Francesca Tomasi received her B.A. from the University of Chicago and is currently a microbiologist.
I had a professor in my undergraduate Cell and Molecular Biology class who often joked about “if quantum biology were a field.” He wasn’t referring to the existing discipline with that name, which seems to apply quantum mechanics to the chemical transformations underlying all biological processes. It was more so an epigram on the nuanced complexities of cellular physiology. Plus, the names of various cell organelles lent themselves quite lithely to puns on some of the classic quantum theories: “Golgi in a Box,” “Schrödinger’s Endoplasmic Reticulum,” “Chromatin Superposition.”
Conveniently, Schrödinger’s Box was something we had all learned about a year earlier in General Chemistry, our professor’s witticisms were not lost on us. Here are the basics:
Everything we know is made up of atoms. These iotas are the building blocks of chemical elements, the most fundamental building blocks of matter. In the nineteenth century, physicists discovered that atoms have their own sort of building blocks: subatomic particles known as electrons, protons, and neutrons (today we posit the existence of several other types of subatomic particles, but for the sake of this article we’ll stick to the main three). The center of an atom is called its nucleus. The nucleus is a tight pack of neutrons (neutrally charged particles) and protons (positively charged particles). These account for the vast majority of an atom’s mass. Now the electrons (negatively charged particles), instead of being confined within it, orbit the nucleus. At first, people thought the atom was like a solar system, with its nucleus as the sun and its electron(s) as its planets, dutifully encircling the sun on a fixed trajectory. However, it turns out an electron’s trajectory is no single flight path at all. Electrons are constantly moving with some momentum (mass times velocity) X at some location Y.
These X’s and Y’s are described using orbitals, mathematical functions describing the electrons orbiting within an atom. The point – at least in our intro-level class – was to know the probability of finding any electron in any specific space surrounding the nucleus of an atom, rather than pinpointing its specific location. You see, quantum mechanics are completely different from the Newtonian mechanics that govern our visible world (Force = Mass x Acceleration, Speed = Distance / Time, and so on). In quantum mechanics, there is a central limit to the exactness with which you can pinpoint the position and the momentum of a moving particle, like an electron. The more you want to be sure about one (the position or the momentum), the less you’ll know about the other. In Newtonian mechanics, position and momentum are used all the time to describe a system of motion. In quantum mechanics, you just can’t use those two together. While we can, in a moving car, easily answer the questions “where are we now and where are we headed?”, in the quantum world, we cannot talk about location and the trajectory of a particle in such a quotidian manner.
This idea, known as Heisenberg's Uncertainty Principle, is fundamental to arriving at Erwin Schrödinger’s paradox because it shows us how weird the quantum world is. Here is what physicists Niels Bohr and Wener Heisenberg said about the uncertainty surrounding measurements in quantum mechanics: During the time in which we do not know the state of an object, it is actually simultaneously in all possible states, so long as we do not peek and check its state ourselves. Measuring the state of an object is what forces it to be restricted to one possibility. Here is what Einstein said: spukhafte Fernwirkung! Here is what Schrödinger said: Ich liebe Katze! Just kidding. He gave us his famous thought experiment.
“One can even set up quite ridiculous cases where quantum physics rebels against common sense. For example, consider a cat is penned up in a steel chamber, along with the following diabolical device (which must be secured against direct interference by the cat). In the device is a Geiger counter with a tiny bit of radioactive substance, so small that perhaps in the course of one hour only one of the atoms decays, but also, with equal probability, perhaps none. If the decay happens, the counter tube discharges and through a relay releases a hammer which shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The first atomic decay would have poisoned it. The wave function for the entire system would express this by having in it the living and the dead cat mixed or smeared out in equal parts.
So while we do not look inside this diabolical device to see whether our poor cat is dead or alive, it is both dead and alive, a bit like this (right).
