Opsin your eyes! Quantum sensors are the colour vision magicians inside our heads

In the fifth instalment of his exclusive monthly series for the South China Morning Post, American theoretical physicist and Nobel laureate Frank Wilczek explores the marvel of vision and why we can credit the quantum world for it. Read his previous articles here. The design of sensors that exploit the strange features of quantum mechanics is a vibrant frontier of modern physics. But nature got there first. Some of the most impressive quantum sensors on Earth have been common for millions of years. They help us to identify objects at great distances; they warn us about unripe food, poisons and predators; they empower us to enjoy the shimmer of jewels and rainbows, or to discern small glittering gold nuggets hidden among common stones. They are the marvellous molecules at the back of our eyeballs, whose response to incoming photons gives us our colour vision. World Quantum Day is a great moment to recognise that vision is a gift to us from the quantum world. To appreciate why and how, it is useful to compare vision with hearing. Both are, fundamentally, ways of detecting vibrations. In hearing, the vibrations – or sound waves – are travelling disturbances of pressure, usually in air. In vision, the vibrations are travelling disturbances in electric and magnetic fields – the class of electromagnetic waves we call light. These two kinds of vibrations occur at vastly different rates. Sounds perceptible to humans are in the range of 20 to 20,000 hertz; that is, the vibrations occur 20 to 20,000 times per second. For reference, dogs hear higher frequencies too, up to 45,000Hz or so; hence the possibility of dog whistles. The electromagnetic vibrations in visible light occur roughly a trillion times faster. Nature has evolved clever mechanical systems that can keep up with, or in technical terms resonate with, perceptible sounds. Indeed, that is how we make the sounds perceptible. We capture and focus incoming sound waves with our oddly shaped outer ears and then transmit them through membranes and tiny bones to fluids in our inner ear. At this point, they meet the tapering basilar membrane where sympathetic vibrations trigger the electric pulse-chains that our brains interpret as speech, music, signals or noise. Together, those transformations of vibratory energy pull off the amazing trick of compressing, for example, tuba-sized sound waves into vibrations that fit snugly inside our heads – what engineers call “impedance matching”. I like to think of the basilar membrane as the keyboard of an astounding piano that runs in reverse: the sounds move the keys. Evolution is an astounding designer, and our auditory system is a brilliant feat of engineering. But no mechanical system can vibrate as fast as light does. So, since sympathetic vibrations were out, nature had to evolve a different strategy. Fortunately, quantum mechanics came to the rescue. Vision is, fundamentally, a quantum sense. According to quantum mechanics, vibratory energy is transmitted in discrete lumps, or quanta, where the energy in a lump is proportional to the frequency of vibration. Albert Einstein proposed that idea in 1905, in the context of electromagnetic vibrations – what we now call photons. A few years later he applied it to vibrations of materials – now known as phonons. Back then, Einstein’s revolutionary ideas were given a very chilly reception. The reticence was perhaps best reflected by Max Planck’s apologetic tone in his otherwise enthusiastic 1913 recommendation of the then young Einstein for election to prestigious Prussian Academy of Sciences: “That he might sometimes have overshot the target in his speculations, as for example in his light quantum hypothesis, should not be counted against him too much.” Yet a few years later, in 1921, Einstein received his only Nobel Prize “especially for his discovery of the law of the photoelectric effect” – that is, for his photon idea. Experiments had vindicated his outrageous predictions. Today, Einstein’s ideas about packets of vibratory energy are a cornerstone of modern quantum theory, tested in thousands of experiments and used in hundreds of applications. The energy in a single photon of light is enough to change the shape of a molecule. That is the crucial effect that our sense of vision rests upon. Specialised cells at the back of our eyes, forming our retinas, contain a few kinds of big molecules, called opsins, that can absorb the energy of photons by bending in response. Such bending triggers electrical pulses that are the raw material from which our brains manufacture our sense of vision. Our perception of colour, properly understood, is a brilliant example of quantum mechanics at work. Most people have three kinds of opsins contained within so-called cone cells, which are densely packed within a small region of the retina called the fovea. These opsins are responsible for vision in daylight or good illumination. They give us our richly detailed daytime colour vision. A fourth kind of opsin is present in so-called rod cells, which are more sparsely distributed over the rest of the retina. Rods support our – monotone – night vision. Because the absorption of light by opsins is governed by the rules of quantum theory, it has the signature properties of quantum processes. Namely, it is all-or-none and probabilistic. Both aspects are crucial in sculpting our perception. By all-or-none, I mean that absorption either happens as a complete process, with a unique endpoint, or not at all. There is no such thing as “slight bending” of the opsin; it either distorts all the way or rests undisturbed. The resulting uniformity and discreteness of the raw signal greatly simplifies the task of interpreting it. Neurons tasked with