ELI5 the tardigrade quantum entanglement experiment

Submitted by night_trotter t3_10jec9x in explainlikeimfive

Here is an article for reference: https://www.space.com/tardigrade-quantum-entangled-experiment

I’m not sure how the tardigrade could be entangled. From my (limited) understanding, entangled particles don’t remain entangled if they are far apart or if something else “touches” one of them, which is not at all how it is explained in the article.

This experiment doesn’t make sense to me with those rules. But I’d like to understand better because I’m really interested in what is happening in the experiment and what exactly this means for science if it’s reproducible and passes peer reviews.

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TactlessTortoise t1_j5jzuw2 wrote

According to the article itself, their experiment seems to be bogus, not much unlike me electrocuting myself on a power outlet and claiming to be a lightning wizard.

What they did was entangle two qubits the same way as always, and then touched the little guys on it to "pair them up" like some sort of quantum bluetooth.

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UntangledQubit t1_j5kjyz0 wrote

This high level description isn't fundamentally different to how you would actually entangle macroscopic objects. We know how to create microscopic superposition states, and we know that entanglement spreads by interaction. That's the point of the Schroedinger's cat experiment - the decaying atom interacts with the cat through the detector+poison vial, and this interaction entangles them, putting both in a very distinct superposition of vastly different states (decayed and not decayed, and dead and not dead).

The question here is whether the tardigrades were meaningfully entangled with the qubit states by this specific interaction (acting as a dielectric on capacitors within the quantum system). The skeptics say that the interaction of the tardigrades with the actual qubit states is so weak, there is effectively no correlation between the two.

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TactlessTortoise t1_j5kl44o wrote

Yes, I didn't mean that the idea in itself was bogus, but they had no way to actually validate that anything happened.

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slinger301 t1_j5lw6bd wrote

Personally, I was getting Star Trek: Discovery vibes here.

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Chromotron t1_j5kblhh wrote

> I’m not sure how the tardigrade could be entangled. From my (limited) understanding, entangled particles don’t remain entangled if they are far apart or if something else “touches” one of them, which is not at all how it is explained in the article.

There is no known or conjectured limit on the distance of entangled particles. We have created and maintained entanglement over kilometers (newest result between two atoms on Earth was 50km; photons in space was 1200km). You can also perfectly well "touch" them; if A, B are entangled, you can entangle B with C without it loosing the entanglement with A. It is unknown what the limit for the amount of entangled atoms is, or if there even is one.

Hence it is potentially possible to entangle a tardigrade. The issue here is the question if that actually happened and many disagree. But the discussion is still up and should be left to those that actually work on such things on a daily basis.

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UntangledQubit t1_j5kmlkv wrote

> From my (limited) understanding, entangled particles don’t remain entangled if they are far apart or if something else “touches” one of them, which is not at all how it is explained in the article.

Entanglement is not an all-or-nothing deal. Entanglement is correlation + superposition. If two particles are in a 'large' superposition (they are simultaneously in multiple states which are very distinct from each other), and their states are highly correlated with each other, they are very entangled. This is what typical spin entangled states look like. One particle is 50% spin up and 50% spin down, the other is 50% up and down, and they are maximally correlated - they will always be spinning in opposite directions. When you observe one particle, you precisely know the state of the other.

However, it's also possible to have very weak entanglement. For example if one particle is in a location superposition, and it interacts with another particle from a distance, their trajectories are now entangled. But this entanglement can be arbitrarily weak. If you observe the location of one particle, you gain a tiny amount of information of the other one - if you thought the location would be clustered around some target, you know that cluster might have shifted a little bit. But it doesn't particularly help you narrow down the location.

There's no strict line between weak entanglement and no entanglement. Extremely weak entanglement still exists in macroscopic interactions. Our classical world arises from continuous weak entanglement of the environment correlating everything's state, so we don't see any disconnected parts of the world existing in large superpositions.

That's what the skeptics are saying. The tardigrade is entangled with the qubit in a very weak sense. If you had a godlike view that could measure the exact quantum state of every particle in the tardigrade's body, you would notice a very slight correlation between them and the state of the qubit. The tardigrades are not in a clean set of two possible states that exactly correspond to the qubit, they are in the same multitude of microstates that continually interact with surrounding objects and some probabilities of those microstates have shifted around just a little bit.

There have been experiments that do actually entangle macroscopic objects, e.g. large mechanical oscillators. The 'Results' section is the most informative - not only does there have to be interaction with some microscopic entangled states, but the states of the oscillators should also be highly correlated, and the statistical properties have to demonstrate that they are in fact in a non-classical superposition. The tardigrade experiment did not do this clear work.

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