24 January 2025
In the future, quantum computers are expected to solve tasks that push even the most powerful supercomputers to their limit. But the world of the smallest particles is governed by its own laws. One of these is decoherence, which states that a quantum mechanical system can lose its properties as soon as it interacts with its environment. Even the smallest disturbance from the outside world can therefore affect a quantum computer's calculations and distort the result. Together with colleagues from Forschungszentrum Jülich, the University of Innsbruck and RWTH Aachen University, Friederike Butt has succeeded in performing logical operations on encoded qubits in a quantum computer using a new type of code switching and correcting errors that occur. The results have been published in Nature Physics.
The main goal is to improve the reliability of quantum computers, which are sensitive to external disturbances. One way to minimise the impact of these damaging effects is to use quantum error correction. This involves combining several physical qubits into one logical qubit in order to detect and correct possible errors. In other words, hardware errors in quantum computers are reduced by redundant storage of quantum information and the use of software. Together with my colleagues, I have developed a method for fault-tolerant switching between two different quantum codes, thus enabling a larger number of fault-tolerant logical operations. This is an important step towards more stable and efficient quantum computers.
Previous error correction codes have specialised in correcting specific types of errors and improving coherence times. The current state of the art is to use auxiliary states to complete the required logical minimal gate set. These states have to be generated in a specific way with a large overhead of additional qubits and operations, and their preparation only succeeds with a certain probability. The breakthrough of our research is that code switching provides predictable control over logical qubits without relying on the probabilistic preparation of these auxiliary states. We have experimentally demonstrated code switching for the first time, thereby implementing a complete universal gate set. In this experiment, code switching enables the construction of logical circuits and the preparation of 12 different logical states that can not be intrinsically and fault-tolerantly reached within a single code.
All the building blocks are now in place to run, in principle, arbitrary algorithms on logical qubits. Our fault-tolerant method for executing a universal gate set has been tested on a setup with ion trap qubits and is in principle flexible. Theoretically, the code can be adapted to other physical Quantum comuter platform. However, the exact implementation depends on the specific properties of the qubits used. The code switching opens up many possibilities for further development and application in different quantum computing systems.
Logical operations in a classical computer are realised by electrical circuits. These circuits are made up of what are called logic gates that link bits using rules such as AND, OR, NOT. Such links allow data to be processed and form the basis of microprocessors. In a quantum computer, logical operations are performed by so-called quantum gates, which manipulate the states of qubits, the computational units of a quantum computer. A distinction is made between physical and logical qubits. Physical qubits are the actual physical units, which can be realised in a variety of ways, for example by means of trapped ions or superconducting circuits. Logical, or encoded, qubits are formed by combining several physical qubits. This distributes the information redundantly and thereby makes the calculations more robust against disturbances.
Friederike Butt studied physics at RWTH Aachen University and is currently doing her PhD under the supervision of Markus Müller, who is a professor at Forschungszentrum Jülich and RWTH Aachen University. She has already completed her Master's thesis in the field of quantum information in Prof. Müller's group. The work at the interface between theory and experiment is particularly exciting for her.
This study was co-authored by Friederike Butt and Ivan Pogorelov from the University of Innsbruck. For more information, see the University of Innsbruck press release.
Pogorelov, I., Butt, F., Postler, L. et al. Experimental fault-tolerant code switching. Nat. Phys. (2025). https://doi.org/10.1038/s41567-024-02727-2
Leader of the Research Group for Theoretical Quantum Technology
