A group of physicists recently placed a microscopic animal known as a tardigrade onto a superconducting qubit, in an attempt to mingle the realms of quantum and classical mechanics. The researchers argue that the tardigrade was entangled at a quantum level, but some scientists say the team’s claims go beyond what they actually achieved.
The results aren’t published in a journal but are currently hosted on the preprint server arXiv.
“I think it’s very cool to start thinking about interfacing quantum things and biology. But with the right claim,” said Claire Aiello, a quantum engineer at UCLA, in a phone call. “I don’t think the experiment qualifies as quantum biology.” On Twitter, physicist Ben Brubaker had similar criticisms.
Quantum entanglement is the phenomenon of two or more particles defining the properties of each other. Quantumly entangled particles are interdependent — knowing something about one particle tells you something about the other — and that would remain true even if the particles were separated by billions of miles. Entanglement happens naturally, but for humans to observe it and better understand quantum mechanics, it must be induced in lab settings.
A tardigrade, also called a water bear or a moss piglet, is a tiny animal that looks like a cross between a caterpillar and the Michelin Man. Tardigrades are extremophiles, which means they can withstand and even thrive in environments most organisms cannot, including the vacuum of space.
The researchers, based in Singapore, Denmark and Poland, chose a tardigrade to try to entangle because of its ability to enter long hibernation to withstand things like searing heat, freezing cold, extraordinarily high pressures, and high levels of ionizing radiation. This hibernation is called cryptobiosis; the animal desiccates, shedding the moisture from its body, and only reanimates when conditions become more manageable.
“The main problem is that systems which we can control well on the quantum level are well isolated from the environment and at very low energies, in other words, extremely cold,” said study co-author Rainer Dumke, a physicist at Nanyang Technological University in Singapore, in an email. “We had to find the right quantum system but also a suitable life form.”
The team put their living subjects (Ramazzottius varieornatus, collected from a Danish roof gutter in 2018) into cryptobiosis. Once they were in that state, the researchers placed the tardigrades (one in each experimental run) on a superconducting qubit — a quantum bit, which, unlike a regular computing bit, can represent 0 or 1 simultaneously. They reported that the tardigrades coupled with the qubit, based on a change in the system’s resonance frequency (the frequency an object naturally vibrates at most excitedly), and posit that the combined tardigrade-qubit system was entangled with a second, adjacent qubit. Those qubits sat side-by-side on a larger silicon chip.
But outside scientists were sceptical that the experiment really showed quantum entanglement. Douglas Natelson, a physicist at Rice University in Texas, wrote in a blog post that the change in resonance frequency was “not entanglement in any meaningful sense,” and that “the tardigrade is no more entangled with the qubits than the underlying silicon chip is.”
Aiello said that quantum biology measures the “endogenous quantum mechanical degrees of freedom that exist in biology”; in other words, the internal dynamics that define quantum behaviour in living things. (For example, some researchers think that birds use quantum mechanics to see the magnetic fields that help them navigate.) The recent research team did not do that, according to Aiello. Instead, they noted a change in resonance frequency of the qubit the tardigrade was placed on, but didn’t measure properties of the tardigrade independent of its interaction with the qubit. The experiment lacked a measure that would confirm that entanglement was occurring as opposed to some other effect, Aiello said. She argued that the title of the paper — “Entanglement between superconducting qubits and a tardigrade” — was misleading and that the interaction between the tardigrade and qubit could have been a classical effect rather than a quantum one.
“One of the criticisms was that we did not produce useful entanglement, which can be exploited, for example, for computing,” Dumke said. “This is true, since we are not able to measure the tardigrade system on its own but only the coupled system.” He added that measuring the tardigrade alone “is beyond our present technological capabilities but certainly something we plan to attempt to do in the future.”
Quantum entangling a tardigrade (which, while tiny, is a whole lot bigger than an atom) would be a huge leap for the field. Particles like photons and atoms are entangled regularly, but going larger than that is an ongoing challenge. In 2007, there was a flurry of excitement at the possibility that photosynthesis is the result of quantum phenomena, but a 2020 study posited that likely wasn’t the case. Before that, bacteria showed hints of quantum behaviour. But even so, no work has yet demonstrated quantum systems working on such macroscopic scales.