Researchers at the University of Innsbruck, together with partners from Sydney and Waterloo, have presented a new diagnostic method for quantum computers. It makes errors in individual quantum bits visible during logical calculation and evaluates them. The new method was demonstrated on an ion trap quantum processor in Innsbruck. It can be used to identify critical error sources -a key to developing more robust, fault-tolerant quantum processors.
In a recently published study, the researchers present a scalable method that can be used to reliably characterize logical quantum operations at the level of the underlying quantum bits. Cycle error reconstruction identifies which physical errors influence the performance of logically encoded gates. "With cycle error reconstruction, we can quantitatively capture the error structure and clearly distinguish between correctable and uncorrectable contributions," emphasizes first author Robert Freund from the Department of Experimental Physics .
From physical to logical quantum bits
The path to fault-tolerant quantum computing leads via logical qubits: because quantum effects are very fragile and physical qubits are therefore prone to errors, several qubits are combined to form a logical quantum bit. This increases the complexity of the diagnosis. "Classic benchmarks are only of limited use for operations between two logical qubits because many physical qubits are involved and errors occur depending on the context," explains Robert Freund.
The new method detects errors over repeated calculation cycles and uses them to reconstruct compact, representative error descriptions for many qubits simultaneously. The process is resource-efficient, scales with the register size, and provides reliable parameters for logically relevant multi-qubit gates. This was demonstrated using a transversal CNOT gate in a 16-qubit ion trap processor at the University of Innsbruck. "The method reveals context-dependent errors and shows where calibration and stabilization are needed to operate logical gates reliably," says Thomas Monz, head of the Innsbruck research group.
Step towards fault-tolerant computing
The analysis revealed context-dependent errors, such as localized dephasing and gate miscalibrations. A statistical model is used to efficiently model error dependencies in order to identify contributions that can be addressed by error correction codes and those that limit logical reliability. This enables precise performance predictions and shows quantum computer hardware development teams where critical sources of error lie and how they can be minimized - a step toward reliable fault-tolerant quantum processors.
In addition to the team from the University of Innsbruck, the Centre for Engineered Quantum Systems at the University of Sydney in Australia and the Institute for Quantum Computing at the University of Waterloo in Canada were also involved in the study. The research was funded by the Austrian Science Fund (FWF), the European Union, and the Federation of Austrian Industries Tyrol, among others.
Publication: Characterizing Physical and Logical Errors in a Transversal CNOT Gate via Cycle Error Reconstruction. Nicholas Fazio, Robert Freund, Debankan Sannamoth, Alex Steiner, Christian D. Marciniak, Manuel Rispler, Robin Harper, Thomas Monz, Joseph Emerson, Stephen D. Bartlett. Phys. Rev. X 16, 011030 DOI: 10.1103/qfwc-584t
