June 23, 2026
Epilogue: measure and meaning
The very first useful thing our simulator did, back in post 0, was turn a vector of amplitudes into a vector of probabilities.
Quantum-Computing
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April 10, 2026
The engine, in full
Across the series I kept saying the simulator was small: a qubit is an array, a gate is a matrix, the whole thing is a few hundred lines.
January 26, 2026
Quantum error correction
Last post was the bad news: a real qubit leaks its coherence into the environment and forgets what it was doing.
November 13, 2025
Noise and decoherence
Last post ended with a warning: the off-diagonal coherences of a density matrix are the fragile part, and a real qubit loses them on its own.
August 30, 2025
Mixed states and the density matrix
Every state in this series so far has been a single vector with definite amplitudes.
June 17, 2025
Scaling the simulator
This post does not add a quantum idea.
April 4, 2025
Shor's algorithm
This is the one everyone has heard of: the algorithm that factors integers in polynomial time and, if a big enough quantum computer is ever built, breaks RSA.
January 20, 2025
Phase estimation
The last post built the Quantum Fourier Transform and promised it was a readout instrument.
November 7, 2024
The Quantum Fourier Transform
The last three posts built circuits whose payoff was a single global fact read out by interference.
August 24, 2024
Grover's search
Deutsch-Jozsa and Bernstein-Vazirani solved artificial promise problems.
June 11, 2024
Circuits and interference
So far the qubits have mostly sat still.
March 29, 2024
Many qubits, and entanglement
In post 0 a qubit was a unit vector in $\mathbb{C}^2$, and everything about it fit in a length-two array.
January 15, 2024
What is a qubit?
I wanted to understand quantum computing properly, which for me means building the thing rather than driving a framework that does the linear algebra in the basement and hands back an answer.