“I don’t like it, and I’m sorry I had anything to do with it,” the physicist Erwin Schrödinger supposedly said of the quantum theory.
He was so sorry that he worked to prove it nonsensical with the most famous thought problem in physics, one that involves putting a cat in a box that would fill with poison if a radioactive atom were to split apart spontaneously. According to the theory, that splitting can be said to have happened only if observed; otherwise, it must be deemed indeterminate. And because the cat’s fate is aligned with the atom’s, Schrödinger’s cat must also be considered neither dead nor alive.
Patent nonsense, concluded Schrödinger. But later researchers found ways to turn the thought problem into real experiments, and these have actually validated the predictions of quantum theory. One experiment used a resonator chilled nearly to absolute zero so that it became “entangled” across two quantum states, vibrating or not. Those two states were then shown to be superposed.
Actually entangling a living creature would be quite a feat for the physicists, perhaps more so for the biochemists. Complex chemical systems don’t normally stand still for inspection, but if you could freeze them quantum-cold you could probe their constituent parts. Some have suggested that biochemical processes, such as photosynthesis, must involve quantum effects; this method could be a way to prove it.
A tardigrade is a good candidate for freezing down to zero in a near-total vacuum. It’s about as tough as an animalcule gets.
To entangle a life-form you have to put it in an extreme vacuum and cool it nearly to absolute zero without killing it. Bacteria have been so entangled. Now a group of scientists say they’ve entangled a tardigrade, commonly called a water bear, a cute critter that’s just barely visible to the naked eye.
The 11 researchers published their work on 15 December in the online preprint server arXiv, which is not peer-reviewed. Among them are Rainer Dumke of the Center for Quantum Technologies, in Singapore, and Tomasz Paterek of the University of Gdansk, in Poland, who in 2019 were honored, so to speak, with an IgNobel Prize for their work on magnetized cockroaches (the results of which bear on methods by which animals navigate).
A tardigrade is a good candidate for freezing down to zero in a near-total vacuum. It’s about as tough as an animalcule gets. Insult the thing and it goes dormant by curling up into a ball, called a tun, in a process known as cryptobiosis. Though some have argued that at least some metabolism must still go on, a tun is perhaps best characterized as a life that’s been put on hold. In 2019, when a bunch of tardigrades were deposited on the moon during the very unintended crash-landing of an Israeli spacecraft, many people speculated that the critters would survive even there. Sadly, experiments involving the firing of nylon bullets later suggested that this didn’t happen.
Dumke and his colleagues came on their current interest in the course of studying superconducting qubits, electronic oscillators that many hope will produce a fundamentally new computer based on quantum effects. They wondered what would happen if they put a dormant tardigrade on top of one of their qubits, bringing the system to near absolute zero.
First, they learned, the tardigrade survived. That alone is a significant finding.
“At this very, very low temperature, almost nothing is moving, everything is in the ground state; it’s a piece of dust,” Dunke tells IEEE Spectrum. “Bring it back to conditions where it can survive, increasing the temperature gently, and the pressure, and it comes back. Some had suggested that in the cryptobiological state, some metabolism is going on. Not so.”
The presence of two superconducting qubits beside the tardigrade strengthens the case for the existence of entanglement—here it appears the creature is in superposition with one |0> qubit and one |1> qubit.
This discovery raises the question of what forces of natural selection might have shaped the tardigrade to be so tough? It seems way overengineered for its normal terrestrial habitats, including moss and lichen.
Second, Dumke and his colleagues argue, they achieved true quantum entanglement between the qubit and the tardigrade. Larger objects have been so entangled, but those objects were inanimate matter. This is a bigger claim—and one that’s harder to nail down.
“We start with a superconducting qubit at energy state 0, comparable to an atom in the ground state; there’s no oscillation—nothing is happening,” Dumke says. “We can use microwaves to supply exactly the right amount of energy for the right amount of time to raise this to level 1; this is like the second orbital in an atom. It is now oscillating.
“Or, and this is the important point, we can add exactly that much energy but supply it for just half the time to raise the system to a quantum state of ½, which is the superposition state. In this state, it is at the same time oscillating and not oscillating. You can do extensive testing to measure all three states.”
Then the workers tested the system under a number of different conditions to determine the quantum state, and they found that the system consisting of the qubit and the tardigrade together occupied a lower energy state than either one alone would have occupied. The researchers concluded that the two things had been entangled.
No need to wait for peer review; in a matter of days, the criticism began to come in.
One critic, Ben Brubaker, a physicist turned journalist, has argued on Twitter that the experiments do not demonstrate what the authors claim. He said there were three possibilities—that quantum entanglement had been achieved with the entire tardigrade, that it had been achieved with a part of it, and that it hadn’t been achieved at all. That last one would imply that any effects were caused by some classical (nonquantum) physical process.
The authors admit that they could not perform the perfect experiment, which would involve measuring the tardigrade and the qubit independently, using two probes. Their tardigrade comes packaged with the qubit, forming a hybrid structure, and so two probes are hard to manage.
A sketch of the experiment—including a photo of the revived tardigrade on the system’s qubit. arXiv
“So you have to construct a model that represents the qubit as a quantum-mechanical system, and if you do it classically you wouldn’t be able to account for all the features,” says Vlatko Vedral, another author, who is a professor of physics at the University of Oxford. “The feature we are talking about is the quantum energy state that the combined system is able to reach. In fact, much of chemistry is based on this kind of thing—the Van der Waals force.”
Kai Sheng Lee, of Singapore’s Nanyang Technological University, says that the criticism of the entanglement claim is at least partially answered in the second part of the arXiv paper, “when we introduce the second qubit.” The presence of two superconducting qubits beside the tardigrade strengthens the case for the existence of entanglement, because here it seems the creature is in superposition with one qubit that’s in the 0 state (sometimes abbreviated |0>) and also with the other qubit, which is in the 1 state (a.k.a. |1>).
“But the major weakness,” Vedral concedes, “is that there is no direct measurements on the tardigrade alone. This is what you need to do to satisfy even the most conspiratorial critic, the one who says we could explain this with classical arguments.”
Can direct measurements of each part in this entanglement triangle ever be made? That question makes Dumke, Vendral, and Lee pause. Finally Dumke takes a stab at it.
“You could try to find a particular resonance frequency inside the tardigrade, then use this frequency to find what leads to a stronger entanglement,” he says.
“Or maybe you could genetically engineer the tardigrade to resonate,” Vendral suggests.
Why the pregnant pause? Maybe they’re thinking about the question. Maybe they’re thinking about how much of their research plan to reveal. Or maybe the two states are superposed.
1 January 2022 Correction: A previous version of this story misspelled Ben Brubaker’s name. Apologies, Mr. Brubaker!
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Philip E. Ross is a senior editor at IEEE Spectrum. His interests include transportation, energy storage, AI, and the economic aspects of technology. He has a master's degree in international affairs from Columbia University and another, in journalism, from the University of Michigan.