From an engineering point of view, quantum computers are very interesting beasts. It’s something new, shiny, you don’t understand it, but it seems to be the future. So you must go there! This is a collection of links that I found very helpful in understanding how quantum computers work and what they can do.

First of all, you need to read this:

Now you’re ready to learn that qubits alone are not very much fun, what you need are gates to make these qubits interact with each other – put 1 qubit into a superposition of 0 and 1, and entangle two or more qubits. Of course Wikipedia is your friend, but I found this lecture from Anuj Dawar to be specially appealing to my need of understanding without getting into all the gory details.

Digging the Bloch-sphere? Know how to say “Hadamard Gate”? When you read “Toffoli Gate”, do you know it’s enough to build an universal quantum computer? OK, let’s move on to some real quantum computers. First of all, according to Federico Carminati from CERN Openlab, none of the existing technologies at here will actually make it into the universal quantum computer! Except, perhaps, the topological qubits from Microsoft. But so far they only exist on paper…

IBM is making progress and replacing the qubit-race with a quantum-volume race. Their goal: doubling the quantum-volume every year. They have a quantum-computer with some qubits readily available on the cloud, so you can play with it. But what does it actually use for the qubits? How are the gates made? I found this explanation a nice level that helped me quench my thirst for understanding.

TLDR: you create a transmon qubit by trapping a Cooper Pair of electrons in a superconductor, control it by applying a certain frequency (5-20GHz) to it. Depending on the frequency you can reset the qubit or apply single-qubit gates to it. The qubits are only linked to their neighbors, and this link is also controlled with the applied frequency. If you want to create an entanglement between two qubits that are far away, you need to use intermediate qubits. So the IBM quantum chips don’t link all qubits with all other qubits, only some direct connections are available. But they are enough to implement a general quantum computer. Their biggest universal quantum computer has 20 qubits.

And Google’s quantum supremacy? Well, as far as I understand, they got 52 qubits together for a PR stunt. But from what I understand, they don’t have a universal quantum computer with 53 qubits.

DWave says they have 1000’s of qubits – what about this? Well, they don’t have a universal quantum computer. “Only” the possibility to do simulated annealing, which is interesting if you want to make a gradient descent in big datasets.

What would you need to crack Ed25519 encryption that is used in current https-connections? With high-quality qubits, 1530 seem to be enough. If you don’t have those (nobody has them yet), then you need something like 20 million noisy qubits. Of them, IBM currently has 20. So if IBM doubles the qubits every year, in 20 years ed25519 can be cracked…