Schrödinger’s Tardigrade, by Sergiy Denysov
So far there is only one animal was involved into quantum physics, that is the famous Schrödinger’s cat. Now some researchers claim that there is one more.
In February 2019, I came across the piece “Quantum theory: the weird world of teleportation, tardigrades and entanglement” [1]: Gröblacher is also interested in experiments involving living creatures … He is currently working on putting a sheet of nitride into a superposition of states … “A superposition state of these membranes would allow us to demonstrate that objects that are visible to the naked eye still behave quantum, and we can really test decoherence – the transition between classical and quantum mechanics,” he says. He hopes to extend the experiment by placing tiny living organisms called tardigrades onto the membrane of silicon nitride, putting them into superposition too.
Targidrates are known for being tough [2]. These micro-animals (their official title) can enter a ‘hibernation state’ of near complete dehydration and metabolism rate decreased by factor 1/1000, and, being in this state, survive an exposure to outer space (almost perfect vacuum), high-intensity radiation of all kinds (including gamma rays), and pressure up to 1200 atmosphere. Therefore, they might hopefully withstand the cryogenic environment required to achieve ground state cooling of the membrane. If there an animal able to survive superposition this must be a tardigrade.
After reading the piece, my immediate thought was: But would the tardy feel the difference between just the groundstate and superposition of the groundstate and the first excited state of the membrane? And in what measurable terms? Or is this difference is simply negligible on the background of the mere exposure to the cryogenic environment?
Another thought back then: The typical size of silicon nitride membranes Gröblacher is dealing with is 0.5 mm. This is also the typical size of tardigrades. I do not know the masses of the membranes and tardigrades but expect them to be comparable. It is not possible to maintain the extra-high quality factor of such membranes after placing on them a tardigrade – unless the membranes are on-purpose designed and curved, with ‘nests’ for tardys (a-la seats of Space Jockeys [3]).
Three years have passed and in 2022 a work with a sensational title, “Entanglement in a qubit-qubit-tardigrade system”, was published in New journal of Physics [4].
The authors claimed that hey set a tardigrade into entanglement with two qubits — and the former has survived it (“The animal is then observed to return to its active form after 420 hours at sub 10 mK temperatures and pressure of 6 × 10−6 mbar, setting a new record for the conditions that a complex form of life can survive“). But had the animal really been quantum entangled?
To prove it, one has to measure the quantum properties of the tardigrade, which the experiment does not do. Instead, a model was used (“The tomographic data shows entanglement in the qubit-qubit-tardigrade system, with the tardigrade modelled as an ensemble of harmonic oscillators or collection of electric dipoles“). Well, the quantum community is sceptical about such a ’proof’.
Also, there is a strong scepticism about that entanglement can be obtained by simply placing a tardy on top of a qubit. After that the qubit is no longer a resonator with well-tuned characteristics so one should not talk about ‘groundstate’ and ‘excited state’. From the point of view of quantum physics, a tardy is a system with macroscopically many degrees of freedom so it decoheres the qubit by working as an external environment. But doesn’t the environment become entangled with the object it is acting on? But is it measurable?
The problem is that the animal is not a quantum object in a sense that its state cannot be described as a superposition of a few basis states. So where is the entanglement? In such situation, the entanglement cannot be located and thus cannot be measured. We could also say that we are entangled with an electron in a oxygen atom of 02 floating somewhere inside out right lung – but it is not measurable and therefore does not exist (at least for quantum physics).
Anyway, the ‘entanglement’ claimed in the paper was not assessed in experiment and therefore the clam is much stronger than the experimental data can support. Schrödinger’s tardigrade is not yet here and Schrödinger’s cat has to wait for a companion.
[2] https://en.wikipedia.org/wiki/Tardigrade
[4] K. S. Lee at al., Entanglement in a qubit-qubit-tardigrade system, New Journal of Physics 24, 123024 (2022); https://iopscience.iop.org/article/10.1088/1367-2630/aca81f#:~:text=In%20particular%2C%20for%20any%20finite,electric%20field%2C%20see%20Appendix%20F%20.
Personally I would love to see more animals enter the quantum world. Let’s hope they eventually succeed!