In quantum eraser experiments, getting information about one entangled photon decides if the second photon behaves classically or quantum (interfere). Optical lengths for these photons chooses time order of these events, so we can delay the "decision" to happen after what it decides about. But in "standard version" of such delayed choice quantum erasure (http://en.wikipedia.org/wiki/Delayed_ch ... tum_eraser ) this decision is made randomly by physics.

I've just found much stronger version - in which we can control this decision affecting earlier events.

Here is a decade old Phys. Rev. A paper about its successful realization (http://grad.physics.sunysb.edu/~amarch/Walborn.pdf ) and here is simple explanation (http://grad.physics.sunysb.edu/~amarch/ ):

We produce two entangled photons - first spin up, second spin down or oppositely.

Photon s comes through double slit on which there are installed two different quarter wave plates changing polarization to circular in two different ways.

Finally there are two possibilities:

u d R L

d u L R

where columns are: linear polarization of p, initial linear polarization of s, circular polarization of s after going through slit 1, circular polarization of s after going through slit 2.

So if we know only the final circular polarization of s, we still don't know which slit was chosen, so we should get interference. But if we additionally know if p is up or down, we would know which slit was chosen and so interference pattern would disappear.

So let us add polarizer on p path - depending on its rotation we can or cannot get required information - rotating it we choose between classical and interfering behavior of s ... but depending on optical lengths, this choice can be made later ...

Why we cannot send information back in time this way?

For example placing s detector in the first interference minimum - while brightness of laser is constant, rotating p polarizer should affect the average number of counts of s detector.

What for? For example to construct computer with time loop using many such single bit channels - immediately solving NP hard problems like finding satisfying cryptokey (used to decrypt doesn't produce noise):

Physics from QFT to GRT is Lagrangian mechanics - finds action optimizing history of field configuration - e.g. closing hypothetical causal time-loops, like solving the problem we gave it.

Ok, the problem is when there is no satisfying input - time paradoxes, so physics would have to lie to break a weakest link of such reason-result loop.

Could it lie? I think it could - there is plenty of thermodynamical degrees of freedom which seems random for us, but if we could create additional constrains like causal time loops, physics could use these degrees of freedom to break a weakest link of such loop.

What is wrong with this picture?

There is a looong discussion on this subject and the opposer there seems to be convinced(?):

http://www.thescienceforum.com/physics/ ... ality.html

Interesting topics from there:

"For clearer picture, let us maybe look at Mach-Zehnder configuration:

Now the whole interference pattern is get by two detectors making the situation simpler:

the path is unknown - everything goes to Ds2,

the path is known - half goes to Ds1.

So if anything goes to Ds1, means interference was destroyed - means the path is well defined.

The path is not known if the polarizer is set to fast axis of one of quater wave plate, is unknown if it is between them (45deg)."

realization:http://singlephoton.wikidot.com/quantum-eraser

"I see also another looking practical possibility I cannot disprove - CPT analogue of laser - lasar (stimulated absorption).

To see that it seems doable, imagine free electron laser - we enforce electron to move on sinus-like curve, emitting photons ... which finally e.g. are absorbed by some target.

Physics is CPT invariant, so let us imagine such tranformation of this picture - excited target emit photons, which fly to the lasar and finally are absorbed by positron going in reverse direction.

So such free electron laser should also work as lasar - but to make it work, it has to (anti-)hit target which is already excited to given specific wavelength - it doesn't occur often.

Imagine we constantly excite the target to required energy (e.g. is sodium lamp) and it is surrounded in all but to the lasar directions by detectors - they usually get the produced light, but if we turn the lasar on, more energy should goin that direction and so we should see a disturbance in energy balance in the lamp-detectors system ... before turning the lasar on by the optic length.

What's wrong also with this picture? "

"For action optimizing Lagrangian mechanics like QFT or GRT, we should think of particles as their trajectories.

For example linear polarizer means that trajectory is fixed in one of two possibilities - like for string, angular tension makes that surrounding feels this tension - surrounding in both directions.

So using the scheme of time-loop computer from the first post is like asking for fixing all polarizations to minimize total tension - action ... but sometimes (like paradoxes) such minimization means that physics had to lie to break this causal loop - for example spoiling statistics we used in hypothetical backtime channel.

Imposing imperfect causal loop constrain to lagrangian mechanics seems to be against intuition, while "standard" quantum computers also do similar stuff (wider explaination).

It is said that their strength is the reversible calculations, while we can also make them classically like

(x,y,z)(x,y,z xor f(x,y))

the problem with trying to reverse such calculations are the auxiliary bits - we don't know how to initialize them...

Here is the real strength of quantum computers - that the measurement is kind of attaching trajectories in the future - for example "selecting" arguments leading to the same value of function in Shor algorithm:

"