An international group of researchers claim, in a paper, “Entanglement between superconducting qubits and a tardigrade”, to have performed an experiment in “quantum biology”. The abstract:
Quantum and biological systems are seldom discussed together as they seemingly demand opposing conditions. Life is complex, “hot and wet” whereas quantum objects are small, cold and well controlled. Here, we overcome this barrier with a tardigrade — a microscopic multicellular organism known to tolerate extreme physiochemical conditions via a latent state of life known as cryptobiosis. We observe coupling between the animal in cryptobiosis and a superconducting quantum bit and prepare a highly entangled state between this combined system and another qubit. The tardigrade itself is shown to be entangled with the remaining subsystems. The animal is then observed to return to its active form after 420 hours at sub 10 mK temperatures and pressure of 6\times 10^{-6} mbar, setting a new record for the conditions that a complex form of life can survive.
Nanoscale Views says, “not so fast”, in a post, “No, a tardigrade was not meaningfully entangled with a qubit”.
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).
In other words, they could have replaced the tardigrade with a snowflake of similar mass and said they were investigating quantum meteorology. Still, tardigrades are tough.