quantum-computing
12 free lessons tagged quantum-computing across Science. Each one is a short sequence of focused steps with narration and a five-question quiz at the end — take them in any order, no signup required.
Why Nobody Deploys It: The Gap Between Proof and Product
QKD has an unconditional security proof and almost no deployment. This lesson covers the authentication bootstrap it cannot solve, distance limits and the trusted node compromise, attacks on real hardware that the proof does not cover, why NSA and NCSC recommend against it, and where quantum genuinely delivers.
Quantum Key Distribution: BB84 and Why Eavesdropping Shows
Quantum cryptography uses physics rather than computational hardness. This lesson covers the no-cloning theorem, the BB84 protocol step by step, why measurement in the wrong basis leaves a detectable trace, the error rate threshold, and the entanglement-based alternative.
Quantum algorithms: where the speedups actually are
What Shor, Grover, and Hamiltonian simulation really promise, how big each speedup is once error-correction overhead is paid, why post-quantum cryptography exists regardless of timelines, and how to evaluate any claimed quantum application.
Quantum error correction: from noisy qubits to logical qubits
Why quantum errors are uniquely hard, how stabilizer codes detect them without destroying the data, what the surface code and code distance mean, and why crossing the threshold turned error correction from theory into the field's central engineering race.
Quantum hardware: how qubits are actually built
The four leading ways to build a qubit, superconducting circuits, trapped ions, neutral atoms, and photons, and the engineering trade-offs between speed, fidelity, connectivity, and scale that define each platform.
Qubits: superposition, measurement, and entanglement
What a qubit actually is, why measurement destroys superposition, how entanglement links qubits, and how the quantum circuit model turns these ingredients into computation.
Post-quantum cryptography: lattices, codes, and the migration
What cryptographic schemes Shor's algorithm threatens, what post-quantum schemes replace them, the math behind lattice-based cryptography, the NIST standardization process and its outputs, and the operational mechanics of a real-world cryptographic migration.
Algorithms where quantum beats classical (and where it doesn't)
Shor, Grover, Hamiltonian simulation, HHL — the catalog of known quantum-algorithmic speedups, what 'speedup' precisely means in each case, and the structural reasons most problems do not gain exponential advantage.
Errors, syndromes, and the surface code
Why classical error correction does not directly transfer to qubits, how stabilizer codes and syndrome measurement work around the no-cloning constraint, the surface code as the leading approach, and the math of physical-to-logical qubit overhead.
Hardware approaches: superconducting, ion, photonic, atomic, spin
Six families of physical qubit implementations and the engineering trade-offs that distinguish them — coherence time, gate time and fidelity, scalability, and control complexity. The numbers behind 'which is best' depend on which metric you care about.
Entanglement, gates, and the circuit model
What entanglement is mathematically, how Bell states are built from Hadamard and CNOT, how quantum circuits compose, and why measurement on entangled subsystems looks correlated regardless of separation.
Superposition and the qubit
The mathematical object behind a qubit — a complex unit vector in a two-dimensional Hilbert space — and why measurement collapses superposition. The structural difference between a quantum state and a classical bit, expressed in math.