# Simulating Closed Timelike Curves through Quantum Optics

1. Jun 21, 2014

### phys_student1

This paper experimentally simulates Closed Timelike Curves (CTC) through quantum optics experiment. Since I have no experience/background in this, I found it hard to understand how exactly the CTC is implemented in the circuit. [Note: I do understand QM, so no need to explain this].

2. Jun 22, 2014

### phys_student1

Is the question not clear?

3. Jun 22, 2014

### DrChinese

It's a pretty sophisticated setup with state of the art theory involved.

4. Jun 23, 2014

### Cthugha

I am not exactly sure what you are asking. Is it about the protocol used or the actual optical setup needed?

In a nutshell: It works a bit like the old joke about how a stonemason creates a statue of a lion: You start with a huge stone cube and just remove everything which does not look like a lion.

The equivalent here is postselection. In quantum teleportation, Alice tries to get an unknown state over to Bob, but they need to exchange one classical bit of information in order to do so: This is the unitarity transformation Bob needs to apply to his side. Now there may be a certain probability that this unitary transformation is simply identity, so Bob does not need to do anything to get the correct state. So in some sense, Bob already had the state which should be teleported to him before the teleportation took place. Postselection now means that the experimentalists just pick all the measurement runs, where the unitary transformation Bob has to apply indeed was the identity operation. This is not controllable, so they just throw many runs of the experiment away - the stonemason analogy so to speak. As in these post-selected cases, the state was already there before the teleportation took place, the authors consider it as analogous to a CTC.

When they try to create a paradoxical situation (grandfather paradox), they just find that it does not work. If they try to end up in a paradoxical situation, the probability that the unitary transformation Bob needs to apply to end up in that state just goes to zero and the paradox will never be realized.

By the way, the journal article in PRL is somewhat better and more precise than the article on ArXiv you linked to. In my opinion the connection to CTC is somewhat handwaving.

5. Jun 23, 2014

### phys_student1

Cthugha, thank you! That was very clear.

I agree that the connection to CTC is not really a rigorous one, which is why I was confused while reading the paper.

P.S. I have (in my university) subscription to APS journals, but I prefer arXiv because some members don't have access.