Quantum ‘Weeping Angel’ Effect Freezes Atoms In Place

Quantum ‘Weeping Angel’ Effect Freezes Atoms In Place

The paradox of Schroedinger’s Cat famously demonstrates that a quantum cat sealed in a box is both alive and dead at the same time until we look inside, at which point it becomes one or the other. Such is the weirdness of quantum mechanics. But if a mere act of observation determines the outcome of an experiment, what happens if we never look away? Answer: time effectively stands still.

That’s the conclusion of a new paper accepted for publication in Physical Review Letters. Cornell University physicists constructed an elaborate experiment to demonstrate that making a series of rapid measurements of atoms — equivalent to looking at the system without blinking — essentially freezes matter in place. It’s a bit like one of Doctor Who‘s Weeping Angels, those creepy statues who are said to be “quantum locked”: they can only move when they’re not being directly observed.

This is the quantum version of one of Zeno’s paradoxes, first proposed by an ancient Greek philosopher named Zeno of Elea, who liked to mess with people’s assumptions. Think of it this way. In order for a Weeping Angel to move from one point to another, it must first cross half the distance to that point. But in order to reach that halfway mark, it must first cross half the distance of that, and then half the distance of that half distance, and so on, ad infinitum. Zeno concluded — tongue very firmly in cheek — that this “proved” the angel could never get from point A to point B, and therefore motion was impossible.

Sometimes thought experiments are best left in the realm of abstract philosophizing. Because of course it is possible for the Weeping Angel to move from A to B (and then zap some poor sod back in time, feeding off the stolen “potential energy”).

But in the subatomic world, where quantum mechanics reigns supreme, something very similar to this paradox actually occurs. Slice time into small enough increments, and everything really does freeze in place. It’s known as the quantum Zeno effect.

Let’s briefly revisit the basics of Schroedinger’s Cat, courtesy of the Big Bang Theory‘s Sheldon Cooper:

It’s the decay of radioactive atoms that matters here, since that’s what triggers the release of the poison — or not. There are two possible states: A (the atoms are not decayed) and B (the atoms are decayed). If we never look in the box, as time passes, a superposition of both states A and B will arise. It’s only when we look inside that this weird superposition collapses into either A or B.

Back in 1977, physicists suggested that if you kept looking in the box continuously, so to speak — i.e., made measurements at such short intervals that you were essentially making a continuous measurement — there would be no decay, because the system has no time to evolve into a superposition. Instead, it keeps collapsing back to its original state. As io9’s Esther Inglis-Arkell wrote a couple of years ago:

Let’s say an atom is very likely to have decayed after three seconds, but very unlikely to have decayed after one. Check on it after three seconds, and it probably will have decayed. But… check on it three times in one second intervals, and it will most likely not have decayed. Every time you check on it, it will revert to its “original” measured state, and the clock will start over.

So a watched quantum pot never boils. And a watched Weeping Angel can’t move.

It’s not just theoretical. The Cornell experiment is just the latest in a series of experiments since then confirming that the quantum Zeno effect really happens. (There’s also an “Anti-Zeno Effect,” whereby staring at the metaphorical quantum pot brings it to a boil more quickly — also experimentally confirmed.)

The Cornell team used lasers to trap a gas of rubidium chilled to super-cold temperatures in a lattice of light. Thanks to the peculiarities of quantum mechanics, every now and then, an atom manages to tunnel out of the trap. But when they repeatedly zapped the atoms with laser pulses at shorter and shorter intervals — the equivalent of looking inside Schroedinger’s box again and again and again — they found this makes it more difficult for trapped atoms to tunnel out. When the intervals become short enough, the atoms make like a Weeping Angel and are effectively frozen in place.

It’s not an exact analogy, of course, since the angels are science fiction. Per the folks at Physics Buzz:

In [Doctor Who], 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.

So remember: DON’T BLINK! Especially if you’re a cloud of rubidium atoms.

[Via Physics Buzz]


Fischer, M.; Gutiérrez-Medina, B.; Raizen, M. (2001) “Observation of the Quantum Zeno and Anti-Zeno Effects in an Unstable System,” Physical Review Letters 87 (4): 040402

Patil, Y.S.; Chakram, S.; and Vengalattore, M. (2015) “Quantum control by imaging: the Zeno effect in an ultra cord lattice gas,” Physical Review Letters, Preprint. [also on arXiv]

Sudarshan, E.C.G. and Misra, B. (1977) “The Zeno’s paradox in quantum theory,” Journal of Mathematical Physics 18(4): 756-763

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