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In a recent paper on the arXiv, a team of researchers from Singapore and elsewhere claim to have established entanglement between a superconducting qubit and a tardigrade (or water-bear), which is a teeny little critter that's known for being able to survive all sorts of extreme conditions (apparently even in the vacuum of outer space), by being able to enter a cryptobiotic state of suspended animation.

There may be scientific value in the study of the extremes tardigrades can be exposed to, e.g. in a dil-fridge, but it's natural to consider the experiment as a very trimmed-down version of Schrödinger's cat.

If we define a Schrödinger's cat experiment as preparing and maintaining coherence of a state $\vert\psi\rangle=\frac{1}{\sqrt 2}(\vert\text{alive}\rangle+\vert\text{dead}\rangle)$ for some macroscopic cat-state, and then measuring in the appropriate basis to prove that the cat is in a coherent superposition of being alive and being dead, then can we say that the tardigrade experiment "also" put a tardigrade into a similar superposition?

Clearly there's some click-baity headlines and perhaps some run-away conclusions, but can we formalize what the Singapore team did, in a way that requires a superconducting quantum computer?

Glorfindel
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Mark Spinelli
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2 Answers2

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I would say that this experiment is not at all similar to preparing such a cat-state, because no measurable property of the tardigrade was ever probed, or rather, that the measurable property is not experimentally justified. The authors make a number of assumptions in order to claim some correlation between the energy state of qubit B and the energy state of the tardigrade, such as:

  • the entire tardigrade is cooled to its ground state
  • the tardigrade is well-modeled by a set of harmonic oscillators
  • these oscillators all couple similarly to the qubit
  • there is never more than 1 photon in the system

which would be pretty extreme claims for something as big and complex as a tardigrade even in the presence of experimental data, which they don't provide. All of this is to assert that if qubit A is entangled with qubit B, and qubit B is correlated with the tardigrade, then the tardigrade must also participate in the entanglement to some degree. But this correlation is entirely assumed, and the effect of the tardigrade on the circuit is never shown to be more than a frequency shift, which is a classical effect. Notice that the tomography circuit only acts on the qubits -- this should hint that one can't directly control the state of the tardigrade nor measure any relevant observable.

PS#1: They use a tardigrade, but it could have been any speck of dust or matter of about the same size.

PS#2: Two of the authors were previous Ig-Nobel Prize winners, so make of that what you will ;)

chrysaor4
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Firstly, they did not prepared tripartite state as they claimed. The tardigrade simply plays role of a dielectric in shunt capacitor on qubit B. As a result, resonance frequecny of qubit B changed due to change in permitivity, hence also capacity (interestingly, the authors discussed this in detail). Therefore, we are left with two qubits. After that the authors simply prepared Bell state (see Fig. 3b) $\frac{1}{\sqrt{2}}(|00\rangle + |11\rangle)$ and carried out a state tomography on it. You can also see this in Fig. 3a, where they operate with density matrices of two-qubits systems, not three-qubits. The tardigrade is simply part of qubit B, not independent entity.

Secondly, they claimed that they broke up Bohr's conjecture on imposibility to study quantum properties of living organisms. However, the tardigrade was in state of "hybernation" during the experiment with no metabolic activity. In other words, the tardigrade was inanimate object during the experiment. It became alive again after the experiment only.

Thirdly, in suplemenet to the article they said that only one tardigrade of three used has been reanimated. This means that in at least two instances of the experiment, they used inanimate matter. Results for all three tardigrades are however the same. Hence, the experiment says nothing about interaction of superconducting qubits and living matter (tardigrades).

Fourthly, they showed that tardigrades can survive extereme environment in a quantum processor (or dilution refrigerator). This is interesting and probably only one relevant result of the experiment.

Finally, the article is probably New Year joke :-)

Martin Vesely
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