Image: D-Wave Systems; Shutterstock/Sunward Art

The dawn of a new era

Quantum physics is one of the most complex areas of physics – and one of the most promising: Researchers are working on a wide variety of technologies based on quants. These technologies could revolutionize science, from medicine to materials research.

The list of hoped-for miracles is long: A sensor that can detect groundwater from orbit. A mind-reading cap. A computer that, within minutes, can solve problems that would take the largest supercomputers decades. These may all become possible thanks to quantum technology. Experts agree that quantum physics has the potential to transform daily life. Human imagination quickly reaches its limits when it comes to the effects quantum physics can achieve (see box). And yet, physicists can control individual atoms, electrons, and photons in their laboratories so precisely that these particles can be used to build lightning-fast computers, extremely precise sensors, and secure communications systems. The most ambitious goal is the quantum computer. It will be able to do everything a normal computer can do – just a lot faster. Its strength is the “qubit”. Today’s computers use the bit as their smallest unit of calculation. Bits can contain either a 0 or a 1. The qubit, on the other hand, uses what is called superposition to calculate using both values at the same time. Each additional qubit doubles the number of pieces of data that can be processed simultaneously. A few hundred qubits will allow more pieces of data to be processed than there are atoms in the universe. Last year, Google demonstrated how fast this miracle machine can do its work. In a few minutes, its quantum chip solved a task that would take a supercomputer millennia – using just 53 qubits.

 

Zoom A glimpse inside. Jülich physicist Markus Jerger prepares cabling to measure the quantum mechanical state of qubits. Image: FZ Jülich/Ralf-Uwe Limbach

There is no practical utility to the calculation performed by Google’s computer, but that is expected to change: Conceivable areas of application for future quantum computers include finding optimum solutions in a haystack of possibilities and creating new algorithms for artificial intelligence that allow much faster learning. At Forschungszentrum Jülich, a computer is being created as part of the OpenSuperQ project. It is the first of its kind in Europe. The special thing about it is that its architecture will be entirely open and accessible – the whole research community will be able to participate in its development and use the computer. The project is being funded by the mammoth European Quantum Flagship initiative, which will provide more than a billion euros over ten years for the development of products based on the rules of the exotic quantum world. Around 5,000 researchers from science and industry are participating in the first 20 projects selected. The objective: Establishing a good starting position for Europe’s scientists and companies in international competition regarding quantum technologies. “The EU commands great scientific excellence in quantum technology,” says Tommaso Calarco, a Jülich physicist and co-initiator of the Flagship project. “The Flagship is intended to help us work with industry in order to transfer this potential to commercial products. Otherwise, there is a risk of the insights initiated in Europe being refined elsewhere to create marketable products,” says Calarco.

Another person who is heavily involved in the Flagship project is his colleague, David DiVincenzo, director of the Division of Theoretical Nanoelectronics at the Peter Grünberg Institute at FZ Jülich. “At the moment, we are testing a very simple quantum chip with two qubits,” DiVincenzo says. Why just two qubits when the Google computer has 53? Europe intends to develop the technology itself. It must therefore start from the beginning, but hopes to catch up quickly. An additional chip with seven qubits is to be installed soon. More will follow. “We need a few years before we can achieve a quantum advantage with this machine,” says David DiVincenzo. By “quantum advantage”, he means initial useful applications – specifically 100-qubit applications that will allow researchers to use OpenSuperQ, primarily to simulate chemical bonds and their reactions, and to do so faster and more precisely than any supercomputer. Industrial companies such as Merck and BASF are already looking into how they might be able to use quantum computers to develop new active ingredients faster and identify materials that are more resistant. But qubits are hard to control. The main problem for the scientists is that after a fraction of a second, qubits lose their ability to store values simultaneously. Quants are very sensitive and easy to disrupt. Superposition is then lost, causing calculation errors. That is why qubits must be stabilized and protected against the outside world as much as possible using large cooling systems. This takes a great deal of effort and money, and its effectiveness is limited. If the qubits were more stable, researchers hope that tens of thousands of them could be linked. And that is the number of qubits experts believe would be necessary for a truly universally usable quantum computer.

This is what Kristel Michielsen is working on at the Jülich Supercomputing Centre. The supercomputer there simulates quantum computers. “We imitate an ideal machine and compare the results with the real one,” says the physicist. The Jülich researchers also simulated Google’s quantum chip to verify the results and determine the quantum computer’s power. One important function of the number games is to research the quantum computer. “It enables us to better understand what qubits do,” says Michielsen. This is an important step towards more stable qubits. Researchers are also hoping to find new materials for qubits. Almost any object that obeys the laws of quantum physics is a candidate. Superconductors (like those in Jülich’s OpenSuperQ) that allow current to flow in both directions and ions that can assume two energy levels at once have proven useful. In the future, however, tiny invisible magnets whose north poles point up and down simultaneously might be used. That is what quantum researcher Wolfgang Wernsdorfer is working on at the Karlsruhe Institute of Technology (KIT). Atomic cores are such magnets. “The electron shells protect them well from the environment”, says Wernsdorfer, who holds a Humboldt Professorship at KIT. “But these magnets generate extremely weak signals that are very hard to read.” He is developing a sort of amplifier for them: Working with his colleagues, he places the magnetic qubit in a plate-shaped molecule from which it generates measurable current. The Karlsruhe researchers see their challenge as equipping large numbers of these magnets with amplifiers. The advantage of such qubits is their miniscule size, Wernsdorfer says. Millions could be concentrated in an extremely small space. 

