Picturing Schrodinger's Cat

[mindwalker_i]

From phys.org:

http://phys.org/news/2014-08-picturing-schrodinger-cat-quantum-physics.html

Researchers from the Institute for Quantum Optics and Quantum Information (IQOQI), the Vienna Center for Quantum Science and Technology (VCQ), and the University of Vienna have developed a fundamentally new quantum imaging technique with strikingly counterintuitive features. For the first time, an image has been obtained without ever detecting the light that was used to illuminate the imaged object, while the light revealing the image never touches the imaged object.

Read more at: http://phys.org/news/2014-08-picturing-schrodinger-cat-quantum-physics.html#jCp

I don't fully understand the experimental setup but from what I gather, entangled photons are created and one photon from each pair is sent to illuminate the object - a stencil of a cat. The other photons are sent to a camera and recorded, and in this way, the photons illuminating the object are not the ones that are used to create the image - in the camera.

There are several (many) proofs that one can't use entanglement to send information, but doesn't this kind of prove that wrong? The information from the stencil or image is getting through the entanglement to the camera. And of course, if that's true, then it can be used to send information faster than light, because entanglement works instantaneously - a concept which gets really interesting in relativistic situations.

[Xipe Totec]

1. Doesn't this kind of prove that wrong? Well, yes, and no. nt

[mindwalker_i]

2. What's you're thinking on it?

It seems pretty straightforward to me that, if one set of photons never touches the image, yet conveys the form of the image, information is coming through entanglement. I probably need a better source that explains the experimental setup better.

[Xipe Totec]

3. It's a joke. Schrödinger's Cat. Get it?

[mindwalker_i]

5. Doh! Sorry

I'm usually better than that

[Xipe Totec]

4. On a more serious note

This is exactly Einstein's argument against quantum mechanics. This 'Spooky action at a distance' which seems to travel faster than light.

But that appears to be exactly what is happening here.

Einstein. Even when he's wrong, he's right.

[mindwalker_i]

6. Yeah, the irony has not been lost on me

Entanglement might have actually been Einstein's greatest discovery, yet it was his greatest attack on quantum theory. Poor guy couldn't lose for winning.

More importantly, to me at least, is that information can be transmitted through entanglement. I need to understand this experiment in more detail - it seems like one photon runs into the template or not, and that information is carried to the entangled partner. I can see how multiple streams of photon pairs can be used to send information by either keeping them all in superposition of momentum or not, but being able to transmit in a single pair - whether one photon hits the template or not - is qualitatively different.

The ramifications of this could be non-trivial.

[Scuba]

7. This conundrum has been solved ...

[mindwalker_i]

8. Hahahaha!

That is an awesome picture!

[caraher]

9. This won't get you superluminal information transmission

This is a 2-crystal downconversion experiment where the object (o in the diagram) is sandwiched between the downconversion crystals. The idler photon interacts with the object (affecting the first signal photon, the one following the path to the upper-right:



The key to recovering the image information is the beamsplitter in the lower right. Single signal photons detected from either of its outputs might come from either crystal NL1 or NL2; you can't tell which path a detected signal photon took. As a result, you get interference, which reveals the effect that the interaction of the first idler had on its "twin" signal photon. But note that this requires that NL2 be pumped with the same laser light that pumped NL1, meaning that all these events have lightlike separation - the information travels at exactly the speed of light!

From the full (paywall-protected) Nature paper (preprint available via arXiv):

The peculiar feature of this interferometer is that no detected photon has taken path d. Yet, in our experiment, it is precisely here where we put the object to be imaged. The key to this experiment is how the signal-source information carried by the undetected idler photon depends on T. For, if T = 0, an idler detected after D3, coincident with a signal count at or , would imply the signal source was NL2. Detection of a signal photon without a coincident idler would imply the signal source was NL1. This which-source information destroys interference because it makes the quantum states overlapping at BS2 distinguishable. If T = 1, the idler photon carries no which-source information. The signal states overlapped at each output of BS2 are then indistinguishable; thus the interference term in equation (2) appears. The above arguments are valid even though the idler photons are not detected, for it is only the possibility of obtaining which-source information that matters in this experiment.

In the excerpt above, T refers to the the transmission properties of the object, so T=0 means opaque, T=1 means transparent.

I think the biggest subtlety for me is the need to overlap the never-detected idlers at NL2. That's required because if you can measure the idlers separately, the signals can be distinguished and no longer interfere.

[mindwalker_i]

10. Thanks for posting this snippet and diagram

The other articles weren't terribly good with the details.

Even though this won't be superluminal, it's really interesting in that it demonstrates using entanglement to carry information. That is a non-trivial result.

[caraher]

11. Absolutely interesting stuff

I knew about "ghost imaging" but this is different. Zeilinger has a flair for these really clever ideas!