the idea that one can make measurements that don’t completely destroy superpositions
That. I did not think that was possible. Like I said, my physics background is pretty weak. I’ve tried reading the quantum physics sequence, but it was really difficult, because it was fairly uninteresting.
Ah, I see. I don’t think reading the sequence would have helped you here, because this is a subtle issue that wasn’t (as far as I recall) covered in the sequence. In fact, it isn’t even covered in most undergraduate courses on QM, so your assumption that measurements must destroy superpositions doesn’t indicate a glaring lack of knowledge of QM.
It is standardly taught that the outcome of a measurement is an eigenvalue, which would mean that (at least within a particular branch) the quantum system “collapses” to a determinate state, and is no longer in a superposition. However, this standard story depends on treating the measurement device itself as a classical system, which is usually not a bad approximation.
But measurement devices are quantum systems, and in the late 80s some theorists demonstrated that this fact lets us obtain information about a quantum system without destroying a superposition. The procedure is called “weak measurement”, and the basis for it is that there is some quantum uncertainty about the reading of the measuring device itself (uncertainty about the position of a pointer on the device, for instance). One can arrange it so that the measuring device is so weakly coupled to the quantum system that any change in the device brought about by interaction with the system is actually smaller than the uncertainty about the device’s reading.
Under this condition, an interaction between the device and the system does not appreciably alter the state of the system. If it was in a superposition, it remains in a superposition. But as a consequence of the weak coupling, reading the device doesn’t actually tell us much about the system, because any effect of the system on the device is swamped by quantum uncertainty. It turns out, however, that if we perform many weak measurements on identically prepared quantum systems, the average of these measurements actually does tell us something about the systems. It tells us the expectation value of the system property we’re measuring.
Anyway, none of this turns QM on its head in any technical sense. The possibility of weak measurements was derived from QM well before any experiments took advantage of the idea. There is some controversial work that builds on the weak measurement idea, but the basic notion of a weak measurement is an uncontroversial consequence of QM.
That. I did not think that was possible. Like I said, my physics background is pretty weak. I’ve tried reading the quantum physics sequence, but it was really difficult, because it was fairly uninteresting.
Ah, I see. I don’t think reading the sequence would have helped you here, because this is a subtle issue that wasn’t (as far as I recall) covered in the sequence. In fact, it isn’t even covered in most undergraduate courses on QM, so your assumption that measurements must destroy superpositions doesn’t indicate a glaring lack of knowledge of QM.
It is standardly taught that the outcome of a measurement is an eigenvalue, which would mean that (at least within a particular branch) the quantum system “collapses” to a determinate state, and is no longer in a superposition. However, this standard story depends on treating the measurement device itself as a classical system, which is usually not a bad approximation.
But measurement devices are quantum systems, and in the late 80s some theorists demonstrated that this fact lets us obtain information about a quantum system without destroying a superposition. The procedure is called “weak measurement”, and the basis for it is that there is some quantum uncertainty about the reading of the measuring device itself (uncertainty about the position of a pointer on the device, for instance). One can arrange it so that the measuring device is so weakly coupled to the quantum system that any change in the device brought about by interaction with the system is actually smaller than the uncertainty about the device’s reading.
Under this condition, an interaction between the device and the system does not appreciably alter the state of the system. If it was in a superposition, it remains in a superposition. But as a consequence of the weak coupling, reading the device doesn’t actually tell us much about the system, because any effect of the system on the device is swamped by quantum uncertainty. It turns out, however, that if we perform many weak measurements on identically prepared quantum systems, the average of these measurements actually does tell us something about the systems. It tells us the expectation value of the system property we’re measuring.
Anyway, none of this turns QM on its head in any technical sense. The possibility of weak measurements was derived from QM well before any experiments took advantage of the idea. There is some controversial work that builds on the weak measurement idea, but the basic notion of a weak measurement is an uncontroversial consequence of QM.
OK, that actually makes a lot of sense. Thanks!