Master equation to boost quantum technologies

As the size of modern technology shrinks down to the nanoscale, weird quantum effects—such as quantum tunneling, superposition, and entanglement—become prominent. This opens the door to a new era of quantum technologies, where quantum effects can be exploited. Many everyday technologies make use of feedback control routinely; an important example is the pacemaker, which must monitor the user's heartbeat and apply electrical signals to control it, only when needed. But physicists do not yet have an equivalent understanding of feedback control at the quantum level. Now, physicists have developed a "master equation" that will help engineers understand feedback at the quantum scale. Their results are published in the journal Physical Review Letters.

"It is vital to investigate how feedback control can be used in quantum technologies in order to develop efficient and fast methods for controlling quantum systems, so that they can be steered in real time and with high precision," says co-author Björn Annby-Andersson, a quantum physicist at Lund University, in Sweden.

An example of a crucial feedback-control process in quantum computing is quantum error correction. A quantum computer encodes information on physical qubits, which could be photons of light, or atoms, for instance. But the quantum properties of the qubits are fragile, so it is likely that the encoded information will be lost if the qubits are disturbed by vibrations or fluctuating electromagnetic fields. That means that physicists need to be able to detect and correct such errors, for instance by using feedback control. This error correction can be implemented by measuring the state of the qubits and, if a deviation from what is expected is detected, applying feedback to correct it.

But feedback control at the quantum level presents unique challenges, precisely because of the fragility physicists are trying to mitigate against. That delicate nature means that even the feedback process itself could destroy the system. "It is necessary to only interact weakly with the measured system, preserving the properties we want to exploit," says Annby-Andersson.

It is thus important to develop a full theoretical understanding of quantum feedback control, to establish its fundamental limits. But most existing theoretical models of quantum feedback control require computer simulations, which typically only provide quantitative results for specific systems. "It is difficult to draw general, qualitative conclusions," Annby-Andersson says. "The few models that can provide qualitative understanding are only applicable on a narrow class of feedback controlled systems—this type of feedback is typically referred to as linear feedback."

'Pen and paper'

Annby-Andersson and his colleagues have now developed a master equation, called a "Quantum Fokker-Planck equation," that enables physicists to track the evolution of any quantum system with feedback control over time. "The equation can describe scenarios that go beyond linear feedback," says Annby-Andersson. "In particular, the equation can be solved with pen and paper, rather than having to rely on computer simulations."

The team tested their equation by applying it to a simple feedback model. This confirmed that the equation provides physically sensible results and also demonstrated how energy can be harvested in microscopic systems, using feedback control. "The equation is a promising starting point for future studies of how energy may be manipulated with the help of information on a microscopic level," says Annby-Andersson.

The team is now investigating a system that makes use of feedback to manipulate energy in "quantum dots"—tiny semiconducting crystals just billionths of a meter across. "An important future direction is to use the equation as a tool for inventing novel feedback protocols that can be used for quantum technologies," says Annby-Andersson.

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Quantum Light Could Probe Chemical Reactions in Real Time

 

One of the things that sets the quantum world apart from our everyday classical one is the capacity for entanglement—when two or more objects share an invisible connection that entwines their fates. Entanglement is the most extreme version of a quantum connection, where measuring one particle can tell you everything you need to know about another. Short of that, particles can still sync up in decidedly quantum ways, where measuring one particle will give you some incomplete information about another. Such quantum correlations can be used to make more precise measurements than classical ones. For example, they can help us detect gravitational waves.


Photons of light don’t often naturally connect in this way. But when they do, quantum-correlated photons could potentially be useful to study materials’ quantum features. Generating this quantum light is tricky business, however, and has so far been largely confined to just a few photons.

Electrons, atoms and molecules, on the other hand, participate in en masse quantum correlations inside of materials all the time. Electrons syncing up inside a metal give rise to superconductivity at low temperatures, for instance, and—physicists speculate—high-temperature superconductivity, exotic fractional electron materials, and more. Now a team of physicists in Israel, Austria, England and the U.S. has found a way to imprint the complex pattern of quantum correlations from such materials onto light. This method can produce bright quantum light at a broad range of frequencies, the team explained recently in Nature Physics.

