GQI is pleased to provide a three part series of articles exploring the transition from NISQ to FTQC in partnership with Dr. Ish Dhand, CEO and co-founder of QC Design. Executive summaries of each of the three parts will be published here on the Quantum Computing Report with the full text published on the QC Design website. The content, intelligence and opinions in this report are those of the author and do not necessarily reflect GQI’s findings or conclusions. At GQI we thrive to publish a variety of viewpoints on our platform to provide our audience with the highest variety of quality analysis. GQI is distributing this information at No Charge and we are grateful to Dr. Dhand for his kind collaboration.

Not a month passes by without a landmark quantum computing demonstration with noisy intermediate scale quantum (NISQ) devices. In the span of a decade, we’ve gone from a handful of qubits in a lab to quantum computers with tens or hundreds of qubits that can be accessed by anyone, anywhere in the world, and that can perform tasks that far outstrip the most powerful of supercomputers. 

Different from NISQ, another field in quantum computing that has seen exciting recent advances is fault tolerant quantum computing (FTQC). My goal in writing this series is to show how these advances are even more important to the future of quantum computing than recent NISQ demonstrations.

This first part digs deeper into the idea that fault-tolerance is what will unlock the true potential of quantum computing. Subsequent parts will explore how the right roadmaps and architectures can accelerate the path to fault-tolerance; the implications of the first logical qubits and gates that were demonstrated recently; and where we’re headed – the fault-tolerance stack, modularity and beyond.

Part 1 – Fault-Tolerance Will Bridge the 10000X Gap to Transformational Applications of Quantum Computing

Executive Summary

Fault tolerance means that we can design quantum circuits to be reliable despite inevitable imperfections in the quantum bits (qubits) and gates. In fault-tolerant quantum computers, the reliable ‘logical’ qubits and gates that run the user’s algorithm in turn comprise a large number of noisy ‘physical’ qubits and gates. This large number of physical qubits and gates connected and controlled in the right manner is what allows turning noisy quantum devices into reliable computing machines. 

This report is how the possibility of computing reliably allows fault-tolerance to deliver the true promise of quantum computing – the promise of not only scientific breakthroughs and faster drug development cycles, but also new materials for batteries and new methods for fertilizer manufacture and carbon capture. These ‘transformative applications’ offer a revolutionary impact on society and the economy, which is why I focus on these applications in this report. 

But why is fault-tolerance needed for these transformative applications? That’s because even the least demanding among these requires quantum circuits with millions of gates. Only a fault-tolerant quantum computer can run circuits with this many gates without the output turning into a structureless stream of random numbers. 

The gate and qubit requirements of these transformative applications have been an active area of research in the last five years with contributions from heavy hitting teams in academia and industry. There has been strong contribution from academic teams in Caltech, ETH, MacquarieU, UMaryland, USherbrooke, UToronto, UVienna, UWashington to name a few; teams building quantum hardware such as those from Google, Microsoft, PsiQuantum, Xanadu; and finally teams exploring their use such as those from BASF SE, Boehringer Ingelheim, Mercedes-Benz, Volkswagen. These so-called resource estimates are providing a concrete target for future fault-tolerant quantum computers in terms of the number of logical qubits and the number of logical gates to be applied on them. 

Here’s a simplified table summarizing the requirements for the number of gates required for the transformative applications mentioned above.

↓ Application \ Requirements →Number of gatesQubit countsRefs.
… scientific breakthrough10,000,000+100+1, 2, 3
… fertilizer manufacture 1,000,000,000+2000+4
… drug discovery1,000,000,000+1000+5
… battery materials10,000,000,000,000+2000+6, 7

Unfortunately, running circuits with these many gates is just not possible with today’s NISQ devices, which are too noisy to run more than a few hundred gates before their output is no longer reliable and useful. Even if noise mitigation techniques are applied, these will require too many repetitions to be feasible and often involve an unfeasibly large number of experimental runs and leverable extrapolation-like methods whose output cannot be trusted because of large and often uncontrolled error bars in the output.

The challenge is in going from the less than 100 gates that can be applied on today’s NISQ devices to 10’s of millions needed for the transformative scientific applications and billions needed for the transformative commercial applications. Fault-tolerance is what will allow us to bridge this 10000X gap and that’s why many of the strongest teams in quantum computing are working on building fault-tolerant quantum computers. 

Bridging the 10000X gap is something that will need progress in three areas: first, the algorithms that are already very sophisticated need to get even smarter and allow performing the same calculations with fewer gates. As discussed below, we’ve already seen multiple orders of magnitude reduction in the costs of several of the transformative applications. Second, the hardware needs to get better so that fault-tolerance is even possible. And finally, we need better fault-tolerance architectures, which tackle imperfections in real hardware and allow squeezing out maximum logical performance from the hardware. These last two points will be the focus of subsequent reports in this series. If we’re able to make substantial progress in these three areas, then it’s only a matter of time before quantum computing delivers the revolutionary impact to society and economy that we are all looking forward to!

The full Part 1 document can be downloaded here.

About the Author

Dr. Ish Dhand is the CEO and co-founder of QC Design, a quantum computing company that designs useful and scalable quantum computers by offering licenses to fault-tolerance architectures and design software. Prior to this, he headed the architecture team at Xanadu. Ish has over 10 years of research and leadership experience in quantum computing including a strong focus on fault-tolerance, quantum advantage, and error suppression.

October 2, 2023