Saturday, December 18, 2021

No, a tardigrade was not meaningfully entangled with a qubit

This week this paper appeared on the arxiv, claiming to have entangled a tardigrade with a superconducting transmon qubit system.  My readers know that I very rarely call out a paper in a negative way here, because that's not the point of this blog, but this seems to be getting a lot of attention, including in Physics World and New Scientist.  I also don't know how seriously the authors were about this - it could be a tongue-in-cheek piece.  That said, it's important to point out that the authors did not entangle a tardigrade with a qubit in any meaningful sense.  This is not "quantum biology".

Tardigrades are amazingly robust.  We now have a demonstration that you can cool a tardigrade in high vacuum down to millikelvin temperatures, and if you are sufficiently gentle with the temperature and pressure changes, it is possible to revive the little creature.  

What the authors did here was put a tardigrade on top of the capacitive parts of one of two coupled transmon qubits.  The tardigrade is mostly (frozen) water, and here it acts like a dielectric, shifting the resonance frequency of the one qubit that it sat on.   (It is amazing deep down that one can approximate the response of all the polarizable bits of the tardigrade as a dielectric function, but the same could be said for any material.)

This is not entanglement in any meaningful sense. If it were, you could say by the same reasoning that the qubits are entangled with the macroscopic silicon chip substrate.  The tardigrade does not act as a single quantum object with a small number of degrees of freedom.  The dynamics of the tardigrade's internal degrees of freedom do not act to effectively decohere the qubit (which is what happens when a qubit is entangled with many dynamical degrees of freedom that are then traced over).  

Atoms and molecules in our bodies are constantly entangling at a quantum level with each other and with the environment around us.  Decoherence means that trying to look at these tiny constituents and see coherent quantum processes related to entanglement generally becomes hopeless on very short timescales.  People still argue over exactly how the classical world seems to emerge from this constant churning of entanglement - it is fascinating.  Just nothing to do with the present paper. 


Unknown said...

You researchers of illegality need to be arrested for putting these damned critters on the open wound of living females without the individuals kbowledge. I have photographed these freakin critters in my you are LIARS.

Anonymous said...

When would you say "meaningful" entanglement would be achieved?
I feel that this article touches on some more fundamental discussions: What does it mean for a creature to be alive? When does approximating something as a dielectric start or stop?

The article did not show 'useful' entanglement. The behavior of the tardigrade was probably not altered when comparing before to after the experiment.

I like the article, because it makes us consider such questions as: Are we, as living beings, special in some way? Are we not deep down just an emergent phenomenon of sufficient dielectrics and chemical reactions stacked on top of each other, that as a sum became sufficiently complex to attribute ourselves things like 'consciousness'?

Douglas Natelson said...

Anon, I realize that I could have been clearer. For entanglement to be meaningful here in the quantum information sense, there would have to be some dynamical internal degree or degrees of freedom of the tardigrade that become(s) entangled with the qubit. An example: In this recent experiment (, there are mechanical resonators that each consist of millions of atoms, but if you consider the center of mass of each, it acts like a quantum harmonic oscillator. It's possible to put the coupled system of two resonators into an entangled state, where (for example) we know that one of the resonators is in its first excited state and the other is in its ground state, but we don't know which is which, until a classical measurement is done. If the authors in the tardigrade paper had somehow coupled to, e.g., the mechanical motion of the whole tardigrade, then I'd buy the entanglement claim. Instead, it sure looks like the tardigrade is no more entangled with the qubits than the underlying silicon chip is.

Suppose there was some two-level defect state in the tardigrade shell, just like there can be two-level systems in the silicon dioxide on the substrate. Would coupling between the qubit and that two-level system mean that it had been entangled with the whole tardigrade? I'd say "no".

I agree that the discussion raises interesting questions. The picture painted by Zurek in the article that I linked is that the classical world emerges from a zillion underlying coupled quantum degrees of freedom. See here too: . This appeals to me as a fan of statistical mechanics and condensed matter, but I'm still not sure I fully understand the end ideas about unitarity.

Anonymous said...

Thank you for clarifying your comment, this makes it much more insightful to me. I agree that it would be amazing if somehow the motion or an action of the tardigrade could be coupled to the quantum system, where one would have to ask the question "is the creature flicking a switch, or is the switch moving the creature to flick it, or both?" This goes however far beyond my knowledge of such systems, and even my imagination.

Thank you again for your answer.

Anonymous said...

Is it accurate to say that they their 3rd system is simply the lowest energy plasmon-like excitation in the tardigrade? (or in the tardigrate-substrate system?)

Anonymous said...

Actuaally I was incorrect there, not plasmon-like, should I say polaron-like excitation?

Anonymous said...

WTF? ��

Unknown said...

To state that the single surviving tardigrade is the first to complete the process is incorrect.
The two that died are. There is only one survivor,therefore there is no partner,as the other two perished.
To assume that death per our understanding limits quantum entanglement is ignorant. Simply because we are unaware of the other two animals' sentient character currently exists does not mean that it doesn't exist outright.