## Jeremy Côté

Bits, ink, particles, and words.

# Virtual CNWW

If you want to learn a topic today, the resources are much more plentiful than even a few decades ago. The internet has given us wonderful resources to learn from, including some which leverage internet technologies to provide animations and teach topics in a much more interactive way. This is particularly true for mathematics and physics, which have been entrenched in dry textbooks that are a chore to read1 for much too long.

1. Though there are some books which are a delight to read. I’m not against the medium, but I am against limiting ourselves to it.

# Pick a State, Any State

Hilbert space is big.

No, not big like the how the Earth is big compared to you. Rather, Hilbert space is astronomically big. Actually, that’s not quite right either. It’s bigger than that. I guess the best adverb I can use is that it’s mathematically big. In a Hilbert space, you tend to have a lot of room to maneuver. (To read more about that, check out my essay, “The Curse of Dimensionality”.)

In the last essay, we saw how to pick a random unit vector in a N dimensional space. This wasn’t for nothing, and now we are going to put that knowledge to use to ask the following question:

If I have a line of N qubits and randomly pick a state from the Hilbert space, what kind of entanglement will it have?

# All in the Corners

As a quantum theorist, I spend a lot of time thinking about high-dimensional spaces. These are the playgrounds for quantum many-body systems, and they are vast. The technical name is a Hilbert space, and it’s the space of complex vectors with the additional structure of a way to put vectors together (called an inner product).

Hilbert space is big (see “The Curse of Dimensionality”), but the usable area for quantum theory is often much smaller. This means we are stuck finding corners of a high-dimensional space that describe physically-relevant phenomena.

A very important tool in my field is entropy. It can characterize the amount of entanglement between quantum states, which lets us talk about systems that produce behaviour far away from our usual ideas.

Recently, my supervisor and I were preparing homework for a class I help teach, and we wanted students to investigate the amount of entanglement1 present in a random quantum state of N qubits. This was a numerical exercise, and to complete it they needed to produce random unit vectors (because quantum states are normalized). This got me thinking about how to actually choose random unit vectors.

1. The measure we were interested in is called the entanglement entropy. It works like this. If you have a quantum state over a composite system (like many particles), you first divide it into two parts. Then, you can compute the reduced density matrix on one of those states, which is just a way to figure out the quantum state on one part by itself. Then, if you calculate the von Neumann entropy of that state, you get the entanglement entropy of the original quantum state. It also turns out that once you separate the composite system into two parts, it doesn’t matter which one you use to calculate the entropy. The result will be the same.