QT/ Energy-efficient quantum computing in magnets
Quantum news biweekly vol. 74, 23rd May — 6th June
TL;DR
- Researchers from Lancaster University and Radboud University Nijmegen have managed to generate propagating spin waves at the nanoscale and discovered a novel pathway to modulate and amplify them.
- Scientists demonstrated a quantum algorithmic speedup with the quantum approximate optimization algorithm, laying the groundwork for advancements in telecommunications, financial modeling, materials science and more.
- Many of today’s quantum devices rely on collections of qubits, also called spins. These quantum bits have only two energy levels, the ‘0’ and the ‘1’. However, spins in real devices also interact with light and vibrations known as bosons, greatly complicating calculations. Researchers now demonstrate a way to describe spin-boson systems and use this to efficiently configure quantum devices in a desired state.
- A technical achievement marks a significant advance in the burgeoning field of observing individual molecules without the aid of fluorescent labels. While these labels are useful in many applications, they alter molecules in ways that can obscure how they naturally interact with one another. The new label-free method makes the molecules so easy to detect, it is almost as if they had labels.
- Lenses are used to bend and focus light. Normal lenses rely on their curved shape to achieve this effect, but physicists have made a flat lens of only three atoms thick which relies on quantum effects. This type of lens could be used in future augmented reality glasses.
- Scientists describe a new method to make very thin crystals of the element bismuth — a process that may aid the manufacturing of cheap flexible electronics an everyday reality.
- Researchers have the first direct evidence that the powerful magnetic fields created in off-center collisions of atomic nuclei induce an electric current in ‘deconfined’ nuclear matter. The study used measurements of how charged particles are deflected when they emerge from the collisions. The study provides proof that the magnetic fields exist and offers a new way to measure electrical conductivity in quark-gluon plasma.
- Nuclear physicists have long been working to reveal how the proton gets its spin. Now, a new method that combines experimental data with state-of-the-art calculations has revealed a more detailed picture of spin contributions from the very glue that holds protons together.
- Using interference between two lasers, a research group has created an ‘optical conveyor belt’ that can move polaritons — a type of light-matter hybrid particle — in semiconductor-based microcavities. This work could lead to the development of new devices with applications in areas such as quantum metrology and quantum information.
- It’s one thing to dream up a quantum internet that could send hacker-proof information around the world via photons superimposed in different quantum states. It’s quite another to physically show it’s possible. That’s exactly what physicists have done, using existing Boston-area telecommunication fiber, in a demonstration of the world’s longest fiber distance between two quantum memory nodes to date.
- And more!
Quantum Computing Market
According to the recent market research report by MarketsandMarkets, the Quantum Computing market is expected to grow to USD 5,3 million by 2029, at a CAGR of 32.7%. The adoption of quantum computing in the banking and finance sector is expected to fuel the growth of the market globally. Other key factors contributing to the growth of the quantum computing market include rising investments by governments of different countries to carry out research and development activities related to quantum computing technology.
According to ‘Quantum Computing Market Research Report: By Offering, Deployment Type, Application, Technology, Industry — Industry Share, Growth, Drivers, Trends and Demand Forecast to 2030’ report, the quantum computing market is projected to reach $64,988 million by 2030. Machine learning (ML) is expected to progress at the highest CAGR, during the forecast period, among all application categories, owing to the fact that quantum computing is being integrated in ML for improving the latter’s use case.
Latest Research
Canted spin order as a platform for ultrafast conversion of magnons
by R. A. Leenders, D. Afanasiev, A. V. Kimel, R. V. Mikhaylovskiy in Nature
Researchers from Lancaster University and Radboud University Nijmegen have managed to generate propagating spin waves at the nanoscale and discovered a novel pathway to modulate and amplify them.
Their discovery could pave the way for the development of dissipation free quantum information technologies. As the spin waves do not involve electric currents these chips will be free from associated losses of energy.
The rapidly growing popularity of artificial intelligence comes with an increasing desire for fast and energy efficient computing devices and calls for novel ways to store and process information. The electric currents in conventional devices suffer from losses of energy and subsequent heating of the environment.
One alternative for the “lossy” electric currents is to store and process information in waves, using the spins of the electrons instead of their charges. These spins can be seen as the elementary units of magnets.
Lead author Dr Rostislav Mikhaylovskiy from Lancaster University said: “Our discovery will be essential for future spin-wave based computing. Spin waves are an appealing information carrier as they don’t involve electric currents and therefore do not suffer from resistive losses.”
