A team of physicists from Cornell has shown that rapid, repeated measurements can freeze matter in place, in a paper recently accepted for publication by Physical Review Letters. The phenomenon, called the quantum Zeno effect (after the Greek philosopher famous for posing tricky questions about arrows and tortoises), limits the quantum tunneling ordinarily exhibited by confined particles.
By using constructive and destructive interference of laser light, the scientists constructed an optical lattice—a standing field of high- and low-energy nodes, to which they introduced atoms of an ultracold rubidium gas. Naturally seeking the lowest-possible energy state, those atoms would get trapped in the potential energy wells, but with some probability of “tunneling” out of them, one of the fundamental peculiarities of quantum mechanics.
Imagine, in two dimensions, a ball rolling between two hills. If the ball isn’t moving fast enough, it can’t make it over the hill, so it’ll roll back down into the valley and up the other slope. If the other slope is of the same height, the ball stays trapped, oscillating between the two. But if we shrink this system down to the quantum level, things start to behave differently; the ball sometimes ends up making it over the hill, even when it didn’t seem to have enough energy to do so. There are a number of ways to explain how this tunneling behavior arises, but it’s perhaps simplest to think of it as a sort of “wind” in the valley. When the system is large, the random air currents around the ball are negligible, but as it shrinks down, their relative effect becomes greater. Eventually, the system becomes small enough that a particle whose energy is close to the “barrier potential” can be blown over the hilltop by a gust of wind, freeing it from its potential well. But by eliciting fluorescence from the atoms trapped in the potential wells of their optical lattice, the researchers found a way to put a stop to that, effectively locking the atoms in place.
When an atom goes from an excited state to the ground state, it can lose energy in the form of a photon, and since the energy levels of an atom are quantized, that photon is always of a certain energy. By bombarding the atoms with photons just below that energy, they induce the atoms to “tunnel” to the fluorescent state. When they fluoresce, the atoms give off more energy than the imaging photon imparted, so the whole process saps energy from the atoms, reducing the probability that they’ll tunnel out of their potential wells. If little enough time elapses between these fluorescence-inducing interactions, the atoms can effectively be frozen in place indefinitely (although the high photon intensity necessary to guarantee this create difficulties, as it can heat the atoms out of the requisite ultracold state).
This is something like the imaginary “quantum lock” effect from the famous TV series Doctor Who, where malicious aliens—the “weeping angels”—can’t move if they’re being looked at. In the television show, though, there’s something about conscious observation that makes this work; the photons bouncing off the angels have to land in someone’s eye to freeze them in place. In reality, however (extrapolating generously from this experiment), such a creature could only move in complete darkness, or perhaps only under certain wavelengths of light. For these atoms, it’s not the photo that freezes them in place, it’s the camera’s flash.