The craziest challenge I took on wasn’t skydiving. Sailing the Amazon on a homemade raft; Scaling Everest; or dig for artifacts on a hill in a Middle Eastern desert around noon in mid-summer.1 The craziest challenge was investigating the possibility that quantum phenomena significantly affect cognition.
Most physicists agree that quantum phenomena are unlikely to affect perception much. Detection takes place in biological systems that have high temperatures, many particles and aqueous components. Such conditions break entanglement (a relationship that quantum particles can share and that can produce correlations stronger than any that can be produced by classical particles).
However, Matthew Fisher, a condensed matter physicist, suggested a mechanism by which entanglement could improve coordinated firing of neurons. Phosphorus nuclei have spins (quantum properties similar to angular momentum) that can store quantum information for a long time when it is in Posner molecules. These molecules can protect the information from decoherence (transmission of quantum information to the environment) via mechanisms described by Fisher.
I cannot verify how correct Fischer’s suggestion is. I am not a biochemist. But I’m a quantum information theorist. This is how I can figure out how Posner’s could process quantum information if Fisher were correct. I took on this task with my colleague Elizabeth Crosson during my doctorate.
Experimentalists have started testing elements of Fischer’s proposal. What if years later they discover that Posners exist in biofluids and protect quantum information for a long time? We need to test whether Posners can share entanglements. Detecting tangles, however, tends to require finer control than you can exert with a stir stick. How can you tell if a mug full of particles has entanglements?
I asked this question to Adam Bene Watts, a graduate student at MIT, and John Wright, then a postdoctoral fellow at MIT and now an assistant professor in Texas. John gave our project his code name. At a meeting one day, he reported that he had seen the film Avengers: Endgame. Did i see it? he asked.
No, I replied. The only superhero movie I’d seen recently was Ant man and the wasp– And that’s because, according to the film’s scientific advisor, the film has rippled over my research.
Go on, said John.
Spiros Michalakis, the Caltech mathematician responsible for this blog, acted as advisor. The film came out during my graduation; During a meeting of our research group, Spiros advised me to watch the film. There was something in there “for you,” he said. “And you,” he added, turning to Elizabeth. I obeyed to hear Laurence Fishburne’s character tell Ant-Man that another character had become entangled in Ant-Man’s brain with the Posner molecules.2
John insisted on calling our research Ant-Man project.
John and Adam study Bell tests. Bell test sounds like a means of checking that the collar your cat is wearing is still ringing. However, the test owes its name to John Stewart Bell, a Northern Irish physicist who wrote a groundbreaking paper in 1964.
For example, suppose you want to check that two particles are entangled. You can conduct an experiment on them as described by Bell. The experiment ends with a measurement of the particles. You repeat this experiment in many trials, using identical copies of the particles in subsequent trials. You collect a lot of measurement results, the statistics of which you calculate. You insert these statistics into a formula compiled by Bell. If the result exceeds a number calculated by Bell, the particles divide.
We needed a variation on Bell’s test. In our experiment, each experiment would involve hordes of particles. The experimenters – tall, clumsy, classical beings that they are – could not measure the particles individually. The experimenters could only record aggregated properties such as the intensity of the phosphorescence emitted by a test tube.
Adam, MIT physicist Aram Harrow, and I, with the help of John, invented such a Bell test. Physical examination A. published our article this month – as a letter and suggestion from an editor, I am pleased to report on it.
For experts: the trick was to make the Bell correlation function nonlinear in the state. We assumed that the particles mostly had pairwise correlations, although our Bell inequality can account for small aberrations. Unfortunately, no one can guarantee that particles mostly only show pairwise correlations. Violating our Bell inequality therefore does not preclude theories about hidden variables. Under reasonable circumstances, a not completely paranoid experimenter can use our test to check for entanglements.
With today’s technology, our macroscopic Bell test can be carried out on photons. However, we are more keen to use the test to characterize lesser-known entities. For example, we have outlined an application to Posner molecules. Recognizing entanglements in chemical systems requires more thought and a lot of headache for experimenters. But our paper speaks to the barrel – and I hope it will flow in the next Ant man Movie. Due to the debut in 2022, the film has the subtitle Quantumania. Sounds almost as crazy as studying the possibility that quantum phenomena affect perception.
1Of these options, I only took the last one.
2In case of confusion, we don’t know that anyone’s brain contains Posner molecules. The film offers speculative fiction.