It has already been known for many years that spins can be kicked out of their equilibrium orientation. After this perturbation, the spins start to precess (i.e. rotate) around their equilibrium position. In magnets neighbouring spins are extremely strongly coupled, forming a net magnetization. Due to this coupling, the spin precession can propagate in the magnetic material, giving rise to a spin wave.
“Observing nonlinear conversion of coherent propagating magnons at nanoscale, which is a prerequisite for any practical magnon-based data processing, has been sought for by many groups worldwide for more than a decade. Therefore, our experiment is a landmark for spin wave studies, which holds the potential to open an entire new research direction on ultrafast coherent magnonics with an eye on the development of dissipation free quantum information technologies.”
The researchers have used the fact that the highest possible frequencies of the spin rotations can be found in materials, in which adjacent spins are canted with respect to each other. To excite such fast spin dynamics, they used a very short pulse of light, the duration of which is shorter than the period of the spin wave, i.e. less than a trillionth of a second. The trick for generating the ultrafast spin wave at the nanoscale is in the photon energy of the light pulse. The material of study exhibits extremely strong absorption at ultraviolet (UV) photon energies, which localises the excitation in a very thin region of only a few tens of nanometres from the interface, which allows spin waves with terahertz (a trillion of Hertz) frequencies and sub-micrometre wavelengths to emerge.
The dynamics of such spin waves is intrinsically nonlinear, meaning that the waves with different frequencies and wavelengths can be converted into each other. The researchers have now for the first time realized this possibility in practice. They achieved this by exciting the system not with only one, but with two intense laser pulses, separated by a short time delay.
First author Ruben Leenders, former PhD student at Lancaster University, said: “In a typical single pulse excitation experiment, we would simply expect the two spin waves to interfere with each other as any waves do. However, by varying the time delay between the two pulses we found that this superposition of the two waves does not hold.”
The team explained the observations by considering the coupling of the already excited spin wave with the second light pulse. The result of this coupling is that when the spins are already rotating, the second light pulse gives an additional kick to the spins. The strength and the direction of this kick depends on the state of the deflection of the spins at the time that this second light pulse arrives. This mechanism allows for control over the properties of the spin waves such as their amplitude and phase, simply by choosing the appropriate time delay between the excitations.
Evidence of scaling advantage for the quantum approximate optimization algorithm on a classically intractable problem
by Ruslan Shaydulin, Changhao Li, Shouvanik Chakrabarti, et al in Science Advances
In a new paper, researchers at JPMorgan Chase, the U.S. Department of Energy’s (DOE) Argonne National Laboratory and Quantinuum have demonstrated clear evidence of a quantum algorithmic speedup for the quantum approximate optimization algorithm (QAOA).
This algorithm has been studied extensively and has been implemented on many quantum computers. It has potential application in fields such as logistics, telecommunications, financial modeling and materials science.
“This work is a significant step towards reaching quantum advantage, laying the foundation for future impact in production,” said Marco Pistoia, head of Global Technology Applied Research at JPMorgan Chase.
The team examined whether a quantum algorithm with low implementation costs could provide a quantum speedup over the best-known classical methods. QAOA was applied to the Low Autocorrelation Binary Sequences problem, which has significance in understanding the behavior of physical systems, signal processing and cryptography. The study showed that if the algorithm was asked to tackle increasingly larger problems, the time it would take to solve them would grow at a slower rate than that of a classical solver.
To explore the quantum algorithm’s performance in an ideal noiseless setting, JPMorgan Chase and Argonne jointly developed a simulator to evaluate the algorithm’s performance at scale. It was built on the Polaris supercomputer, accessed through the Argonne Leadership Computing Facility (ALCF), a DOE Office of Science user facility. The ALCF is supported by DOE’s Advanced Scientific Computing Research program.
“The large-scale quantum circuit simulations efficiently utilized the DOE petascale supercomputer Polaris located at the ALCF. These results show how high performance computing can complement and advance the field of quantum information science,” said Yuri Alexeev, a computational scientist at Argonne. Jeffrey Larson, a computational mathematician in Argonne’s Mathematics and Computer Science Division, also contributed to this research.
To take the first step toward practical realization of the speedup in the algorithm, the researchers demonstrated a small-scale implementation on Quantinuum’s System Model H1 and H2 trapped-ion quantum computers. Using algorithm-specific error detection, the team reduced the impact of errors on algorithmic performance by up to 65%.