“Imagine having quantum light that you could see with your eyes,” says Ido Kaminer, an electrical and computer engineer at Technion–Israel Institute of Technology and senior author of the study. “That would be amazing, and it would also have many advantages for applications of quantum science that you wouldn’t consider otherwise.”

The researchers’ idea builds on an existing process for creating bursts of bright light. This process, known as high harmonic generation, involves shining a bright laser beam onto a gas of atoms, or more recently a solid crystal, such as zinc oxide, the active ingredient in many mineral sunscreens. The gaseous atoms or solids absorb the laser light and in turn emit light at higher harmonics: if the input light is like a middle C on a piano, the emitted light is comparable to many C notes hundreds of octaves up.

The emissions combine to produce light pulses that pass by in the tiniest fraction of a second—a billionth of a billionth. If directed at electrons, atoms or molecules, these short bursts can be used to capture high-frame-rate videos.

For their new study, the researchers aimed to understand how quantum correlations inside a source material, be it a gas or a mineral, would impact the quantum properties of the light bursts coming out, if at all. “High harmonic generation is a very important area. And still, until recently, it was described by a classical picture of light,” Kaminer says.

In quantum mechanics, figuring out what’s going on with more than a few particles at the same time is notoriously difficult. Kaminer and Alexey Gorlach, a graduate student in his lab, used their COVID-imposed isolation to try to make progress on a fully quantum description of light emitted in high harmonics. “It’s really crazy; Alexey built a super complex mathematical description on a scale that we’ve never had before,” Kaminer says.

Next, to fully incorporate the quantum properties of the material used to generate this light, Kaminer and Gorlach teamed up with Andrea Pizzi, then a graduate student at the University of Cambridge and now a postdoctoral fellow at Harvard University.

“This is a very beautiful mathematical framework to attack the very tricky mesoscopic world,” says Elena del Valle, a light-matter interaction expert and a physicist at the Autonomous University of Madrid, who was not involved with the work. “Mesoscopic” refers to anything that combines a medium number of particles: more than a few but not so many that individual behavior is completely irrelevant. Here, that means the many photons and their quantum correlations.

The researchers’ results spell out precisely how the quantum correlations of the source will translate into quantum correlations of emitted light.

If such quantum light is successfully generated in experiments, there are two main ways it could be of practical use. First, it can give insight into the material that generated it. “Quantum properties are at the core of a lot of things, like high-temperature superconductors,” Kaminer says. “And this would teach you something you couldn’t see otherwise.”

Second, quantum light can be used as a source, especially in the case of x-ray imaging. In this realm, correlated light could pick up on extra quantum information that would be inaccessible otherwise. “Once you get to the x-ray regime,” Kaminer says, “then you can use it for imaging materials, going through samples.”

Atoms and materials used for high harmonic generation today don’t have any interesting quantum properties to speak of, Kaminer says, and so do not produce quantum light. To choose a material to work with and create this light in the lab, the scientists are aiming to team up with an experimental group. They warn that a real implementation might not be straightforward.

“From here to the experiment there will still be some hard work, innovative engineering and theoretical developments,” Pizzi says. But researchers have several promising experimental ideas, and Pizzi and his collaborators, as well as others in the field, are optimistic. “Putting all this together for a few atoms, under a strong pulsed excitation, is not science fiction at the moment,” del Valle says. If realized, this technique could allow scientists to glimpse matter’s full quantum complexity like never before.

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The multiverse: How we're tackling the challenges facing the theory

 

The idea of a multiverse consisting of "parallel universes" is a popular science fiction trope, recently explored in the Oscar-winning movie "Everything Everywhere All At Once." However, it is within the realm of scientific possibility.



It is important to state from the start that the existence (or not) of the multiverse is a consequence of our present understanding of the fundamental laws of physics—it didn't come from the minds of whimsical physicists reading too many sci-fi books.


