Lauren Fuge
In a pre-peer-reviewed paper recently released on ArXiv, a team of physicists claims to have done something extraordinary: place a tardigrade in a state of quantum entanglement with a pair of qubits.
According to the mind-bending weirdness of quantum physics, quantum entanglement means that two things or systems cannot be described separately.
Very simplistically this means that anything that happens to the tardigrade also happens to the qubits, or vice versa.
Previously, this has only been done with particle-sized objects – so if true, this would be the first time that an animal has ever been quantum entangled.
The paper triggered an avalanche of media reportage, tweets and online coverage – tardigrade meets quantum world is an exciting headline, after all.
But there’s one problem: other physicists are not convinced by the claims.
“The authors did not entangle a tardigrade with a qubit in any meaningful sense,” writes Rice University physics professor Douglas Natelson in a blog post. “This is not ‘quantum biology’.”
Others echo the view that the tardigrade did not achieve true quantum entanglement, with some saying instead that the researchers only showed classical interaction between the tardigrade and the qubit.
“The qubit is an electrical circuit and putting the tardigrade next to it affects it through the laws of electromagnetism we’ve known about for more than 150 years,” explains science writer and physicist Ben Brubaker on Twitter.
“Putting a speck of dust next to the qubit would have a similar effect.”
Get it? Me neither.
Let’s unpack the experiment.
The researchers – led by Kai Sheng Lee from Nanyang Technological University, Singapore – aimed to entangle a quantum system with a biological system. This is a difficult task, as they write in their paper, because: “Life is complex, ‘hot and wet’, whereas quantum objects are small, cold and well-controlled.”
To overcome this barrier, they chose to use tardigrades in their experiment – microscopic creatures that are incredibly robust, able to survive in all kinds of hostile environments by entering a state kind of like suspended animation. For them, a place like the unforgiving vacuum of space or the surface of the moon is a breeze, so why not the super-cold temperatures of a quantum computer?
So the team collected a tardigrade species called Ramazzottius varieornatus off a rooftop in Denmark. They froze three of them to 10 millikelvin above the chilling temperature of absolute zero, and also lowered the pressure to just 0.000006 millibars.
The tardigrades looked dead, but they weren’t – their metabolism dropped to zero and they entered a state of dehydrated cryptobiosis, able to survive like that for years.
Then, the team attempted to entangle the tardigrades with two superconducting transmon qubits, which are used in quantum computing. They went through three experimental sequences, each with a different tardigrade.
Essentially, they placed a tardigrade on top of the capacitive parts of one of the two already coupled transmon qubits. Since a frozen tardigrade is mostly water, the team claimed that it acted like a conductor of electric current (albeit a poor one), shifting the resonance frequency of the qubit it was on down.
The team say that this is evidence of entanglement.
“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,” they write in their paper.
After 420 hours of experiments, one of the tardigrades was successfully unfrozen (though whether it was returned to the rooftop home it was kidnapped from remains unclear). The other two were not quite so lucky.
“Our present investigation is perhaps the closest realisation combining biological matter and quantum matter available with present-day technology,” the team writes. “While one might expect similar physical results from inanimate object with similar composition to the tardigrade, we emphasise that entanglement is observed with entire organism that retains its biological functionality post experiment.”
But others disagree that the tardigrade entered the quantum world, saying that this was not true entanglement.
“If it were, you could say by the same reasoning that the qubits are entangled with the macroscopic silicon chip substrate [on which the qubits sit],” writes Natelson in response.
Essentially, just because the tardigrade altered the qubit’s frequency doesn’t mean it had become entangled – to figure that out, the quantum properties of the tardigrade would have to be measured.
In a comprehensive Twitter thread, Brubaker argues that the researchers only achieved classical, non-quantum interaction between the tardigrade and the qubit.
“The wording of the paper very strongly suggests claims much stronger than the data can support,” he says.
Other scientists – like Jay Gambetta, who works on quantum computing at IBM – think that the researchers simply placed a very cold tardigrade onto a qubit and it didn’t actually shift the resonance frequency of the qubit; he suggests that perhaps it was the temperature change of the system that changed the frequency.
Either way, we’ll have to wait for peer review. The paper will undergo a rigorous review process by other physicists before it is published in a journal. It will be interesting to see if the current claims are downgraded, and if so whether the story will appear again in the media – without the buzzwords.
But quantum weirdness aside, the experiment did show something pretty cool: it’s a new test of a tardigrade’s (near) invincibility.
The research exposed tardigrades to the most extreme and prolonged conditions they have ever experienced – near absolute zero for more than 17 days, completely halting the tardigrades’ internal biology.
Then, one was successfully revived by gently returning it to normal atmospheric pressure and temperature.
While the other two didn’t make it, this still reinforces how hardy and impressive these little critters are.