“Our long-standing partnership with JPMorgan Chase led to this meaningful and noteworthy three-way research experiment that also brought in Argonne. The results could not have been achieved without the unprecedented and world leading quality of our H-Series Quantum Computer, which provides a flexible device for executing error-correcting and error-detecting experiments on top of gate fidelities that are years ahead of other quantum computers,” said Ilyas Khan, founder and chief product officer of Quantinuum.
Fast Quantum State Preparation and Bath Dynamics Using Non-Gaussian Variational Ansatz and Quantum Optimal Control
by Liam J. Bond, Arghavan Safavi-Naini, Jiří Minář in Physical Review Letters
Many of today’s quantum devices rely on collections of qubits, also called spins. These quantum bits have only two energy levels, the ‘0’ and the ‘1’. However, spins in real devices also interact with light and vibrations known as bosons, greatly complicating calculations. In a new publication, researchers in Amsterdam demonstrate a way to describe spin-boson systems and use this to efficiently configure quantum devices in a desired state.
Quantum devices use the quirky behaviour of quantum particles to perform tasks that go beyond what ‘classical’ machines can do, including quantum computing, simulation, sensing, communication and metrology. These devices can take many forms, such as a collection of superconducting circuits, or a lattice of atoms or ions held in place by lasers or electric fields.
Regardless of their physical realisation, quantum devices are typically described in simplified terms as a collection of interacting two-level quantum bits or spins. However, these spins also interact with other things in their surroundings, such as light in superconducting circuits or oscillations in the lattice of atoms or ions. Particles of light (photons) and vibrational modes of a lattice (phonons) are examples of bosons.
Unlike spins, which have only two possible energy levels (‘0’ or ‘1’), the number of levels for each boson is infinite. Consequently, there are very few computational tools for describing spins coupled to bosons. In their new work, physicists Liam Bond, Arghavan Safavi-Naini and Jiří Minář of the University of Amsterdam, QuSoft and Centrum Wiskunde & Informatica work around this limitation by describing systems composed of spins and bosons using so-called non-Gaussian states. Each non-Gaussian state is a combination (a superposition) of much simpler Gaussian states.
“A Gaussian state would look like a plain red circle, without any interesting blue-red patterns,” explains PhD candidate Liam Bond. An example of a Gaussian state is laser light, in which all light-waves are perfectly in sync. “If we take many of these Gaussian states and start overlapping them (so that they’re in a superposition), these beautifully intricate patterns emerge. We were particularly excited because these non-Gaussian states allow us to retain a lot of the powerful mathematical machinery that exists for Gaussian states, whilst enabling us to describe a far more diverse set of quantum states.”
Bond continues: “There are so many possible patterns that classical computers often struggle to compute and process them. Instead, in this publication we use a method that identifies the most important of these patterns and ignores the others. This lets us study these quantum systems, and design new ways of preparing interesting quantum states.”
The new approach can be exploited to efficiently prepare quantum states in a way that outperforms other traditionally used protocols. “Fast quantum state preparation might be useful for a wide range of applications, such as quantum simulation or even quantum error correction,” notes Bond. The researchers also demonstrate that they can use non-Gaussian states to prepare ‘critical’ quantum states which correspond to a system undergoing a phase transition. In addition to fundamental interest, such states can greatly enhance the sensitivity of quantum sensors.
While these results are very encouraging, they are only a first step towards more ambitious goals. So far, the method has been demonstrated for a single spin. A natural, but challenging extension is to include many spins and many bosonic modes at the same time. A parallel direction is to account for the effects of the environment disturbing the spin-boson systems. Both of these approaches are under active development.
Label-free detection and profiling of individual solution-phase molecules
by Lisa-Maria Needham, Carlos Saavedra, Julia K. Rasch, et al in Nature
Scientists at the University of Wisconsin-Madison have developed the most sensitive method yet for detecting and profiling a single molecule — unlocking a new tool that holds potential for better understanding how the building blocks of matter interact with each other. The new method could have implications for pursuits as varied as drug discovery and the development of advanced materials.
The technical achievement, marks a significant advance in the burgeoning field of observing individual molecules without the aid of fluorescent labels. While these labels are useful in many applications, they alter molecules in ways that can obscure how they naturally interact with one another. The new label-free method makes the molecules so easy to detect, it is almost as if they had labels.
“We’re very excited about this,” says Randall Goldsmith, a UW-Madison professor of chemistry who led the work. “Capturing behaviors at the level of single molecules is an amazingly informative way of understanding complex systems, and if you can build new tools that grant better access to that perspective, those tools can be really powerful.”
While researchers can glean useful information from studying materials and biological systems at larger scales, Goldsmith says that observing the behavior of and interactions between individual molecules is important for contextualizing that information, sometimes leading to new insights.
