# Review of Quantum Country

Yesterday, I wrote about how Quantum Country is helping me understand what books are for. As a recap, QC is a series of essays about quantum computing. You should think of it like a text book—they are not trying to write a pop-sci, hand-wavy, high-level overview, but a serious science/math book suitable for a college level class. Basic linear algebra is a pre-requisite to understanding it.

The book is also an exercise in a new form they call the “mnemonic medium”. Embedded throughout the text are interactive flash cards that test you on the material you just read. Depending on how you rate your own memory, they will used spaced-repetition algorithms to show you cards more or less often—the aim is to show you a card just before you will forget the answer. Using this system, you should be able to remember the content indefinitely (and need to visit and test yourself less often over time).

Does this work? I was tempted to review the book by just listing out the facts I remember, but honestly I think that would be boring to read. I did do that exercise, and I can remember quite a bit right now, but I just finished the book last week. I read it fairly slowly over six weeks and did the cards as prompted. I also did a few of the proposed exercises (which involve linear algebra), but not all of them.

In addition to the mechanics of quantum computing, I do think I also understand the higher level “point” of it. I don’t think I could make my own circuit to do something, but I could do the math to analyze a circuit. I have also gone on youtube and watched the more challenging presentations on quantum computing (the similarly math-y ones) and was able to follow them quite easily. If the “What is it for?” for Quantum Country is to teach the reader quantum computing, I can say that it was successful at that for me.

Here is a motivating example of why you’d want to know about quantum computing. In quantum computing, the fundamental unit of computation is a qubit (or quantum bit). They are quite a bit harder to make than our classical bits, but a system of N of them can represent 2^N states, and a state change on that system (affecting every qubit) can be done in constant time.

We are currently extremely limited in the number of qubits we can make and keep stable. Reading the result of the system is noisy and imperfect. However, we have still been able to do computations that are impossible on classical computers (just because of sheer size). This feat is called quantum supremacy, and has been achieved by a few research groups.

The essays include a description of a quantum computing search algorithm that can (probabilistically) find a value in an unordered list in O(π/4*(sqrt(n))) time (instead of O(n)). Just knowing that that is possible and the basics of how it is done was worth the read to me (even though I can’t say I fully understand it).

So, if you can multiply matrices, I would say to read Quantum Country enough to see if it interests you. I would at least try to get through some of the embedded flash cards to experience the mnemonic medium.