But could we use it for communications? Or not?
If the answer is yes another "problem" raises: if the "receiver" arm would be shorter than the "trasmitter" one, you receive infos BEFORE they are sent... A sort of impossible time machine...
(See John Cramer Transactional Interpretation for details)
Yes, it can be. And for some VERY secure communication, too. I had worked out a device some years ago (which may be buried on the forum here somewhere already), based on quantum entanglement to transmit binary information.
The basic problem you face is that we cannot deal with single photons, since the "wire" to conduct them would have to be smaller than the smallest atom. That's just geometry--a photon has no net spatial displacement, being represented as a linear vibration (birotation). So you are always dealing with an
aggregate of photons (wave functions). And that's the problem--the quantum entanglement, being temporal, has no correspondence between the geometry of one half to the other, in the entanglement. All you know is that if you alter one entangled photon, the other will change. But since you are dealing with an aggregate, you start with a random pattern--so when you alter that random pattern, the random pattern on the other side changes, but to another random pattern. Probability takes over, so the net "change" is zero. It happens, but without geometric correlation, you can't really tell it happened.
The fix... Nehru, in his papers on birotation, brought up the Zeeman effect and discusses
circularly polarized photons. Photons are normally linearly polarized (planar). A major characteristic of the circularly polarized photon is that it is a rotation--not a vibration--and therefore possesses
torque. And that's how you do it... treat a linearly polarized photon as a "0" and a circularly polarized one as a "1" and you can transmit binary information. On the receiver side, the pattern being received is still geometrically random, but now the photons--rather than changing linear orientation--are showing up with or without a net torque. Circularly polarized photons also refract differently in a crystal, creating in "e-beam" (extraordinary), so it should be fairly easy to detect.
The problem I ran into in building such a device was the photon source, which needs to send photons in opposite directions. Emission tends to be in random directions, because of the atomic structure altering the coordinate time of the photon flight path.
The device, itself... take a quantum entanglement generator so photons are emitted down opposing fiber optic cables, so you have entangled pairs traveling in opposite directions. At some fixed distance (your "arm" question) you put the transmitter, which is just a device to alter the polarization between linear and circular. On the other side, at just a bit more (1 natural unit) past the distance as the transmitter is from the source, you put your receiver, which is basically a crystal beam splitter to split off the circular from linear polarized photons. And now you have an instantaneous, binary communication system that is 100% secure. The photons between the source and receiver/transmitter are just random. The transmitter modifies the polarization at a specific distance, causing a corresponding change at the receiver side at the same distance, where the data is pulled off, and the photons continue on, but as all "zeros" (linear polarization).
The "problem" you mention never occurs, because the photons have
no independent motion--they are carried by the progression at the speed of light (unity). It gives a one-to-one correspondence between space and time. If you travel at 1 mph for 1 hour, you go 1 mile. If you go 1 mile, you've triaveled 1 hour. A shorter receiver arm will just give you zeros... the altered entanglement has not occurred yet. There is no causality involved.