qubit specific measurement:
At the end of a computation, the result is read out by specific measuring qubits. The measurement represents an interaction between qubits and the measurement apparatus and this is an irreversible process.
For any process modifying the quantum states of qubits, the final measurement of those states is of fundamental importance when performing computations. If our system allows for non-destructive projective measurements, then, in principle, this can be used for state preparation. Measurement is at the foundation of all quantum algorithms, especially in concepts such as quantum teleportation. Measurement techniques that are not 100% efficient are typically repeated to increase the success rate. Examples of reliable measurement devices are found in optical systems where homodyne detectors have reached the point of reliably counting how many photons have passed through the detecting cross-section. More challenging is the measurement of quantum dots, where the energy gap between the |01>+|10>} and |01>-|10> (the singlet state) is used to measure the relative spins of the 2 electrons.
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