The results of the world’s smallest double slit experiment are in. The experiment itself, is one of the basic proofs of the wave nature of light and of matter (deBroglie waves). The question this experiment was raising is what happens to the wave nature of an electron when it is probed at the very smallest level? Is it still there? Or does a particle nature start to dominate – so that we can think of wave-nature as only a macroscopically observed phenomenon?

From the write-up:

“The wave nature of the electron means that in a double slit experiment even a single electron is capable of interfering with itself. Double slit experiments with photoionized hydrogen molecules at first showed only the self-interference patterns of the fast electrons, their waves bouncing off both protons, with little action from the slow electrons.

‘From these patterns, it might look like the slow electron is not important, that double photoionization is pretty unspectacular,’ says Weber. The fast electrons’ energies were 185 to 190 eV (electron volts), while the slow electrons had energies of 5 eV or less. But what happens if the slow electron is given just a bit more energy, say somewhere between 5 and 25 eV? As Weber puts it, ‘What if we make the slow electron a little more active? What if we turn it into an ‘observer?”

As long as both electrons are isolated from their surroundings, quantum coherence prevails, as revealed by the fast electron’s wavelike interference pattern. But this interference pattern disappears when the slow electron is made into an observer of the fast one, a stand-in for the larger environment: the quantum system of the fast electron now interacts with the wider world (e.g., its next neighboring particle, the slow electron) and begins to decohere. The system has entered the realm of classical physics.

Not completely, however. And here is what Belkacem calls ‘the meat of the experiment’: ‘Even when the interference pattern has disappeared, we can see that coherence is still there, hidden in the entanglement between the two electrons.’ (emp. added)

Although one electron has become entangled with its environment, the two electrons are still entangled with each other in a way that allows interference between them to be reconstructed, simply by graphing their correlated momenta from the angles at which the electrons were ejected. Two waveforms appear in the graph, either of which can be projected to show an interference pattern. But the two waveforms are out of phase with each other: viewed simultaneously, interference vanishes.

If the two-electron system is split into its subsytems and one (the ‘observer’) is thought of as the environment of the other, it becomes evident that classical properties such as loss of coherence can emerge even when only four particles (two electrons, two protons) are involved. Yet because the two electron subsystems are entangled in a tractable way, their quantum coherence can be reconstructed. What Weber calls ‘the which-way information exchanged between the particles’ persists.”

Read the rest here.

Or in the words of the Matrix, “There is no spoon.”