What really are light and matter? Superconducting quantum circuits could reveal the answer
The terms “light” and “matter” are well-known in everyday life, but at the quantum scale their definitions are less clear-cut.
You come around from a well-earned snooze late on Sunday morning. Breakfast is next on the day’s itinerary. You amble downstairs at a leisurely pace, but with purpose, motivated by the prospect of fresh coffee and warm buttered toast. You flick on the toaster and kettle. Electricity surges through high resistance elements to produce heat that toasts your bread and boils your water. Leaning against the kitchen counter, you check the morning news on your smartphone. Powered by your phone’s battery, intricately designed circuitry performs high-frequency computations that depend on your inputs. Wireless electromagnetic signals provide communication with your home’s internet router, which in turn communicates with a distant server via electromagnetic signals sent through a vast infrastructure of cables. The end product of these interactions is visible light produced by your smartphone’s screen to be received by your eyes. The information it holds is transported to your brain in the form of electrical impulses travelling along your optic nerve.
Your toast pops up and the kettle has boiled. Just moments later, still glancing at the day’s latest developments, you sit down to enjoy the fruits of the morning’s labour and a question forms in your mind; how is all this possible? Nearly every aspect of the world we have built around us is based on our understanding of the interaction between matter and the electromagnetic field that permeates all space and time. It might therefore come as a surprise that, at the most fundamental level, light and matter remain somewhat mysterious.
The term “Light” can be taken as short for electromagnetic radiation of which visible-light comprises only a narrow band of frequencies. Around 150 years ago the famous Scottish physicist James Clerk Maxwell unified the known physical laws that govern light and matter, producing what is now a pillar of modern physics. Maxwell’s electrodynamics possesses unparalleled mathematical richness and beauty combined with an equal measure of practical utility. It is the prototypical example of our most fundamental class of theories, so-called gauge-field theories, which are characterised by their possession of a certain mathematical freedom referred to as “gauge-freedom”. Consider, for example, an everyday battery that supplies around 1.5 Volts of electricity. This 1.5 Volts is the potential difference between the battery’s positive and negative terminals, and yet only this difference is measurable. We are free to choose any numbers we like as the values of the potentials at the terminals themselves as long as these numbers differ by 1.5. This is a simple example of gauge-freedom, and physicists refer to choosing specific values for the potentials at the battery terminals as “choosing a gauge”.
Despite the unprecedented success of electrodynamics in advancing both our understanding of reality and our technological capabilities, over the past century we have come to learn that at the most elementary level a new kind of physical theory is required to explain what we observe. Enter quantum theory; as mysterious as it is powerful, when combined with electrodynamics, it provides the fundamental theory aptly known as quantum electrodynamics (QED). QED is the most accurate scientific theory ever constructed. Understanding it is imperative to manipulating light-matter interactions for next-generation quantum technologies. Quantum computers for example, will rely on a phenomenon called entanglement, the stronger-than-classical correlations that can occur between two or more quantum systems. Photons are quanta of light energy that can be used to mediate entanglement between different, even distant material systems allowing networked quantum information processing.
The definitions of entanglement and of photons are predicated on the idea that we can break-up overall quantum systems into constituent parts called quantum subsystems, such as ``light” and ``matter”. However, quantum theory’s inherently probabilistic nature imposes specific mathematical constraints on the ways in which we can do this. Combined with the gauge-freedom of electrodynamics, a most intriguing twist occurs in QED; different choices of gauge can provide us with different definitions of the subsystems.
As an analogy, let’s imagine we are dealing instead with a quantum theory of fruit. Our system consists of four kinds of fruit; apples, pears, strawberries and raspberries. There are many versions of our fruit theory, each one corresponding to a different choice of gauge. Each version gives us the same predictions for any given observable property, for example, the number of strawberries at a certain point in time, or the positions of the raspberries. Yet the quantum properties of the fruit system have to be understood through its division into two subsystems, one subsystem of red fruit and the other of green fruit. In one version of the theory strawberries and raspberries are red, while apples and pears are green. On the other hand, as most people are aware, some apples are red, or are half red and half green. There is a different choice of gauge for which apples are classed as red and only pears are classed as green. Since red and green are being defined differently in these two different versions of the theory, they provide us with different predictions for quantum properties, such as the entanglement between the red and green subsystems. This effect can be viewed as a form of relativity, in the sense that theoretically, the quantum subsystems can only be defined relative to a choice of gauge.
The same effect is essentially what occurs in QED, a fact which is surely quite important? Indeed, it is important, or at least, it is important now. The reason the effect has largely gone unnoticed so far, is that light-matter interactions are naturally weak in comparison to the energies that characterise the subsystems themselves. If the interactions between your red and green fruits are weak enough, then it turns out not to really matter where you put your apples. Whatever your choice of classification, you will get the same predictions for all practical purposes. However, in many systems of current importance light-matter interactions are enhanced. For example, in superconducting circuits, which are a major platform for realising quantum computation, ultra-strong light-matter interactions are frequently encountered.
So, in what way do the definitions of light and matter differ between gauges? To answer this, we need to know how their interaction is usually understood. Elementary processes involve material systems emitting, absorbing, or exchanging photons. Unlike the situation in classical physics, in QED the vacuum of free space is not strictly empty, because its energy fluctuates and is only zero on average. This means that the interaction between a material subsystem and the quantum electromagnetic field is always present. A physical atom, for example, is not just a “bare” atom, but a “dressed” one, which is surrounded by a cloud of so-called virtual photons that rapidly flit in and out of existence, continually being emitted and then reabsorbed. The pioneers of QED, such as Richard P. Feynman, developed a diagrammatic method to systematically represent these processes, but the true physical processes that occur in reality are always infinite combinations of these elementary ones. The “bare” atom and the “bare” electromagnetic field can be understood as hypothetical subsystems that would exist if we could assume that no interactions were present. Feynman diagrams then represent interactions between these hypothetical subsystems, which are a handy theoretical tool for calculating an approximation of a complicated and infinite mathematical series.
As the name suggests, the virtual photons surrounding an atom aren’t directly measurable under normal circumstances. If they were, they would also have to carry usable energy, which would put a miraculous end to the problem of fulfilling global energy demands. But what if virtual photons could be converted into real ones?
Artificial light-matter systems can be engineered using superconductors and afford a high-degree of experimental control. They can also realise ultra-strong light-matter interactions, which are distinguished by the property that even when the overall system occupies its lowest energy states there is a high population of virtual photons together with a high degree of light-matter entanglement. Virtual photons belonging to the cloud tied to the (artificial) atom are emitted and reabsorbed on very short timescales, but by switching-on and-off ultra-strong interactions fast enough (which requires energy), virtual photons could be released from the atom’s grip. If, after a virtual emission event, the interaction has ceased before the photon has a chance to be reabsorbed then the virtual photon must “detach” and should then be detectable as a real one. Of course, this assumes that virtual photons are there in the first place!
What varies between gauges is the description of the “bare” subsystems and it has recently been shown that there are choices of gauge in which the properties that usually distinguish ultra-strong light-matter interactions are highly suppressed, at least in the lowest energy states. The picture then provided is one of a system that appears much as it would in the ordinary weak-interaction regime, with negligible photon population and entanglement in the lowest energy state of the combined system.
These findings clearly sharpen a quite profound question: what really are light and matter at the quantum scale? If nothing else, perhaps it’s food for thought during your next late Sunday morning breakfast.
Original research
