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Quantum from Scratch · Episode 12 of 19

Quantum circuit

What Is a Quantum Circuit? How to Read One

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What I say in the video

This is what actually runs on a quantum computer — not code, not an app, this: a diagram made of lines and boxes.

Each line is a qubit, each box is an instruction — flip it, mix it, tangle it up with another qubit — and read left to right, you're watching a qubit's state get built one move at a time, like sheet music except the notes are physics.

This is called a quantum circuit, and here's the part that matters: every algorithm you already know from this series — Grover's, the ones running on Google's and IBM's machines right now — is just a longer version of this exact diagram.

So when a headline says "a quantum computer ran an algorithm," this picture is literally what got run.

Learn to read one of these, and you stop watching quantum computing from the outside.

In plain words

What actually runs on a quantum computer? Not an app, and not the code itself: a quantum circuit.

How to read a quantum circuit in this Short:

  1. Each horizontal line is a qubit.
  2. Each box is a gate, an instruction: flip it (X gate), mix it into superposition (Hadamard gate), or entangle it with another qubit (CNOT gate).
  3. Read left to right, like sheet music. You're watching the qubit's state get built one step at a time.
  4. Algorithms like Grover's search are written as longer versions of the same diagram, then compiled to the machine's native gates and run on gate-based hardware from IBM and Google.

Note: this is the gate-based model. Quantum annealers (D-Wave) solve optimisation problems without circuits.

What I simplified

Quick precision: this is gate-based quantum computing (IBM, Google). Quantum annealers like D-Wave don't run circuits; they solve optimisation problems a different way. That was Ep 9. Source for "read it like sheet music": IBM Quantum Learning, "Running quantum circuits".

Sources (6)
  1. IBM Quantum Learning, "Running quantum circuits"quantum.cloud.ibm.com
  2. IBM Quantum, Basics of Quantum Information, "Circuits"quantum.cloud.ibm.com
  3. Google Quantum AI, Cirq conceptsquantumai.google
  4. quantum.cloud.ibm.com
  5. quantum.cloud.ibm.com
  6. dwavequantum.com

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