Quantum Holography

Buzz Skyline writes "Physicists succeed where psychics fail. Researchers from Boston University propose a quantum holography system that can construct 3d images of objects sealed in closed containers. Could it lead to quantum luggage scanners at the airport?"

Comments

Re:Schroedinger's Cat

[FrozedSolid]

We are observing the object, period. If it's observed, then it completely destroys the principle.

Duh :)

Re:No luggage scanning here

[Strange_Attractor]

Don't dismiss it too hastily!
The specially constructed sphere is the easiest, and thus first, configuration to be tried in the lab.

All they need to do is require all interior luggage surfaces be built from these time-reporting light sensors (and maybe prevent travellers from putting anything else in the bag if they have a bomb), and voila! No time-and-cost-prohibitive bomb-sniffing machinery!

Re:This could be used to fight terrorism

[FlowerPotAdmin]

Think for a moment which is worse: to have the government searching you in broad daylight, where everyone can witness, or to give that government the power to search you at any time it wishes, with neither consent nor notification?

I for one do not feel the same way. In my mind, this kind of "secret" searching is just as degrading, and does not provide as much protection for the citizen.

Faster than light communication

[pete-classic]

I don't really understand quantum entanglement but . . .

Couldn't you use it to communicate instantly over any amount of distance?

Imagine:

You are at point A (say, earth) and I'm at point C (say, a spacecraft) and we have a buoy, at point B, precisely half way between us. Let's say that you and I are one light-year apart, and that buoy has been splitting a beam of photons between the point where I am and the point were you are for the last six months.

You have a photoreceptor oriented 90 degrees out from the beam, and I have a mirror at 45 degrees, hooked up to a solenoid. I type you a message in morse code on a switch that controls current to the solenoid. You see it in real-time.

I'm sure that either 1. there is a really good reason why this won't work in theory or 2. someone else has proposed it.

Can someone give me a reference either way?

-Peter

Re:Faster than light communication

[Mr. Slippery]

Couldn't you use it to communicate instantly over any amount of distance?

No, you couldn't. :-)

Your mirror scenario wouldn't be making any measurements on the incoming photons, I don't see that it has anything to do with entanglement.

Let's look at another example that gets closer to - but turns out not to be - instantaneous communication. It's been a while since I studied this, so real physicists please correct me, but I think I remember the gist of it.

We'll use polarization as an example. Quick review: every photon is polarized at some angle. If it hits a detector that's at the same angle, it passes though; a detector at 90 degrees to its angle, it's blocked; and at some angle in between, it may or may not pass through, but if it does it will now have the new angle of the detector (i.e., a 45 degree photon hitting a 0 degree piece of polarized material has a 50% chance of being blocked at a 50% chance of passing with its polarization at 0 degrees).

The polarization vector is a quantum superposition of the 0 degree and 90 degree states. If two photons are entangled, and one gets measures and "snaps to" one of these states, its entangled partner always "snaps to" the same state. (Or maybe it always snaps to the opposite state. I forget. Doesn't matter for this example.)

Let's say that our entangled photon source is sending out beams that are polarized at 45 degrees (i.e., in a superposition of 0 and 90 degrees). The sender - call her Alice - sets her polarization detector to either 0 degrees (to transmit a "dot") or 90 degrees (to transmit a "dash"), and her photon randomly snaps to one of these polarizations. If it happens to snap to the matching one, it passes thru the polarization detector.

A light-year away, the matching photon in the detector belonging to the receiver (call him Bob) spookily snaps to the same polarization direction. Bob's all set to make a measurement, but which way should be set his polarization detector? If he sets it at 45 degrees, then regardless of whether the photon is at 0 or 90 it has a 50/50 chance of passing through, so he'll see half the photons pass. If he sets it at 0, the incident photon has (from Bob's perspective, not knowing whether the next bit of the message is a "dot" or a "dash") a 50/50 chance of being polarized at 0 at 90 degrees, so he'll see half the photons pass. Same if he sets it at 90.

Even though the photons were linked, and each instantaneously "knew" what was happening with the other one, no information can be recovered from the beam, because what the photons do is still random.

(However, by changing this around a little bit Alice and Bob can generate an unbreakable cryptographic key - search Google for "quantum cryptography".)

Re:Faster than light communication

[redcliffe]

If you change an entangled photon in any way, the other one instantly changes the same way. That's what confused Einstien. Quantum nonlocality, you change one half of the pair here, and instantly the other half changes the same. That's what entanglement and teleportation is all about, IIRC, but IANAP.

David

P.S. Of course, I could be wrong. Any quantum physicist out there wish to confirm this. I'm sure this is what they did in the Bell experiment.

Re:Schroedinger's Cat

[autocracy]

If I shoot a bullet through the center of 3 pieces of paper all one inch a part at signifigant speed, I KNOW it will hit the forth one that I can't see if it's an inch away. Physics. And you can damned well bet that it changed things. (Note: I do believe in the theory of quantum *).

Re:Meow

[Faramir]

No. At this point, there is no established physical way to "break" the uncertainty principle. I'm not sure why it is not the "uncertainty law" at this point--perhaps this is a subtlety of the way the principal was derived. Nevertheless, it has the tenacity of a law of nature, and will not "break". The equation will not be violated--the uncertainty (or change) in position squared times uncertainty (or change) in momentum squared will alwasy be greater than or equal to Plank's constant divided by 2 (dx^2 * dp^2 >= hbar)

Yes and No

[ArcSecond]

There is a really good book I read about stuff like this a few years ago, called "Schrodinger's Kittens". To sum it up, the guy says there is ANOTHER way to look at quantum weirdness, other than the "Standard/Copenhagen" model.

The idea is that since photons travel at the speed of light (duh), they don't experience "time", and can actually make a "choice" about the path they are going to take, so that an entangled pair of photons "agree" as to where they will be in the future. This has the affect of looking to us like there is an "instantaneous transmission" of information from one to the other, which would violate causality from the standard view. The "choice" is the ability of the photon to "feel out" all the possible paths it could take, and select an event in the future to which to tie itself to. This might be interpreted as a basis for "fate", which is fine by me, since that's the way I lean anyhow.

Obviously (as IANAP) this explanation is worded poorly and not really an accurate representation of the weird math involved. But, while information is "traveling back in time", I don't think there is a practical way to use this effect as a communications medium. Maybe you could have four entangled beams (two each for two observers)?

Not entirely new...

[Pig Hogger]

It is possible now to view hidden objects by ultrasonic holography... I believe that this process has even been (gasp!) patented more than 25 years ago...

The object has to be at the bottom of a pool filled with some opaque liquid; a transducer is immersed, bathing the object with ultrasound. Sound waves reflect on the object, and they form an interference pattern on the surface, which is lit by coherent light, thus forming a virtual image of the object.

One caveat, though... Given the ***BIG*** difference of wavelength, the virtual image appears to be quite far, and has to be viewed with a telescope...