Physicists Demonstrate "Weeping Angel" Effect

BBC / Doctor Who.

Physicists from Cornell University have demonstrated experimentally that matter can be “quantum locked,” or frozen in place, with repetitive, rapid measurements. In their paper, recently published in the Physical Review Letters, the team explains how the quantum Zeno effect can restrict the quantum tunneling of confined particles.

We know that the quantum world is weird, and the concept of quantum tunneling is no exception. This phenomenon, made possible by quantum mechanics, shows how trapped particles can tunnel through a barrier that it would otherwise not be able to cross. Imagine you are on one hill and you want to roll a ball over top an adjacent hill. The ball has to have enough energy to make it over the hill; if not, it will roll back down and be trapped in the valley below. In the realm of quantum mechanics, objects behave differently, and the ball sometimes makes it over the hill even though it lacks the required energy. How is this possible? Quantum particles can exhibit wave-like behaviors, and as such, a particle’s wave function can pass through the barrier, allowing the particle to “tunnel through.”  

For this experiment, the team trapped atoms of ultracold rubidium gas in an optical lattice. The rubidium atoms naturally seek out the lowest possible energy states, and would become trapped in the energy wells of the lattice – just like the ball between the two hills – but still have the potential to escape thanks to quantum mechanics. The researchers discovered they could prevent the tunneling, and essentially freeze the atoms in place, by repeatedly zapping the atoms with lasers. To perform the experiments, the team developed a new imaging technique that allowed them to observe the rubidium atoms while not affecting their quantum state. “It took a lot of dedication from these students and it has been amazing to see these experiments be so successful,” Mukund Vengalattore, assistant professor of physics, said in a statement. “We now have the unique ability to control quantum dynamics purely by observation.”

We learned from Schrodinger's famous thought experiment that a hypothetical cat could simultaneously be alive and dead. It wasn’t until we looked in the box and observed the system that we knew the outcome. In this new experiment, we find out what happens if we never stop observing. The team’s research shows that if the atoms are repeatedly zapped with lasers at shorter and shorter intervals – as if under continuous observation – the atoms can be frozen in place. This effect, known as the quantum Zeno effect, basically says we can “freeze” matter if we never stop observing it. If you’re a Whovian, this might sound a bit familiar.

In the science fiction series, "Doctor Who," an imaginary race of aliens – known as the Weeping Angels – kill their victims “kindly” by transporting them back in time to live out their lives while the angels feed off the resulting potential energy. They are unique as they exhibit a quantum locked effect: The angels can’t move as long as you are looking at them. So don’t blink.

The team’s research does not show us that weeping angels exist (what a scary thought that would be), but that the principle behind their movement can be demonstrated in the lab. According to Physics Central, “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.”

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