At the Garching campus, TUM researchers are helping to shape the era of quantum technology. Photo credit: Kai Neunert / BAdW
Electrons rotating to the right and left at the same time. Together, the particles change their state, even if they are far apart. Interesting phenomena like this are completely commonplace in the world of quantum physics. Researchers at the TUM Garching campus are using them to build quantum computers, high-sensitivity sensors, and the internet of the future.
"We're cooling chips to thousandths of a degree above absolute zero, colder than outer space," said Rudolf Gross, professor of technical physics and director of technology. He stands in front of a delicate-looking device with golden discs, connected by cables: a cooling system for a special chip, which makes use of the singular laws of quantum physics.
For about two decades, WMI researchers have been working on quantum computers, a technology based on the scientific revolution that took place 100 years ago, when quantum physics introduced a new way of observing physics. Today, it has become the basis of what Professor Gross calls a "new era of technology".
To shape this emerging era, Garching's researchers are investigating ways to harness the rules of quantum physics, as well as the associated risks and potential benefits of quantum technology to society.
Manipulating individual atoms
"We encounter quantum physics every day," Gross said. For example, when we see a stove burner element emitting a red light. In 1900, Max Planck discovered the formula for radiation emitted by objects of different temperatures. This means that he has to assume that the emitted light is made up of tiny packages of energy, called quanta. In the years that followed, quantum physics continued to evolve, fundamentally changing our understanding of the microscopic world. New technologies take advantage of the special properties of atoms and electrons, such as lasers, magnetic resonance tomography scanners, and computer chips.
The technology of the first quantum revolution controlled a large number of particles. At the same time, physicists can manipulate individual atoms and photons, and can produce objects that conform to the laws of quantum physics. "Today, we can create tailor-made quantum systems," Gross said. The principles of quantum physics, which have not yet been implemented in almost any technology, can be used for the so-called second quantum revolution.
The first of these principles is superposition: quantum objects can take on parallel states that are mutually exclusive in classical frames of reference. For example, electrons can rotate to the right and left at the same time. Superposition states can also interact with each other, similar to intersecting waves, in that they either reinforce or cancel each other out – this is the second principle: quantum interference.
Catch the unthinkable
The third phenomenon is entanglement. Two particles can have joint quantum states, even if they are several kilometers away from each other. For example, if we measure the polarization of a given photon, then the measurement of the entangled partner is immediately determined, as if the space between the two photons did not exist.
As peculiar as these concepts may sound, they are just as important to technological advancements. Classical computers have one drawback: they process information sequentially, one step at a time. "Even a growing supercomputer can't master all the tasks at hand," Gross said, because the complexity of some tasks can increase dramatically.
For example, the number of possible travel routes between multiple cities increases with each potential stop. There are six possible routes between the four cities, while the figure for 15 cities is more than 40 billion. Thus, in the feasible time, the task of quickly finding the shortest route using a classical computer becomes very complex or even unsolvable.
The principle of superposition makes the task of a quantum computer much easier: it uses qubits or qubits, which can handle bit values 0 and 1 at the same time, rather than sequentially. A large number of qubits, connected to each other by quantum interference or entanglement, can process an incredible number of combinations in parallel, so highly complex tasks can be solved very quickly.
Qubits: Miniature circuits
Back to WMI: here we find a silver vacuum chamber in which metal atoms are deposited precisely on a palm-sized silicon wafer. The high-purity metal layers formed on these wafers form the basis of miniature circuits. When supercooling makes circuits superconducting, the electricity they carry oscillates at different frequencies corresponding to different energy levels. The two lowest levels are used as qubit values 0 and 1. The chip in one of the cooling systems contains six qubits, which is enough for research purposes.
However, quantum computers require hundreds of qubits to solve real-world problems. In addition, each qubit should be able to perform as many computational steps as possible to implement an algorithm that is relevant to the practical purpose. But qubits quickly lose their superposition state, even after the slightest disturbance, such as a material defect or electrical fog — "a huge problem," Gross said.
