Measurement
_|_|2, for amplitudes of all states matching an output bit-pattern, gives the probability that it will be read.
Example: 0.316|00›+0.447|01›+0.548|10›+0.632|11› The probability to read the rightmost bit as 0 is |0.316|2+|0.548|2=0.4
Measurement during a computation changes the state of the system but can be used in some cases to increase efficiency (measure and halt or continue).