Entangled Photon Repeater = FTL Communication?

Allow me to create a scenario and ask questions first, then subsequently make an admission of ignorance.

Assume we create a device (the source or repeater) that produces two beams of entangled photons at a steady, pulsing rate. Each beam is focused on a target at a far-away station. The source and stations are not moving relative to one another. Each station has an operator who knows the rate of pulses.

One station is closer to the source than the other. This closer station is active. After tuning the target to locate the pulse, the operator moves the target aside, allowing pulses to pass normally. Now, the operator can choose to move the target to catch (absorb) a pulse.

The further station is just a receiver, and the operator simply observes each pulse.

This is intended to be a one-way, discrete communication device. The active station is the message sender. A two-way device could involve two sources, each source closer to a different station, and two targets at each station.

Question 1) If the active operator chooses to catch a pulse, does that interupt the pulse that would arrive at the receiver station?

Question 2) If yes to Q1, could this be used for faster than light (FTL) communication?

Question 3) If no to Q1, why not?

Question 4) If yes to Q1 and no to Q2, why not?

Admissions of Ignorance
* You may notice I do not know much about quantum entanglement. I am trying to learn.
* I wonder if there are inherent contradictions in my assumed scenario.
* Notice I am not trying to measure particular properties of the entangled photon, rather just its arrival at the receiver target.
* One key assumption of mine is that absorbing a photon has an instantaneous affect on the entangled pair, be it annihilation or significant alteration.
 

Filip Larsen

Gold Member
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I'm no expert, but as I understand it entangled particles do not locally behave any differently than non-entangled particles, it is only the global "state", that is, the globally combined result of a measurement made on each of the entangled particles that statistically will reveal that they were entangled. So, in short, entanglement does not provide for FTL communication.

Others here may offer a better explanation.
 

DrChinese

Science Advisor
Gold Member
7,190
1,008
Allow me to create a scenario and ask questions first, then subsequently make an admission of ignorance.

Assume we create a device (the source or repeater) that produces two beams of entangled photons at a steady, pulsing rate. Each beam is focused on a target at a far-away station. The source and stations are not moving relative to one another. Each station has an operator who knows the rate of pulses.

One station is closer to the source than the other. This closer station is active. After tuning the target to locate the pulse, the operator moves the target aside, allowing pulses to pass normally. Now, the operator can choose to move the target to catch (absorb) a pulse.

The further station is just a receiver, and the operator simply observes each pulse.

This is intended to be a one-way, discrete communication device. The active station is the message sender. A two-way device could involve two sources, each source closer to a different station, and two targets at each station.

Question 1) If the active operator chooses to catch a pulse, does that interupt the pulse that would arrive at the receiver station?

Question 2) If yes to Q1, could this be used for faster than light (FTL) communication?

Question 3) If no to Q1, why not?

Question 4) If yes to Q1 and no to Q2, why not?
Welcome to PhysicsForums, Drewbeenius! You have asked a good question.

1) You can effectively transmit the state measured from Alice (your "active" target) to Bob (your passive target). That doesn't do much for you as Bob has no way to decode what that state was. So Alice can choose to measure, say, polarization at 0 degrees but Bob won't know the difference between that and a measurement by Alice at 45 degrees.

2) So no, there is no FTL communication channel present.
 
Thank you for your replies, they have spurred my thinking.

I now think a fundamental part of my question was buried in my assumptions at the bottom of the original posting. I can rephrase my question as follows:
If photons A & B are entangled, and photon A is absorbed, does that cause something to happen to photon B?

If so, what happens to photon B and when?

* My assumptions were that a causal relationship does exist between the entangled photons, and that actions upon one of them immediately alter the other. Perhaps this is not the case.
* Maybe rather than describing quantum entanglement as "spooky action at a distance," it could be called spooky knowledge at a distance.
 

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