Complex correction procedures would then have to be used to correct these errors, but these processes would require thousands of additional qubits. Experts expect that this will take many years to materialize. However, when the number of qubit errors decreases, the initial application may already work, if not eliminate.
"An important source of error is unwanted interaction between qubits," said Dr. Kirill Fedorov of WMI. His remedy was to distribute the qubits across multiple chips and entangle them with each other. In WMI's basement, Fedorov pointed to a tube with a diameter of a twig that led from one quantum computer to another. These tubes contain microwave conductors that make qubits interact with each other. This approach could make it possible for thousands of qubits to work together in the future.
Ultra-sensitive quantum measurements are more accurate
Eva Weig, a professor of nano- and quantum sensor technology, has a different take on this lack of perfection. "The fact that quantum states are so sensitive to everything is also an advantage," she said. Even the slightest magnetic field, pressure change, or temperature fluctuation can measurably alter the quantum state. "This can make the sensors more sensitive, more precise, and enable them to achieve better spatial resolution," Weig said.
She wanted to use relatively large objects as mechanical quantum sensors. Even nanostructures made up of millions of atoms can enter the quantum ground state, as first demonstrated by researchers at the University of California in 2010. Eva Weig is building on this discovery. "I wanted to build nanosystems that were easy to control to measure the smallest forces.
In the lab, the physicist showed off the chips her team had made in their own cleanroom. Above is what she calls a "nanoguitar", invisible to the naked eye: tiny strings, 1,000 times thinner than a human hair, vibrating at radio frequency. Weig's team is trying to place these nanooscillators in a defined quantum state. These strings can then be used as quantum sensors, for example to measure the forces present between individual cells.
The road to the quantum internet
Andreas Reiserer, a professor of quantum networks, wants to use another aspect of quantum systems to facilitate the quantum internet: a particle's quantum state is destroyed when it is measured, meaning that the information it contains can only be read out once. Therefore, any attempt at interception will inevitably leave a trace. If there is no such trace, the communication can be trusted. "Quantum cryptography is cost-effective and can already support intercept-resistant communications today," he said.
But the scope of this technology is still limited. According to Reiserer, fiber optic components are ideal for using light to transmit quantum information. But the glass absorbs some of the light every kilometer it travels. After about 100 km, communication is no longer possible.
As a result, Reiserer's team is working on so-called quantum repeaters, or storage units for quantum information, that are spaced along the fiber-optic network approximately every 100 kilometers. If it is possible to entangle each quantum repeater with its nearest neighbor, then the information sent can be passed on without loss. "In this way, we want to be able to cross distances on a global scale," says Reiserer. "Then it is possible to connect devices around the world to form a 'quantum supercomputer'.
The Munich-based team wanted to miniaturize quantum repeaters, simplifying them and making them suitable for mass production by putting them on computer chips. The chip contains an optical fiber in which erbium atoms are embedded. These atoms are used as qubits that can buffer information. However, Reiserer acknowledges that this requires cooling down to as low as 4 Kelvin (i.e. about -269°C), adding that more research is needed before practical feasibility can be achieved.
Social risk
The arrival of quantum technology in everyday life also brings with it some risks. Error-correcting quantum computers can crack today's traditional encryption programs, such as those that could compromise the security of online banking. "The good news is that there are already new encryption programs that are safe against attacks by quantum computers," said Urs Gasser, professor of public policy, governance and innovative technologies and head of TUM's "Quantum Society Lab." Legal scholar Gasser added that the transition will take several years, so it is necessary to start now.
"The cost of being late can even exceed the cost of being late for AI," Gasser warns. The Quantum Society Lab focuses on the ethical, legal, and societal implications of emerging quantum technologies. This includes, for example, questions about how to include people in the debate around new technologies, or whether only rich countries can better plan their cities through quantum optimization.
"The second quantum revolution is a paradigm shift that will have far-reaching social, political and economic implications," Professor Gasser said. "We must shape this revolution in the best interests of society.