Just What Physicists Wanted to See: Nothing

"Nothing" is a hard thing to experiment with. While the nature of nothingness is among the most deeply strange and counterintuitive properties of the universe, to observe nothing in action—to measure and probe nothing, to quantify nothing—requires the usage of, well, something. And something in nothing is no longer nothing. It's something.

A group of physicists based at Chalmers University of Technology in Sweden and the University of Waterloo have devised a new way to probe the quantum vacuum (nothing, such as it is) using a sort of mirror. As implemented via a one-dimensional superconducting waveguide, it's not quite a mirror in the sense we're used to, but the basic idea of viewing an object via its reflection is similar enough.

Here, the object in question is an artificial atom placed within an engineered vacuum, where its spontaneous radiation emissions reveal, in a sense, an image of that vacuum.

First, understand that there is no nothing. Nothing does not exist. It turns out that nothing, a perfect vacancy of matter and energy, violates some pretty key principles of quantum mechanics. It's easiest to see this via the lens of the uncertainty principle, which forbids perfect (simultaneous) knowledge of both a particle's position and momentum. That means "nothing" is a forbidden certainty in an uncertain universe. Thems the rules, as they say.

So, the upshot of this is that particles just appear, effervescing out of the void just long enough to actually exist but not long enough to violate another fundamental feature of physics, which is that energy can be neither created nor destroyed. These are vacuum fluctuations, and they're obviously pretty interesting to physicists. This is also known as zero-point energy.

An important revelation of recent years is that these fluctuations are non-trivial. They play important roles in more familiar physical processes, such as the lifetimes of excited states of atoms, the small blurrings among electron energy levels known as the Lamb shift, and some other electron behaviors. The particle fizz of the vacuum can even be seen macroscopically as the Casimir effect, in which virtual photons (the fluctuations) exert a pressure on parallel plates.

The "real" effects of zero-point energy were at first controversial. "In the intervening years, however, the idea that the vacuum itself is physical gained increasing credence with a growing number of striking vacuum phenomena predicted such as Hawking radiation, the Unruh effect, and the Casimir effects," the Chalmers physicists write in a paper published this week in Nature Physics (links are mine). "In recent years, these vacuum effects have even started to have technological impacts, contributing to stiction in nanomechanics, and decoherence in superconducting qubits. This has led to an increasing interest in engineering the vacuum."

Here's how the artificial atom scheme works: An atom or molecule in an excited state—where an electron at its lowest possible energy level in an atom is briefly kicked upward to a higher energy state—will usually undergo a near-immediate decay back to its stable ground state, kicking out excess energy as photons (light particles) along the way. This decay is kickstarted with help from quantum fluctuations. So, an atom undergoing radiative decay (note: not radioactive decay) will give off light, and this light will contain information about the vacuum.

It's been difficult to take advantage of this, however, as we're talking about very small amounts of light. The Chalmers group solves this by adding in a waveguide capable of shuffling away incoming photons with 99 percent efficiency. It also adds in a new level of control over the movements of the atom and waveguide, ensuring no interference from motion-noise in the signal.

"Although the structure of the vacuum fluctuations cannot be directly measured with a classical probe, such as a voltmeter, they can be measured by observing the effect of the vacuum fluctuations on a quantum probe, such as an atom or qubit," the paper explains. "The decay rate of an excited state … is proportional to the strength (spectral density) of electromagnetic fluctuations near the frequency that are present in the atom's environment."

And there you have it: Nothing.