There are different versions of the multiverse. The first and perhaps most popular version comes from quantum mechanics, which governs the world of atoms and particles. It suggests a particle can be in many possible states simultaneously—until we measure the system and it picks one. According to one interpretation, all quantum possibilities that we didn't measure are realized in other universes.
Eternal inflation

The second version, the cosmological multiverse, arises as a consequence of cosmic inflation. In order to explain the fact that the universe today looks roughly similar everywhere, the physicist Alan Guth proposed in 1981 that the early universe underwent a period of accelerated expansion. During this period of inflation, space was stretched such that the distance between any two points were pushed apart faster than the speed of light.

The theory of inflation also predicted the existence of the primordial seeds which grew into cosmological structures such as stars and galaxies. This was triumphantly detected in 2003 by observations of tiny temperature fluctuations in the cosmic microwave background, which is the light left over from the Big Bang. It was subsequently measured with exquisite precision by the space experiments WMAP and Planck.

Due to this remarkable success, cosmic inflation is now considered the de facto theory of the early universe by most cosmologists.

But there was a (perhaps unintended) consequence of cosmic inflation. During inflation, space is stretched and smoothed over very large scales—usually much larger than the observable universe. Nevertheless, cosmic inflation must end at some point, else our universe wouldn't have been able to evolve to what it is today.

But physicists soon realized that if inflation really is true, some regions of space-time would continue to inflate even as inflation ended in the others. The regions that continue to inflate can be considered a separate, inflating universe. This process continues indefinitely, with inflating universes producing even more inflating universes, creating a multiverse of universes.

This phenomenon is dubbed "eternal inflation." First described by physicists Paul Steinhardt and Alex Vilenkin in 1983, eternal inflation remained a curious artifact of cosmic inflation until the early 21st century, when it was combined with an idea from string theory to produce a controversial yet compelling explanation of why our physical laws are what they are today.

String theory is not yet proven, but it is presently our best hope for a theory of everything—uniting quantum mechanics and gravity. However, physically realistic string theories must possess ten or more dimensions (rather than our normal three spatial dimensions plus time). Thus, to describe our present universe, six or more of these dimensions must be "compactified"—curled up in a such way that we can't see them.

The mathematical procedure for this is known. The problem (some might say the feature) of this process is that there are at least 10500 ways to do this compactification—and this mind-bogglingly huge set of possibilities is called the "string landscape." Each compactification will yield a different set of physical laws, potentially corresponding to a different universe. This begs two crucial questions: where are we in the string landscape, and why?

Eternal inflation provides an elegant answer to the first question: each inflating universe of the multiverse realizes a different point in the string landscape, so all possible physical laws can exist somewhere in the multiverse. But why is our universe so great at producing intelligent life like us? Well, some universes should, statistically speaking, be like ours—and we live in the universe in which our physical laws are the ones we observe.

However, this view is highly controversial—many argue it is not a scientific argument and it has spurred an intensive inquiry.

Testability

The obvious challenge with the multiverse is its observability. Suppose it does exist, is it then possible to observe the other universes, even in principle? For the quantum multiverse, the answer is no—different universes don't communicate. But in the inflationary multiverse, the answer is "yes, if we are lucky."

Since the different universes occupy the same physical space, neighboring universes could in principle collide with each other, possibly leaving relics and imprints in our observable universe. A research collaboration led by Hiranya Peiris of University College London and Matthew Johnson of the Perimeter Institute showed that such collisions should indeed leave imprints on the cosmic microwave background (light left over from the Big Bang) that can be searched for—although so far, these signatures have not been found.

The next challenge is theoretical. Some theorists have suggested that most of the universes in the string landscape are actually mathematically inconsistent—unable to exist in the way our universe does. They instead exist in a swampland of solutions—and in particular, solutions of string theory which permit cosmic inflation seem to be difficult to find.

There is deep disagreement among string theorists and cosmologists on whether string theory can describe inflation, even in principle. This conundrum is both vexing and exciting—it suggests that one of the two ideas is wrong, either of which will lead to a revolution in theoretical physics.

