Credit: Journal of Physical Chemistry Chemical Physics

If you had asked us six months ago, when we thought fault tolerant quantum computers would become available, we would have estimated some time around the 2030 timeframe. We would have indicated that between now and 2030, available quantum computers would all be NISQ based and that a few users would be able to figure out how to use these NISQ machines to achieve quantum advantage for a few applications during this period.

But over the past few months, we have seen a large number of technical papers and roadmaps that indicate what we will call early Fault Tolerant Quantum Computers (FTQC), will start appearing 2-3 years earlier than we thought and these could be available as early as 2027 or 2028. We have recently reported in the Quantum Computing Report by GQI on several developments towards this goal including articles on activities at QuEra, Alice & Bob, Infleqtion, Pasqal, Nord Quantique, Google, and IBM. And we are aware of many other organizations that are also heavily researching fault tolerant quantum computing.

Now we don’t expect these early FTQC machines to be the million qubit monsters that could be used for running Shor’s algorithm. But think it is quite possible to see machines with a few thousand physical qubits and a few hundred high-fidelity logical qubits within the next 3-5 years that could be used for quantum advantage. And this will result in an interesting scenario because we may see NISQ processor and early FTQC processors in the market at the same time. So end users in this timeframe will need to decide which path they want to follow.

Certainly, none of the technical results from the companies mentioned above represents the final answer. Each one of those early research results lacks key characteristics that make what they currently have not enough to be used for a production machine. But each new development is a stepping stone that gets closer to the end goal. GQI expects to see many more papers on error correction appearing over the next few months and years. There are three key challenges we are watching to see if the researchers can overcome to achieve the early FTQC machines can become a reality.

Challenge 1: Code scaling for systematically suppressing logical errors
Production algorithms working on a FTQC will require the machine to run through thousands or millions of levels without an error. This will require a very low logical error rate and requires ensuring that as the codes get bigger (e.g. provide greater distance between the codewords), the logical error rates continue to improve.

Challenge 2: Universal fault-tolerant circuits with realistic clock times
It is not sufficient to achieve quantum advantage if an error correction architecture only implements a set of the Clifford gates (gates including H, S, CNOT, and others that can be generated by combining these together). A universal gate set that can perform any quantum calculation requires an additional gate, a non-Clifford gate such as the T-gate. Most of the demonstrations so far cannot do this without a universal gate set they will not be able to processor algorithms show an advantage over classical processing.

Challenge 3: Platform capable of scaling to commercially relevant size
Sometimes, a technical approach works when you try it on a small scale, but it falls apart when you try to implement it on a larger scale. So researchers need to demonstrate the scalability of an error correction algorithm to make sure it is still practical at larger sizes. Problems that could arise include issues with crosstalk, a slow-down of gate speed, connectivity limitations, cooling, etc.

So it will be an exciting time in the quantum world as we hear more news about progress being made in fault tolerant quantum computers. GQI has created a report that provides more details about these challenges called The Era of Logical QC. For information on how you can obtain this report, click here.

March 16, 2024