# Quantum ComputingA universe in a photon

In my last post, From Farnam Street to Quantum Computing, I

threatenedpromised to write about.

Quantum Country, a new kind of book that happens to be about quantum computing.

This morning I meandered over to Quantum Country and learned something so delightful and astounding (to me) that I had to write about it.

## The short form

You can encode either a zero or a one in a classical bit.

In a quantum bit (qubit) you can encode an almost infinite amount of information.

Really!

One qubit in a photon could contain the universe.

Of course, there’s a practical problem. But never mind. It’s possible.

## The details

In everyday computing, a bit has a value 0 or 1. In quantum computing, a qubit has a value

`α∣0⟩+β∣1⟩`

Where `∣0⟩`

is the `computational basis state,`

analogous to the classical 0 state, and `∣1⟩`

is the computational basis state analogous to 1, and α and β are complex numbers. α and β can take on any values, provided that

∣α∣2+∣β∣2=1.

What do α and β mean?

Well, that’s tricky.

But one interpretation is that they give the probabilities that, when measured, the qubit will yield a classical 0 or 1.

The probabilities are, respectively ∣α∣2 and ∣β∣2.

α could anything. It could be a string of digits like 0.113117097110116117109032099111109112117116097116105111110032105115032097119101115111109101

which could be decoded to the string `quantum computing is awesome.`

Alpha could also be a much, much, much longer string that could encode the entire current contents of the World Wide Web.

The point is: a single qubit could encode anything.

In theory.

## The problem

There are just three problems.

The first: it’s exponentially (or worse) difficult to encode increased amounts of information in a qubit.

The second: once you’ve got it a known state, it’s tough to keep it there.

And finally, once you’ve got the information encoded, it’s impossible to find out what it was.

But never mind that! See this qubit here? It could contain all the secrets of the universe.

Isn’t that cool?

## Actually, there’s something there…

Like matter and energy, information can’t be created.

As David Deutsch says, knowledge is a particular kind of information. So all the knowledge that the world has obtained so far—whether in books or in DNA—must have been present at the beginning of the universe, in the remnants of the big bang.

All the knowledge of the universe—all the knowledge that could ever be obtained, inefficient encoded—must have been there in the beginning. It’s just taken all this time to decode it.

Deutsch argues that we can construct a world like our own out of any sufficiently large (multiple-star-sized) amount of hydrogen.

And in that world, intelligent creatures could evolve.

And they could do science and collect knowledge comparable to and transcending our own.

So all the knowledge that we have and could obtain must be encoded in any sufficiently large amount of hydrogen.

Must be.

I’m not saying it would be easy to decode it.

But it’s possible.

And it’s amazing. Because, as I’ve written before, I’m Star Stuff and We are each at the center of the universe, made of knowledge, in bodies of once-living stars.

And here I am, surrounded by all the knowledge that has ever been and will ever be.

And maybe even made out of it.