Computers Math

Quantum computer: One of the obstacles for progress in the quest for a working quantum computer has been that the working devices that go into a quantum computer and perform the actual calculations, the qubits, have hitherto been made by universities and in small numbers. But in recent years, a pan-European collaboration, in partnership with French microelectronics leader CEA-Leti, has been exploring everyday transistors — that are present in billions in all our mobile phones — for their use as qubits. The French company Leti makes giant wafers full of devices, and, after measuring, researchers at the Niels Bohr Institute, University of Copenhagen, have found these industrially produced devices to be suitable as a qubit platform capable of moving to the second dimension, a significant step for a working quantum computer. The result is now published in Nature Communications.
Quantum dots in two dimensional array is a leap ahead
One of the key features of the devices is the two-dimensional array of quantum dot. Or more precisely, a two by two lattice of quantum dots. “What we have shown is that we can realize single electron control in every single one of these quantum dots. This is very important for the development of a qubit, because one of the possible ways of making qubits is to use the spin of a single electron. So reaching this goal of controlling the single electrons and doing it in a 2D array of quantum dots was very important for us,” says Fabio Ansaloni, former PhD student, now postdoc at center for Quantum Devices, NBI.
Using electron spins has proven to be advantageous for the implementation of qubits. In fact, their “quiet” nature makes spins weakly interacting with the noisy environment, an important requirement to obtain highly performing qubits.
Extending quantum computers processors to the second dimension has been proven to be essential for a more efficient implementation of quantum error correction routines. Quantum error correction will enable future quantum computers to be fault tolerant against individual qubit failures during the computations.
The importance of industry scale production
Assistant Professor at Center for Quantum Devices, NBI, Anasua Chatterjee adds: “The original idea was to make an array of spin qubits, get down to single electrons and become able to control them and move them around. In that sense it is really great that Leti was able to deliver the samples we have used, which in turn made it possible for us to attain this result. A lot of credit goes to the pan-European project consortium, and generous funding from the EU, helping us to slowly move from the level of a single quantum dot with a single electron to having two electrons, and now moving on to the two dimensional arrays. Two dimensional arrays is a really big goal, because that’s beginning to look like something you absolutely need to build a quantum computer. So Leti has been involved with a series of projects over the years, which have all contributed to this result.”

advertisement

The credit for getting this far belongs to many projects across Europe
The development has been gradual. In 2015, researchers in Grenoble succeeded in making the first spin qubit, but this was based on holes, not electrons. Back then, the performance of the devices made in the “hole regime” were not optimal, and the technology has advanced so that the devices now at NBI can have two dimensional arrays in the single electron regime. The progress is threefold, the researchers explain: “First, producing the devices in an industrial foundry is a necessity. The scalability of a modern, industrial process is essential as we start to make bigger arrays, for example for small quantum simulators. Second, when making a quantum computer, you need an array in two dimensions, and you need a way of connecting the external world to each qubit. If you have 4-5 connections for each qubit, you quickly end up with a unrealistic number of wires going out of the low-temperature setup. But what we have managed to show is that we can have one gate per electron, and you can read and control with the same gate. And lastly, using these tools we were able to move and swap single electrons in a controlled way around the array, a challenge in itself.”
Two dimensional arrays can control errors
Controlling errors occurring in the devices is a chapter in itself. The computers we use today produce plenty of errors, but they are corrected through what is called the repetition code. In a conventional computer, you can have information in either a 0 or a 1. In order to be sure that the outcome of a calculation is correct, the computer repeats the calculation and if one transistor makes an error, it is corrected through simple majority. If the majority of the calculations performed in other transistors point to 1 and not 0, then 1 is chosen as the result. This is not possible in a quantum computer since you cannot make an exact copy of a qubit, so quantum error correction works in another way: State-of-the-art physical qubits do not have low error rate yet, but if enough of them are combined in the 2D array, they can keep each other in check, so to speak. This is another advantage of the now realized 2D array.
The next step from this milestone
The result realized at the Niels Bohr Institute shows that it is now possible to control single electrons, and perform the experiment in the absence of a magnetic field. So the next step will be to look for spins — spin signatures — in the presence of a magnetic field. This will be essential to implement single and two qubit gates between the single qubits in the array. Theory has shown that a handful of single and two qubit gates, called a complete set of quantum gates, are enough to enable universal quantum computation. More

