, which is far more powerful than the electromagnetic force.

To interact strongly with quark-gluon plasma, only having electric charge is not enough, Li said. You dont expect it to interact very strongly with quark-gluon plasma.

He said a variety of theories were offered to explain Yangs unexpected findings.

One proposed explanation is that the photon-photon interaction will look different not because of quark-gluon plasma, but because the two ions just get closer to each other, Li said. Its related to quantum effects and how the photons interact with each other.

If quantum effects had caused the anomalies, Yang surmised, they could create detectable interference patterns when ions narrowly missed one another but photons from their respective light clouds collided.

So the two ions, they do not strike each other directly, Yang said. They actually pass by. Its called an ultraperipheral collision, because the photons collide but the ions dont hit each other.

The Compact Muon Solenoid experiment at the European Organization for Nuclear Researchs Large Hadron Collider. Credit: CERN

Theory suggested quantum interference patterns from ultraperipheral photon-photon collisions should vary in direct proportion to the distance between the passing ions. Using data from the LHCs Compact Muon Solenoid (CMS) experiment, Yang, Li and colleagues found they could determine this distance, or impact parameter, by measuring something wholly different.

The two ions, as they get closer, theres a higher probability the ion can get excited and start to emit neutrons, which go straight down the beam line, Li said. We have a detector for this at CMS.

Each ultraperipheral photon-photon collision produces a pair of particles called muons that typically fly from the collision in opposite directions. As predicted by theory, Yang, Li and colleagues found that quantum interference distorted the departure angle of the muons. And the shorter the distance between the near-miss ions, the greater the distortion.

Li said the effect arises from the motion of the colliding photons. Although each is moving in the direction of the beam with its host ion, photons can also move away from their hosts.

The photons have motion in the perpendicular direction, too, he said. And it turns out, exactly, that that perpendicular motion gets stronger as the impact parameter gets smaller and smaller.

This makes it appear like somethings modifying the muons, Li said. It looks like one is going at a different angle from the other, but its really not. Its an artifact of the way the photons motion was changing, perpendicular to the beam direction, before the collision that made the muons.

Yang said the study explains most of the anomalies he previously identified. Meanwhile, the study established a novel experimental tool for controlling the impact parameter of photon interactions that will have far-reaching impacts.

We can comfortably say that the majority came from this QED effect, he said. But that doesnt rule out that there are still effects that relate to the quark-gluon plasma. This work gives us a very precise baseline, but we need more precise data. We still have at least 15 years to gather QGP data at CMS, and the precision of the data will get higher and higher.

Reference: Observation of Forward Neutron Multiplicity Dependence of Dimuon Acoplanarity in Ultraperipheral Pb-Pb Collisions at sNN=5.02TeV by A.M. Sirunyan et al. (CMS Collaboration), 17 September 2021, Physical Review Letters.DOI: 10.1103/PhysRevLett.127.122001

LHC and CMS are supported by the European Organization for Nuclear Research, the Department of Energy, the National Science Foundation and scientific funding agencies in Austria, Belgium, Brazil, Bulgaria, China, Colombia, Croatia, Cyprus, Ecuador, Estonia, Finland, France, Germany, Greece, Hungary, India, Iran, Ireland, Italy, South Korea, Latvia, Lithuania, Malaysia, Mexico, Montenegro, New Zealand, Pakistan, Poland, Portugal, Russia, Serbia, Spain, Sri Lanka, Switzerland, Taiwan, Thailand, Turkey, Ukraine and the United Kingdom.

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