New method prevents quantum computers crash

Jülich, 9 September 2020 – Quantum information is fragile, which is why quantum computers must also be able to correct errors. But what if whole qubits are lost? Researchers at Forschungszentrum Jülich and RWTH Aachen University, in collaboration with Universities of Innsbruck and Bologna, are presenting a method in the journal Nature that allows quantum computers to keep going even if they lose some qubits along the way.

Qubits—the carriers of quantum information—are prone to errors induced by undesired environmental interactions. These errors accumulate during a quantum algorithm and thus correcting them becomes a key requirement for reliable use of quantum computers. Similar to the classical computer, quantum computers require error correction.

By now, quantum computers can withstand a certain amount of computational errors, such as bit flip or phase flip errors. However, in addition to computational errors, qubits might get lost altogether from the quantum register of qubits. Depending on the type of quantum computer, this can be due to actual loss of particles, such as atoms or ions, or due to quantum particles transitioning for instance to unwanted electronic states, so that their state is no longer recognized as a qubit. When a qubit gets lost, the information in the remaining qubits becomes scrambled and unprotected, rendering this process into a potentially fatal type of error.

Detect and correct lost qubits in real time

The theoretical quantum technology research group by Markus Müller at the Institute for Quantum Information at RWTH Aachen University and the Peter-Grünberg-Institute at Forschungszentrum Jülich, in collaboration with the experimental team by Rainer Blatt at the University of Innsbruck and Davide Vodola at the University of Bologna, has now developed and implemented advanced techniques that allow a trapped-ion quantum computer to adapt in real-time to loss of qubits and to maintain protection of the fragile stored quantum information.

“In trapped-ion quantum computers, ions hosting the qubits can be trapped for very long times, even days”, says Roman Stricker. “However, our ions are much more complex than a simplified description as a two-level qubit captures. This offers great potential and additional flexibility in controlling our quantum computer, but unfortunately it also provides a possibility for quantum information to leak out of the qubit space due to imperfect operations or radiative decay”.

Using an approach developed by the theorists around Müller, the collaboration has now demonstrated that such leakage can be detected and corrected in real-time. Müller emphasizes that “combining quantum error correction and correction of qubit loss and leakage is a necessary next step towards large-scale and robust quantum computing.”

Widely applicable techniques

The researchers had to develop two key techniques to protect their quantum computer from the loss of qubits. The first challenge was to detect the loss of a qubit in the first place: “Measuring the qubit directly was not an option as this would destroy the quantum information that is stored in it”, explains Philipp Schindler from the University of Innsbruck. “We managed to overcome this problem by developing a technique where we used an additional ion to probe whether the qubit in question was still there or not, without disturbing it”, explains Martin Ringbauer.

The second challenge was to adapt the rest of the computation in real-time in case the qubit was indeed lost. This adaptation is crucial to unscramble the quantum information after a loss and maintain protection of the remaining qubits. Thomas Monz, senior scientist in the Innsbruck group, emphasizes that “all the building blocks developed in this work are readily applicable to other quantum computer architectures and other leading quantum error correction protocols.” Alternative approaches for quantum computers, using in particular solid state-based qubits such as superconducting circuits or quantum dots, are a focus area of theoretical as well as experimental research at Aachen and Jülich.

Part of the research was financed by the European Union under the Quantum Technology Flagship project AQTION.

Newly developed methods ensure that the loss of individual qubits does not disrupt a quantum computer.

Copyright: Harald Ritsch

Original publication:

Experimental deterministic correction of qubit loss
Roman Stricker, Davide Vodola, Alexander Erhard, Lukas Postler, Michael Meth, Martin Ringbauer, Philipp Schindler, Thomas Monz, Markus Müller, Rainer Blatt
Nature (published online 9 September 2020), DOI: 10.1038/s41586-020-2667-0

Further information:

Peter Grünberg Institute, Theoretical Nanoelectronics (PGI-2/IAS-3)

Contact:

Prof. Dr. Markus Müller
Peter Grünberg Institute, Theoretical Nanoelectronics (PGI-2/IAS-3)
Tel: +49 2461 61-3137
Email: markus.mueller@fz-juelich.de

Press contact:

Dr. Regine Panknin, Press officer
Tel: +49 2461 61-9054
Email: r.panknin@fz-juelich.de