I consider simulating a coin flip (aka creating a so-called “even superposition”) to be the hello world of quantum computing. In a quantum programming language called Quil, this looks like

    DECLARE result BIT
    H 0
    MEASURE 0 result

This will put a 0 or a 1 in result with equal probability. ‘H 0’ is performing a Hadamard operation on qubit 0. The MEASURE allows us to extract a classical bit value out of a qubit.

Others might say the “Bell pair” is the hello world, and that looks like this:

    H 0
    CNOT 0 1

This is a bit harder to explain, because it requires understanding both superposition and entanglement.

I am just being annoyingly pedantic, but I do not think a Clifford circuit (both of your examples fall in this category) is a good quantum hello world, because they are trivial to simulate efficiently on a classical computer. But I do not have better suggestions.

I don't think a "hello world" type of program/circuitry(?) needs to be unique to a quantum computer. It just needs to be expressible with a quantum circuit.

Similar to how you can have hello world in both ruby and python, but arrive at them with different syntax and compilers.

And displaying "Hello World" can be done trivially with a pencil and paper. That doesn't make doing so in a classical programming language devoid of value.

Sure, but there is value in a "Hello World" being focused on what is special about the hardware/language. Look at Haskell and OCaml tutorials for instance: they all explicitly say stuff like "let us start with the 'Hello World' of functional languages, namely, implementing the Fibonacci (or factorial) function recursively".

Isn't that the case for literally anything you could run on the current state of the art quantum computers?

The troll answer of simulating a qbit using a qbit excluded.

I think part of the problem (although not all of it) is that quantum computers are at a phase that's analogous to writing a "Hello World" program in very rudimentary assembly. Even ignoring the quantum part, there's still a big gap.

I guess the closest thing to a quantum hello world would be a simple quantum circuit—each line represents a qubit register, and various types of gates operate on the qubits.

To see quantum circuits that do something useful, check out the "Example Circuits" links that show up in the Quirk simulator: https://algassert.com/quirk

via https://algassert.com/2016/05/22/quirk.html via https://news.ycombinator.com/item?id=11752421

Quantum teleportation is a somewhat simple but non-trivial procedure: https://qiskit.org/textbook/ch-algorithms/teleportation.html

A controlled-NOT gate (CNOT) can be thought of as addition mod 2: https://en.wikipedia.org/wiki/Controlled_NOT_gate

I think a simple simulator would go a long way, using random numbers in a language like Python. It doesn't have to be fast or efficient, just be able to proceed through some basic operations while we watch it evolve.

Check @ahelwer comment on this page. The youtube video is excellent.