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It's Possible to Keep Schrodinger's Cat Alive Without Ever Opening The Box

Just keep shaking it.

MIKE MCRAE
19 JUN 2017
 

Researchers have used an artificial atom to show it's possible to keep Schrödinger's cat alive indefinitely, but also accelerate its demise, all without needing to look inside its box.

Using classic analogies like this might seem simplistic or weird, but this work has huge implications for science. It actually reveals how reality operates on a fundamental level, and could also lead to better tools for physicists to use in quantum engineering.

 

The scientists at Washington University in St. Louis set out to explore whether it's necessary to actually collect information from a quantum system – or in simple terms, look at a particle – in order to affect its behaviour, or if disturbing it will be enough.

Spoiler alert: they've found that you really don't need to look.

Some history: A cat, a box and Zeno effects

For those of you who don't know much about Schrödinger's cat, here's the basics. According to the Copenhagen interpretation of quantum mechanics, a physical object, such as an atom, doesn't have defined properties until we measure them.

In response, physicist Erwin Schrödinger proposed a thought experiment. He argued that if that was true, we could put a radioactive material into a small container next to a Geiger counter, link the counter to a hammer, and put the hammer over a flask of acid ready to smash it the moment the atom decays.

If the whole thing is put inside a box with a cat, we can't measure the atom's properties, so as far as we know, the atom has both decayed and not decayed. As a consequence, the cat is alive and dead until we look.

 

This is the story most people have heard. But there's a twist.

In 1974, researchers wondered, "Does the lifetime of an unstable system depend on the measuring apparatus?"

In what has become known as the quantum Zeno effect, physicists ask what would happen if we constantly watched an unstable atom? Would it decay?

According to the Zeno effect, if constantly measured, it would never emit a single particle of radiation.

This was actually demonstrated for the first time in 1989 in an experiment conducted by the US National Institute of Standards and Technology, taking the thought from a peculiar hypothesis to strange reality.

Just under a decade later, the opposite of the Zeno effect was proposed – an anti-Zeno effect. Frequently measuring a radioactive atomic nucleus could also speed up its decay, depending on how it was done.

This clip below might help it make more sense:

One of the big questions is what exactly does "measurement" mean?

To measure something like a radioactive atom, something needs to interfere with it so information of some sort can come out. In doing so, the atom's multiple possibilities collapse into a single outcome, the one we can see.

But is this collapse the cause of the Zeno effect? Or can the likelihood of the atom's decay be sped up or slowed down without causing it to collapse into an absolute state?

 

Back to now: Zeno versus anti-Zeno

This all brings us back to the experiment conducted at Washington University.

To determine if it's the prodding or the transfer of information at work behind both of the Zeno and anti-Zeno effects, the researchers used a device that behaved for most purposes like an atom with multiple energy states.

This "artificial atom" could test a hypothesis on how energy states called electromagnetic modes could be responsible for these effects.

 

"Atomic decay rates depend on the density of possible energy states, or electromagnetic modes, at a given energy," said researcher Kater Murch.

"In order for the atom to decay, it must emit a photon into one of these modes. More modes means more ways to decay, and therefore faster decay."

By the same token, fewer modes means fewer options to decay, which can explain why this atomic watched pot would never boil.

Murch and his team managed to manipulate the number of modes in their artificial atom before using standard measurements to check its state once every microsecond, increasing or decreasing their artificial atom's "decay".

"These measurements constitute the first observation of the two Zeno effects on a single quantum system," said Murch.

To see if it was the observing or the interference that was ultimately responsible, the researchers did what's called a quasi-measurement, which is basically creating the interference without actually resulting in a collapse of the atom's state.

The team weren't sure what they'd find.

"But days of data taking conclusively showed that the quasi-measurements caused the Zeno effects in the same way as the usual measurements," said Murch.

That means it's the disturbance of the measuring and not the actual measurement itself that gives rise to the Zeno effect and anti-Zeno effect.

Knowing this could provide new ways of controlling quantum systems using Zeno dynamics.

So what does all of this mean for poor old Schrödinger's cat?

"The Zeno effect says that if we check on the cat, we reset the atom's decay clock, keeping the cat alive," said researcher Patrick Harrington.

"The twist, however, is that because the Zeno effects have to do with disturbance and not information, it isn't even necessary to look inside the box to provoke them. The same effects will occur if you just shake the box."

This research is published in Physical Review Letters.

 

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