Quantum Coherence

Coherence is coherence, a coherence in space and time. Atoms that vibrate in phase, flocks of birds that continue to form a unit, a football team at its best, a wave that washes up on the beach. In physics, coherence is used to indicate the possibility of interference. The light coming from two slits in a double slit experiment is coherent because the slits act as equally phased vibrating sources of light. These coherent light waves are then able to cancel each other out at certain spots behind the double slit – destructive interference – or to amplify each other – constructive interference.

Complex quantum state wave and interference

In quantum physics, the state of a system is defined by probability waves where the square of the amplitude (the deflection from the middle position ) represents the probability of finding the system in a particular state when measured. The quantum mechanical description of the quantum wave is a complex vector depending on space and time. In order to correctly calculate the course of the probability waves in time and place, it is necessary to assume coherence. Quantum waves show coherence, in other words are coherent, just like waves on a water surface. And coherence, if you really think about it, is a curious property for non-material waves. The image below is therefore an image of a coherent non-material thing. Coherence, on the other hand, in waves in water or air or another element is a purely physical event where pushing and pulling forces are exerted on matter.

Coherent development of the quantum wave Ψ(a) along a single axis, the x-axis. The imaginary (Im) and real (Re) components are not physical spatial coordinates. @ Wikimedia Commons – Maschen

In this context, read Quantum Phases and Quantum Coherence by Dr Mae-Wan Ho.

Quantum waves do exhibit interference because of the cohesion of its medium, just like physical waves do. Or rather, because they show interference, the quantum wave medium, although it is not material, must be coherent. Where there is constructive interference – a maximum intensity – the probability of finding the system in a certain state when measured is maximum. Where there is destructive interference this probability becomes zero. The fact that the quantum wave theory works 100% correctly implies that all quantum systems exhibit quantum coherence.

Coherent photons

A beam of laser photons is coherent, which lends laser light its special properties. The laser photons all move in one single coherent quantum wave, unlike the light of, for example, an incandescent source like a light bulb. In a light bulb, all photons go their own individual course, each ‘guided’ by their own individual quantum wave. So those incandescent photons are not coherent. But as soon as their individual quantum wave passes through a double slit, two coherent expanding quantum waves are created for each individual photon. Hence the interference pattern, a pattern of dark and light bands, minima and maxima, which we will not see normally with incoherent sunlight without special measures, such as a double slit.

Biological quantum coherence

According to research in the last decades, biological systems appear to exhibit an exceptional amount of quantum coherence, both in the microscopic and in the larger sense. In fact, it seems that every biological system is one single quantum coherent whole. This certainly applies to single-celled organisms and probably also to a system made up of many quantum coherent cells. A good example is photosynthesis by chlorophyll which is something I cover extensively in my book.

I briefly repeat here what I describe in the book in Chapter 6 – seven critical experiments, 7: photosynthetic efficiency. Chlorophyll converts sunlight into the energy that the plant needs in the production of biomass, a process in which carbon dioxide is absorbed and oxygen is released. Groups of chlorophyll molecules in a cell’s chloroplast form rows of antennae for sunlight. As soon as a chlorophyll antenna receives a photon, a free electron is released that has to bring its energy to elsewhere, the reaction center. This free electron must do so very quickly, otherwise it will recombine with a positive ‘hole’ that was created simultaneously with the release of the electron. A free electron plus positive hole is called an exciton. Such an exciton was hypothesized to follow a random path until it arrived at the reaction center. See figure below.

The random walk path, from chorophyll antenna to the reaction center, a material exciton would travel in the chloroplast. © nemosciencemuseum.nl

The following figure – click for animation – is good example to get an idea of the abominable efficiency of such a random walk to reach a certain destination.

Random walk in two dimensions @ Wikimedia Commons


How that exciton manages to find its way so quickly and efficiently was therefore a mystery until it was discovered that quantum coherence plays a major role here. The chlorophyll antennas, the electron and the ‘hole’ form together a quantum coherent system. The resulting interference makes it possible to create maximum intensity at the place where the electron has to release its energy, exactly where it can be used most efficiently, the reaction center. Compare this with a football team where the real team spirit rears its head at a certain point in the game and the suddenly coherent team starts to score. Chlorophyll has been around for 2.5 billion years on Earth, so 2.5 billion years ago the chloroplasts in plant cells already had the right configuration of chlorophyll antennae for each other. Actually even 3.5 billion years ago, because those chloroplasts were formed because cyanobacteria, which could then already convert sunlight into oxygen for 1 billion years, fused symbiotically with single-celled organisms that were not yet able to do this, to form together the first plant cells. This is called symbiogenesis. Every plant now has in every cell those chloroplasts, the offspring of those cyanobacteria. Isn’t it stunningly clever to achieve that particularly finely tuned quantum coherence so quickly, 1 billion years after the creation of the earth, by chance mutations?


In my book and also on this website I put forward chlorophyll as an argument from biology against the material multiversa hypothesis. If the probability of the exciton showing up in the reaction center is as low as I initially estimated, then at any moment astronomical numbers of split-off universes should arise where the exciton didn’t arrive in time and so their plants wouldn’t grow and flower. Then it would be very coincidental if we were to find ourselves constantly in those extremely improbable universes where photosynthesis always happened to go so well. That was when I actually knew less about quantum coherence. Quantum coherence in biological systems disproves that argument. It ensures through constructive interference of coherent quantum waves, with their maximum intensity exactly in the reaction center, that the probability that the exciton will appear there with its full load of energy is very high. Impressive, to say the least.