Everything is a wave, until you look.
Quantum mechanics is the most precisely tested theory in science, and also the strangest. Particles spread out like waves, distant particles share outcomes, and looking changes the result. Quantum computers turn that strangeness into a new kind of computation.
The double-slit experiment
Fire particles one at a time at two slits. Each lands as a single dot, yet together the dots build stripes: an interference pattern, as if each particle went through both slits as a wave. Richard Feynman called it the only mystery of quantum mechanics.
A qubit you can rotate
A classical bit is 0 or 1. A qubit can be any point on a sphere: the north pole is |0⟩, the south pole is |1⟩, and everywhere else is a superposition. Gates rotate the arrow; measuring snaps it to a pole at random, weighted by how close it was.
Drag the sphere to turn it
Spooky action: the Bell test
Einstein hoped entangled particles carried hidden instructions from the start. In 1964 John Bell found a way to test that: any “hidden instructions” theory gives correlations of at most 2 in the CHSH test, while quantum mechanics predicts about 2.83. Experiments side with quantum mechanics, which won Aspect, Clauser and Zeilinger the 2022 Nobel Prize.
Grover’s search: finding a needle faster
Find one marked item among 16 by looking at them one at a time and you need 8 tries on average. Lov Grover’s 1996 algorithm needs about 3. It starts with every item equally likely, then repeats two steps that pump probability into the right answer.
Tap a bar to choose which item is marked
The strange rules, one by one
| Interpretation | What happens when you measure? | Cost |
|---|---|---|
| Copenhagen (Bohr, Heisenberg, 1920s) | The wavefunction “collapses” to one outcome. Don’t ask what happens in between. | Never says what counts as a measurement |
| Many-worlds (Everett, 1957) | Nothing collapses. Every outcome happens, each in its own branch of reality. | An unimaginable number of unseen worlds; probability is hard to explain |
| Pilot wave (de Broglie 1927, Bohm 1952) | Particles always have positions, guided by a real wave. Fully deterministic. | Instantaneous influences across space; hard to square with relativity |
| Objective collapse (GRW, 1986) | Collapse is a real, random physical process that big objects trigger quickly. | Changes the theory slightly; experiments are hunting for it |
| QBism and relational views | The wavefunction is information or belief, not a physical thing. | Leaves open what the world is like underneath |
From strange physics to a new kind of computer
A quantum computer isn’t a faster ordinary computer. It is a machine for steering interference. For a few problems that is dramatically better; for most everyday tasks it offers nothing.