We're used to being told that the fundamental oddities of quantum mechanics don't really pertain to the world we live in. They're there, of course, but not at the scales we interact with. I can safely say that this chair is here and only here and not both here and there and everywhere in-between all at once because the chair is an almost unfathomably vast collection of quantum particles that, together, behave in all of the deterministic ways we're used to and depend upon.
Nonetheless, physicists have been enthusiastically pushing the boundaries of this classical/quantum boundary. In a paper posted recently to the arXiv pre-print server, a pair of researchers from Purdue University and Tsinghua University have proposed an experiment in which a bacterium, a living object that would seem to be very much so a part of our classical deterministic world, is put into a superposition of states in a real-life version of the Schrödinger's cat thought experiment. So: a single bacterium simultaneously occupying multiple quantum states.
To be clear, this wouldn't be a sudden jump from, like, single electrons to creatures each composed of around 1011 atoms. There's been a steady progression of increasing complexity in superposition experiments: from electrons to protons to atoms to molecules to, finally, tiny mechanical systems. Quantum coherence even seems to serve as a component of photosynthesis functions in some plants, just as a natural way of doing things.
Trapping molecules and silica microspheres using beams of photons is reasonable enough—what's known as optical tweezing—and allows for objects to be super-cooled (by restricting their motion to an extreme degree) and placed in quantum superpositions (again: two states at once), the same idea employed with a living creature wouldn't work so well. The absorption characteristics of a bacterium would allow the subject to heat up to the point that the superposition would likely be destroyed, sinking the experiment.
The experiment proposed in the new paper uses a more traditional means of cooling—with actual cold, that is—to bring the microorganism's energy state to its absolute minimum (ground state). Here, its center-of-mass motion is coupled to the motions of a tiny mechanical oscillator membrane integrated into a superconducting circuit.
The researchers explain that the superposition would originate in a superconducting quantum bit, or qubit, attached to the LC circuit, and might consist of simultaneous clockwise and anticlockwise currents. That superposition would then induce a tiny current in the LC circuit that sets up a microwave oscillation with energy about halfway between the circuit's ground and first excited state. This would excite the mechanical oscillator to vibrate simultaneously in the ground and first excited states, and create a vibration-based superposition of the organism.
The researchers offer a second variation of the experiment using mostly the same apparatus. This involves coupling one particular internal state of the microorganism manifested via the small charged molecules known as free radicals. The electron spins within these particles take the place of the oscillating membrane in the first variation, and so the center-of-mass motion of the bacterium is entangled with the electron spin state of a particle.
"In order to couple the internal spins states of a microorganism to the center of mass motion of the microorganism, a magnetic field gradient is applied," the researchers explain. This gradient comes courtesy of a ferromagnetic tip held above the bacterium. Different electron spin states can then be achieved by varying the distance between the tip and the subject (and so the field).
The first experiment is more realistic to actually implement, but the second offers some potential real-world utility should it actually be brought to life. An example of this would using such a set-up to detect the spin states characteristic of free radicals, perhaps revealing defective DNA or proteins. It's a thought, anyway.
In any case, bacteria would seem to be more or less the fundamental limit of the bio-entanglement concept because it requires extremely cold temperatures. Bacteria can survive being frozen and thawed, but cats, not so much.