Because qubits are so sensitive, they can be used for extremely sensitive, precise, miniaturized sensors – another goal of quantum technology. Their small size allows atoms to be used in inaccessible spots, such as inside the body, where special sensors based on quantum technology could be used to map tumors, for example. The great advantage is that all particles of a given type are identical and react to the same to stimuli. Therefore, a “quantum sensor” does not have to be regularly calibrated because nature adjusts it.

Arne Wickenbrock of the Helmholtz Institute Mainz uses nitrogen atoms embedded in diamonds for his sensors. The atom is like a compass needle that reacts to tiny magnetic fields. “This sensor is the most sensitive in the world by volume,” Wickenbrock says. The diamond shell shields the atom against interfering environmental influences, so it could even work inside the human body. “It would then be possible to take brain wave measurements that are precise enough for computers to be controlled by thought alone,” Wickenbrock continues. And because the atomic compass needle’s orientation can be determined precisely, Wickenbrock’s research team hopes to pave the way for high-precision navigation devices that are accurate to the millimeter. They would be able to keep autonomous vehicles in their lanes when satellite contact is interrupted – in tunnels, for instance. Quantum sensors achieve what is probably the peak of precision in measuring gravity.

 

Zoom The measuring precision and long-term stability of the transportable quantum gravimeter, the QG1, developed in Hannover is much greater than in classical gravimeters. Image: Institute of Quantum Optics, Leibniz University Hannover

Scientists are convinced that atoms serving as gravitation sensors are sensitive enough to detect changes in groundwater levels from space. Such a sensor would use the wave nature attributed to atoms in quantum physics. The particles’ wavelength is one ten-thousandth the wavelength of light. Broadly speaking, the wave acts as an extremely fine ruler that can be used to measure the length of the path an atom takes. The tiniest variations in gravity change the length of the path taken by atoms in free fall within a certain amount of time. This allows extremely small variations in gravitation to be detected.

“We are developing a gravimetric quantum sensor that is ten times more precise than the best sensors currently available,” says Wolfgang Ertmer of the Institute for Satellite Geodesy and Inertial Sensing of the German Aerospace Center (DLR). In purely mathematical terms, this would allow the gravitational pull of a human body to be measured at the distance of one kilometer. This precision opens up fascinating possibilities, Ertmer says, such as analyzing the interior of the planet Mars. Various materials beneath the planet’s surface affect local gravitation. However, he is even more interested in the possibility of solving unsolved physics puzzles, such as whether the laws of gravitation apply to extremely small masses. This knowledge could help close a gap in concepts of the physical world that has been known for a long time: So far, no one has been able to reconcile quantum physics with Einstein’s theory of gravity. Combining two models in a single system often leads to completely new insights. This would mean that quantum technology could change not only everyday life, but the very way we perceive the world.

What are quants?

Quants are everywhere. When you read this text, light quants, or photons, reach your eye, allowing you to perceive the text. The term “quant” refers generally to elementary particles – that is, particles that cannot be further subdivided. There is one thing all quants have in common, and this gives them their name: Their state is quantized, which means that they only occur in certain strictly defined sizes and energy levels. The famous term “quantum leap” refers to a quant changing from its previous state to the next higher or lower state. Quantum physics describes the behavior of these particles. This area is so complex that only specialists can comprehend it – primarily because it completely contradicts common sense and human experience. For instance, quants cannot be assigned a precise position or an exact direction of movement. None of their properties can be predicted exactly. An electron, for instance, can rotate clockwise and counter-clockwise at the same time. It does not assume a fixed position or specific properties until it is measured (that is, observed) – as though it knows when it is being watched. The famous “Schrödinger’s Cat” thought experiment helps explain this. Imagine a cat locked in an opaque box. The box contains a device that can kill the cat, and the device is controlled by the completely random decay of a radioactive atom. Is the cat alive or dead? The question cannot be answered until the observer opens the box. Until then, the cat is both alive and dead.

According to quantum physics, particles can be in an incredible number of different states. They can even be in two places at once. This phenomenon is called “superposition” and is key to quantum technology. An example is the quantum computer: A conventional computer has bits that can assume one of two states: 0 or 1. A quantum computer can be in both states at once; and with two qubits, not only can it be in two states, but rather two times two different states. At 50 qubits, the number of possible states is more than a quadrillion. The number of possible states drives the number of calculations that can be performed.

Another rule of quantum physics is wave-particle duality. Quants live double lives, so to speak: Sometimes they behave like particles, sometimes like waves – it depends on the type of measurement being applied to them. The following experiment illustrates this: If a single electron is accelerated towards a plate with two slits, classical physics would predict that it would only be detectable behind the left or the right slit. What actually happens is that an interference pattern forms as though waves have hit the slits, making the pattern denser or sparser through superposition. This mysterious property of quants can also be useful in a variety of areas, such as precise navigation devices that require no satellites.

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Themenseite Quantentechnologie

08.07.2020 , Christian Meier

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