# Quantum entanglement and affecting the particles [duplicate]

I am trying to grasp some aspects of the quantum entanglement, but the existing resources (including some of the links here) seem a bit confusing. I am trying to find an answer to the following questions.

If two particles are entangled and then separated, will affecting one of them affect the other (for example, the particle is placed in a field that would set some property of it in a specific direction), or will it disentangle the system? If affecting one particle affects the other, then how is it not possible to use this effect to transfer information (once; for example by affecting the spin of the first particle to be up the other particle would have its spin down)?

Is affecting the particle equivalent to measuring the property affected (for example the spin)?

When a property of the particle is measured, does the particle get entangled with the measuring apparatus? More precisely, does interaction imply entanglement of a sort?

I apologize if the questions are trivial or nonsensical, but I am asking as a layman in the field.

## marked as duplicate by Norbert Schuch, user36790, ACuriousMind♦, Qmechanic♦ quantum-mechanics StackExchange.ready(function() { if (StackExchange.options.isMobile) return; $('.dupe-hammer-message-hover:not(.hover-bound)').each(function() { var$hover = $(this).addClass('hover-bound'),$msg = $hover.siblings('.dupe-hammer-message');$hover.hover( function() { $hover.showInfoMessage('', { messageElement:$msg.clone().show(), transient: false, position: { my: 'bottom left', at: 'top center', offsetTop: -7 }, dismissable: false, relativeToBody: true }); }, function() { StackExchange.helpers.removeMessages(); } ); }); }); Jun 2 '16 at 15:52

• No, entanglement is not an active link of any sort, and it can't be used to transfer information. Entanglement is the most general description of any correlation in which the quantum mechanical description by pure states is applicable. The correlation/entanglement exists from the moment of the interaction of the subsystem in the past and nothing is "communicating" at the moment of the measurement. See e.g. physics.stackexchange.com/q/3158 – Luboš Motl Jun 2 '16 at 4:36

If two particles are entangled and then separated, will affecting one of them affect the other?

If the state space for particle $i$ is $H_i$ and $U$ is a unitary operator acting on $H_1$, then $U$ acts on $H_1\otimes H_2$ (that is, the state space of the entangled pair) as $U\otimes 1$.

If affecting one particle affects the other, then how is it not possible to use this effect to transfer information (once)?

This appears to be a non sequitur. How exactly would you transfer information?

Is placing the particle in a field (or affecting it) equivalent to measuring its corresponding property? In any event, one does not "place particles in fields". Fields permeate all of spacetime.

What does "its corresponding property" mean?

When a property of the particle is measured, does the particle get entangled with the measuring apparatus? More precisely, does interaction imply entanglement of a sort?

If you're thinking of a wave function collapse,then you can equally well model the collapse as taking place at the moment of interaction with the measuring apparatus, or as the particle becoming entangled with the measuring apparatus and then the collapse happening later. This is a theorem of von Neumann.

• Thanks for your answer. I edited the post to clarify the questions. – Igor Ševo Jun 1 '16 at 15:53
• One can certainly create a field that affects the particle. Saying that this is not the same as "placing a particle in a field" is splitting hairs. – Peter Shor Jun 3 '16 at 15:32