Computers, whether classical or quantum, can make mistakes. In classical computers, these errors are generally minor and easily handled by redundant data storage, error-checking systems, and correction algorithms. However, when we venture into the realm of quantum computing, we face unique challenges. Quantum systems are fundamentally different from classical systems, as they rely on the delicate properties of quantum bits (qubits), which obey quantum mechanics and behave in unpredictable ways.
In classical computing, redundancy plays a crucial role in error detection and correction. If one copy of a data state is compromised, a backup or multiple copies can quickly flag and repair the error. This is not so in quantum computing. The act of measurement and copying a quantum state presents an inherent challenge due to the no-cloning theorem. This theorem asserts that an unknown quantum state cannot be duplicated, which prevents the simple use of redundancy that works in classical computers. Additionally, once a quantum state is measured, it collapses, and no further quantum information can be extracted from it in its original form.
As a result, detecting errors and ensuring computational correctness in quantum systems becomes vastly more difficult. However, researchers have devised a sophisticated method to address this problem—quantum error correction (QEC), a process by which quantum information is distributed across several quantum bits in such a way that if one or more qubits experience errors, the information can still be retrieved and processed correctly.
Redundant Storage Through Entangled Qubits
The key to quantum error correction is entanglement, a phenomenon in quantum mechanics where two or more qubits become interdependent, and the state of one qubit is directly tied to the state of the others, even across vast distances. By entangling qubits, quantum information can be stored redundantly across several entangled qubits. This redundancy allows errors to be corrected, as one faulty qubit in an entangled system can be identified and repaired using the correct information from the other qubits in the group.
However, while the concept seems promising, there are significant hurdles to overcome. One of the primary difficulties with quantum error correction is the requirement that any given quantum operation, or “gate,”—which is a fundamental part of performing calculations on a quantum computer—must be implemented without inducing errors in the system. Unfortunately, the very nature of quantum error correction limits the implementation of certain quantum gates on a single set of encoded qubits. A quantum error correction code can protect qubits from errors, but certain codes make the implementation of particular logic gates more difficult or inefficient.
Markus Müller’s Quantum Computing Breakthrough
In 2022, a breakthrough in quantum error correction was achieved by a team of researchers led by Thomas Monz from the University of Innsbruck’s Department of Experimental Physics and Markus Müller from the Department of Quantum Information at RWTH Aachen University, as well as from the Peter Grünberg Institute at Forschungszentrum Jülich in Germany. The team successfully demonstrated that fault-tolerant quantum computation could be achieved on a universal set of quantum gates by implementing error correction on quantum bits. This was a critical step in showing that it is possible to efficiently correct quantum errors while performing complex calculations on a quantum computer.
Their work demonstrated how errors in quantum computers could be corrected dynamically during a computation by shifting between different quantum error correction codes. This enabled the performance of different operations across multiple quantum states, without being constrained to one specific error correction scheme. The method involves using a set of two distinct error correction codes that enable the quantum computer to shift between them depending on the demands of the logic gate operations being implemented. This ensures that all types of gate operations, necessary for full computational functionality, can be performed with minimal error propagation.
The Role of Different Quantum Error Correction Codes
One of the fundamental challenges in quantum error correction is that no single correction code works effectively for all operations. Quantum gates, which perform the equivalent of classical computational operations on qubits, are notoriously difficult to implement efficiently, especially as computational systems grow in complexity. The reason for this difficulty is that not all quantum error correction codes allow the same range of quantum gates to be executed smoothly. Some codes are better suited to certain types of operations than others, meaning that the full range of quantum logic gates needed to perform complex quantum computations can’t always be handled by a single code.
Thus, the need to combine error correction codes arises. The collaborative team led by Monz and Müller has tackled this challenge by showing how a quantum computer can seamlessly switch between two different correction codes while executing quantum gates. When one code becomes inefficient or encounters limitations while executing a specific gate operation, the quantum computer can automatically transition to the second code, ensuring that no error propagation occurs and the calculation can proceed without interference.
This innovation opens the door to more efficient, scalable quantum computations. By employing this method, it becomes feasible to apply a universal set of quantum gates on a quantum computer without worrying about the limitations of any one error correction code.
Key Developments and Contributions
The development of this dual-code error correction method is significant in two ways:
1. Error-Tolerant Switching: The ability to dynamically switch between two correction codes on-demand during an ongoing computation is a remarkable breakthrough in maintaining error-tolerant quantum computations. The research proves that quantum computers are capable of conducting complex and freely programmable computations without the error propagation that otherwise inhibits their reliability.
2. Combining Experimental and Theoretical Expertise: The success of this method comes from an efficient collaboration between the experimental team at the University of Innsbruck, which works with ion-trap quantum computers, and the theoretical framework proposed by Markus Müller’s team. Combining practical, experimental know-how with solid theoretical foundations allowed the researchers to overcome the significant barrier posed by coding incompatibilities in error correction.
In their experiment, Friederike Butt, a doctoral student from Müller’s group, played a pivotal role in developing and implementing the quantum circuits that were needed to test this method. As a part of this work, Butt worked alongside Monz’s research group in Innsbruck to create the setup on which this innovative approach was based.
This collaborative effort culminated in the demonstration that two combined quantum error correction codes could be used to realize universal sets of quantum gates on a trapped-ion quantum computer—an experimental system where qubits are created by individual ions trapped in electromagnetic fields. Such systems are widely studied because they promise the ability to implement accurate quantum logic gates and error correction schemes that can protect quantum information.
Implications for Quantum Computing
The practical implications of this development are far-reaching. Achieving fault-tolerant quantum computing is one of the most pressing challenges facing the field today. Quantum computers rely on extremely delicate qubits, and any tiny disturbance, such as noise from the environment or imperfect measurements, can quickly compromise the integrity of the computational results. Quantum error correction is the key to protecting computations from these issues, and the method developed by Monz, Müller, and their colleagues moves us significantly closer to solving this problem.
By enabling efficient error correction in a way that can adapt to different types of gate operations, this research demonstrates that quantum computers can evolve beyond the rudimentary quantum operations that we can currently perform to execute large, complex computations required for real-world applications. It represents a leap forward in the realization of fault-tolerant quantum computing—a crucial step toward solving highly complex scientific and technological problems, ranging from cryptography to simulations of quantum systems and even quantum machine learning.
Conclusion
In summary, the work by Thomas Monz, Markus Müller, and their teams represents a critical breakthrough in the field of quantum error correction, paving the way for more reliable and scalable quantum computers. By utilizing dual quantum error correction codes, the team has demonstrated a novel method that could revolutionize quantum computing by making it more practical for a wider array of applications. This experiment, published in Nature Physics, highlights the enormous potential for the future of quantum technology and ensures that the dream of error-free quantum computations may one day become a reality.
As the quantum field continues to progress rapidly, the implications of these findings are enormous, and they represent one of the many advances required before quantum computers can outperform their classical counterparts in fields ranging from simulations of molecular dynamics to AI development, material design, and beyond.
Reference: Experimental fault-tolerant code switching, Nature Physics (2025). DOI: 10.1038/s41567-024-02727-2. On arXiv: DOI: 10.48550/arxiv.2403.13732