The reason quantum computers are prone to errors lies in the qubits they use. Whereas a conventional computer processes information using bits – zeros and ones – a quantum computer uses qubits, which can behave as a combination of zero and one. This is the quantum-mechanical principle of superposition. Multiple qubits can also be linked in a special way, a phenomenon known as entanglement. These properties could allow quantum computers to perform certain calculations far more efficiently than conventional computers.
However, qubits are exceptionally fragile. Heat, vibrations, electromagnetic interference and imprecise control can all cause errors. Moreover, a qubit cannot simply be measured continuously to check whether it is still working correctly, as measuring it changes its quantum state.
The longer and more complex a calculation becomes, the more errors can accumulate. “We are now at a stage where we can create qubits that are good enough for us to detect errors in them,” says Jonas Helsen. “This has led to the development of early fault-tolerant quantum computers, or FTQCs, which are designed to detect and correct errors during a computation. But the process is still far from fully developed.”
Test programmes
Methods for detecting and correcting these errors are still in their infancy. Helsen has spent years investigating what can go wrong with qubits. Among other things, he develops test programmes that assess the quality of physical qubits and identify what they do and do not do well. The results can then be used to improve the equipment, after which it is tested again. “I will now bring this approach to the world of fault-tolerant quantum computing.”
The ERC Starting Grant will allow Helsen to spend five years developing methods that contribute to reliable fault-tolerant quantum computers. The first step is to identify and understand the problems that cause errors. Do they originate in the qubits themselves, in interactions with the qubits, during the error-correction process, or in all three? “I will also work on a more theoretical question: how can we detect these errors, and what is the most efficient algorithm for doing so?”
Simulation
Finally, Helsen aims to create a reliable ‘digital twin’ of a quantum computer that can be used for testing. This requires quantum-mechanical systems to be simulated. That is another major challenge, he explains, because “quantum computers are difficult to simulate precisely because they can do things that conventional computers cannot”. Helsen will therefore also develop algorithms that can mimic the behaviour of qubits.
“Methods for analyzing FTQCs will become very important,” says Helsen. “The theoretical models for quantum error correction have existed for thirty years. Only now is the hardware becoming good enough for us to put them into practice.”
About the ERC Starting Grant
The European Research Council (ERC) supports frontier research across Europe by awarding individual grants to outstanding researchers conducting fundamental research. One of these funding schemes is the ERC Starting Grant, which is intended for researchers who obtained their PhD between two and seven years ago. They can receive up to €1.5 million over five years to establish their own research programme or research group.
