Quantum computing has undergone a remarkable transformation in recent years, with the development of noisy intermediate scale quantum (NISQ) devices. These quantum computers, which have grown from just a few qubits in a lab to machines with tens or hundreds of qubits, are able to perform tasks far surpassing even the most powerful supercomputers. However, there is another aspect of quantum computing that holds even greater promise for the future: fault tolerant quantum computing (FTQC).
FTQC is a field that focuses on designing quantum circuits that are reliable despite imperfections in the quantum bits (qubits) and gates. In fault-tolerant quantum computers, the user’s algorithm is run on ‘logical’ qubits and gates, which are made up of a large number of noisy ‘physical’ qubits and gates. It is this combination of interconnected and controlled physical components that transforms noisy quantum devices into reliable computing machines.
The true potential of quantum computing lies in the concept of fault-tolerance. The ability to compute reliably opens the door to transformative applications in various fields, including scientific breakthroughs, faster drug development cycles, new materials for batteries, and innovative methods for fertilizer manufacture and carbon capture. These applications have the potential to bring about revolutionary changes in society and the economy.
But why is fault-tolerance necessary for these transformative applications? The answer lies in the complexity of quantum circuits. Even the simplest of these applications require circuits with millions of gates. Only a fault-tolerant quantum computer can handle circuits of this scale without producing meaningless and random outputs.
Extensive research has been conducted in the past five years to determine the gate and qubit requirements for these transformative applications. Contributions from esteemed academic institutions such as Caltech, ETH, Macquarie University, UMaryland, USherbrooke, UToronto, UVienna, and UWashington, as well as quantum hardware builders like Google, Microsoft, PsiQuantum, and Xanadu, have shaped these resource estimates. Companies like BASF SE, Boehringer Ingelheim, Mercedes-Benz, and Volkswagen are also exploring the potential applications of quantum computing. These resource estimates have set concrete targets for the future development of fault-tolerant quantum computers in terms of logical qubits and logical gates.
To illustrate the scale of these transformative applications, consider the following requirements for the number of gates:
– Scientific breakthrough: 10,000,000+ gates
– Fertilizer manufacture: 1,000,000,000+ gates
– Drug discovery: 1,000,000,000+ gates
– Battery materials: 10,000,000,000,000+ gates
Unfortunately, today’s NISQ devices are unable to handle circuits with such a large number of gates. The noise in these devices makes it impossible to run more than a few hundred gates before the output becomes unreliable and unusable. Although noise mitigation techniques can be applied, they require an impractical number of repetitions and often yield untrustworthy results due to large and uncontrolled error margins.
Bridging the 10000X gap between the gate requirements of current NISQ devices and the transformative applications necessitates advancements in three key areas. Firstly, algorithms need to become more efficient, allowing the same calculations to be performed with fewer gates. Significantly, there have already been orders of magnitude reduction in the cost of several transformative applications. Secondly, hardware improvements are needed to enable fault-tolerance. Lastly, fault-tolerance architectures must be developed to mitigate imperfections in real hardware and extract maximum performance from the system.
By making progress in these three areas, quantum computing will eventually deliver the revolutionary impact on society and the economy that we eagerly anticipate.
For more information on fault-tolerance and its role in quantum computing, you can read the full document on the GQI website [link to the full document].
What is fault-tolerant quantum computing?
Fault-tolerant quantum computing refers to the design of quantum circuits that are reliable despite imperfections in the quantum bits (qubits) and gates. In fault-tolerant quantum computers, logical qubits and gates, made up of interconnected and controlled physical qubits and gates, are used to run algorithms.
Why is fault-tolerance important for transformative applications?
Transformative applications, such as scientific breakthroughs and faster drug development cycles, require quantum circuits with a large number of gates. Only fault-tolerant quantum computers can handle these complex circuits without producing unreliable outputs.
What are the gate requirements for transformative applications?
The gate requirements vary depending on the application. For example, scientific breakthroughs may require 10,000,000+ gates, while battery material research may demand 10,000,000,000,000+ gates.
Why can’t NISQ devices handle circuits with a large number of gates?
NISQ devices are too noisy to run circuits with many gates. The noise in these devices leads to unreliable outputs. Although noise mitigation techniques can be applied, they often involve unfeasible repetitions and yield untrustworthy results due to large error margins.