The Second Quantum Revolution – From Entanglement to Quantum Computing and Other Super-Technologies
In Douglas Adams‘ parody of intergalactic life The Hitchhiker’s Guide to the Galaxy, one reads at the beginning of the second book:
There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened.
The physics of the 20th century can hardly be described more fittingly. Around 1900, physical concepts such as fields and waves, the invisible force of gravity, and entropy were already quite bizarre and difficult to grasp for a broad audience. Not all these phenomena could be seen or touched, but they were calculable and predictable and reflected what people were experiencing in their everyday lives. Despite their abstractness, they were still quite real in comparison to the mental constructs physicists had to develop to understand the nature of the atomic world (as well as the vastness of the universe).
The triumph of the totally bizarre began with the observation that at the level of atoms certain quantities cannot take just any value. For example, the radiated energy of certain bodies only assumes fixed, and in fact discrete values. It is so to speak packaged in what physicists were to call quanta (from the Latin word quantum – that much). If the rules of the micro world were also valid in „our“ world, one would only be able set the temperature in one’s living room at 10, 20, or 30°C, while all values in-between would simply not exist. A short time later, physicists realized that light has a dual nature: sometimes it is a wave, while another time it may be a particle. The same holds for the electron, as was observed shortly afterwards. But how can a spatially localized particle simultaneously be a spatially extended (de-localized) wave? In the world of classical science, where white is always white and black is exactly black, this „wave-particle duality“ seemed like an outrageous provocation.
By the end of the 19th century, physicists had just become accustomed to the idea that their theories would soon provide a complete understanding of everything in the world. What felt like a moment later, they were suddenly forced to say goodbye to 250-year-old physical truths and more than 2,500-year-old philosophical certainties. They had to deal with more and more seemingly impossible circumstances. Quantum entities can be in several states at the same time, for example, they can be in several places at once. And then quantum entities do not even possess objectively defined properties: their properties can only be specified with probabilities, the results of measurements depend on the observer, and their quantum states (wave functions) simply decay outside any window of time. And finally there is the strangest of all quantum phenomena: the entanglement of spatially separated particles. Even when they are far apart, two particles can be coupled together as if by magic. The bottom line is that the nature and properties of quantum entities are highly abstract and can no longer be reconciled with the way we perceive and think about things in our everyday lives.
However, despite all these imponderables, today’s quantum theories predict the outcome of experiments and natural phenomena with an accuracy unsurpassed by any other theory in science. Here is another counter-intuitive manifestation that contradicts any everyday experience: something that is indefinite and elusive is nevertheless a process that can be calculated 100 percent accurately.
Because we can calculate ever more exactly what is going on at the atomic level, we are able to gain more and more control over the microcosm. Applications of quantum physics have long since become a concrete part of our lives. Electronics, digital technologies, lasers, mobile phones, satellites, televisions, radio, nuclear technology, modern chemistry, medical diagnostics – all these technologies are based on the laws of quantum theory. From modern chemistry to solid-state physics, from signal processing to modern imaging systems in medicine – everywhere we meet these today. Everyday we trust their laws when we get into a car (relying on the on-board electronics), start up our computer (which consists of integrated circuits, i.e. electronics based on quantum phenomena), listen to music (CDs are read out by a laser, a pure quantum phenomenon), make X-ray or MRI images of our body, let us be guided by GPS or communicate using our mobile phone. According to various estimates, between one-quarter and one-half of the gross national product of industrialized nations today is directly or indirectly based on inventions based on quantum theory. So we completely rely on quantum technology, even if the theory behind it –in our everyday understanding – describes a world with very uncertain and unstable manifestations and seemingly paradoxical properties.
Only in recent years have physicists begun to realize that quantum physics can ensure a significant supply of as yet unexploited technological capabilities. The renowned quantum physicist Rainer Blatt predicts another „century of quantum technology“ for the 21st century, enough to fundamentally change both our economy and society. We are just at the beginning of our understanding of the possibilities arising from this revolution, Blatt believes.Concretely: We are at the brinkof another breath-taking technological development, a new, second quantum revolution!
What characterises this second quantum revolution? Physically speaking the (first) quantum revolution of the 20th century is essentially based on the properties of large ensembles of quantum particles and the possibilities for controlling them: steered flow of many electrons, targeted excitation of a large number of photons, and the measurement of the nuclear spin of numerous atoms. Concrete examples are the tunnel effect in modern transistors, the coherence of photons in lasers, the spin properties of the atoms in magnetic resonance tomography, Bose–Einstein condensation, or the discrete quantum leaps in an atomic clock.The emerging second generation of quantum technologies is based on something completely new: the directed preparation, control, manipulation, and subsequent selection of states of individual quantum particles and their interactions with each other. Of crucial importance here is one of the strangest phenomena in the quantum world, which already troubled the founding fathers of quantum theory. This is entanglement. Probably the most exciting technology of the second quantum revolution is the quantum computer, which could outnumber today’s computer by a million fold in terms of speed and computational efficiency.
My new book “The Second Quantum Revolution”, to appear in October 2018 with Springer (German edition in August 2018), takes the reader into the completely crazy, fabulous, and incredible world of the quantum. On this journey, we will first take a look at the world of the quantum technologies that have already started to shape our today’sworld. We will quickly realize whywe are at the beginning of another breath-taking development. In the second and the third parts of the book, we take a closer look at the strange discoveries in the quantum world, which, as will be explained in the fourth part, also strongly shaped the philosophical, spiritual, and religious thinking of the 20th century. The fifth part then takes us to the much disputed very core of the quantum world, which at the same time represents the basis of several exciting future quantum technologies:the phenomenon of entanglement, on which physicists only in the last few years have been able to get their hold on.. Here, as we shall see, we find solutions to some of the most challenging contradictions that Einstein, Bohr, and their colleagues were struggling so hard (und ultimately unsuccessfully) to resolve. In the last chapter, we shall venture some suggestions as to how new quantum technologies willconcretely shape our future.