[olePigeon]

Technology Review: Blogs: arXiv blog: How to Create Quantum Superpositions of Living Things

Yehaw! Next step is bacteria, then amoeba, then... who knows.

Quick background:

Particles can be in a superposition state, meaning, they're both positive and negative simultaneously. Only after you observe the particle (measure it) do you find out which one it is. Erwin Schrödinger came up with a cool thought experiment to explain the phenomenon. Assuming you can keep the objects secluded from all external forces and observation, put a cat, some poison, and a geiger counter in a box. There is a 50% chance the radioactive substance will decay fast enough and the cat will live, and a 50% the radioactive substance won't decay fast enough and the cat dies. However, with the result unknown, the cat is in a superposition... it is both alive and dead simultaneously. Not until we open the box and look does the cat manifest in one state only, either living or dead.

Particles in a superposition state are also said to be in a "cat state," hence Schödinger's Cat. In an equally interesting and related phenomenon, we also have quantum entanglement, or Spooky Action at a Distance. But I'll save that for another thread.

I'm aware that viruses are not technically alive, however, as it relates to this experiment, they can at least determine if the virus is "alive" or "dead."

[wallinbl]

The cat is actually alive or actually dead. The fact that we egotistical humans don't know the answer doesn't void the reality of the answer.

I have no idea whether the particles exhibit this behavior, but it's safe to say that the cat doesn't.

[olePigeon]

Originally Posted by wallinbl 

The cat is actually alive or actually dead. The fact that we egotistical humans don't know the answer doesn't void the reality of the answer.

Egotism has nothing to do with it. That's how quantum physics works. The cat is both alive and dead simultaneously. It's not a philosophical question.

Originally Posted by wallinbl 

I have no idea whether the particles exhibit this behavior, but it's safe to say that the cat doesn't.

All known particles can exhibit this behavior, therefor the cat can as well.

[wallinbl]

Cat != quantum particle.

[olePigeon]

Originally Posted by wallinbl 

Cat != quantum particle.

The cat is composed of many particles, all of which can exist in a superposed state. Given the proper conditions, the entire cat will be in a superposition.

That's the point of the experiment I linked to.

[olePigeon]

Keep in mind I'm a complete layman when comes to physics, so I'm trying to explain it as best I can.

[OreoCookie]

Oh boy, the most misunderstood thought experiment in physics

Originally Posted by olePigeon 

Egotism has nothing to do with it. That's how quantum physics works. The cat is both alive and dead simultaneously. It's not a philosophical question.

That's false, the cat is either alive or dead with a probability that's essentially 1: first of all, everything is quantum mechanical: humans, the box, our measurement apparatus, whatnot. The superposition is destroyed by interaction of the cat with its surroundings (the box and everything else inside as well as outside the box). The technical term for this is decoherence. Even if you perturb the Schrödinger cat superposition state just a tiny bit, the cat consists of more than 10^23 atoms. Even if the probability that one atom is in a superposition is 99 %, the probability that a sizeable number of atoms (say, 10^10 -- which is just a tiny fraction of the cat) is already just 0.99^{10^10} is already so small that you can take it to be 0.

The common misconception that is perpetuated through popular literature and media is that the fact of us humans looking at it changes something: as if the moon isn't there if we're not looking or there are clouds in the sky. Us looking is just a special form of interaction of one system (= us, human) with the experiment (cat in the box with poison). There are many other interactions that happen even if we're not looking.

Originally Posted by olePigeon 

All known particles can exhibit this behavior, therefor the cat can as well.

Yes, true. Ditto for all other objects. Just the probability that you will observe it is miniscule. So tiny that you will not see it.

There are experiments with larger molecules, e. g. bucky balls (C60), and seeing quantum interference is already very tricky. That's only 60 atoms! There are plenty of new experiments which can be used to probe and move the boundary (e. g. the cavity QED experiments mentioned in the reference) and it's truly fascinating that people can reproduce pretty much any thought experiments dreamt of in the 1930s There is a French group in Paris who can manufacture a cavity (that's basically just a box with small holes and two very, very, very, very, very, very, very, very, very, very, very, very good mirrors for microwaves) which can be used to trap single photons. They decay in something like 0.03 seconds! Sounds small, but enough time so that the photon can bounce off the mirrors more than a billion times without being absorbed!

