Breakthroughs in error correction
Quantum computers are significantly more susceptible to perturbations than conventional computers, and so as well to computational errors. The correction of these is an essential hurdle to the breakthrough of quantum computers. In international teams, Jülich researchers led by Prof. Markus Müller from the Peter Grünberg Institute (PGI-2/IAS-3) and RWTH Aachen University have developed two promising solutions.
Prof. Müller, what are these solutions?
Firstly, an international team has presented a method on a superconducting quantum computer chip that automatically compensates for errors while storing quantum information. What’s special about this: the technique, which was implemented by the team led by Prof. Andreas Wallraff of ETH Zurich, detects and corrects both basic types of error, the bit flip and the phase error. Previous procedures were not able to do so.
This procedure was described as a milestone.
Thanks to this technique, we can be sure that the results of our quantum calculations are correct, meaning that they can effectively be used in practice. The experiments at ETH Zurich are impressive and confirm how great the potential of quantum error correction techniques is.
How does the method work?
The quantum information of a qubit is actually lost when you read it out – which is also the case when you want to check whether an error has occurred. However, the method doesn’t use individual qubits, but several together. A chip with 17 superconducting qubits was developed for this purpose. Nine qubits form what’s called a logical qubit, that is, the unit with which an error-corrected quantum computer calculates. The remaining eight qubits detect errors without disturbing the information stored in the logical qubit by the readout process. To do this, they are measured repeatedly and quickly. This measurement information allows one to deduce which errors most likely occurred and and where they occurred on the chip. The effects of the detected errors can then be corrected.
What part did you have in the development?
My team contributed characterization techniques we had previously explored in our group, in our group, to assess the quality of the logical qubit. Our techniques also made it possible to distinguish and evaluate the nature of errors that may occasionally occur despite the high quality of the experiment.
What is the other solution?
It identifies errors already during the calculations of the quantum computer. Together with colleagues from the University of Innsbruck, we have shown how an algorithm can be programmed on a quantum computer in such a way that any quantum errors that occur don’t falsify the result of the calculations – which means error-tolerant quantum computing. For this purpose and for the first time, we have realized a universal set of computational operations on two logical quantum bits of an ion trap quantum computer. Any desired quantum algorithms can be constructed from these operations – an important step towards making error-corrected quantum computers freely programmable and usable for a wide variety of practical problems.
Interview conducted by Tobias Schlößer
You can find the complete interview at: go.fzj.de/effzett-quantum-error-correction
Photo: Forschungszentrum Jülich/Sascha Kreklau