On measurement and quantum nondemolition

Christopher Monroe argues that the concept of quantum nondemolition has little or no value in understanding quantum mechanics, vis-a-vis the entanglement approach, described by Wojciech Zurek, ¹ wherein no quantum state is ever “demolished,” but rather becomes so entangled with the environment that its quantum properties may no longer be discerned.

However, Monroe demonstrates how easy it is to make an error in a quantum argument: “When a single photon strikes a photomultiplier tube” has already begged the question. Yes, a single photon has been registered, but whether there was only one or an indefinite number with a well-defined phase is not known—the single photon state has been projected out and the preceding state “demolished.”

Thus, while one may well emit another photon and “get the same answer again and again,” the process is not the same as, say, that of slowing light to a stop in a medium—which may well work for a state of a single photon or an indefinite number of photons. I suggest that a little more thinking is in order.