What does measuring a qubit do?
Reading a qubit. It returns 0 or 1 at random, with chances set by the squared amplitudes, and ends the blend.
After a measurement the qubit stays at the result it gave, so measuring again gives the same answer. The blend it had before is gone.
Because each result is random, one measurement tells you little about the amplitudes. Running the same computation many times and counting the results shows the odds, though never the sign of an amplitude.
In the lessonsSee it in a simulation
Related
- QubitThe basic unit of a quantum computer: it can be 0, 1, or a blend of both until it is measured.
- AmplitudeThe number a qubit holds for each possible result. Squaring it gives the chance of that result.
- SuperpositionA qubit state that is a blend of 0 and 1, with an amplitude for each.
- No-cloning theoremThe rule that an unknown quantum state can't be copied.