The Measurement Problem

Earlier where we introduced quantum measurements, we mentioned what is called the “measurement problem.” The difficulty is trying to find a theoretical description, ideally within the framework of standard quantum mechanics, for what happens when a qubit (or any other quantum entity) interacts with a measurement device.

As we have seen, the probability interpretation of quantum mechanics (the Born rule) comes into play with measurements. Although we haven’t said much about how quantum states evolve in time, we have implicitly assumed that if I produce a qubit in a superposition state and launch the qubit to Bob, the state stays the same unless some interaction with other qubits intervenes. If those interactions do occur, we describe the changes in a state via matrices that represent reversible processes. The problem is that measurements, as normally construed, are irreversible. The measurement postulate asserts that the appropriate state for the qubit after a measurement has been made is the basis state associated with the observed value of that property—spin-up or spin-down, for example. But the postulate does not describe a mechanism by which that happens. How do we account for the irreversibility? That is one aspect of the measurement problem. The measurement problem continues to generate spirited debates among scientists and philosophers.