What is quantum error correction
New method protects quantum computers from failures
Dr. Christian Flatz Public Relations Office
University of Innsbruck
Quantum information is fragile, which is why quantum computers need to be able to correct errors. But what if entire qubits are lost? Researchers from the University of Innsbruck, in collaboration with colleagues from RWTH Aachen University and the University of Bologna, now present a method in the journal Nature with which quantum computers can continue to calculate even if they lose some qubits.
The carriers of quantum information, the so-called qubits, are prone to errors caused by undesirable interactions with the environment. These errors accumulate during a quantum calculation, and their correction is a key requirement for the reliable use of quantum computers. Similar to the classical computer, the quantum computer also needs a functioning error correction.
In the meantime, quantum computers can deal with a certain number of calculation errors, such as bit-flip or phase-flip errors. In addition to these errors, however, qubits can also be completely lost from the quantum register. Depending on the type of quantum computer, this can be due to the actual loss of particles such as atoms or ions, or to the fact that quantum particles change into undesired energy states, for example, which are no longer recognized as qubits. If a qubit is lost, the information in the remaining qubits becomes illegible and unprotected. This process can turn into a potentially devastating mistake for the result of the calculation.
Detect and correct loss in real time
A team of physicists led by Rainer Blatt from the Institute for Experimental Physics at the University of Innsbruck, in collaboration with theoretical physicists from Germany and Italy, has now developed and implemented advanced methods that allow their ion trap quantum computer to adapt to the loss of qubits in real time Maintain protection of fragile quantum information. “The ions that the qubits store can be trapped in our quantum computer for a very long time, even days,” says Roman Stricker from Rainer Blatt's team. “However, our ions are much more complex than the simplified description as a two-stage qubit suggests. This offers great potential and additional flexibility in the control of our quantum computer, but unfortunately also means that quantum information is lost due to imperfect arithmetic operations or decay processes. ”With one of the theory group around Markus Müller at RWTH Aachen University and Forschungszentrum Jülich The approach developed in collaboration with Davide Vodola from the University of Bologna has shown that such a loss can be recognized and corrected in real time. Müller emphasizes that "the combination of quantum error correction and the correction of qubit losses is a necessary next step in the direction of large and robust quantum computers".
Widely applicable methods
The researchers had to develop two key techniques to protect their quantum computer from losing qubits. The first challenge was to even detect the loss of a qubit: "Direct measurement of the qubit is not an option, as this would destroy the quantum information stored in it," explains Philipp Schindler from the University of Innsbruck. “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 interfering with it,” explains Martin Ringbauer. The second challenge was to adjust the rest of the computation in real time in the event that a qubit was actually lost. This adaptation is crucial in order to decrypt the quantum information after a loss and to protect the remaining qubits. The head of the Innsbruck team, Thomas Monz, emphasizes that “all of the building blocks developed in this thesis are easily applicable to other quantum computer architectures and other leading protocols for quantum error correction”.
The research was financed by the Austrian Science Fund FWF, the Austrian Research Promotion Agency FFG and the European Union.
Institute for Experimental Physics
University of Innsbruck
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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 2020 doi: https://doi.org/10.1038/s41586-020-2667-0, [arXiv: 2002.09532]
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