## Tuesday, 26 January 2016

### 2 - Storing Quantum Information

This is part of a project to get people involved with quantum error correction. See here for more info.

Storing Information

Last time we used error correction to help us send a message without getting messed up by errors. Here we will consider saving a message, for which we can also use error correction.

Suppose we have an important meeting, and need to remember the details of the appointment. Naturally we can write it down, but the paper can get lost or dirty. Is the information secure enough? If not, we can use error correction.

Let's use the repetition code, just like last time. We take multiple pieces of paper and write the time of the meeting on each. They are all then put in a safe place.

On the day of our meeting we gather up the pieces of paper. Maybe a couple have gotten lost, and some are too dirty to read. But the majority are probably fine. So we can get the information back using the same decoding as last time.

What if the meeting is in a year? More time means more errors. More of the pieces of paper will get lost, dirty or otherwise messed up. If it happens to too many, we won't be able to get the information back. What can we do?

We can expect that there won't be too many errors in just a week. So every week we can collect the pieces of paper, see what the majority is saying and replace everything else. Then no error will survive for long. The information will stay secure for a year or even more.

Quantum Information

Our current information technology* uses normal kinds of information like numbers, letters and pictures. The information technology of the future will use a new kind of information alongside this: quantum information. This follows the unusual rules of quantum mechanics. With quantum information, new methods for computation, communication and cryptography will become possible.

The magic of quantum information needs quantum superpositions. Normal information, for example, is either '0' or '1'. When it is 0 it is not 1, and vice-versa. We call this a bit. With quantum mechanics it can be 0 and 1 at the same time, and the two can influence each other. This is a quantum superposition. It continues until we measure it. Then it must decide which to be. We call such a quantum bit, a qubit.

A qubit is more complicated than a bit. They allow more complicated information technology. But they are fragile. So how can be protect our quantum information? With quantum error correction!

The problem of quantum error correction

As we've seen, storing normal information means looking at it often enough to catch the errors as they happen. With quantum information, that is not a good idea. As soon as we look at our qubits, the quantum superposition is gone. So instead of making things better, we've made them worse. We need to be careful to only measure exactly what is needed to catch the errors, and nothing more. But how to do this? We'll find that out soon. But next we'll look at exactly what this quantum stuff is all about.

* That's what IT stands for, by the way. It's not 'Internet Things'.