Quantum of Solace

In Feynman’s version of the double-slit experiment, single photons are emitted at intervals. Over time, they show a classic interference pattern on the back wall. If we place a detector by each slit, to observe the passage of the photon, the back wall gets two overlapping bell-curves with no interference (a blob). A variation of this experiment can be performed by using polarizers as the “detector” by the slits. They provide data on which path (slit) the photons take.

Quantum Entanglement

The quantum world is full of complementary variables such as position/momentum and wave/particle. The equations describing quantum mechanics are such that precise knowledge of one complementary variable precludes knowledge of the other–regardless of the measurement technique. Even if we could devise a way of accurately measuring position without affecting momentum, the momentum variable would still be imprecise. This is quantum uncertainty as defined by the Copenhagen interpretation, subject to Heisenberg’s Uncertainty Principle.

The authors provide a recently-developed take on what is happening in the double-slit experiment. By observing the a quantum variable, it becomes “entangled” with the measurement variable. For example by using a polarizer as the measuring device, the polarization variable becomes associated with the position variable such that neither can be considered separate from the other.

Thus we see that even a completely transparent act of observation has an effect on quantum variables. They are entangled with the observation variables. This may be the reason that we see the weird quantum effects previously attributed to quantum uncertainty or multiverses. (I am not describing this as well as I could, and I hope I’m not making any errors.)

Quantum Erasure

This new concept of entanglement leads to the idea of quantum erasure. The authors of the article created a polarization setup using entangled photons. Photon A goes through the double-slit. Photon B goes through a polarizer to a detector. If the polarizer for photon B provides which-path information, then the back wall for photon A will be a blob. If the polarizer for photon B does not provide which-path information, then the back wall for photon A will be an interference pattern. Thus, the quantum collapse of photon A is completely dependent upon photon B. This is quantum erasure. By manipulating photon B, the experiment erases quantum possibilities for photon A.

In accordance with quantum weirdness, of course, the authors can’t let it rest at that. They discovered that it doesn’t matter which photon arrives first. If photon A goes through the slit and hits the back wall detector before photon B gets to the polarizer, the result is the same. So somehow photon A has “knowledge” of the future observation of photon B’s path.

Summary

The article presents two new aspects of quantum mechanics. The first is the replacement of quantum uncertainty by quantum entanglement. The second is quantum erasure. Both of these will play a significant role in the future of quantum theory.

This post, which appeared back on Jul 21, 2003 referred to a The July-August 2003 edition of American Scientist [[*NOT* Scientific American]] has an interesting article on quantum erasure. (Note that the online article is available only to subscribers or members of Sigma Xi.)

So, what’s a Quantum of Solace (here for the movie trailer? Well, it was the title of an Ian Fleming short story and is apparently a description of the emotional state one exists in at the time of losing love…at least that’s how 007 actor Daniel Craig seemed to describe it on TV this week. So, Fleming presumably was describing the smallest amount of solace one feels at such times, perhaps the antithesis of la petite morte…who knows?