“When you see how nations interact with each other, it all comes down to interactions between individuals,” says Goldsmith. “You wouldn’t even think of understanding how groups of people interact with each other while ignoring how individuals interact with each other.”
Goldsmith has been chasing the allure of single molecules since he was a postdoctoral researcher at Stanford University more than a decade ago. There, he worked under the chemist W.E. Moerner, who received the Nobel Prize in chemistry in 2014 for developing the first method of using light to observe a single molecule.
Since Moerner’s initial success, researchers around the world have devised and refined new ways to observe these tiny bits of matter. The method that the UW-Madison team developed relies on a device called an optical microresonator, or microcavity. As its name suggests, the microcavity is an extremely tiny space where light can be trapped in both space and time — at least for a few nanoseconds — where it can interact with a molecule. Microcavities are more commonly found in physics or electrical engineering laboratories, not chemistry labs. Goldsmith’s history of combining concepts from disparate scientific fields was recognized in 2022 with a Polymath award from Schmidt Futures.
Microcavities are built from incredibly small mirrors fashioned right on top of a fiber optic cable. These fiber optic mirrors bounce the light back and forth many times very quickly within the microcavity. The researchers let molecules tumble into the cavity, let the light pass through it, and can not only detect the molecule’s presence, but also learn information about it, such as how fast it moves through water. This information can be used to determine the molecule’s shape, or conformation.
“Conformation at the molecular level is incredibly important, particularly for thinking about how biomolecules interact with each other,” says Goldsmith. “Let’s say you have a protein and you have some small-molecule drug. You want to see if the protein’s druggable, which is to say, ‘Does the drug have some kind of major interaction with the protein?’ One way you might be able to see that is if it introduces a conformational change.”
There are other ways to do that, but they require large amounts of sample material and time-consuming analyses. With the newly developed microcavity technique, Goldsmith says, “we can potentially build a black-box tool to give us the answer in tens of seconds.”
The team, which included Lisa-Maria Needham, a former postdoctoral researcher who is now a laboratory director at the University of Cambridge, has filed a patent for the device. Goldsmith says the device and methods will now be refined over the next couple of years. In the meantime, he says he and his collaborators are already thinking about the many ways it could be useful.
“We’re excited about many other applications in spectroscopy,” he says. “We hope we can use this as a stepping stone to other ways to learn about molecules.”
Temperature-Dependent Excitonic Light Manipulation with Atomically Thin Optical Elements
by Ludovica Guarneri, Qitong Li, Thomas Bauer, Jung-Hwan Song, Ashley P. Saunders, Fang Liu, Mark L. Brongersma, Jorik van de Groep in Nano Letters
Lenses are used to bend and focus light. Normal lenses rely on their curved shape to achieve this effect, but physicists from the University of Amsterdam and Stanford University have made a flat lens of only three atoms thick which relies on quantum effects. This type of lens could be used in future augmented reality glasses.
When you imagine a lens, you probably picture a piece of curved glass. This type of lens works because light is refracted (bent) when it enters the glass, and again when it exits, allowing us to make things appear larger or closer than they actually are. We have used curved lenses for more than two millennia, allowing us to study the movements of distant planets and stars, to reveal tiny microorganisms, and to improve our vision.
Ludovico Guarneri, Thomas Bauer, and Jorik van de Groep of the University of Amsterdam, together with colleagues from Stanford University in California, took a different approach. Using a single layer of a unique material called tungsten disulphide (WS2 for short), they constructed a flat lens that is half a millimetre wide, but just 0.0000006 millimetres, or 0.6 nanometres, thick. This makes it the thinnest lens on Earth!
Rather than relying on a curved shape, the lens is made of concentric rings of WS2 with gaps in between. This is called a ‘Fresnel lens’ or ‘zone plate lens’, and it focuses light using diffraction rather than refraction. The size of, and distance between the rings (compared to the wavelength of the light hitting it) determines the lens’s focal length. The design used here focuses red light 1 mm from the lens.
A unique feature of this lens is that its focussing efficiency relies on quantum effects within WS2. These effects allow the material to efficiently absorb and re-emit light at specific wavelengths, giving the lens the built-in ability to work better for these wavelengths.
This quantum enhancement works as follows. First, WS2 absorbs light by sending an electron to a higher energy level. Due to the ultra-thin structure of the material, the negatively charged electron and the positively charged ‘hole’ it leaves behind in the atomic lattice stay bound together by the electrostatic attraction between them, forming what is known as an ‘exciton’. These excitons quickly disappear again by the electron and hole merging together and sending out light. This re-emitted light contributes to the lens’s efficiency.
