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

Schroedinger's Cat

[gblues]

Great! Now we'll be able to tell Schroedinger once and for all whether his stupid cat is dead or not.

Nathan

Re:Schroedinger's Cat

[Faramir]

This is a good question, and there have already been several good answers. However, I don't feel like they've really answered your question.

Far from destroying the uncertainty principal, the article indicates that one of the "spooky" things about quantum holography is, essentially, the exploitation of the uncertainty principle.

Now, as to direct observation and the uncertainty principle: perhaps these should be explained for the casual /.'ers out there.

The uncertainty principle says that we cannot know exactly both the position and momentum at the same time. Momentum is a combination of mass and velocity. Mass often remains constant, so sometimes this is stated as "position and velocity" instead. Now, I used the word "exactly", and I meant just that. We can have a good idea of both numbers, but the more exact one measurement is, the less exact the other measurement will be. Basically, think of it this way: if we take a probe, like the tip of a pencil, and move it around till we find exactly were a particle is, we'll find it. But we'll also hit it and change its momentum.

Now, all observations require some kind of probe, be it pencils, electrons, or photons (light). A related feature of quantum mechanics is that the equations we use to determine where a particle (or wave, they're the same thing at this level) is going (the famous Schrodinger equations) don't actually tell us where a particle is going--only where its likely to go. So we don't even know how to say where it is going to go. In fact, it is considered that a particle does not have just one specific path until the particle has been measured.

In our case, that measurement--that is, the observation of the photons--occurs at the wall of the chamber. And from this data, convoluted equations work backwards to figure out what the photons bounced off of.

Hope that helps...

No luggage scanning here

[Rob Parkhill]

The article seems to imply that you need a specially constructed sphere to make this work. One that lets light in at a specific point, and allows no light out. It also is built in such a way to detect when a photon hits the inside surface. Just take a look at the diagram.

So unless someone is stupid enough to try and sneak a bomb onto a plane in one of these spheres, it's not much use to the security guards.

Dunno if their idea works...

[jd]

...but I get to check two boxes in Slashdot Buzzword Bingo. Just a few more to go....

Re:More important implmentations

[Syberghost]

No more exploratory surgery. Quickly detect cancer growths.

Yeah, I can see it now:

"After putting you in this big sphere and exposing you to massive amounts of electromagnetic radiation, we've determined that you do indeed have skin cancer."

Damnit!

[nanojath]

Yeah yeah, it's all funny but it ticks me off that nobody is pointing out that The principle illustrated in Schroedingers "cat" thought experiment are NOT THE SAME as the Heisenberg Uncertainty Principle. In fact, it ticks me off that nobody knows what the Uncertainty Principle is really about and people constantly confuse it with the whole indeterminate quantum particle state and whether does in fact create quantum indeterminacy on the macro scale (if a tree falls in the forest...) issue. Heisenberg's Uncertainty Principle establishes a mathematically defined absolute uncertainty balanced between the momentum and position of a quantum scale particle. The corresponding thought experiment would be the gamma ray microscope.

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".)