Many famous experiments have shown that the simple act of observing a quantum system can change the properties of the system. This phenomenon, called the "observer effect," appears, for example, when Schrödinger's cat becomes either dead or alive (but no longer both) after someone peeks into its box. The observation destroys the superposition of the cat's state, or in other words, collapses the wave function that describes the probabilities of the cat being in each of the two states.

In a new paper, physicists have further investigated exactly how measurements affect , which in this context is equivalent to the extent to which a system is in a superposition. Previous studies have shown that, when a quantum system is left alone to evolve without any outside interference, its degree of entanglement tends to increase. That is, tend to drift over time into states with a large degree of quantum superposition.

On the other hand, making a measurement on an entangled state tends to decrease its entanglement. This happens because a measurement on a spin state (for example) collapses that spin into a definite state, which causes that spin to become disentangled from the other spins, whose states remain in a superposition. This reduces the amount of entanglement in the system overall.

In the new paper, the physicists have demonstrated via and theoretical arguments that, when measurements are made at a rate that exceeds a critical value, a measurement-induced phase transition occurs. This causes the system to sharply transition from an "entangling" phase, in which the amount of entanglement grows continuously over time, to a "disentangling" phase, in which some entanglement still exists, but its growth rate drops to zero.

The physicists, Brian Skinner at MIT, Jonathan Ruhman at MIT and Bar-Ilan University, and Adam Nahum at Oxford University, have published their paper on the phase transition for entanglement in a recent issue of Physical Review X.

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