The scientists detected a clear peak in lens efficiency for the specific wavelengths of light sent out by the excitons. While the effect is already observed at room temperature, the lenses are even more efficient when cooled down. This is because excitons do their work better at lower temperatures.
Another one of the lens’s unique features is that, while some of the light passing through it makes a bright focal point, most light passes through unaffected. While this may sound like a disadvantage, it actually opens new doors for use in technology of the future. “The lens can be used in applications where the view through the lens should not be disturbed, but a small part of the light can be tapped to collect information. This makes it perfect for wearable glasses such as for augmented reality,” explains Jorik van de Groep, one of the authors of the paper.
The researchers are now setting their sights on designing and testing more complex and multifunctional optical coatings whose function (such as focussing light) can be adjusted electrically. “Excitons are very sensitive to the charge density in the material, and therefore we can change the refractive index of the material by applying a voltage,” says Van de Groep. The future of excitonic materials is bright!
Exceptional electronic transport and quantum oscillations in thin bismuth crystals grown inside van der Waals materials
by Laisi Chen, Amy X. Wu, Naol Tulu, Joshua Wang, Adrian Juanson,et al in Nature Materials
In a study, scientists from the University of California, Irvine describe a new method to make very thin crystals of the element bismuth — a process that may aid the manufacturing of cheap flexible electronics an everyday reality.
“Bismuth has fascinated scientists for over a hundred years due to its low melting point and unique electronic properties,” said Javier Sanchez-Yamagishi, assistant professor of physics & astronomy at UC Irvine and a co-author of the study. “We developed a new method to make very thin crystals of materials such as bismuth, and in the process reveal hidden electronic behaviors of the metal’s surfaces.”
The bismuth sheets the team made are only a few nanometers thick. Sanchez-Yamagishi explained how theorists have predicted that bismuth contains special electronic states allowing it to become magnetic when electricity flows through it — something essential for quantum electronic devices based on the magnetic spin of electrons.
One of the hidden behaviors observed by the team is so-called quantum oscillations originating from the surfaces of the crystals.
“Quantum oscillations arise from the motion of an electron in a magnetic field,” said Laisi Chen, a Ph.D. candidate in physics & astronomy at UC Irvine and one of the lead authors of the paper. “If the electron can complete a full orbit around a magnetic field, it can exhibit effects that are important for the performance of electronics. Quantum oscillations were first discovered in bismuth in the 1930s, but have never been seen in nanometer-thin bismuth crystals.”
Amy Wu, a Ph.D. candidate in physics in Sanchez-Yamagishi’s lab, likened the team’s new method to a tortilla press. To make the ultra-thin sheets of bismuth, Wu explained, they had to squish bismuth between two hot plates. To make the sheets as flat as they are, they had to use molding plates that are perfectly smooth at the atomic level, meaning there are no microscopic divots or other imperfections on the surface. “We then made a kind of quesadilla or panini where the bismuth is the cheesy filling and the tortillas are the atomically flat surfaces,” said Wu.
“There was this nervous moment where we had spent over a year making these beautiful thin crystals, but we had no idea whether its electrical properties would be something extraordinary,” said Sanchez-Yamagishi. “But when we cooled down the device in our lab, we were amazed to observe quantum oscillations, which have not been previously seen in thin bismuth films.”
“Compression is a very common manufacturing technique used for making common household materials such as aluminum foil, but is not commonly used for making electronic materials like those in your computers,” Sanchez-Yamagishi added. “We believe our method will generalize to other materials, such as tin, selenium, tellurium and related alloys with low melting points, and it could be interesting to explore for future flexible electronic circuits.”
Next, the team wants to explore other ways in which compression and injection molding methods can be used to make the next computer chips for phones or tablets.
“Our new team members bring exciting ideas to this project, and we’re working on new techniques to gain further control over the shape and thickness of the grown bismuth crystals,” said Chen. “This will simplify how we fabricate devices, and take it one step closer for mass production.”
Observation of the Electromagnetic Field Effect via Charge-Dependent Directed Flow in Heavy-Ion Collisions at the Relativistic Heavy Ion Collider
by M. I. Abdulhamid, B. E. Aboona, J. Adam, J. R. Adams, G. Agakishiev, et al in Physical Review X
Data from heavy ion collisions give new insight into the electromagnetic properties of quark-gluon plasma ‘deconfined’ from protons and neutrons.
