This content originally appeared on DEV Community and was authored by Juan Gómez
“Fault-Tolerant Quantum Computers” might sound like jargon, but what we’re really talking about are universally useful quantum computers—machines that can do something meaningful for real-world problems.
Let me share the excitement about what have been recently published because I genuinely believe it is big. Bottom line: We know how to build a quantum computer that is going to be universally useful. One that will be able to function despite all the problems that naturally arise from the quantum world. One that can correct information fast enough so it can be used to solve a non-negligible range of problems way better than our classical counterparts.
This is an attempt to explain in plain English all the nitty gritty details, so you can explain to your colleagues while taking some cañas on a Thursday after work.
Correcting noise
I’ve already talked about it in the past, but let me insist, because this is the key to understanding why we are doing what we are doing.
Whenever we try to act upon our qubits so we can get a computational advantage of their quantum properties, qubits tend to disorientate over time and we can’t get anything meaningful out of them. So yes, we need a way to fix this otherwise these expensive machines are worthless. So far, we haven’t found a way to keep them completely focused long enough, and yes, it’s probably not possible, but that is ok. Did you know that our classical transistor-based hardware is actually not perfect either?. Yes, it makes mistakes from time to time too. The difference, though, is that we found a way a long time ago to correct these mistakes so computers, phones and fridges are useful nowadays.
We think there’s a way to fix our qubits, too. We made some experiments about it, and we are confident that this can be done. That was not the case not very long ago, even though we kept the faith, faith was not going to fix the qubits – science will.
What is more interesting, the techniques used for correcting errors in our qubits are heavily inspired by those used in classical hardware. Everything is about using several qubits to represent a piece of information, and some other auxiliary qubits to help in the process. This is the reason why you would hear about this concept of “logical qubits”, vs “physical qubits”. A logical qubit is error-corrected and useful, but it requires several physical qubits – which play different roles – to make that possible.
Surface Codes
There’s a myriad of techniques to correct qubit errors, each one has its pros and cons. Most of the industry has put a lot of focus on a family of error correction techniques called “Surface codes”, but one of the major problems is the huge amount of physical qubits required to protect a logical qubit, something in the order of thousands, but, in the other hand, they work very well with noisy qubits.
LDPCQ & Gross Codes
Scalability (number of qubits) matters if the goal is to build a quantum computer that can solve real-world practical problems. One promising family of codes for building large-scale fault-tolerant quantum computers is the LDPCQ (Low-Density Parity-Check Quantum) codes. Within this family, there’s a notable subfamily known as the “Gross code,” named after Nobel Prize–winning physicist David Gross.
It offers several attractive properties, but also comes with tradeoffs:
- It doesn’t require too many physical qubits to operate, just 12 or 24 (depending on the use case)! so it scales very well
- It’s relatively easy to build specialized hardware that reduces latency.
On the flip side:
- Detecting and interpreting errors is more challenging
- It requires way lower error rates in the physical qubits
- The hardware architecture is also tailored for this technique
Is a very strong commitment, so it better works as expected!.
As you can imagine, making such an important decision requires extensive experimentation, prototyping, and an organization designed for rapid hypothesis validation. This gives us high confidence in the chosen path. More importantly, it allows us to quickly adjust course if needed.
These new fault-tolerant quantum computers are not the end of the journey–they’re the beginning of the truly useful era for quantum computing.
They are not initially going to solve all kinds of problems… I know what you are thinking… Don’t worry, your bitcoins are going to be safe for many years to come.
They are definitely going to solve some of today’s intractable problems, though.
I can’t wait to see how this technology unfolds—and how it shapes science, industry, and everyday life.
For me, it’s already having a great impact :).
This content originally appeared on DEV Community and was authored by Juan Gómez