Ron Cohen

17 ביוני 2022

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To those of you who are **familiar **with Quantum Computing, and to those who are just **curious**, I would like to introduce you to the fascinating world of **Quantum Error Correction.**

The big buzz around Quantum Computing is very nice, but what many people don't know, is that Quantum Computing, just like Classical Computing, **will not be possible without the core idea of Error Correction.**

**Noise and Errors**

Our world is noisy, and noise is causing communication to **suffer from errors**. Think of yourself trying to hear your roommate while you are taking a shower, or try talking on the phone while your friend is shredding paper.

In classical communication, noise is added to 0 / 1 bits and may lead to **incorrect 0 / 1 identification**. The solution to these errors is **encoding**. For example, you can replace every 0 with four zeros. In this way, even with some low chance of 0-1 flipping, the **decoder **will understand if the origin was 0 or 1.

Qubits (quantum bits) are suffering from similar issues. Qubit, __like a bit__, is a creature that stores a basic unit of 0/1 information. __Unlike a bit__, a qubit can store both **0 and 1 at the same time** (a **superposition **of 0 and 1), where each of the states is stored with some **probability of being determined **as 0 or 1 at the time of measuring the qubit.

When noise is added to a qubit, those 0 / 1 probabilities are changed. The values of those 0/1 probabilities are **crucial for quantum computation**. The interesting algorithms that are using those qubits are **relying **on those probabilities information. Therefore we cannot allow for errors to misrepresent these crucial probabilities.

**How is it affecting the whole Quantum Computer?**

The noisier a quantum computer is, the **more qubits it will need to compensate for its noise**. Think of the classical repetition code that I mentioned just earlier, where the addition of extra bits helped us mitigate the introduction of noise.

Ok, but how many more qubits? It may reach up to **thousands of times more than the silent case**! Let's see an example. You might have heard of Shor's Algorithm which is a quantum solver that can crack RSA encryption by decomposing numbers into primary factors.

To decompose a 2000 bits number, in ~24 hours Shor's algorithm will need a total amount of ~300,000 silent qubits (logical qubits). For today's noise rate of 1/1000, each one of these 300,000 **logical qubits will be replaced by 3,600 physical qubits**, which will result in **~1 Billion (1Giga) physical qubits!**

It might sound huge, but don't forget that a classical computer hard disk contains ~x1000 times bit than 1 Billion. Anyway, the only reason that those algorithms are **even possible, is only thanks to the Quantum Error Correction schemes**.

**The Ugly Duckling**

Quantum Computing has received much attention around the globe. **The spotlight is focused mainly on advanced applications** like simulating molecules, solving optimization problems, financial use cases, and quantum machine learning.

*"The quantum community proceedes under the assumption that it already has a fault-tolerant quantum computer, while the reality is that we are far from it."*

The community assumes that each logical qubit is one silent physical qubit, somewhat overlooking the reality of noise. **This wrong way of thinking has huge implications on products are designed**, whether it is hardware, algorithm, or software.

Founders have invested more than 100,000,000$ to develop applications that may (or may not) be relevant just in more than 10 years. Yet **there is a severe lack of knowledge, courses, and infrastructures** among researchers, engineers, and developers about **the foundation concepts surrounding the creation of a fault-tolerant quantum computer.**

The **ugly duckling of error correction needs more attention.** Don't forget who saved the chicks at the end of the story.

Soon, quantum computer companies will start to publish that they have a fully connected quantum computer with more than 1000 physical qubits. Don't forget that this is **equivalent to only one logical qubit!** In order to build useful architectures for a quantum computer, those **companies will have to dive into the world of error correction and include it as a part of their solution.**

**So, are you a chick or a duck? **Would you like **to be among the ducks** that know how to build a **fault-tolerant quantum computer**? In the next article, I will explain the **fundamentals **of Quantum Computing Theory that are a prerequisite **to understanding how Quantum Error Correction works**.

*Stay tuned... Don't be a chick...*

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