Technology

Sounds like we are getting closer to finding the question to 42.

Doesn't sound all that useful to have an answer with knowing then question.

Computer: Yes but only after 2:30pm. And on Tuesday, obviously.

Me: Er?

It's great you learned about entanglement! There's a bit more to it though :) (also sorry, this got long, I got a bit carried away)

Have you had any linear algebra at all? It's the best way I can explain it without introducing any "quantum" terminology.

Basically, with our normal digital computers we operate on bits that are 0 or 1. We read those bits, store those bits, shift them, flip them, etc. Quantum computers, on the other hand, are more like analog computers, but with a twist.

Quantum bits (qubits) are not represented by a single binary digit. Instead, they are represented by two complex numbers (a vector) [a, b] with |a|^2 + |b|^2 == 1. You can represent this visually as a point on the surface a sphere with radius 1. A qubit's state can be any point on that sphere. That's the notion of superposition. When measuring a qubit's state however, nature forces us to choose just one axis along which the qubit can point. So let's say we choose the up/down axis, and we make the qubit point somewhere perpendicular to that, we will measure either a 0 or a 1 with 50% probability.

Entanglement means two (or more) qubits are strongly correlated. To represent the state of two qubits, let's extend our vector to four complex numbers [a, b, c, d] (still with norm 1).

• [1, 0, 0, 0] means "qubit A and qubit B are both pointing up on their spheres",
• [0, 0, 0, 1] means "A and B are pointing down".
• [0, 1, 0, 0] means "A points up and B points down"
• [0, 0, 1, 0] means "A points down and B points up"

And any combination of those is possible. This state for example: [1/√2, 0, 0, 1/√2] means: "A and B point up together and A and B point down together". A and B are entangled. Nature doesn't let us observe this directly, but we can measure it statistically by repeating an experiment over and over. And the measurement results should show that A and B always point in the same direction (in a perfect world, that is). And while we can't ever observe entanglement directly, we know that these states are possible and that we can use them.**

So how do we use these qubits? We rotate them and make their rotations correlate and then rotate them some more and... we measure. And repeat this a bunch of times. In mathematics world this means we multiply these vectors above with so-called unitary matrices. In the physics world this means we send microwaves and/or RF waves and/or laser light at the qubits.

What's the use of all this? Well, for one it's much closer to how nature actually works and we can use it e.g. to simulate complex chemicals. Plenty of use-cases in machine learning are also being researched (as you can imagine, large matrices with floating point numbers map quite well to qubits).

As for what this article means though... to non-researchers, not much. Qubits are analog and thus they (and operations on them) have errors. Especially today's qubits. This means you need 1000s of qubits that still must be of much higher quality than today's qubits to do anything remotely practically useful (and we try not to think about this too much...).

** Side-note: I should also mention the small mistake in your explanation. Changing one qubit's state doesn't change it's entangled counterpart. Let's stick with the example entangled state. If I rotate A to point, let's say, along the "left/right" axis, then the state could be "left/up and right/down". I.e., only A's state rotates. If this were otherwise, faster-than-light communication would be possible.