4 ms·
> the distinction between a quantum system being measured or unitarily evolved When you try to describe a part of an entangled quantum system, independently of
by mathgenius 7y ago
> the distinction between a quantum system being measured or unitarily evolved
When you try to describe a part of an entangled quantum system, independently of the whole, you will be forced to use probabilities in your description. This is quantum measurement.
- codethief 7y agoI don't think I would call this a "measurement". A measurement, in the Copenhagen interpretation, is a fundamentally non-unitary process. What you're describing is simply tracing out the environment, so that we end up with a mixed state. The complete density matrix, however, is still being governed by the Von-Neumann equation and therefore evolving unitarily. So I assume what you're saying is that we don't need to model measurements separately since their results would be described by statistical ensembles and, at the level of a subsystem, you can't really tell the difference between a genuinely entangled state and a statistical ensemble, anyway?