Schrödinger's thought experiment tells us several things. One, the microscopic and macroscopic worlds are governed by very different principles. Two, the observer in a quantum scenario has everything to do with the quantum scenario itself. Three, quantum objects exist in something called "superposition," or many states - in this case, exemplified by the simultaneous existence of a cat which is both dead and alive. Four, kind of like point two, the observer collapses this superposition.
So what is all this physics talk doing on a microbe blog? A few months ago, scientists in China and the US announced that they are going to perform a variation on Schrödinger’s gruesome (and at the same time not gruesome) scenario. Instead of a box with radioactive poison, they are using a highly sophisticated quantum oscillator. Instead of a cat, they’re using a microbe. Instead of asking where this bacterium is going to be and when, the proposers of the study are asking “can microbes be in superposition?”
Superposition posits that a physical system exists in two or more states until a third party comes to measure it. Physicists in recent years have been able to create superposition states using electrons, atoms, and some molecules (all inanimate objects). What about a live object? Can a bacterium be simultaneously dead and alive? Researchers at the National Institute of Standards and Technology in Colorado recently built a tiny mechanical oscillator to examine quantum properties. If "tiny" to you is the millimeter scale, you may not quite grasp the tininess of this oscillator, which is 15 micrometers (10-6 meters) across and 100 nanometers (10-9 meters) thick. This aluminum disc acts as the upper plate of a capacitor within a superconducting inductor-capacitor circuit. (Superconductivity refers to conducting an electric current with next to no resistance – such principles have already been applied to MRI equipment, high-speed magnetic-levitation trains, and many other devices). The oscillator, back in 2011 (two years before I had uttered the term "Golgi in a Box" for the first time), was successfully put in its quantum "ground state" (lowest energy state) by ultra-cooling it and subjecting its mechanical vibrations to microwave radiation.
So we have an oscillator that can potentially place something in a quantum superposition. This something would need to be extremely small – so small that its mass should have a negligible effect on the oscillator’s vibrations. Tongcang Li from Purdue University in Indiana and Zhang-Qi Yin from Tsinghua University in China settled on a 0.02 pico-gram (10-12 grams) mycoplasma bacterium (for reference, the oscillator weighs about 48 picograms). Since adding any sort of glue would add a significant mass to the oscillator, the researchers will rely on van der Waals forces (a type of chemical attraction between molecules) to keep the bacterium attached to the oscillator. The oscillator plus mycoplasma will then be induced (they think) in a superposition state using simultaneous clockwise and counterclockwise currents and a superconductor attached to the circuit.
Of course, since this procedure requires extreme cooling, the mycoplasma will have to be frozen to one one hundredth of a degree above absolute zero (zero Kelvin, the lowest theoretically possible temperature, is a point at which the motion of a particle due to heat is minimal. Heat equals kinetic energy). Li speculates that this is feasible, as bacteria are often frozen at extremely low temperatures for years at a time, only to be thawed and metabolically reactivated with no problem.
The superposition should then create a very small current that sets up a microwave oscillation with an energy that is halfway between the circuit’s ground state and first excited state (higher energy than the ground state). The oscillator would then be prompted to vibrate simultaneously in both states (because you can’t be in a “half” state – quantum particles can only take on certain discrete values of energy), creating a vibration-based superposition of the microbe!
Schrödinger’s mycoplasma is still some ways away from testing. Doctors Li and Yin are recruiting collaborators with very specific experience using all the different mechanical components of the oscillator system. They also need to find a way to ensure that the aluminum disc plus bacterium attached to their oscillator is in fact in a superposition along with the oscillator. Also, no part of this experiment actually seems to suggest the mycoplasma simultaneously being dead and alive at any point. Instead, it seems to be a play on energy levels of the matter composing the microbe. But if this pans out, Schrödinger's cat may turn out not to be as ridiculous as he made it out to be. Erwin, after all, was making fun of superposition’s absurdity when he came up with his famous thought experiment.