How quantum computers can become more reliable and powerful

If we wanted to summarize Christopher Eichler’s field of research in one sentence, we could say that Schrödinger’s cat is chasing its tail: In order to make quantum computers more reliable and powerful, faulty operations must be detected and corrected. On the one hand, this requires redundancy and scaling up to larger quantum processors. On the other, the individual components on the increasingly complex processors must be improved. This poses a great challenge, because the quantum states are extremely sensitive to external disturbances caused by electrical and magnetic fields, heat or material defects. “But there is no way around these two phases of development,” says the Chair of Experimental Physics at FAU. “We want to build better processors but we also want to perfect the methods of error correction.”

Christopher Eichler and his team are researching the future of quantum computing. At the same time, he is speaker for the QuMeCo project, which represents FAU in the Munich Quantum Valley.

Inspecting black boxes without opening them

For the layperson, quantum computers are a mystery because they use superpositions rather than the conventional binary states of zero and one. Quantum bits, in short: qubits, can be in two states at the same time – as long as they are not observed. Erwin Schrödinger described this effect in 1935 in a thought experiment involving a cat in an opaque box, where we only learn whether the cat is alive or dead when we decide to open the box. “Qubits are, in a way, black boxes that we can’t open without interrupting the computing process,” explains Eichler.

But how can errors be detected in closed boxes? The quantum researchers resort to a trick: they use a kind of quantum scale to compare the states of neighboring qubits without observing them individually. Christopher Eichler’s research group is working on these measurement methods. “This is where redundancy comes into play,” explains the physicist. “We need to distribute quantum information over a number of qubits, in the form of so-called entangled states. Only then can we measure deviations and identify them as errors without interrupting the computing process.”

Eichler explains why such control mechanisms – and better hardware – are so important using the example of ammonia synthesis, a fundamental process for fertilizer production: “This is one of the most energy-intensive processes in the chemical industry. Even the smallest improvements would lead to significant savings,” he says. For the development of new catalysts, however, the processes at the molecular level would have to be better understood – with simulations that conventional computers cannot perform. “In order to get sufficiently accurate computational results, quadrillions of quantum operations are needed. Currently, the quantum computer makes a mistake about every 1000 operations. To close this gap, we need quantum error correction.”

Research group with a start-up spirit

Christopher Eichler was appointed Chair of Experimental Physics at FAU in September 2021. Anyone who has an ear for dialects might detect a slight Swiss accent, even though he was born and raised near Mainz. “This comes from my long time at ETH Zurich, where I did my doctorate in 2013 and researched until last year.” Zurich is one of the most renowned locations for quantum research worldwide – and the place where fundamental breakthroughs in error correction procedures were achieved. From 2014 to 2016, Eichler moved to Princeton University in the USA as a Dicke Fellow. “There is an incredible momentum there with a lot of optimism.”

Eichler brought this spirit to Erlangen, comparing his research group to a start-up. He moved to FAU because he can build something new here and also rely on an excellent academic and technical environment – for example, the Max Planck Institute for the Physics of Light with one of the best-equipped university cleanrooms in Germany. “With all the fascination for theory – in the end, the processors also have to be developed and manufactured, and with superconducting quantum computers we are working not only at the micro level, but also at the nanoscale. This can only be done in very few locations.” The 39-year-old also shares his enthusiasm with his students; teaching is just as important to him as research.

Flagship project at Munich Quantum Valley

FAU is one of the founding members of Munich Quantum Valley and has been involved in the Bavarian initiative since January 2023 with the flagship project “Quantum Measurement and Control for the Enablement of Quantum Computing and Quantum Sensing”, in short: QuMeCo. Christopher Eichler is the speaker for the project, which brings together eleven scientists from Erlangen – eight from Light and Matter at the Department of Physics and three from the Department of Electrical Engineering at FAU. “In addition to optimizing the qubits and correcting errors, the consortium also deals with control electronics,” says Eichler. “We want to develop microwave circuits that are as close as possible to the quantum chips and enable control pulses in the nanosecond range. The components must have an extremely low power consumption in order to exclude unwanted interactions with the qubits.”

Other focal points of the project are sensor technology and imaging. The researchers experiment with novel quantum light sources and detectors, benefiting from the special characteristics of entangled photons. They are also using color centers – crystal defects which introduce additional light absorption – as highly sensitive quantum sensors to depict electrical and magnetic fields and electro-chemical and photo-chemical reactions of molecules at an entirely new level of optical resolution. As a result, researchers can gain insights into spectrums that are otherwise extremely hard to access, for example in order to gain a better understanding of biological processes. A further potential application is optimizing electrolytes and ionic fluids in order to increase the efficiency and lifespan of battery cells.

“The participation in Munich Quantum Valley demonstrates FAU’s special expertise and at the same time offers us the opportunity to network with other initiatives in the field of quantum research in Bavaria,” explains Christopher Eichler. “We all share the fascination of being able to use the astonishing laws of the quantum world in applications.”

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