Scientists have the first direct evidence that the powerful magnetic fields created in off-center collisions of atomic nuclei induce an electric current in “deconfined” nuclear matter. This is a plasma “soup” of quarks and gluons that have been set free, or “deconfined,” from nuclear matter — protons and neutrons — in the particle collisions. The magnetic fields in deconfined nuclear matter are a billion times stronger than a typical refrigerator magnet, but their effects can be hard to detect. This new study’s evidence is from measuring the way particles with an electric charge are deflected when they emerge from the collisions. The study provides proof that the powerful magnetic fields exist. It also offers a new way to measure the electrical conductivity in the quark-gluon plasma (QGP).
Scientists can infer the value of the QGP’s electrical conductivity from how much the electromagnetic field deflects charged particles such as electrons, quarks, and protons. The stronger a particular type of deflection is, the stronger the conductivity. Conductivity is an important property of matter, but scientists have not been able to measure it in QGP before. Understanding the electromagnetic properties of the QGP may help physicists unravel the mysteries of the phase transition between QGP and ordinary nuclear matter made of protons and neutrons. The work will also aid in explorations of other magnetic effects in the QGP.
Off-center collisions of atomic nuclei at the Relativistic Heavy Ion Collider (RHIC), a Department of Energy particle accelerator user facility at Brookhaven National Laboratory, should generate powerful magnetic fields. That’s because some of the non-colliding positively charged protons are set swirling as the nuclei sideswipe one another at close to the speed of light. The fields are expected to be stronger than those of neutrons stars and much more powerful than Earth’s. But measuring magnetic fields in the QGP is challenging because this deconfined nuclear matter doesn’t last very long. So, instead, scientists measure the QGP’s properties indirectly, for example by using RHIC’s STAR detector to track the impact of the magnetic field on charged particles streaming from the collisions.
The STAR physicists saw a pattern of charged-particle deflection that could only be caused by an electromagnetic field and current induced in the QGP. This was clear evidence that the magnetic fields exist. The degree of deflection is directly related to the strength of the induced current. Scientists will now use this method to measure the conductivity of the QGP. That, in turn, may help them unravel mysteries of the phase transition between deconfined quarks and gluons and composite particles such as protons and neutrons.
Gluon helicity from global analysis of experimental data and lattice QCD Ioffe time distributions
by J. Karpie, R. M. Whitehill, W. Melnitchouk, C. Monahan, K. Orginos, J.-W. Qiu, D. G. Richards, N. Sato, S. Zafeiropoulos in Physical Review D
Nuclear physicists have long been working to reveal how the proton gets its spin. Now, a new method that combines experimental data with state-of-the-art calculations has revealed a more detailed picture of spin contributions from the very glue that holds protons together. It also paves the way toward imaging the proton’s 3D structure.
The work was led by Joseph Karpie, a postdoctoral associate in the Center for Theoretical and Computational Physics (Theory Center) at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility. He said that this decades-old mystery began with measurements of the sources of the proton’s spin in 1987. Physicists originally thought that the proton’s building blocks, its quarks, would be the main source of the proton’s spin. But that’s not what they found. It turned out that the proton’s quarks only provide about 30% of the proton’s total measured spin. The rest comes from two other sources that have so far proven more difficult to measure.
One is the mysterious but powerful strong force. The strong force is one of the four fundamental forces in the universe. It’s what “glues” quarks together to make up other subatomic particles, such as protons or neutrons. Manifestations of this strong force are called gluons, which are thought to contribute to the proton’s spin. The last bit of spin is thought to come from the movements of the proton’s quarks and gluons.
“This paper is sort of a bringing together of two groups in the Theory Center who have been working toward trying to understand the same bit of physics, which is how do the gluons that are inside of it contribute to how much the proton is spinning around,” he said.
He said this study was inspired by a puzzling result that came from initial experimental measurements of the gluons’ spin. The measurements were made at the Relativistic Heavy Ion Collider, a DOE Office of Science user facility based at Brookhaven National Laboratory in New York. The data at first seemed to indicate that the gluons may be contributing to the proton’s spin. They showed a positive result. But as the data analysis was improved, a further possibility appeared.
“When they improved their analysis, they started to get two sets of results that seemed quite different, one was positive and the other was negative,” Karpie explained.
While the earlier positive result indicated that the gluons’ spins are aligned with that of the proton, the improved analysis allowed for the possibility that the gluons’ spins have an overall negative contribution. In that case, more of the proton spin would come from the movement of the quarks and gluons, or from the spin of the quarks themselves.