Finally, the very premise of cosmic inflation is now being challenged. The raison d'etre of cosmic inflation is that, regardless of how the early universe looked, inflation would dynamically drive the cosmos to the smooth universe we see today. However, it has never been rigorously investigated whether cosmic inflation can actually begin in the first place.

This is because the equations describing the beginning of the process are too complicated to solve analytically. But this question is now being rigorously tested by several research groups around the world, including my own at King's College London, where the power of modern high performance computing is brought to bear on solving these formerly intractable equations. So watch this space.

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Scientists open door to manipulating 'quantum light'

 

For the first time, scientists at the University of Sydney and the University of Basel in Switzerland have demonstrated the ability to manipulate and identify small numbers of interacting photons—packets of light energy—with high correlation.



This unprecedented achievement represents an important landmark in the development of quantum technologies. It is published today in Nature Physics.

Stimulated light emission, postulated by Einstein in 1916, is widely observed for large numbers of photons and laid the basis for the invention of the laser. With this research, stimulated emission has now been observed for single photons.

Specifically, the scientists could measure the direct time delay between one photon and a pair of bound photons scattering off a single quantum dot, a type of artificially created atom.

"This opens the door to the manipulation of what we can call 'quantum light'," Dr. Sahand Mahmoodian from the University of Sydney School of Physics and joint lead author of the research said.

Dr. Mahmoodian said, "This fundamental science opens the pathway for advances in quantum-enhanced measurement techniques and photonic quantum computing."

By observing how light interacted with matter more than a century ago, scientists discovered light was not a beam of particles, nor a wave pattern of energy—but exhibited both characteristics, known as wave-particle duality.

The way light interacts with matter continues to enthrall scientists and the human imagination, both for its theoretical beauty and its powerful practical application.

Whether it be how light traverses the vast spaces of the interstellar medium or the development of the laser, research into light is a vital science with important practical uses. Without these theoretical underpinnings, practically all modern technology would be impossible. No mobile phones, no global communication network, no computers, no GPS, no modern medical imaging.

One advantage of using light in communication—through optic fibers—is that packets of light energy, photons, do not easily interact with each other. This creates near distortion-free transfer of information at light speed.

However, we sometimes want light to interact. And here, things get tricky.

For instance, light is used to measure small changes in distance using instruments called interferometers. These measuring tools are now commonplace, whether it be in advanced medical imaging, for important but perhaps more prosaic tasks like performing quality control on milk, or in the form of sophisticated instruments such as LIGO, which first measured gravitational waves in 2015.

The laws of quantum mechanics set limits as to the sensitivity of such devices.

This limit is set between how sensitive a measurement can be and the average number of photons in the measuring device. For classical laser light this is different to quantum light.

Joint lead author, Dr. Natasha Tomm from the University of Basel, said, "The device we built induced such strong interactions between photons that we were able to observe the difference between one photon interacting with it compared to two."

"We observed that one photon was delayed by a longer time compared to two photons. With this really strong photon-photon interaction, the two photons become entangled in the form of what is called a two-photon bound state."

Quantum light like this has an advantage in that it can, in principle, make more sensitive measurements with better resolution using fewer photons. This can be important for applications in biological microscopy when large light intensities can damage samples and where the features to be observed are particularly small.

"By demonstrating that we can identify and manipulate photon-bound states, we have taken a vital first step towards harnessing quantum light for practical use," Dr. Mahmoodian said.

"The next steps in my research are to see how this approach can be used to generate states of light that are useful for fault-tolerant quantum computing, which is being pursued by multimillion dollar companies, such as PsiQuantum and Xanadu."

Dr. Tomm said, "This experiment is beautiful, not only because it validates a fundamental effect—stimulated emission—at its ultimate limit, but it also represents a huge technological step towards advanced applications."

"We can apply the same principles to develop more-efficient devices that give us photon bound states. This is very promising for applications in a wide range of areas: from biology to advanced manufacturing and quantum information processing."

The research was a collaboration between the University of Basel, Leibniz University Hannover, the University of Sydney and Ruhr University Bochum.

