Quantum twitter was taken by storm by an unusual paper, Entanglement between superconducting qubits and a tardigrade. What started as a bit of whimsical fun, quickly soured as some news outlets credulously took the pre-print at face value. To their credit, at least a few sites did display an appropriate level of skepticism, which seems to occur when journalists seek comment from physicists. There have been multiple twitter threads more or less explaining why this experiment doesn’t show what it purports to show. For instance:
This post is not intended to rehash the perfectly correct explanations of others, but more to discuss why (most) physicists on Quantum Twitter have this unanimous and visceral response to the claims made in the paper. The reaction seems to say something about our standards of evidence, explanatory power, and what is necessary to ‘explain’ something.
Classical Primacy
One requirement we have for accepting quantum entanglement claims is that the data should not be able to be explained classically. The tardigrade paper conspicuously fails to satisfy this requirement, because it relies on a transmon frequency shift due to the presence of the tardigrade (and filter paper). As many commenters have pointed out, this is easily described by a change to the dielectric that the transmon capacitor paddles ‘see’. The effects of adding a new dielectric are routinely predicted classically with standard physical simulation software.
In principle, it is also possible to work out this frequency shift from a quantum mechanical perspective. Describing the tardigrade as a collections of quantum harmonic oscillators, then defining a coupling between it and the transmon. The entangled system has a frequency that is just slightly shifted from the bare qubit frequency.
I think it is safe to say that, to explain why a piece of dielectric (tardigrade) would change the frequency of a transmon, the overwhelming majority of physicists would accept the classical explanation over the quantum mechanical one.
Why should they? I think we would all agree that quantum mechanics is more fundamental than classical mechanics. When faced with what appears to be a perfectly fine quantum mechanical explanation, under what circumstances do we reject it in favor of the classical option?
Details, Details
The answer must be in the details of how the experiments were performed, the conclusions to be made, and what exactly we’re trying to explain. One way to immediately accept the quantum mechanical version of events is to directly measure entanglement. It should be sufficient to measure the correlations between the entangled systems, and perhaps ensure that they violate the Bell inequalities. But even this statement is loaded with detail. In our tardigrade example, how do we know what the “system” is? Is it some collective vibrational mode of the tardigrade itself? Is it some microscopic sub-system that has a frequency close to that of the transmon? Does it still count if the transmon is entangled with some microscopic element of the tardigrade?
What if the best case for us is not available? We cannot directly measure the correlations between our entangled systems. Can we infer something about the presence of entanglement based on our collective knowledge of quantum mechanics?
Let’s revisit the frequency shift example above. The coupling between the transmon qubit and some collection of quantum harmonic oscillators (the tardigrade) can be written as:
Here, g describes the strength of coupling between the transmon and tardigrade. In principle we should be able to use this to calculate a frequency shift of the transmon-tardigrade system, compared to the transmon alone. In practice it it not clear how to do this. Primarily, because we have no idea how many oscillators the tardigrade should have. Or even what g should be.
We are quickly bogged down by unknowns in the quantum tardigrade system. Conversely, treating the tardigrade classically simplifies things substantially. We assign the tardigrade a plausible dielectric constant based on measured values of the same for the proteins that make up the organism itself. A few minutes of numerical simulation on off-the-shelf software and we’ve got a decent estimate for the expected transmon frequency!
The explanatory power of classical physics is clearly superior to quantum mechanics, here. In fact, trying to start with a quantum approach would have been impossible here, raising more questions than answers. Instead, it’s far easier to abstract away thorny questions like “what is g ?” by leveraging our knowledge of simple capacitors and dielectrics.
It’s Entanglement All the Way Down
Fine, fine. So quantum mechanics runs into some difficulties in terms of actually predicting anything here. It should still be valid to describe the tardigrade as a system of quantum harmonic oscillators in general. We don’t need to know the details to know that this is a valid way to think of things, and that means that the qubit and tardigrade are entangled!
It’s true, we could describe a tardigrade or almost anything else as some collection of harmonic modes. And these modes could interact with some qubit, which could mean that they are entangled.
I think of this as the ‘everything is entangled’ argument, and I find it pretty unsatisfactory. It could be a true statement, but it is useless and lacks meaning. I think, once again, we run into the problem of explanatory power. The ‘everything is entangled’ argument has none. It doesn’t explain anything in a useful or insightful way, nor does it allow for useful predictions.
Principle of Maximum Explanation
When I think about it, I am not sure I can identify of any features of entanglement that lend themselves to particularly good indirect probes. A compelling demonstration of entanglement pretty much always requires a measurement of each part of the entangled system1. It is this unusual feature of an unusual phenomenon that leads us into strange situations where we might prefer ‘less fundamental’ (classical) descriptions over ‘more fundamental’ (quantum) descriptions2.
Conveniently, this makes for a nice heuristic when confronted with the next headline that “For the first time, X has been entangled with Y!!!” Ask your closest tame physicist the following:
Was a direct measurement of both X and Y performed?
If not, is there a way to explain the data without invoking entanglement?
Really, you’d want to track each state involved in the entanglement very closely.
There’s also something to be said here about the ‘fragility’ of quantum states. It’s so easy to lose quantumness and return to classical mechanics, that you have to work hard to make high quality quantum systems.