# Can you observe a photons polarization or change in polarizati without disturbing it?

1. Feb 23, 2012

anybody?

2. Feb 23, 2012

### DrChinese

Re: can you observe a photons polarization or change in polarizati without disturbing

Yes, you can.

a) If you knew it before, and measured the same thing again: If it is entangled (in some other basis than polarization obviously), it will remain entangled. And it is generally not otherwise disturbed.

b) Using a wave plate, you can twist the polarization by any amount. If it is polarization entangled, it will remain entangled.

3. Feb 23, 2012

### Mr_Physicist

Re: can you observe a photons polarization or change in polarizati without disturbing

Change? Yes, easily. You can rotate the polarization of a photon with a half or quarter waveplate without otherwise disturbing the photon. Observe? Yes and no. If you have a single (non entangled) photon in an unknown polarization state, you cannot determine its polarization without disturbing it, as when you measure its polarization it ends up in whatever state it was measured to be in. However, if you have two entangled photons, you can do a measurement on one without disturbing the other, then you can know its state without doing a direct measurement. However, you still disturb the joint two photon state. Also, for a single photon, you can perform weak measurements, which tell you something but not everything about the state of the photon.

4. Feb 24, 2012

### San K

Re: can you observe a photons polarization or change in polarizati without disturbing

Since we have changed/twisted, hence measured (?), the polarization by the wave plate, have we not broken the entanglement?

Last edited: Feb 25, 2012
5. Feb 25, 2012

### Mr_Physicist

Re: can you observe a photons polarization or change in polarizati without disturbing

Rotating the polarization, which includes changing horizontal to vertical or even linear to circular or elliptical, is not a measurement, only a rotation. It will change the state, but entanglement is preserved. If you send one or both photons in an entangled pair (entangled in polarization, at least) through a polarizer, then that is a projection and will destroy the entanglement.

6. Feb 25, 2012

Re: can you observe a photons polarization or change in polarizati without disturbing

so you cant back and forth messages through entanglement. thats the only answer i was after. if you could it would spacedock einstein and take a dump on relativity/information theory

7. Feb 25, 2012

### San K

Re: can you observe a photons polarization or change in polarizati without disturbing

if we can change the polarization of A from say horizontal to vertical, and entanglement is preserved then polarization of B will also change instantaneously to vertical?

that way information can be passed to B via manipulation of A FTL (faster than light), which is not possible. so somewhere along this chain of thought, something is not right.......

8. Feb 25, 2012

### Mr_Physicist

Re: can you observe a photons polarization or change in polarizati without disturbing

Your error is in the first sentence. Rotating the polarization of photon A does not rotate photon B. Photon A will still be entangled with photon B, but the relationship will be different, i.e. where in the previous case both photons would be found to be in the same polarization state, if you rotate one photon they will both be found to be in different, but still correlated, states. Entanglement is preserved, but doing a measurement or projection on one photon will not affect the other photon in any way that allows FTL communication. Does this make sense? If no I can write out an example.