[olePigeon]

Originally Posted by OreoCookie 

That's false, the cat is either alive or dead with a probability that's essentially 1: first of all, everything is quantum mechanical: humans, the box, our measurement apparatus, whatnot.

I always understood the cat as representing a single particle in a controlled experiment, so the cat is essentially alive and dead (positive/negative spin.) It's just easier for nitwits like me to understand the concept; kinda like using oranges and apples instead of numbers when doing math.

Originally Posted by OreoCookie 

Even if you perturb the Schrödinger cat superposition state just a tiny bit, the cat consists of more than 10^23 atoms. Even if the probability that one atom is in a superposition is 99 %, the probability that a sizeable number of atoms (say, 10^10 -- which is just a tiny fraction of the cat) is already just 0.99^{10^10} is already so small that you can take it to be 0.

In a controlled experiment (assuming you have the tools), you can set all particles to a superposed state. That's what they hope to accomplish in the linked experiment. All particles in the virus will be set and excited at the same time. Not saying it'll be easy, as you've pointed out.

Originally Posted by OreoCookie 

Us looking is just a special form of interaction of one system (= us, human) with the experiment (cat in the box with poison). There are many other interactions that happen even if we're not looking.

Yes, but the idea of "looking" at the cat is easy for many people to understand, even if they misconstrued the looking as literal. I made that distinction by using observing/measuring.

Originally Posted by OreoCookie 

There are experiments with larger molecules, e. g. bucky balls (C60), and seeing quantum interference is already very tricky. That's only 60 atoms! There are plenty of new experiments which can be used to probe and move the boundary (e. g. the cavity QED experiments mentioned in the reference) and it's truly fascinating that people can reproduce pretty much any thought experiments dreamt of in the 1930s There is a French group in Paris who can manufacture a cavity (that's basically just a box with small holes and two very, very, very, very, very, very, very, very, very, very, very, very good mirrors for microwaves) which can be used to trap single photons. They decay in something like 0.03 seconds! Sounds small, but enough time so that the photon can bounce off the mirrors more than a billion times without being absorbed!

Don't forget about using particle spin for quantum computing.

[OreoCookie]

Originally Posted by olePigeon 

I always understood the cat as representing a single particle in a controlled experiment, so the cat is essentially alive and dead (positive/negative spin.) It's just easier for nitwits like me to understand the concept; kinda like using oranges and apples instead of numbers when doing math.

Well, that's the essential difference: the cat isn't microscopic.
The point of the Schrödinger cat gedanken experiment is that you couple the state of something microscopic (the atom that may or may not decay after the preset amount of time) with the state of something macroscopic: the atom really is in a superposition which you can measure (= see). The cat is always macroscopic and thus follows different rules: the cat's superposition decays even after the slightest perturbation from the environment (e. g. a few gas molecules or photons that hit the cat). Sort of like supercooled water freezes immediately after you perturb it a little.

This is what happens to the cat. New experiments allow us to see exactly at which point decoherence destroys the alive/dead superposition.

Originally Posted by olePigeon 

In a controlled experiment (assuming you have the tools), you can set all particles to a superposed state. That's what they hope to accomplish in the linked experiment. All particles in the virus will be set and excited at the same time. Not saying it'll be easy, as you've pointed out.

That's right, just as with supercooled water, there is no doubt a lot can be done after careful preparation. The biggest issue is to minimize interactions with the environment. Practically this means you cool down your sample and do the experiment in vacuum.

Don't get me wrong, I'm very, very stoked to see things like this realized in experiment. I've heard a mini course by a French top researcher and all of my colleagues were just flabbergasted of what they can do. Really.

Why I'm writing so much text is to fight the perpetuated myth of Schrödinger's cat. Although I don't like to quote him in this context, Steven Hawking has said that if somebody asks him about Schrödinger's cat again, he'll go and get a gun. The misconception of the thought experiment concerns `what is and isn't a measurement' and microscopic vs. macroscopic.

Originally Posted by olePigeon 

Yes, but the idea of "looking" at the cat is easy for many people to understand, even if they misconstrued the looking as literal. I made that distinction by using observing/measuring.

This is the second misconception: there is no difference whatsoever between a `measurement' and an `interaction.' Measurements are just interactions we humans taylor to our liking. What destroys the superpositions are any sort of interactions.