The carriers of quantum information, the qubits, are susceptible to errors caused by undesirable interactions with the environment. These errors accumulate during a quantum calculation. Their correction is a central requirement for the reliable use of quantum computers. Similar to conventional computers, quantum computers also require a functioning error correction.
Today’s quantum computers can certainly handle a certain number of arithmetical errors, for example bit flip or phase flip errors. However, qubits from the quantum register can also be lost, and there has been no protection against the potentially serious arithmetical errors resulting from this until now. Depending on the type of quantum computer, qubit losses can be due to the actual loss of particles such as atoms or ions, or to the fact that quantum particles, for example, enter undesirable electronic states so that their state is no longer recognised.
The Research Group for Theoretical Quantum Technology led by Prof. Markus Müller from RWTH Aachen University and the Jülich Peter Grünberg Institute, in collaboration with experimental physicists around Rainer Blatt from the University of Innsbruck and Davide Vodola at the University of Bologna, has now developed and implemented methods to protect against the loss of qubits. The new technique enables an ion trap quantum computer to adapt in real time to the loss of qubits and to maintain the protection of the fragile quantum information.
“In trapped ion quantum computers, the particles that store the qubits can be trapped for very long periods of time, even days,” says Roman Stricker from the team of experimental physicists in Innsbruck. “However, our ions are much more complex than the simplified description as a two-stage qubit would suggest. This offers great potential and additional flexibility in controlling our quantum computer, but unfortunately also leads to the loss of quantum information due to imperfect computing operations or decay processes”.
Using an approach developed by the theory group around Markus Müller, the collaboration with the Innsbruck researchers has shown that such a loss can be detected and corrected in real time. Müller stresses that “the combination of quantum error correction and the correction of qubit losses is a necessary next step towards large and robust quantum computers”.
To protect their quantum computer from qubit loss, the scientists had to develop two key techniques. The first challenge was to detect the loss of a qubit at all: However, direct measurement of the qubit is not an option, as this would destroy the quantum information stored in it. “We were able to overcome this problem by developing a technique in which we use an additional ion to check whether the qubit in question is still present or not, but without disturbing it,” explains Martin Ringbauer from the University of Innsbruck.
The second challenge was to adjust the rest of the calculation in real time if a qubit is actually lost. This adjustment is crucial to decipher the quantum information after a loss and to protect the remaining qubits. Thomas Monz, Senior Scientist in the Innsbruck team, emphasises that “the building blocks developed in this work are also applicable to other quantum computer architectures and other leading protocols for quantum error correction”.
Alternative approaches for quantum computers, in which, for example, solid-state qubits are realised by superconducting circuits or quantum dots, are mainly pursued in Aachen and Jülich, both theoretically and experimentally.
The research has been funded by the European Union under the quantum technology flagship project AQTION, among others.
More information: https://www.fz-juelich.de/pgi/pgi-2/EN/Home/home_node.html
Image: © Universität Innsbruck