The lead authors are Dr. Natasha Tomm from the University of Basel and Dr. Sahand Mahmoodian at the University of Sydney, where he is an Australian Research Council Future Fellow and Senior Lecturer.

The artificial atoms (quantum dots) were fabricated at Bochum and used in experiment performed in the Nano-Photonics Group at the University of Basel. Theoretical work on the discovery was carried out by Dr. Mahmoodian at the University of Sydney and Leibniz University Hannover.

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Quantum engineers design new tool to measure spins in materials with high precision

 

In a paper published over the weekend in the journal Science Advances, Associate Professor Jarryd Pla and his team from UNSW School of Electrical Engineering and Telecommunications, together with colleague Scientia Professor Andrea Morello, described a new device that can measure the spins in materials with high precision.

"The spin of an electron—whether it points up or down—is a fundamental property of nature," says A/Prof. Pla. "It is used in magnetic hard disks to store information, MRI machines use the spins of water molecules to create images of the inside of our bodies, and spins are even being used to build quantum computers.

"Being able to detect the spins inside materials is therefore important for a whole range of applications, particularly in chemistry and biology where it can be used to understand the structure and purpose of materials, allowing us to design better chemicals, drugs and so on."

In fields of research such as chemistry, biology, physics and medicine, the tool that is used to measure spins is called a spin resonance spectrometer. Normally, commercially produced spectrometers require billions to trillions of spins to get an accurate reading, but A/Prof. Pla and his colleagues were able to measure spins of electrons in the order of thousands, meaning the new tool was about a million times more sensitive.

This is quite a feat, as there are a whole range of systems that cannot be measured with commercial tools, such as microscopic samples, two-dimensional materials and high-quality solar cells, which simply have too few spins to create a measurable signal.

The breakthrough came about almost by chance, as the team were developing a quantum memory element for a superconducting quantum computer. The objective of the memory element was to transfer quantum information from a superconducting electrical circuit to an ensemble of spins placed beneath the circuit.

"We noticed that while the device didn't quite work as planned as a memory element, it was extremely good at measuring the spin ensemble," says Wyatt Vine, a lead author on the study. "We found that by sending microwave power into the device as the spins emitted their signals, we could amplify the signals before they left the device. What's more, this amplification could be performed with very little added noise, almost reaching the limit set by quantum mechanics."

While other highly sensitive spectrometers using superconducting circuits had been developed in the past, they required multiple components, were incompatible with magnetic fields and had to be operated in very cold environments using expensive "dilution refrigerators," which reach temperatures down to 0.01 Kelvin.

In this new development, A/Prof. Pla says he and the team managed to integrate the components on a single chip.

"Our new technology integrates several important parts of the spectrometer into one device and is compatible with relatively large magnetic fields. This is important, since measure the spins they need to be placed in a field of about 0.5 Tesla, which is ten thousand times stronger than the earth's magnetic field.

"Further, our device operated at a temperature more than 10 times higher than previous demonstrations, meaning we don't need to use a dilution refrigerator."

A/Prof. Pla says the UNSW team has patented the technology with a view to potentially commercialize, but stresses that there is still work to be done.

"There is potential to package this thing up and commercialize it which will allow other researchers to plug it into their existing commercial systems to give them a sensitivity gain.

"If this new technology was properly developed, it could help chemists, biologists and medical researchers, who currently rely on tools made by these large tech companies that work, but which could do something orders of magnitude better."

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High-performance photon detectors to combat spies in the quantum computing age

 

How can we combat data theft, which is a real issue for society? Quantum physics has the solution. Its theories make it possible to encode information (a qubit) in single particles of light (a photon) and to circulate them in an optical fiber in a highly secure way. However, the widespread use of this telecommunications technology is hampered in particular by the performance of the single-photon detectors.
A team from the University of Geneva (UNIGE), together with the company ID Quantique, has succeeded in increasing their speed by a factor of twenty. This innovation, published in the journal Nature Photonics, makes it possible to achieve unprecedented performances in quantum key distribution.