Meanwhile, the HadStruc collaboration had been addressing the same measurements in a different way. They were using supercomputers to calculate the underlying theory that describes the interactions among quarks and gluons in the proton, Quantum Chromodynamics (QCD).
To equip supercomputers to make this intense calculation, theorists somewhat simplify some aspects of the theory. This somewhat simplified version for computers is called lattice QCD. Karpie led the work to bring together the data from both groups. He started with the combined data from experiments taken in facilities around the world. He then added the results from the lattice QCD calculation into his analysis.
“This is putting everything together that we know about quark and gluon spin and how gluons contribute to the spin of the proton in one dimension,” said David Richards, a Jefferson Lab senior staff scientist who worked on the study.
“When we did, we saw that the negative things didn’t go away, but they changed dramatically. That meant that there’s something funny going on with those,” Karpie said.
Karpie is lead author on the study. He said the main takeaway is that combining the data from both approaches provided a more informed result.
“We’re combining both of our datasets together and getting a better result out than either of us could get independently. It’s really showing that we learn a lot more by combining lattice QCD and experiment together in one problem analysis,” said Karpie. “This is the first step, and we hope to keep doing this with more and more observables as well as we make more lattice data.”
The next step is to further improve the datasets. As more powerful experiments provide more detailed information on the proton, these data begin painting a picture that goes beyond one dimension. And as theorists learn how to improve their calculations on ever-more powerful supercomputers, their solutions also become more precise and inclusive. The goal is to eventually produce a three-dimensional understanding of the proton’s structure.
“So, we learn our tools do work on the simpler one-dimension scenario. By testing our methods now, we hopefully will know what we need to do when we want to move up to do 3D structure,” Richards said. “This work will contribute to this 3D image of what a proton should look like. So it’s all about building our way up to the heart of the problem by doing this easier stuff now.”
Non-reciprocal band structures in an exciton–polariton Floquet optical lattice
by Yago del Valle Inclan Redondo, Xingran Xu, Timothy C. H. Liew, Elena A. Ostrovskaya, Alexander Stegmaier, Ronny Thomale, Christian Schneider, Siddhartha Dam, Sebastian Klembt, Sven Höfling, Seigo Tarucha, Michael D. Fraser in Nature Photonics
Using interference between two lasers, a research group led by scientists from RIKEN and NTT Research have created an ‘optical conveyor belt’ that can move polaritons — a type of light-matter hybrid particle — in semiconductor-based microcavities. This work could lead to the development of new devices with applications in areas such as quantum metrology and quantum information.
For the current study, the scientists used the interference between two lasers to create a dynamic potential energy landscape — imagine a landscape of valleys and hills, in constant repeating motion — for a coherent, laser-like state of polaritons known as a polariton condensate. They achieved this by introducing a new optical tool — an optical conveyor belt — to enable the control of the energy landscape, concretely, the lattice depths and the interactions between neighboring particles. By further tuning the frequency difference between the two lasers, the conveyer belt moves at speeds of the order of 0.1 percent of the speed of light, driving the polaritons into a new state.
Non-reciprocity — a phenomenon where system dynamics are different in opposing directions — is a crucial ingredient for creating what is known as an artificial topological phase of matter. Topology is the mathematical classification of objects by counting the number of number of ‘holes’, e.g. a donut or a knot may have a finite number of holes, while a ball has none. Quantum materials can also be engineered with a non-zero topology, which in this case is more abstractly embedded into the band structure. Such materials can exhibit behavior such as dissipationless transport, meaning that they can move without energy loss, and other exotic quantum phenomena. It is extremely challenging to introduce non-reciprocity into engineered optical platforms, and this simple, extendible experimental demonstration opens new opportunities for emerging quantum technologies incorporate functional topology.
The research group, including first author Yago del Valle Inclan Redondo, and led by Senior Research Scientist Michael Fraser, both from RIKEN CEMS and NTT Research, together with collaborators from Germany, Singapore and Australia, have conducted a study in this direction. Fraser says, “We have created a topological state of light in a semiconductor structure by a mechanism involving rapid modulation of the energy landscape, resulting in the introduction of a synthetic dimension.” A synthetic dimension is a method of mapping a non-spatial dimension, in this case time, into a space-like dimension, such that the system dynamics can evolve in a higher number of dimensions and become better suited to realizing topological matter. This work extends upon a technique developed by the group, published last year, which similarly used temporally modulated lasers to drive the rapid rotation of polariton condensates.