This summer, when bars and restaurants and stores began to reopen across the United States, people headed out despite the continuing threat of COVID-19.
As a result, many areas, including the St. Louis region, saw increases in cases in July.
Using mathematical modeling, new interdisciplinary research from the lab of Arye Nehorai, the Eugene & Martha Lohman Professor of Electrical Engineering in the Preston M. Green Department of Electrical & Systems Engineering at Washington University in St. Louis, determines the best course of action when it comes to walking the line between economic stability and the best possible health outcomes.
The group — which also includes David Schwartzman, a business economics PhD candidate at Olin Business School, and Uri Goldsztejn, a PhD candidate in biomedical engineering at the McKelvey School of Engineering — published their findings Dec. 22 in PLOS ONE.
The model indicates that of the scenarios they consider, communities could maximize economic productivity and minimize disease transmission if, until a vaccine were readily available, seniors mostly remained at home while younger people gradually returned to the workforce.
“We have developed a predictive model for COVID-19 that considers, for the first time, its intercoupled effect on both economic and health outcomes for different quarantine policies,” Nehorai said. “You can have an optimal quarantine policy that minimizes the effect both on health and on the economy.”
The work was an expanded version of a Susceptible, Exposed, Infectious, Recovered (SEIR) model, a commonly used mathematical tool for predicting the spread of infections. This dynamic model allows for people to be moved between groups known as compartments, and for each compartment to influence the other in turn.

advertisement

At their most basic, these models divide the population into four compartments: Those who are susceptible, exposed, infectious and recovered. In an innovation to this traditional model, Nehorai’s team included infected but asymptomatic people as well, taking into account the most up-to-date understanding of how transmission may work differently between them as well as how their behaviors might differ from people with symptoms. This turned out to be highly influential in the model’s outcomes.
People were then divided into different “sub-compartments,” for example age (seniors are those older than 60), or by productivity. This was a measure of a person’s ability to work from home in the case of quarantine measures. To do this, they looked at college degrees as a proxy for who could continue to work during a period of quarantine.
Then they got to work, developing equations which modeled the ways in which people moved from one compartment to another. Movement was affected by policy as well as the decisions an individual made.
Interestingly, the model included a dynamic mortality rate — one that shrunk over time. “We had a mortality rate that accounted for improvements in medical knowledge over time,” said Uri Goldsztejn, a PhD candidate in biomedical engineering. “And we see that now; mortality rates have gone down.”
“For example,” Goldsztejn said, “if the economy is decreasing, there is more incentive to leave quarantine,” which might show up in the model as people moving from the isolated compartment to the susceptible compartment. On the other hand, moving from infectious to recovered was based less on a person’s actions and can be better determined by recovery or mortality rates. Additionally, the researchers modeled the mortality rate as decreasing over time, due to medical knowledge about how to treat COVID-19 becoming better over time.

advertisement

The team looked at three scenarios, according to Schwartzman. In all three scenarios, the given timeline was 76 weeks — at which time it assumed a vaccine would be available — and seniors remained mostly quarantined until then.
If strict isolation measures were maintained throughout.
If, after the curve was flattened, there was a rapid relaxation of isolation measures by younger people to normal movement.
If, after the curve was flattened, isolation measures were slowly lifted for younger people.
“The third scenario is the case which was the best in terms of economic damage and health outcomes,” he said. “Because in the rapid relaxation scenario, there was another disease spread and restrictions would be reinstated.”
Specifically, they found in the first scenario, there are 235,724 deaths and the economy shrinks by 34%.
In the second scenario, where there was a rapid relaxation of isolation measures, a second outbreak occurs for a total of 525,558 deaths, and the economy shrinks by 32.2%.
With a gradual relaxation, as in the third scenario, there are 262,917 deaths, and the economy shrinks by 29.8%.
“We wanted to show there is a tradeoff,” Nehorai said. “And we wanted to find, mathematically, where is the sweet spot?” As with so many things, the “sweet spot” was not at either extreme — total lockdown or carrying on as if there was no virus.
Another key finding was one no one should be surprised to hear: “People’s’ sensitivity to contagiousness is related to the precautions they take,” Nehorai said. “It’s still critical to use precautions — masks, social distancing, avoiding crowds and washing hands.” More

A research team has for the first time experimentally proved a century old quantum theory that relativistic particles can pass through a barrier with 100% transmission. More

