Good headphones shut out interference or even prevent it from intruding in the first place – the error correction methods that Jülich researchers are developing for quantum computers work along similar lines.
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Everyone makes mistakes, they say. However, it becomes annoying when errors accumulate. This is exactly what happens in today’s quantum computers: their computing units, the quantum bits, are extremely susceptible to interference and thus to errors. Researchers at Jülich are looking for ways to find and remedy these.
The record hunt has begun: while the first quantum computers run with some 50 qubits, US companies such as IBM and Google are already planning systems with thousands of quantum bits, or qubits for short. Germany’s quantum roadmap is aiming for 500 qubits, in any case.
Picture above: Good headphones shut out interference or even prevent it from intruding in the first place – the error correction methods that Jülich researchers are developing for quantum computers work along similar lines.
For Prof. Frank Wilhelm-Mauch, however, head of Quantum Computing Analytics at Jülich’s Peter Grünberg Institute (PGI-12), the number of qubits alone does not provide the right tools to measure the performance of a quantum computer, considering the state of technology. This is because the formula “many qubits means high performance” is not true. The fault lies with the computing units, which are susceptible to errors: even the smallest of interferences can cause qubits to lose their quantum state – and thus their special computing capabilities. For this reason, all known types of quantum computers have to be isolated from the environment – some by ultra-high vacuum, some by temperatures close to absolute zero at – 273 degrees Celsius. Any minute deviation as may occur, for example, during programming can disrupt the system.
In order to still be able to perform calculations, a large part of the computing units have previously been used to diagnose and correct errors. “Of the 50 or more qubits currently achieved by leading quantum computers, 12 at most are used for applications,” says the researcher. What’s more, the error rate increases ever more rapidly the more complex a calculation is. At some point, there are so many faulty qubits that the system no longer functions properly. “Therefore, it is currently more important to reduce the error probability than to increase the number of qubits,” Frank Wilhelm-Mauch is convinced.
But even the recognition of errors is a problem, because the qubits must not be measured while a quantum computer is computing. This, as well, would disturb and destroy the quantum properties that are important for further information processing. What complicates matters further is the fact that there are different types of quantum errors.
There is the so-called bit flip error, for example. To understand it, you can think of a qubit as a pointer that can indicate any direction. For comparison: the bit of a conventional calculator only points vertically upwards, corresponding to the number 1, or vertically downwards, corresponding to the number 0. In the case of the bit flip error, the “pointer” suddenly indicates the opposite direction. If the pointer rotates around its vertical axis, experts speak of a phase flip error. In both cases, the qubit provides incorrect information.
“It is currently more important to reduce the error probability than to increase the number of qubits.”
Prof. Frank Wilhelm-Mauch
Over the last two decades, researchers have developed various computational methods to detect these types of errors. They do not directly observe the qubits that are needed for computing, but use auxiliary qubits as warning lights that indicate when a qubit has “flipped out”, so to speak. The procedures also make it possible to correct the errors immediately after detecting them.
Another type of error occurs when a qubit disappears from a collection of qubits. Here, a quantum particle is either no longer recognized as such by the others or even lost altogether. “We were the first to develop a method to detect and correct this type of error using auxiliary qubits without disturbing the calculation,” says Prof. Markus Müller from PGI-2, who is an expert in theoretical quantum technology. Cooperation partners at the University of Innsbruck have demonstrated with a small ion trap quantum computer that the method works in practice. Markus Müller assumes that it can be combined with methods for correcting bit flip and phase flip errors.
In a way, all of the above methods work similarly to active noise cancellation in headphones: these block out noise disturbances from outside by means of anti-noise. This way, only the selected music is heard. In the case of qubits, a detected interference is followed by a computational operation that removes the interference. What remains is the information in its pure, original form.
A completely different approach was presented by Martin Rymarz and Prof. David DiVincenzo, head of PGI-2, together with partners from the University of Basel and QuTech Delft: they have designed a circuit that is intended to passively protect the qubits from interference – a headphone, so to speak, that does not even allow external noise to reach the ear and thus, in the best case, manages without anti-sound. At the core of this circuit is a gyrator, an electrical component with two ports that couples current at one port with voltage at the other.
“In superconducting quantum computers, active error correction could be omitted or at least made less complex thanks to our circuit,” says doctoral researcher Martin Rymarz. He is convinced that this would greatly simplify the construction of a superconducting quantum computer with a large number of qubits. DiVincenzo is aware that this concept may still be a little ahead of its time, but he is optimistic: “Given the expertise available, we see the possibility of testing our proposal in the lab in the foreseeable future.”
Frank Frick
For more about Jülich quantum research at: fzj.de/quanten
Also check out the cover story of the effzett 1-21 issue at: effzett.fz-juelich.de/en/1-21/in-superposition