Using this simple experimental scheme involving the interference between two lasers, the scientists were able to organize polaritons in precisely the right dimensions to create an artificial band structure, meaning that the particles organized into energy bands like electrons in a material. By tuning the dimensions, depth and speed of the polariton optical lattice, control over the band structure is achieved. Thanks to this rapid motion, the polaritons see a different potential energy landscape depending on whether they are propagating with or against the flow of the lattice, an effect which is analogous to the Doppler shift for sound. This asymmetric response of the confined polaritons breaks time-reversal symmetry, driving non-reciprocity and the formation of a topological band structure.
“Photonic states with topological properties can be used in advanced opto-electronic devices where topology might greatly improve the performance of optical devices, circuits, and networks, such as by reducing noise and lasing threshold powers, and dissipationless optical waveguiding. Further, the simplicity and robustness of our technique opens new opportunities for the development of topological photonic devices with applications in quantum metrology and quantum information,” concludes Fraser.
Entanglement of nanophotonic quantum memory nodes in a telecom network
by C. M. Knaut, A. Suleymanzade, Y.-C. Wei, D. R. Assumpcao, P.-J. Stas, Y. Q. Huan, B. Machielse, E. N. Knall, M. Sutula, G. Baranes, N. Sinclair, C. De-Eknamkul, D. S. Levonian, M. K. Bhaskar, H. Park, M. Lončar, M. D. Lukin in Nature
It’s one thing to dream up a quantum internet that could send hacker-proof information around the world via photons superimposed in different quantum states. It’s quite another to physically show it’s possible.
That’s exactly what Harvard physicists have done, using existing Boston-area telecommunication fiber, in a demonstration of the world’s longest fiber distance between two quantum memory nodes to date. Think of it as a simple, closed internet between point A and B, carrying a signal encoded not by classical bits like the existing internet, but by perfectly secure, individual particles of light.
The groundbreaking work, was led by Mikhail Lukin, the Joshua and Beth Friedman University Professor in the Department of Physics, in collaboration with Harvard professors Marko Lončar and Hongkun Park, who are all members of the Harvard Quantum Initiative, alongside researchers at Amazon Web Services. The Harvard team established the practical makings of the first quantum internet by entangling two quantum memory nodes separated by optical fiber link deployed over a roughly 22-mile loop through Cambridge, Somerville, Watertown, and Boston. The two nodes were located a floor apart in Harvard’s Laboratory for Integrated Science and Engineering.
Quantum memory, analogous to classical computer memory, is an important component of an interconnected quantum computing future because it allows for complex network operations and information storage and retrieval. While other quantum networks have been created in the past, the Harvard team’s is the longest fiber network between devices that can store, process and move information.
Each node is a very small quantum computer, made out of a sliver of diamond that has a defect in its atomic structure called a silicon-vacancy center. Inside the diamond, carved structures smaller than a hundredth the width of a human hair enhance the interaction between the silicon-vacancy center and light.
The silicon-vacancy center contains two qubits, or bits of quantum information: one in the form of an electron spin used for communication, and the other in a longer-lived nuclear spin used as a memory qubit to store entanglement (the quantum-mechanical property that allows information to be perfectly correlated across any distance). Both spins are fully controllable with microwave pulses. These diamond devices — just a few millimeters square — are housed inside dilution refrigeration units that reach temperatures of -459 Fahrenheit.
Using silicon-vacancy centers as quantum memory devices for single photons has been a multi-year research program at Harvard. The technology solves a major problem in the theorized quantum internet: signal loss that can’t be boosted in traditional ways. A quantum network cannot use standard optical-fiber signal repeaters because copying of arbitrary quantum information is impossible — making the information secure, but also very hard to transport over long distances.
Silicon vacancy center-based network nodes can catch, store and entangle bits of quantum information while correcting for signal loss. After cooling the nodes to close to absolute zero, light is sent through the first node and, by nature of the silicon vacancy center’s atomic structure, becomes entangled with it.
“Since the light is already entangled with the first node, it can transfer this entanglement to the second node,” explained first author Can Knaut, a Kenneth C. Griffin Graduate School of Arts and Sciences student in Lukin’s lab. “We call this photon-mediated entanglement.”
Over the last several years, the researchers have leased optical fiber from a company in Boston to run their experiments, fitting their demonstration network on top of the existing fiber to indicate that creating a quantum internet with similar network lines would be possible.
“Showing that quantum network nodes can be entangled in the real-world environment of a very busy urban area, is an important step towards practical networking between quantum computers,” Lukin said.
A two-node quantum network is only the beginning. The researchers are working diligently to extend the performance of their network by adding nodes and experimenting with more networking protocols.
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