Anyone entering the world of quantum physics must prepare themself for quite a few things unknown in the everyday world: Noble gases form compounds, atoms behave like particles and waves at the same time and events that in the macroscopic world exclude each other occur simultaneously.
In the world of quantum physics, Reinhard Dörner and his team are working with molecules which — in the sense of most textbooks — ought not to exist: Helium compounds with two atoms, known as helium dimers. Helium is called a noble gase precisely because it does not form any compounds. However, if the gas is cooled down to just 10 degrees above absolute zero (minus 273 °C) and then pumped through a small nozzle into a vacuum chamber, which makes it even colder, then — very rarely — such helium dimers form. These are unrivaledly the weakest bound stable molecules in the Universe, and the two atoms in the molecule are correspondingly extremely far apart from each other. While a chemical compound of two atoms commonly measures about 1 angstrom (0.1 nanometres), helium dimers on average measure 50 times as much, i.e. 52 angstrom.
The scientists in Frankfurt irradiated such helium dimers with an extremely powerful laser flash, which slightly twisted the bond between the two helium atoms. This was enough to make the two atoms fly apart. They then saw — for the very first time — the helium atom flying away as a wave and record it on film.
According to quantum physics, objects behave like a particle and a wave at the same time, something that is best known from light particles (photons), which on the one hand superimpose like waves where they can pile upor extinguish each other (interference), but on the other hand as “solar wind” can propel spacecraft via their solar sails, for example.
That the researchers were able to observe and film the helium atom flying away as a wave at all in their laser experiment was due to the fact that the helium atom only flew away with a certain probability: With 98 per cent probability it was still bound to its second helium partner, with 2 per cent probability it flew away. These two helium atom waves — Here it comes! Quantum physics! — superimpose and their interference could be measured.
The measurement of such “quantum waves” can be extended to quantum systems with several partners, such as the helium trimer composed of three helium atoms. The helium trimer is interesting because it can form what is referred to as an “exotic Efimov state,” says Maksim Kunitski, first author of the study: “Such three-particle systems were predicted by Russian theorist Vitaly Efimov in 1970 and first corroborated on caesium atoms. Five years ago, we discovered the Efimov state in the helium trimer. The laser pulse irradiation method we’ve now developed might allow us in future to observe the formation and decay of Efimov systems and thus better understand quantum physical systems that are difficult to access experimentally.”

Story Source:
Materials provided by Goethe University Frankfurt. Note: Content may be edited for style and length. More

Researchers have developed a new microscopy method that allows scientists to see the building blocks of ‘smart’ materials being formed at the nanoscale. More

Chemists at the University of Bonn (Germany) have synthesized extremely unusual compounds. Their central building block is a silicon atom. Different from usual, however, is the arrangement of the four bonding partners of the atom, which are not in the form of a tetrahedron around it, but flat like a trapezoid. This arrangement is usually energetically extremely unfavorable, yet the molecules are very stable. Their properties are completely unknown so far; researchers now want to explore them. The results will be published in the Journal of the American Chemical Society, but are already available online.
Like its relative carbon, silicon generally forms four bonds with other atoms. When it does, the result is usually a tetrahedron. The silicon atom is located in the center, its bonding partners (the so-called ligands) at the tetrahedral corners. This arrangement is most favorable energetically. It therefore arises quasi automatically, just as a soap bubble is usually spherical.
Researchers led by Prof. Dr. Alexander C. Filippou of the Institute for Inorganic Chemistry at the University of Bonn have now constructed silicon-containing molecules that are as unusual as a cube-shaped soap bubble. In these, the four ligands do not form a tetrahedron, but a distorted square, a trapezoid. They lie in one plane together with the silicon. “Despite this, the compounds are so stable that they can be filled into bottles and stored for weeks without any problems,” explains Dr. Priyabrata Ghana, a former doctoral student who has since moved to RWTH Aachen University.
Molecular exotics are unusually stable
The researchers themselves were surprised by this unusual stability. They discovered the reason by modeling the molecules on the computer. The ligands also form bonds with each other. In the process, they form a solid framework. This appears to be so strong that it completely prevents the trapezoidal arrangement from “snapping” into a tetrahedron. “Our computer calculations indicate that there is no structure for the molecules that would be more energetically favorable than the planar trapezoidal shape,” emphasizes Jens Rump, a doctoral student at the Institute for Inorganic Chemistry.
The researchers grew crystals of the substances and then blasted them with X-rays. The X-ray light is scattered by the atoms and changes its direction. These deviations can therefore be used to calculate the spatial structure of the molecules in the crystal. Together with spectroscopic measurements, this method confirmed that ligands and silicon are indeed in the same plane in the new molecules.
Although the synthesis of the exotic compounds must be carried out under inert gas, it is otherwise comparatively simple. Producing the starting materials, on the other hand, is complex; one of them was first synthesized only just over ten years ago and has already been the source for the synthesis of several novel classes of silicon compounds.
The influence of the unusual structure on the properties of silicon, an important element for the electronics industry, is completely unclear at the moment. At any rate, for a long time it was considered completely impossible to produce such compounds.

Story Source:
Materials provided by University of Bonn. Note: Content may be edited for style and length. More

A new study by neuroscientists demonstrates that the brain does not remap itself even with long-term bionic limb use, posing challenges for the development of realistic prosthetic limbs. More

Atomic force microscopy (AFM) allows to obtain images and movies showing proteins at work, however with limited resolution. The developed BioAFMviewer software opens the opportunity to use the enormous amount of available high-resolution protein data to better understand experiments. Within an interactive interface with rich functionality, the BioAFMviewer computationally emulates tip-scanning of any biomolecular structure to generate simulated AFM graphics and movies. They greatly help in the interpretation of e.g., high-speed AFM observations. More