# Possibility of superluminal signals using quantum entanglement

1. Jan 13, 2013

### kochanskij

Superluminal signals using quantum entanglement

Please consider this thought experiment. Would this provide a means for communicating faster than light or is there an error somewhere in it?

SUMMARY: Person B can instantly know whether or not Person A did measurements on his particles by looking for an interference pattern with his entangled partner particles.

DETAILS:
A source sends entangled particle pairs in opposite directions. Each particle and its partner are in a superpostion of spin-up and spin-down states. As a particle reach Person A, he can choose to do a spin measurement or let it go by unmeasured. If he lets it go by, then its entangled partner will remain in a superposition. If he does a measurement, then its partner will instantly collapse to a definite spin state. Person B wants to know if his particle is in a superposition or a definite spin state. (He doesn't care if it is spin-up or spin-down.) So he puts a device in its path that uses a magnetic field to deflect it upward if its spin is up, downward if its spin is down, and both ways if it is in a superposition. The device brings the two paths out through slits that are very close together. Just as in the standard double slit experiment, if the particle followed both paths, it will interfere with itself and hit a screen at a spot allowed by the interference pattern mathematics.

Person B can not see an interference pattern with only one particle, so lets use batches of 1000 particles each. Person A receives particles one at a time every millisecond. He lets 1000 pass by to transmit a "1". He measures 1000 in a row to transmit a "0". Person B lets 1000 entangled partner particles pass through his double slit device, one at a time, and hit a screen. If he sees interference fringes, he writes down a "1". If he sees two spots behind the slits but no fringes, he writes down a "0". He then clears the screen and does the same thing with the next batch of 1000 particles.

The two scientists could be any distance apart, even many light-years. The collapse of the B particle's wavefunctions will occur instantly when (and if) person A does a measurement. So a binary message could be sent faster than light. This experiment could actually be done in a modern physics lab using high speed electronic devices.

Where is the error in this plan? Or could information really be transmitted faster than light, in violation of special relativity?

2. Jan 13, 2013

### Staff: Mentor

You can't build such a device. Apply a magnetic field to a particle in a superposition of spin up and spin down, and you'll get one of two results: up or down.

3. Jan 14, 2013

### StevieTNZ

So a big list of 0101010101 refers to what, exactly?

4. Jan 17, 2013

### kochanskij

Much thanks to Nugatory for the simple direct answer and to Al for the reference to a detailed explanation.

I didn't really think that I discovered a superluminal signaling device, but I couldn't find where it went wrong. Now I understand and agree with you.

It seems entanglement can't be used to send faster than light messages because:
1. the transmitting person can not control the spins of his particles (They are random)
2. the receiving person can not determine whether his particles are in a superposition state or a definite state just before he makes a measurement (Their wavefunction is not an observable property)

It is tantalizing that entangled particles seem to send superluminal info to their partners, but the laws of quantum mechanics conspire to prevent us from doing it !

5. Jan 17, 2013

### Fredrik

Staff Emeritus
Actually, the magnet will only correlate the Sz eigenstates with approximate px eigenstates, i.e. it will put the particle in a superposition of "moving to the left with spin up" and "moving to the right with spin down".

(Maybe it's left where I said right and vice versa. I didn't think that part through, because it's not very interesting).

6. Jan 17, 2013

### nanosiborg

It doesn't seem that way to many (dare I say most?) people.

7. Jan 17, 2013

### San K

Person B will get a blob and will never get an interference pattern in all cases. The interference pattern is buried/hidden/embedded inside the blob.

To get (separate/filter) the interference pattern; B needs to compare with A.

There is way to get "partial" interference pattern but it does not help in transmitting information (FTL) either.

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