using signals from different cones “know” exactly what they’re dealing with. In seeing the world as we do, we humans, gifted by quantum theory, have been using digital information processing long before we knew what we were doing. By probabilistic, I mean that we cannot predict precisely whether a given photon, impinging on a given opsin, will be absorbed or not. Under quantum theory, we can only compute probabilities. For each opsin there is a sensitivity curve that describes how the probability for absorption of a photon depends on the photon’s frequency. The three foveal opsins are commonly called blue, green and red detectors, after the colours of their peak sensitivities. But this is misleading, because the sensitivities are broadly tuned: thus, the green detector also responds significantly to red, and vice versa. Vision scientists prefer to use S, M, L – designating short, medium and long wavelengths – as opposed to blue, green and red. Whatever you call them, the fact that colour vision involves just three opsins is profoundly consequential. It is why we can synthesise beams with all possible perceived colours using just three kinds of sources. This is why our computer and digital TV screens can give the impression of realistic colours using pixels of just three kinds of LEDs, and our digital printers use just three kinds of ink to achieve the same effect. It is interesting – and to me, inspiring – to note that fundamental work establishing these principles was done in the mid-nineteenth century by the same James Clerk Maxwell who explained the electromagnetic nature of light. Using spinning tops to combine the colours of paper strips arranged in annular rings, he showed that strips of three fixed colours, combined in different proportions, could match the perception of any colour whatsoever. Another way of interpreting trichromatic vision is to propose that we are all severely colour-blind. The yellow-looking beam you get by combining red and green is very different, physically, from the “pure” yellow beam you would get from that part of a rainbow, even though they look the same. It is as if, in music, we could not hear the difference between the sounds produced by pure E versus a two-note chord of C and G. We can hear the individual notes within a musical chord, but colours collapse together. This defect is another consequence of our quantum vision. We do not keep track of the intensities in different frequencies of light separately, but only through their combined influence on how often three different opsins fire. Since there is a continuum of possible frequencies, and three is much less than infinity, we lose a lot. Can we get the information back? Yes, in several different ways. The most powerful, if not the most convenient, is scientific spectroscopy. The strategy, dating back to Isaac Newton, passes an incoming light beam through a prism. Since photons of different frequencies are deflected differently by the glass, this separates the beam and reveals its detailed intensity profile. This pattern of intensities, called the spectrum of the beam, reveals much about the quantum processes that produced it. Using quantum theory, we can often identify the spectral fingerprints of specific atoms and molecules in their native environments. In this way we learn about the temperature, magnetism and chemical composition of stellar atmospheres, for example. One vibrant frontier of spectroscopy today is the close examination of the dim light coming to us from planets outside the solar system, known as exoplanets, looking for signs of life – or maybe even industrial processes. Another, radically different approach is to create new kinds of opsins, with different sensitivity profiles, and then somehow persuade cone cells to take them up. Nature has already experimented in this direction. People with deuteranomaly – the most common form of “colour blindness” – have a mutant form of the M opsin, with a different sensitivity profile. It is probable that many people – especially the daughters of deuteranomalous males – have both the normal and the mutant forms of the opsin, coming from their two X chromosomes. These are tetrachromats – people with superior colour vision. They sometimes become designers or artists. In the animal world, many creatures have more and different opsins. Pigeons use five, and mantis shrimps can have a dozen or more. Genetically engineered monkeys, with extra opsins, have acquired colour vision superpowers. In the future, humans might do this too. Together with Nathan Newman and other colleagues at Arizona State University, and artists Denise Milan and Penny Cagney, I’ve been exploring a simpler, less invasive way to widen our colour perception. When we view objects in reflected light – which is the usual case – both the objects and the light source factor into what we see. By varying the illumination source in controlled, interactive, adjustable ways we can bring out different features of a scene. For example, we can emphasise the blue of the sky, or the yellow in a field of daffodils. Over time, we can begin to appreciate how tetrachromats experience the world, or pigeons, or even mantis shrimps. More generally, we open rich new possibilities for interacting with works of visual art. Let us give thanks for the quantum sensors inside our heads – and for our ability to understand them.

Opsin your eyes! Quantum sensors are the colour vision magicians inside our heads

.css-1c6uqr6{color:inherit;font-weight:inherit;font-size:inherit;font-family:inherit;line-height:inherit;overflow-wrap:break-word;}The science behind the colours of a rainbow is a reminder of the wonders of the original quantum designer

The science behind the colours of a rainbow is a reminder of the wonders of the original quantum designer