Quantum Physics from Zero
The real physics inside quantum computers, from zero: energy packets, the double slit, amplitudes, measurement, uncertainty, spin, atoms, tunneling, entanglement and decoherence — each experiment explained simply and modelled with a circuit you run.
"Quantum" means a fixed, countable amount. In 1900 Max Planck found that glowing objects only trade energy in small packets, and the size of each packet is Planck's constant times the frequency: E = h × f. Energy comes in steps, like a staircase, not smoothly like a ramp. That one idea is the root of every quantum computer.
Light travels and spreads like a wave, but it is always detected in whole packets called photons. In 1905 Albert Einstein used Planck's packets, E = h × f, to explain the photoelectric effect: one photon knocks out one electron, and only if that photon carries enough energy. Brighter light means more photons, not bigger ones.
Send particles through two slits one at a time and each lands as a single dot, yet thousands of dots build up bright and dark stripes, the mark of waves that add and cancel. If anything records which slit each particle used, the stripes vanish. The pattern needs the two paths to stay unrecorded so they can interfere.
Quantum physics gives each possible outcome an amplitude, a number that can be positive, negative, or even complex. The chance of seeing that outcome is the amplitude squared. This is the Born rule, found by Max Born in 1926. Because amplitudes can be negative, two paths can cancel, which plain probabilities never can.
A superposition is one definite quantum state whose amplitudes are spread over more than one outcome. It is not "0 and 1 at the same time," and it is not a hidden coin that already landed. You can prove the difference: a superposition can be turned back into a certain answer by interference, while a random coin cannot.
A measurement is a physical interaction that leaves a lasting record, such as a detector click. It gives one outcome at random, with chances set by the Born rule, and afterwards the system matches that outcome, so measuring again the same way repeats the answer. It is the record that matters, not a human mind.
Some pairs of properties, such as position and momentum, cannot both have sharp values in the same state. Werner Heisenberg found this in 1927; the precise rule is that the spread in position times the spread in momentum is at least ħ/2. It is not just clumsy measuring. It is built into how quantum states work, and qubits show it with two clashing questions.
In 1922 Otto Stern and Walther Gerlach sent silver atoms through an uneven magnet and saw them split into exactly two spots, not a smear. The cause is spin, a built-in magnetism of the electron that always measures as one of two values along any axis you pick. Magnets in a row show that a new question erases the answer to an old one, just like a qubit.
An atom can only hold certain energies, its energy levels, so it gives off and takes in light only at matching colors. Shine light tuned to the gap between two levels and the atom swings smoothly from the lower level to the upper one and back, a Rabi oscillation. That controllable swing is exactly how qubits made from atoms and ions are steered.
A quantum particle can cross a barrier it does not have the energy to climb, because its wave does not stop dead at the wall. The wave fades inside the barrier, and if the barrier is thin some of it leaks out the far side. Tunneling powers the Sun, the scanning tunneling microscope, and the Josephson junctions inside superconducting qubits.
Two particles are entangled when their shared state cannot be split into a separate state for each one. Their results are then linked more strongly than any pre-set plan could explain, which John Bell turned into a testable limit in 1964. Experiments by Clauser, Aspect, Zeilinger and others broke that limit, earning the 2022 Nobel Prize, yet entanglement never lets anyone send a message faster than light.
Decoherence happens when a system's surroundings, such as air, light and heat, keep picking up records of its state. Each record acts like a tiny which-path detector, so the amplitudes can no longer cancel and the system behaves like an ordinary object. Big things decohere almost instantly, which is why a cat is never seen in superposition, and slowing decoherence is the central engineering problem of building qubits.