An Experimentally Tested Surface Code

Forty years after Richard Feynman’s “Simulating Physics with Computers” lecture, the race toward the quantum computer is well underway.

Alexandre Blais, Scientific Director at the Institut quantique (IQ) and professor in the Department of Physics at the Université de Sherbrooke and his group aim to improve each component of this major technology based on superconducting circuits. In 2004, Prof. Blais, Steven Girvin, Rob Schoelkopf and Andreas Wallraff proposed an architecture allowing to consider the development of a quantum computer. This approach is now being developed by an ever-growing number of academic research groups and companies including Google, IBM, Amazon, and the IQ.

Despite the advances made thus far, a daunting challenge remains: Due to the fragility of quantum effects, quantum computers tend to make many more errors than our current computers. Those in the business agree that error correction codes are the next hurdle to overcome in ensuring the reliability of these new computers. With their colleagues from ETH Zürich led by Prof. Andreas Wallraff, Prof. Blais’ group, including Élie Genois, Catherine Leroux, and Agustin Di Paolo, tackled the problem, and the results of their work are documented in an article titled Realizing Repeated Quantum Error Correction in a Distance-Three Surface Code, a text now available online.

A Significant Step Toward the Realization of the Quantum Computer

“First steps had been taken by the community, the milestone we are now crossing is significant, carrying out several cycles of correcting a surface code which is considered to be the most promising of the error-correcting codes. This is the first time that these theoretical principles have been successfully implemented in the laboratory on a superconductor-based quantum computer,” explains Prof. Blais.

The surface code is an error correction code that uses topological characteristics of a qubit network to protect against errors. Using a superconducting circuit consisting of 17 physical qubits, the team encoded quantum information on the device and applied up to 16 cycles of error correction.

Prof. Blais’ team worked in collaboration with Andreas Wallraff’s group at ETH Zürich to achieve these results. The Swiss group carried out the experiment while the IQ group of theorists validated the experimental results using detailed numerical simulations of the experiment.

Élie Genois, doctoral student in Prof. Blais’ group, explains the challenges of their approach: “Simulating a quantum system of this size is a major challenge. It was therefore necessary to build a model which is able to give reasonable results using numerical computing resources which are also reasonable. To model the qubits, we set up a numerical model which allows us to simulate them efficiently. This method provides a lot of information on how the correction code should behave and allows us to check whether we are able to understand what is going on in the experiment.”

The excellent agreement between the numerical simulation of the surface code and the experiment makes it possible to use these simulations to identify the elements that can be improved: “We are now looking to improve operations and obtain better measurement and more precise qubit control since that will lead to an even more efficient error correction. This is crucial since a reliable quantum computer depends on the quality of the physical qubits,” adds the doctoral student.

The Next Steps

Now that the theoretical and experimental models are validated, a clearer path is rising for the next steps: “We are not yet at the point where we can simply scale this architecture, there is still research to be done. However, this model guides us toward the next experimental decisions and allows us to see where we must invest to obtain the most significant gain,” concludes Prof. Blais.

Decades of development were required to bring our traditional computers to market. Prof. Blais reminds us that the results presented in this study represent one more step toward the quantum computer: “The quantum computer is not for tomorrow and there is still a lot of work to do, and every step we take brings us closer to the goal. Achieving this surface code was a prerequisite for the quantum science field,” says the holder of the Rutherford Medal from the Royal Society of Canada in recognition of his achievements.

This development is part of a completely natural timeline: challenges in fundamental research must be resolved, all the components must be improved, and then we must know how to operate them. These are all challenges for the IQ scientists.