The first multicellular organism said to be "quantum entangled" set the internet alight. Physicists aren't quite so sure.
Last week, reports trickled out that a tardigrade, one of the world's hardiest creatures, did something extraordinary: It became the first multicellular organism to be "quantum entangled" with a qubit and survive. The tardigrade's invincibility is the stuff of legend, so it isn't all that surprising to hear it may have endured an encounter with the quantum world.
The reports stemmed from a research paper uploaded to the preprint study repository arXiv. The paper, which is yet to be peer-reviewed, set off a flurry of tweets, online commentary and screenshots of a paywalled New Scientist piece accompanied by a sense of bewilderment and excitement. The tardigrade, it seemed, had added another notch to its belt.
But had it? Had the animal really been quantum entangled? As physicists began to weigh in online, the answer became a little messier. Consensus seemed to, quite quickly, snuff out the idea of tardigrade 'tanglement.
"Given that the results of the entanglement process could be reproduced describing the impact of the tardigrade classically -- that is, without entanglement -- I think that is the case here," said John Bartholomew, a quantum hardware developer at the University of Sydney.
Let's try to explain.
The tardigrade is a microscopic invertebrate known to survive some of the most extreme conditions humans have thrown at it. The vacuum of space? Easy mode for tardigrades. Freezing? Radiation? Tardigrades survive that, too -- thanks to their ability to enter a state akin to suspended animation, known as the "tun state."
Tun tardigrades look pretty much dead... but they aren't. Their metabolism drops to almost zero (they become "ametabolic") and they can remain in that state, some experiments have suggested, for decades. Those characteristics make the critters a good choice for this particular experiment because quantum computers require extremely cold temperatures to operate.
The quantum world is really strange, and to understand this experiment we have to (try to) understand that world. It's a place where our understanding of physics begins to break down.
Quantum entanglement is a weird quirk of this world which can see two things -- like electrons -- linked in a way that essentially means they cannot be described separately. It's a difficult, mind-bending concept and, after speaking to multiple experts about this, I still struggle to understand it.
A qubit is a quantum system, and it has two possible states, kind of like a classic computer bit can be a 0 or a 1. If you link up two qubits, you can create a new two-qubit system where the qubits might exist in both states at once. You could have qubit A be a 0/1 and qubit B be a 0/1 but it would be impossible to say what state either qubit is in before measuring them.
Confused? Well, that's OK. What you need to know is that in this preprint study, the researchers claim they were able to entangle a qubit with a tardigrade. In their experiment, they created this two-qubit system described above and placed a tardigrade over the top of one qubit (B). They left the other qubit (A) tardigrade-free. Placing the tardigrade on top of qubit B, the researchers say, shifted its frequency down. This, they suggest, is evidence of entanglement.
"It is clearly entanglement, although not maximally entangled," Rainer Dumke, the quantum physicist at Nanyang Technological University in Singapore who led the study, told CNET. Dumke says it's entangled, but you could not use the tardigrade itself as a qubit in your quantum computer.
Other physicists aren't as confident about the tardigrade's journey to the quantum world.
"For entanglement to be meaningful ... you have to have some internal state that you're looking at," says Douglas Natelson, a professor of physics at Rice University in Texas. Natelson documented his thoughts in a short blog post. "This is not 'quantum biology'," he suggested.
Simply put, the presence of the tardigrade on top of qubit B might alter the qubit's frequency, but other quantum physicists believe this does not mean the microscopic crawler has become entangled with the qubit. To speak to entanglement, you'd want to measure the quantum properties of the tardigrade, which the experiment does not do.
Other scientists online have suggested you could just as easily replace the tardigrade with dust in the experiment and you'd see a similar effect on the qubit. Dumke explains that because the tardigrade is in its tun state, it behaves essentially the same way that a dust particle would and so, this is entanglement.
If all of that went over your head (and to be fair, I get it), then perhaps the simplest way to understand this whole quantum tardigrade (quantardigrades?) mess comes via experimental physicist Ben Brubaker, who wrote a comprehensive thread on the subject: "The wording of the paper very strongly suggests claims much stronger than the data can support," he wrote.
Or another way to look at it: Perhaps "entanglement" -- the idea of it -- is being applied differently by different physicists here. This is where scientists and science communicators need to proceed with caution. If this isn't "maximal entanglement" and isn't "entanglement in a meaningful way," perhaps it's too much to call it entanglement at all and shout that from our headlines and across social media?
And given that it's a preprint, there's still a chance this paper doesn't get published in its current form. Once it is reviewed by other experts in the field, in an official capacity, the entanglement debate will be resolved.
Though the entanglement aspect seems a little misleading, there is another impressive aspect of the experiment that should have tardigrade fans once again applauding the little beast's near-invincibility.
"We are extremely fascinated by the tardigrade itself," notes Dumke. "This is an extraordinary lifeform that can survive absurd conditions."
The team reported that, in the experiment, the tardigrade survived the most extreme conditions it has ever been subject to. Operating quantum systems requires ultra-cold temperatures, barely above absolute zero (minus 459 degrees Fahrenheit). In this case, the plucky little animal's temperature sat at this temperature and at extremely low pressures for over 17 days.
The team reasons this shows metabolic processes are completely stopped in tun state tardigrades. At such low temperatures, chemical reactions become impossible. The tardigrade is essentially frozen in time: Its internal biology has stopped.
Once the experiment was over, the team returned the tardigrade to normal atmospheric pressure and temperature and rehydrated the invertebrate in water. What some of the reports are missing is the fact this process was performed three times, each with a different tardigrade. Only once was the tardigrade successfully revived, when the return from extremely low pressure to room pressure (and temperature) occurred "gently."
While the quantum entanglement headlines are exciting, they are, unfortunately, misleading. The real headline here is that scientists have shown once again just how hardy a tardigrade can be -- and perhaps, Bartholomew says, this paper could open the door to incorporating the tardigrade into other quantum experiments in the future.