I have waited for this development since 2009 (see old post below).
The fact that a macroscopic, living organism can be placed in a superposition state may come as a shock to many people, including a number of physicists.
Related posts: Gork revisited 2018 , Feynman and Everett
If a tardigrade can exist in a superposition state, why can't you?
Are you in a superposition state right now?
Is there some special class of objects that "collapse wavefunctions"? (Copenhagen) ... It's ridiculous, absurd. In any case we now know that tardigrades are not in that class.
K. S. Lee et al.
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 6×10−6 mbar, setting a new record for the conditions that a complex form of life can survive.
From the paper:
In our experiments, we use specimens of a Danish population of Ramazzottius varieornatus Bertolani and Kinchin, 1993 (Eutardigrada, Ramazzottiidae). The species belongs to phylum Tardigrada comprising of microscopic invertebrate animals with an adult length of 50-1200 µm . Importantly, many tardigrades show extraordinary survival capabilities  and selected species have previously been exposed to extremely low temperatures of 50 mK  and low Earth orbit pressures of 10−19 mbar . Their survival in these extreme conditions is possible thanks to a latent state of life known as cryptobiosis [2, 13]. Cryptobiosis can be induced by various extreme physicochemical conditions, including freezing and desiccation. Specifically, during desiccation, tardigrades reduce volume and contract into an ametabolic state, known as a “tun”. Revival is achieved by reintroducing the tardigrade into liquid water at atmospheric pressure. In the current experiments, we used dessicated R. varieornatus tuns with a length of 100-150 µm. Active adult specimens have a length of 200-450 µm. The revival process typically takes several minutes.
We place a tardigrade tun on a superconducting transmon qubit and observe coupling between the qubit and the tardigrade tun via a shift in the resonance frequency of the new qubit-tardigrade system. This joint qubit-tardigrade system is then entangled with a second superconducting qubit. We reconstruct the density matrix of this coupled system experimentally via quantum state tomography. Finally, the tardigrade is removed from the superconducting qubit and reintroduced to atmospheric pressure and room temperature. We observe the resumption of its active metabolic state in water.
Note Added: I wrote this post the day after getting a Covid booster and a shingles vaccine, so I was a little zonked out and was not able to look at the details at the time.
The authors claim that the B qubit states B0 and B1 are entangled with two different internal states of T (tardigrade): B0 T0 , B1 T1. Then they further entangle B with the other qubit A to make more complex states.
In the supplement they analyze the density matrix for this d=8 Hilbert space, and claim to have measured quantities which imply tripartite entanglement. The results seem to depend on theoretical modeling -- I don't think they made any direct measurements on T.
They do not present any uncertainty analysis of the tripartite entanglement measure π.
The line in the main body of the paper that sounds convincing is We reconstruct the density matrix of this coupled system experimentally via quantum state tomography (see Fig 3), but the devil is in the details:
... a microscopic model where the charges inside the tardigrade are represented as effective harmonic oscillators that couple to the electric field of the qubit via the dipole mechanism... [This theoretical analysis results in the B0 T0 , B1 T1 system where T0 T1 are effective qubits formed of tardigrade internal degrees of freedom.]
... We applied 16 different combinations of one-qubit gates on qubit A and dressed states of the joint qubit B-tardigrade system. We then jointly readout the state of both qubits using the cavity ...
Some commentary online is very skeptical of their claims, see here for example.
More (12/23/2021): One of the co-authors is Vlatko Vedral, a well-known theorist who works in this area. His recent blog post Entangled Tardigrades is worth a look.
After thinking a bit more, the B0 T0 , B1 T1 description of the system seems plausible to me. So, although they don't make direct measurements on T (only on the combined B-T system), it does seem reasonable to assert that the tardigrade (or at least some collective degree of freedom related to internal charges of it) has been placed into a superposition state.
See this 2009 post: Schrodinger's virus
If the creature above (a tardigrade arthropod) can be placed in a superposition state, will you accept that you probably can be as well? And once you admit this, will you accept that you probably actually DO exist in a superposition state already?
It may be disturbing to learn that we live in a huge quantum multiverse, but was it not also disturbing for Galileo's contemporaries to learn that we live on a giant rotating sphere, hurtling through space at 30 kilometers per second? E pur si muove!
Of course there is no wavefunction collapse, only unitary evolution.
Many people are confused about this -- they have not recovered from what they were taught as beginning students. They still believe in the Tooth Fairy ;-)
You are Gork!
Gork is a robot name made up by Sidney Coleman for his talk Quantum Mechanics, In Your Face! (video, Gork @40m or so). Before the word entanglement became fashionable, Sidney summarized this talk to me in his office as "Quantum Mechanics is just a theory of correlations, and we live in this tangle of correlations." He may not have said "tangle" -- I am not sure. But he was describing the Everett formulation, trying not to scare a young postdoc :-)
R. Buniy and S. Hsu
arXiv:2011.11661, to appear in Foundations of Physics
For any choice of initial state and weak assumptions about the Hamiltonian, large isolated quantum systems undergoing Schrodinger evolution spend most of their time in macroscopic superposition states. The result follows from von Neumann's 1929 Quantum Ergodic Theorem. As a specific example, we consider a box containing a solid ball and some gas molecules. Regardless of the initial state, the system will evolve into a quantum superposition of states with the ball in macroscopically different positions. Thus, despite their seeming fragility, macroscopic superposition states are ubiquitous consequences of quantum evolution. We discuss the connection to many worlds quantum mechanics.
2021 witnessed other demonstrations of macroscopic entanglement: Quantum entanglement of two macroscopic objects is the Physics World 2021 Breakthrough of the Year.
... Quantum technology has made great strides over the past two decades and physicists are now able to construct and manipulate systems that were once in the realm of thought experiments. One particularly fascinating avenue of inquiry is the fuzzy border between quantum and classical physics. In the past, a clear delineation could be made in terms of size: tiny objects such as photons and electrons inhabit the quantum world whereas large objects such as billiard balls obey classical physics.
Over the past decade, physicists have been pushing the limits of what is quantum using drum-like mechanical resonators measuring around 10 microns across. Unlike electrons or photons, these drumheads are macroscopic objects that are manufactured using standard micromachining techniques and appear as solid as billiard balls in electron microscope images (see figure). Yet despite the resonators’ tangible nature, researchers have been able to observe their quantum properties, for example, by putting a device into its quantum ground state as Teufel and colleagues did in 2017.
This year, teams led by Teufel and Kotler and independently by Sillanpää went a step further, becoming the first to quantum-mechanically entangle two such drumheads. The two groups generated their entanglement in different ways. While the Aalto/Canberra team used a specially chosen resonant frequency to eliminate noise in the system that could have disturbed the entangled state, the NIST group’s entanglement resembled a two-qubit gate in which the form of the entangled state depends on the initial states of the drumheads. ...