# Quantum entanglement and information transfer

I know it was stated many times that information transfer using entanglement is not possible and I am most probably wrong but I would be glad if you can at least point me to where I am mistaken.

If we entangle only 2 particles it does not gives us much. However lets say we entangle 4 pairs of particles.

Now we have 2 observers Alice and Bob which have 4 particles each (one of each entangled pair). Lets say particle one will be used to pass the data and particles 2,3,4 to idicate whether the data is correct.

So for example Alice and Bob, both agree beforehand that if 2,3,4 have for example all spin up (spin down for the other observer) then then data transfer occurs and spin of particle 1 indicates correct value. Otherwise it indicates that transfer is not happening (break between 2 messages).

I know now that there is something called Zeno Quantum Effect, that tells us if we do the measurement of particles frequently enough we have a great chance of them returning to a state we measured at the beginning. I don't know if that is already possible or not but let's assume that we are able to measure so frequently that over 0,1 second we are able to get the same measurement of spin 99% of times.

My idea is as follows. Let's say Alice wants to transfer spin up to Bob. She measures particle one until she gets spin up and after that measures so frequently to experience Zeno effect and maintain same measurement results for as long she can.

Then she has to indicate that the value is correct and Bob can read the correct data by forcing the other 3 particles to be all spin-up (as agreed with Bob). I assume she would also use Zeno effect here.

On the other side Bob does observes to detect if paritcles 2,3,4 have all spin down on his side. If they do, then it means he can assume with great probability that spin of particle 1 is what Alice intended him to get.

Now Alice stops measuring particles so Bob gets inconsistent results and knows that there is a break in communication. Alice repeats everyhting again but this time maybe passing other spin in particle one.

This way Bob can read bits of information from Alice one by one in 0,2 secs period. Maybe we could increase this frequency or use much more particles and get even faster transfer?

Wouldn't this be possible using multiple particle system and Zeno effect?

• There are so many problems with this that it's hard to know where to start. Entanglement doesn't work the way you think it does; it is not a magic portal which instantaneously sends arbitrary measurements on one particle to another: instead, entanglement is a set of strange non-classical correlations which can (only!) be observed when we bring measurements of both systems back together and correlate them. The quantum Zeno effect doesn't work the way you think it does either; it protects a system which wants to tunnel into a lower energy from quantum tunneling, by rapid measurement. – CR Drost Oct 12 '15 at 19:42
• @ChrisDrost I think the Zeno effect is not just relevant when a system "wants to tunnel into a lower energy" (in fact I'm not sure what you mean by that). For a system to move to a lower energy it needs to dump the extra energy somewhere else, so you're talking about an incoherent effect. The Zeno effect happens in perfectly coherent systems too (except for the measurement causing the Zeno effect itself) such as Rabi oscillations. – DanielSank Oct 13 '15 at 0:15
• @DanielSank: Yeah, I guess more accurately I meant "prevents a system which is in a non-stationary state from moving in the direction it wants," or so, but as I understood you need a potential barrier and some degree of isolation to prevent the state from "slowly falling down the gradient" during the times you're not measuring. – CR Drost Oct 13 '15 at 18:31

## 1 Answer

After the first measurement, Alice and Bob's further results are uncorrelated. They go from being $| \downarrow \uparrow \rangle + |\uparrow \downarrow \rangle$ to just $|\uparrow \downarrow \rangle$ or $| \downarrow \uparrow \rangle$, and this is no longer an entangled state (since we can write it as a product). Things that Alice does to her qubit after measuring it don't affect Bob's. Otherwise, we don't need to use tricky Zeno effect anything--she could just flip it over with a $\pi$-pulse or something. They only get one measurement on each qubit, essentially, so this idea that Bob can "keep testing his qubit" doesn't work. It's also not clear to me what adding the extra qubits achieve, is it just redundancy?

• So is it that the entanglement last only until first measurement? I thought that if particles are entangled every time particleA is measured, particleB will have opposite spin when also observed. – RobertPorter Oct 12 '15 at 19:41
• @RobertPorter Nope. Entanglement is pretty fragile, and tends not to last. Remember, in QM measurement changes the state of a particle. In this case, you started in an entangled state, the gist of which is just that A and B will have opposite spins when measured (or always the same spin). Once you measure, you go to a state where both particles have the definitive spin, and this new state has no entanglement. – zeldredge Oct 12 '15 at 19:44
• If you prefer the many-worlds interpretation, you can of course also view the measurement as entangling a measurement apparatus with the entangled system. Entanglement, in general, destroys quantum coherence effects: or more precisely it propagates that coherence out to the bigger system. The entanglement effects of the 2 particle system are coherence effects which are destroyed by entangling with the measurement apparatus, propagating out to the bigger system. – CR Drost Oct 12 '15 at 19:48