Quantum Computing: an interactive course

Published 8 Nov 20265 min read

Quantum Computing is an interactive quantum computing course that explains how quantum computers work, from a single qubit to the algorithms that make them interesting. Its 25 short sections cover qubits and amplitudes, measurement and collapse, the Bloch sphere, quantum gates, two qubits and CNOT, entanglement, interference, Grover's search, Shor's algorithm and encryption, noise and error correction, today's hardware and what quantum computers cannot do.

It opens with a simulated qubit: apply an H gate to give it a 50/50 chance, then measure it 200 times and watch the results fill a histogram. Every later idea builds on that picture.

What the course covers

Sections	Topic	What you do
1 to 5	Qubits, amplitudes, chances and measurement	Turn amplitudes into chances, then predict what measuring gives
6 to 9	The Bloch sphere and quantum gates	Sort states by result, then build a circuit that reaches a target state
10 to 13	Two qubits, CNOT and entanglement	Trace a short circuit, then predict an entangled pair
14 to 15	Interference	Add the amplitudes of two paths, then square for the chance
16 to 19	Grover's search and Shor's algorithm	Count Grover's rounds, then sort security methods by how a quantum computer affects them
20 to 22	Noise, decoherence and error correction	Read parity checks and find the flipped qubit
23 to 25	Hardware today, and what quantum computers cannot do	Match each kind of hardware to how it holds its qubits

Qubits and measurement

A qubit's state is described by two amplitudes, one for each result, 0 and 1. Squaring an amplitude gives the chance of that result. Measuring gives one result at random, with those chances, and the qubit then stays as that result. The Bloch sphere pictures any state as an arrow, and gates turn the arrow.

\left|\psi\right\rangle=0.6\left|0\right\rangle+0.8\left|1\right\rangle

What makes it quantum

With two qubits, a CNOT gate after an H gate creates entanglement: measuring one qubit tells you the other's result. Interference is the real source of speed. Amplitudes, unlike chances, can be negative, so paths to wrong answers can cancel while paths to right answers add up.

Algorithms, and limits

Grover's algorithm finds one marked item among N in about √N rounds instead of up to N checks. Shor's algorithm could factor large numbers quickly, which would break RSA encryption; that is why NIST has published new post-quantum encryption standards. The course is honest about the limits: qubits are fragile, error correction needs many physical qubits for each reliable one, and a quantum computer does not simply try every answer at once.

Try a question from the course

For the state above, the amplitude of 0 is 0.6 and the amplitude of 1 is 0.8. Square each to get the chances: 0.6² = 0.36 and 0.8² = 0.64. So measuring gives 0 with a 36% chance and 1 with a 64% chance, and the two always add to 1.

Now work out the chances for a new state, then press Mark to check your answer in the course.

Press Mark to have your answer marked in the Quantum Computing course.

Who this course is for

Curious learners who keep hearing about quantum computers and want to know what they really do.
Physics, maths and computing students who want an intuitive start before a university course.
Developers and IT professionals who need to understand what quantum computing means for encryption.

What you need to know first

Squaring numbers, square roots and simple probability. No physics degree or linear algebra is needed: the course uses the Bloch sphere and short calculations instead of matrices. The Probability course helps if chances are new.

How long it takes

There are 25 sections and most take a few minutes. Learners without a plan can open two sections of a course a day, in order, so the whole course takes 13 days. A plan removes the daily limit and lets you go straight to any topic, such as entanglement or Shor's algorithm.

Guided courseCryptocurrenciesSee the cryptography quantum computers threaten at work: hashes, digital signatures and blockchains.

Common questions

Is this quantum computing course free?

You can take every section without paying, two sections a day, in order. A plan removes the daily limit and opens any section straight away.

What is a qubit?

The basic unit of a quantum computer. Like a bit it gives 0 or 1 when measured, but before measurement its state has two amplitudes that set the chance of each result.

What is superposition?

A qubit state with non-zero amplitudes for both 0 and 1. It does not mean the qubit is secretly both; it means measuring it gives one result at random, with chances set by the amplitudes.

What is quantum entanglement?

A link between qubits where their measurement results are correlated in a way no separate states can explain. In a Bell state, measuring one qubit tells you what the other will give.

Will quantum computers break encryption?

A large, error-corrected quantum computer running Shor's algorithm could break widely used public-key encryption such as RSA. No such machine exists yet, and new post-quantum encryption standards have been published to replace the methods at risk.

Do I need advanced maths for this course?

No. You need squares, square roots and simple chances. The course pictures states on the Bloch sphere instead of using matrices.

Revision cards

Say the answer first, then flip the card to check it.

Where to go next

Learn it
Quantum Computing
Qubits, superposition and the computers of tomorrow
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