Buying a train ticket, booking a taxi, getting a meal delivered: these are all transactions carried out daily via mobile applications. These are based on payment systems involving an exchange of secret information between the user and the bank. To do this, the bank generates a public key, which is transmitted to their customer, and a private key, which it keeps secret. With the public key, the user can modify the information, make it unreadable and send it to the bank. With the private key, the bank can decipher it.


This system is now threatened by the power of quantum computers. To resolve this, quantum cryptography—or quantum key distribution (QKD)—is the best option. It allows two parties to generate shared secret keys and transmit them via optical fibers in a highly secure way. This is because the laws of quantum mechanics state that a measurement affects the state of the system being measured. Thus, if a spy tries to measure the photons to steal the key, the information will be instantly altered and the interception revealed.

Current limitations


One limitation to the application of this system is the speed of the single-photon detectors used to receive the information. In fact, after each detection, the detectors must recover for about 30 nanoseconds, which limits the throughput of the secret keys to about 10 megabits per second. A UNIGE team led by Hugo Zbinden, associate professor in the Department of Applied Physics at the UNIGE Faculty of Science, has succeeded in overcoming this limit by developing a detector with better performance. This work was carried out in collaboration with the team of Félix Bussières from the company ID Quantique, a spin-off of the university.


"Currently, the fastest detectors for our application are superconducting nanowire single-photon detectors," explains Fadri Grünenfelder, a former doctoral student in the Department of Applied Physics at the UNIGE Faculty of Science and first author of the study. "These devices contain a tiny superconducting wire cooled to -272°C. If a single photon hits it, it heats up and ceases to be superconducting for a short time, thus generating a detectable electrical signal. When the wire becomes cold again, another photon can be detected."

Record performance


By integrating not one but fourteen nanowires into their detector, the researchers were able to achieve record detection rates. "Our detectors can count twenty times faster than a single-wire device," explains Hugo Zbinden. "If two photons arrive within a short time in these new detectors, they can touch different wires and both be detected. With a single wire this is impossible." The nanowires used are also shorter, which helps to decrease their recovery time.


Using these sensors, scientists were able to generate a secret key at a rate of 64 megabits per second over 10 km of fiber optic cable. This rate is high enough to secure, for example, a videoconference with several participants. This is five times the performance of current technology over this distance. As a bonus, these new detectors are no more complex to produce than the current devices available on the market.


These results open up new perspectives for ultra-secure data transfer, which is crucial for banks, health care systems, governments and the military. They can also be applied in many other fields where light detection is a key element, such as astronomy and medical imaging.

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An innovative twist: Tubular nanomaterial of carbon makes ideal home for spinning quantum bits

 

Scientists are vigorously competing to transform the counterintuitive discoveries about the quantum realm from a century past into technologies of the future. The building block in these technologies is the quantum bit, or qubit. Several different kinds are under development, including ones that use defects within the symmetrical structures of diamond and silicon. They may one day transform computing, accelerate drug discovery, generate unhackable networks and more.

Working with researchers from several universities, scientists at the U.S. Department of Energy's (DOE) Argonne National Laboratory have discovered a method for introducing spinning electrons as qubits in a host nanomaterial. Their test results revealed record long coherence times—the key property for any practical qubit because it defines the number of quantum operations that can be performed in the lifetime of the qubit.

Electrons have a property analogous to the spin of a top, with a key difference. When tops spin in place, they can rotate to the right or left. Electrons can behave as though they were rotating in both directions at the same time. This is a quantum feature called superposition. Being in two states at the same time makes electrons good candidates for spin qubits.

Spin qubits need a suitable material to house, control and detect them, as well as read out information in them. With that in mind, the team chose to investigate a nanomaterial that is made from carbon atoms only, has a hollow tubular shape and has thickness of only about one nanometer, or a billionth of a meter, roughly 100,000 times thinner than the width of a human hair.

"These carbon nanotubes are typically a few micrometers long," said Xuedan Ma. "They are mostly free of fluctuating nuclear spins that would interfere with the spin of the electron and reduce its coherence time."

