Friday, November 09, 2007

The "Simplest" 2-Slit and Decoherence Experiment

While the big news this week is the major report out of the Auger Collaboration, the other significantly as important paper in the same issue as Science is a very elegant and astounding report.

The Simplest Double slit: Interference and Entanglement in Double Photoionization of H2, D. Akoury et al., Science v.318, p.949 (2007).

Abstract: The wave nature of particles is rarely observed, in part because of their very short de Broglie wavelengths in most situations. However, even with wavelengths close to the size of their surroundings, the particles couple to their environment (for example, by gravity, Coulomb interaction, or thermal radiation). These couplings shift the wave phases, often in an uncontrolled way, and the resulting decoherence, or loss of phase integrity, is thought to be a main cause of the transition from quantum to classical behavior. How much interaction is needed to induce this transition? Here we show that a photoelectron and two protons form a minimum particle/slit system and that a single additional electron constitutes a minimum environment. Interference fringes observed in the angular distribution of a single electron are lost through its Coulomb interaction with a second electron, though the correlated momenta of the entangled electron pair continue to exhibit quantum interference.

A review of this work can also be found at PhysicsWorld (free registration is required) and at PhysOrg.

What they essentially did is this. They use the H2 molecule as a "double slit". The different between the regular double-slit experiment is that the regular experiment typically uses plane waves, whereas here, you essentially get spherical waves originating from each of the H atom in the molecule. Still, the physics is the same and you get angular interference pattern. They used photons of energies 240 eV and 160 eV to cause a double-photoionization of the H2 molecules, resulting in 2 electrons, but with varying energy.

When they use photons of 240 eV, the electron tagged as "1" comes out at between 185 and 190 eV, while the second electrons comes out with less than 5 eV. Due to the widely different kinetic energy, electron 1 doesn't really "see" electron 2, so electron 1 essentially can be described via single-particle physics. This is where you get the expected interference pattern as the experiment is repeated many times.

But when they use photons of 160 eV, they get two different cases. The first case is electron 1 emitted with energy 110 eV and electron 2 emitted with energy < 1 eV. They get roughly the same result as before. However, for the 2nd case where electron 1 has energy of 95 eV and electron 2 has energy of between 5 and 25 eV, the interference pattern almost completely disappear! In this second case electron 1 has now "seen" electron 2, and has sufficiently coupled to electron 2 to demolish the single-particle description of it. In essence, electron 2 has caused a decoherence of the single-particle description of electron 1, cause electron 1 to behave classically.

What this report is saying is that it only requires ONE particle and one interaction to induce a decoherence of a single-particle picture. Electron 2 is sufficient to be the "environment" that electron 1 couples to to induce such decoherence. Interestingly enough, while the single-particle picture is no longer valid for electron 1, both electron 1 and 2 are now "entangled" and their correlated momenta actually can still exhibit quantum interference. The system has evolved into a 2-particle state.

I love, LOVE clever experiments like this!


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