Measurement changes everything
Lesson 3 of 23 in the course “Quantum”. Free, with animations and a quiz.
Measurement gives one answer
Before measurement a particle has amplitudes for different outcomes. At the moment of measurement exactly one outcome comes up, at random, with the chance “amplitude squared”. And here’s the key part: the state changes afterwards. If you got 0, the particle is now simply 0, and measuring again gives 0 again. The amplitudes of the other options are gone. Physicists call this collapse. For qubits it means: a computation has to be set up so the right answer comes up almost every time.
Randomness that can’t be hacked
An ordinary computer’s random numbers come from a formula. Know the starting number and you can repeat the whole chain. That’s why they’re called pseudo-random. Quantum measurement gives randomness with no formula: nobody knows the outcome, not even nature in advance. In 2020 Samsung released the Galaxy A Quantum with a quantum random number generator to protect data. And in 2025 Quantinuum and the bank JPMorgan used a quantum computer to get random numbers with a mathematical proof that nobody faked them.
Try it at home: sunglasses and a screen
Polarised sunglasses only let through light that vibrates in one direction. Many screens, especially LCDs, give off exactly that kind of light. Put the glasses on and tilt your head: at some angle the screen goes dark. Now the physicists’ trick. Two filters crossed at right angles block everything. Slip a third one in between at 45°, and light gets through again, about 12.5%. An extra filter added light! Each filter is a measurement, and it changes the light’s state.
The world peeks all the time
It’s not only instruments that measure. An air molecule, heat, a stray photon: every such touch leaves a trace of the particle’s state in the world. That’s decoherence: the quantumness leaks out. At room temperature a qubit loses it almost instantly. That’s why Google’s and IBM’s superconducting chips run in fridges at hundredths of a degree above absolute zero. That’s around −273.14 °C, about 200 times colder than open space.
Schrödinger’s cat is a joke
In 1935 Erwin Schrödinger came up with a cat in a box that’s “both alive and dead” to show how absurd quantum theory sounds in the world of big things. A real cat is warm and huge; decoherence would kick in instantly. But physicists keep chasing records anyway. In 2023 at ETH Zürich a crystal weighing 16 micrograms, about a grain of sand, was put into a superposition of two vibrations. Nobody has found the edge of the quantum world yet.
Check yourself
- A qubit was measured and gave 0. What will a second measurement right after give?
Correct answer: 0 again. After measurement the other options’ amplitudes are gone. The qubit is now simply 0, so a repeat gives 0. - Why are qubits kept near absolute zero?
Correct answer: Heat “measures” the qubit and wipes out the quantumness. Thermal jiggling and stray photons keep touching the qubit. Each touch is an information leak, which is decoherence. Cold slows it down dramatically. - What happens to the light in the three-filter experiment?
Correct answer: Some light gets through again. The middle filter “measures” the light at 45° and changes its state. After it, the light has a chance of passing the last filter: about 12.5% in the end.