Ma is a scientist in Argonne's Center for Nanoscale Materials (CNM), a DOE Office of Science user facility. She also holds appointments at the Pritzker School of Molecular Engineering at the University of Chicago and Northwestern-Argonne Institute of Science and Engineering at Northwestern University.

The problem the team faced is that carbon nanotubes by themselves cannot maintain a spinning electron at one site. It moves about the nanotube. Past researchers have inserted electrodes nanometers apart to confine a spinning electron between them. But this arrangement is bulky, expensive and challenging to scale up.

The current team devised a way to eliminate the need for electrodes or other nanoscale devices for confining the electron. Instead, they chemically alter the atomic structure in a carbon nanotube in a way that traps a spinning electron to one location.

"Much to our gratification, our chemical modification method creates an incredibly stable spin qubit in a carbon nanotube," said chemist Jia-Shiang Chen. Chen is a member of both CNM and a postdoctoral scholar in the Center for Molecular Quantum Transduction at Northwestern University.

The team's test results revealed record long coherence times compared to those of systems made by other means—10 microseconds.

Given their small size, the team's spin qubit platform can be more easily integrated into quantum devices and permits many possible ways to read out the quantum information. Also, the carbon tubes are very flexible and their vibrations can be used to store information from the qubit.

"It is a long way from our spin qubit in a carbon nanotube to practical technologies, but this is a large early step in that direction," Ma said.

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Quantum chemistry: Molecules caught tunneling

Tunneling reactions in chemistry are difficult to predict. The quantum mechanically exact description of chemical reactions with more than three particles is difficult, with more than four particles it is almost impossible. Theorists simulate these reactions with classical physics and must neglect quantum effects. But where is the limit of this classical description of chemical reactions, which can only provide approximations?



Roland Wester from the Department of Ion Physics and Applied Physics at the University of Innsbruck has long wanted to explore this frontier. "It requires an experiment that allows very precise measurements and can still be described quantum-mechanically," says the experimental physicist. "The idea came to me 15 years ago in a conversation with a colleague at a conference in the U.S.," Wester recalls. He wanted to trace the quantum mechanical tunnel effect in a very simple reaction.


Since the tunnel effect makes the reaction very unlikely and thus slow, its experimental observation was extraordinarily difficult. After several attempts, however, Wester's team has now succeeded in doing just that for the first time, as they report in the current issue of the journal Nature.
Breakthrough after 15 years of research

Roland Wester's team chose hydrogen—the simplest element in the universe—for their experiment. They introduced deuterium—a hydrogen isotope—into an ion trap, cooled it down and then filled the trap with hydrogen gas. Because of the very low temperatures, the negatively charged deuterium ions lack the energy to react with hydrogen molecules in the conventional way. In very rare cases, however, a reaction does occur when the two collide.

This is caused by the tunnel effect: "Quantum mechanics allows particles to break through the energetic barrier due to their quantum mechanical wave properties, and a reaction occurs," explains the first author of the study, Robert Wild. "In our experiment, we give possible reactions in the trap about 15 minutes and then determine the amount of hydrogen ions formed. From their number, we can deduce how often a reaction has occurred."

In 2018, theoretical physicists had calculated that in this system quantum tunneling occurs in only one in every hundred billion collisions. This corresponds very closely with the results now measured in Innsbruck and, after 15 years of research, for the first time confirms a precise theoretical model for the tunneling effect in a chemical reaction.
Foundation for a better understanding

There are other chemical reactions that might exploit the tunnel effect. For the first time, a measurement is now available that is also well understood in scientific theory. Based on this, research can develop simpler theoretical models for chemical reactions and test them on the reaction that has now been successfully demonstrated.

The tunnel effect is used, for example, in the scanning tunneling microscope and in flash memories. The tunnel effect is also used to explain the alpha decay of atomic nuclei. By including the tunnel effect, some astrochemical syntheses of molecules in interstellar dark clouds can also be explained. The experiment of Wester's team thus lays the foundation for a better understanding of many chemical reactions.

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