Schrodinger’s Bacterium: Superposition of Living Things

What unites chlorophyll, the way birds find their way south for winter, and possibly the most famous thought experiment of all time? That would be superposition.

Superposition is a concept within quantum theory in which an object capable of being in either of two states (a particle that could be red or blue, say) is simultaneously in both those states until it is observed. In other words, the particle is both red and blue until someone performs an experiment to find out which it is, at which point it settles down to being definitely either red or blue. Or the universe splits into two universes, in one of which the scientist discovers the particle to be blue, and in the other finds a red particle, or whatever other school of thought happens to float your quantum-mechanical boat. Essentially, performing the experiment forces an object into one definite state – usually known as ‘collapsing the wavefunction’. But until this happens, the object is perfectly happy to be both at once.

The best-known example of this is the thought experiment known as Schrödinger’s Cat. Trapped in a box with a single radioactive atom and an ingenious device which will deal out some form of gruesome death as soon as it detects the atom decay, the cat is, to say the least, in an uncomfortable position. It seems inevitable that sooner or later, the atom will decay and trigger a swift, painful end for kitty. However, remember that by closing the box, the scientist has deliberately avoided making a measurement of the atom – so it will happily remain in a state of superposition, both decayed and undecayed, until the box is opened.

Where does this leave the cat? Well, there are two ways you could find the cat when you open the box – alive or dead. If the atom has decayed, it will be dead; if it has not, then the scientist is due to be confronted with a very angry ball of fur when time is up. But what about before the box is opened, when the atom is both decayed and not decayed? Following the logic through, the cat must be simultaneously alive and dead. Hence Schrödinger’s Cat has come to symbolise the fundamental weirdness at the heart of quantum theory – alive yet not alive, fate hanging in suspension for days, weeks, months, years, however long it takes for the experimenter to decide to check on it.

This is why physics doesn’t have ethics committees

(Image here, credit Robert Couse-Baker/Flickr)

So far, though, superposition in practice has been limited to things much smaller than the average cat. Light particles, atoms, molecules… Chlorophyll, the chemical which allows plants to capture energy from sunlight, is suspected to exploit this quantum weirdness to do its job: the energy-harvesting process is long and complex, but centres around electrons zipping between molecules, transferring energy. This is easier the closer together the molecules are; however, if they get too close, they may end up overlapping, meaning they will lose the energy the electrons need to pass along.

It’s therefore been proposed that the way chlorophyll gets around this issue is by taking advantage of superposition: when one molecule receives an energy boost from incoming sunlight, it can ‘share’ this excited state to some extent with its neighbours, via superposition. Then, instead of electrons having to take the relatively laborious route between molecules, their collective state can simply fluctuate, shifting the excitation back and forth between them and so allowing the energy to reach its destination in a much shorter time frame.

Oh chlorophyll, you scamp

(Image here, credit scifun.org)

It’s also thought that birds could be using superposition as part of their navigation system. Though experiments suggest that observation of the Sun and even smell are also integral to the avian satnav, birds are known to rely partly on somehow measuring the magnetic field of their surroundings and using this to orient themselves. It’s thought that this happens via the local magnetic field strength determining the chance that each member of a particular quantum-entangled particle pair in the bird’s eye will take up particular states; their individual states then affect their interaction with certain light receptors in the eye, allowing the bird to ‘see’ the strength of the magnetic field.

Just your average tiny feathered quantum physicist

(Image here, credit unknown)

Chlorophyll is a fairly complex molecule, which makes the fact it’s capable of superposition remarkable, but its structure has nothing on a full-blown living organism. So this makes the subject of today’s post especially exciting: a pair of researchers have suggested a way to put a living organism into a state of superposition. Granted, it’s only a bacterium, but if carried out this would be the first experiment of its kind. The plan hinges on a previous experiment in which a very tiny vibrating aluminium membrane was put into a state of superposition; put the bacterium on that membrane, the thinking goes, and it would effectively be in two places at once. Hello, Schrödinger’s Bacterium.

However, in order to allow the experiment to take place without heat disrupting the delicate quantum states involved, it would need to be conducted close to absolute zero. At this temperature, the bacterium’s internal processes will virtually grind to a halt, raising the question of whether or not it should truly be considered a ‘living’ organism (though it would be possible to revive the bacterium after the experiment by thawing it out). This experiment, though radical, still has some way to go to live up to the sheer excitement factor of Schrödinger’s Cat.

On the plus side, it’s thought that the techniques used could prove beneficial to medicine – entanglement of living cells in this way could allow for the study of quantum states in their component molecules, and hence more detailed analysis of biological samples.  (Note for the more physics-inclined: previous techniques tended to focus on detecting the existence of single electron spins, whereas this methodology would allow the quantum states themselves to be detected, and, moreover, manipulated, allowing more information to be gained from studying the sample.)

Header image from here.

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