Paul van Gerven
27 January

Silicon-based quantum computers could piggyback on decades of miniaturization and manufacturing experience in the semiconductor industry, but the technology has struggled with high error rates. Now, researchers at Qutech have demonstrated silicon qubits that operate above the long-standing goal of more than 99 percent fidelity.

In the fall of 2019, Google researchers claimed the world’s first demonstration of quantum supremacy. Their 53-qubit Sycamore quantum computer based on superconducting electronic circuits took 200 seconds to perform a calculation that the researchers estimated would take a top-of-the-line supercomputer 10,000 days to complete.

The claim has been challenged by IBM, but even if it holds up, there may be trouble down the line. Since superconducting qubits are relatively large, the technology may not be ideal for scaling to the number of qubits required to build a quantum computer of practical value. According to most estimates, such a machine will need hundreds of thousands, if not millions, of qubits onboard.

There’s a more compact alternative to superconducting circuits: qubits based on the spin of electrons that are confined to so-called semiconductor quantum dots. These structures, formed at the interface of two semiconductor materials, shield the trapped electrons from the environment, but not so much that they can’t be manipulated anymore. If silicon is used as the base semiconductor, scaling is facilitated by the possibility of piggybacking on decades of semiconductor manufacturing practice.

But ‘silicon qubits’ aren’t as sophisticated as superconducting qubits yet. One major issue is their reliability. Instability and randomness are inherent to the quantum world, rendering all types of quantum technology prone to errors. Error-correcting mechanisms can compensate for that, but only up to a point: the qubits need to achieve a certain level of stability for it to work. In that respect, silicon qubits have been trailing superconducting qubits – there’s a good reason for Google to use the latter for its quantum supremacy demonstration.

Now, researchers from the Delft-based research institute Qutech have closed the gap a little. In a Nature paper, they demonstrate quantum operations in silicon devices with fidelities above the threshold of one of the most popular quantum error-correcting codes. “Now that this important barrier has been surpassed, semiconductor qubits have gained credibility as a leading platform, not only for scaling but also for high-fidelity control,” comments Xiao Xue, lead author of the study. “We’re optimistic that the individually demonstrated advantages of semiconductor spin qubits can be combined into a fault-tolerant and highly integrated quantum computer,” adds lead researcher Lieven Vandersypen.

Qutech 2 qubit gate
Artist impression of a two-qubit quantum gate in action. Credit: Marieke de Lorijn for Qutech

Easily checked

The basic building blocks of quantum circuits are quantum gates, the equivalents of classical logic gates in digital systems. Quantum gates operate on qubits, leveraging key aspects of quantum mechanics such as entanglement and superposition to perform the ‘magic’ of quantum computing. To realize single-qubit and two-qubit logic gates, both single-qubit control and two-qubit interactions are required.

The Qutech researchers demonstrated single-qubit and two-qubit gates with fidelities exceeding 99.5 percent, above the threshold to implement proper error correction. To demonstrate their setup can do something useful, they had it calculate the ground-state energy of molecular hydrogen. Being the simplest and smallest possible molecule, this calculation can also be performed classically, so the quantum answer is easily checked.

Open problem

Thanks to the breakthrough, silicon-based quantum dots have joined superconducting circuits and a few other technologies as a viable quantum computing platform. The next step will be to add more gates while maintaining high fidelities.

This is far from trivial, Virginia Tech physicists Ada Warren and Sophia Economou explain in a commentary. The Qutech researchers had to carefully fine-tune the system to achieve their results. Adding even a single qubit will destroy that delicate balance. “The next experimental milestone for this system would therefore be to build a larger array of quantum dots hosting two-qubit gates with fidelities as high as those demonstrated. A further breakthrough for such a system would be the demonstration of quantum error correction,” Warren and Economou write.

At some point, however, adding qubits to the array will do more harm than good. This is because it will become too difficult to calibrate and control a large system with multi-qubit interactions. Overcoming this problem will likely involve swapping quantum dots for different microstructures and employing a modular, networked architecture. “The details of how large these modules need to be and how they would be connected are an open problem from both a theoretical and an experimental point of view,” the commentators caution.