Quasi particle of sound “cut in two”

When Alice walked through the mirror, she never imagined that she would discover a wonderful world whose rules constantly defy intuition. In a way, physicists are explorers of a wonderful world like Lewis Carroll’s, where quantum mechanics leads to amazing and counterintuitive phenomena. His laws explain, for example, how a particle can simultaneously find itself in two different states or pass through two different paths at the same time, as if it were “cut in two”! These tricks have long been perfected for photons and particles of light. But can we do the same thing with phonons, the “sound particles”? Andrew Cleland’s team has just hit that performance. Paves the way for hybrid quantum computing devices.

The idea is surprising because, unlike photons, phonons are not elementary particles. We are talking instead of quasiparticles, because phonons actually describe collective behavior usually 10¹⁵ that vibrate in matter and form, for example, a sound wave. However, a quasiparticle behaves in many ways like a single, indivisible particle. It is possible to conduct quantitative experiments on such things.

But what does the idea of ​​cutting a particle into two parts consist of? A good starting point is the famous experience of the encyclopedic scientist Thomas Young, who was born only two hundred and fifty years ago. This English doctor, passionate about Egyptology, especially left his mark in physics with his “Young cracks”. In practice, this amounts to illuminating a beam of light on a screen pierced by two minute parallel slits very close together. On the second screen placed behind the slits, a motif consisting of alternating light and dark bands is then molded. This result can be easily understood if we consider that light is a wave. Passing through the two slits, the light interferes with itself: depending on the position on the screen, the intensity of the light coming from one of the slits is added to or subtracted from the other. This experiment was crucial to the debate against the particle nature of light against those who defended the wave nature.

But with the advent of quantum mechanics since the 1920s, physicists understood that light is in fact subject to wave-particle duality and can therefore be interpreted, depending on the situation, as a wave or a group of photons. But, in this context, what happens if we bombard Young’s slits with the emitted photons one by one? Each photon that crosses the slits seems to arrive at the downstream screen in a random position…but if we wait long enough for the photons to accumulate, we see that we find the interference pattern again. To explain this, we must admit that each photon passes through the two slits and interferes with itself: it is as if it were cut in two! More precisely, quantum physics suggests that the photon exists in what is called a “superposition of states”. A particle is described with a function consisting of the state in which it passes through the left slit and the state in which it passes through the right slit. These two components interfere with each other.

Another way to achieve Young’s slit principle is to use a semi-reflective mirror. The latter has a 50% probability of reflecting the photon and 50% of letting it pass (we are talking about “transmission”). Thus, the photon that reaches the mirror finds itself in a superposition of the state in which it is transmitted and the state in which it is reflected.

A Michelson interferometer (famous for its use to prove the invariance of the speed of light) or a Mach-Zehnder interferometer uses a semi-reflective mirror and an array of mirrors to create an interferometer. These elements are found in many optical devices that exploit the laws of quantum physics. Furthermore, in 2000 Emmanuel Neel, Raymond Laflamme, and Gerard Milburn showed that it is possible to perform quantum computing with single photon sources, quasi-reflecting mirrors, and single photon sensors. The mirrors are then arranged to build logic gates to perform operations on the photons. This field of research is booming.

These manipulations performed on photons are probably It applies to any kind of quantum object, even to phonons. But one item was missing from the toolbox of physicists fiddling with phonons: the semireflective mirror. However, this is exactly what Andrew Cleland’s team has just developed.

The experimental device consists of two superconducting qubits that act as single photon sources. Each source is then linked to a transducer capable of converting a photon into a single phonon, or conversely converting a phonon into a photon. Between the two transducers, the researchers placed a “comb” of sixteen metal teeth through which phonons pass. Each tooth reflects approximately 3% of the vocal energy. Thus, any phonon emitted by one superconductor has a 50% chance of reaching the other and a 50% chance of returning to its starting point. The researchers were then able to show that the phonon was indeed in a superposed state.

The team then performed an interference experiment consisting of simultaneously injecting a phonon from the left and another from the right. The two phonons interact in the comb and both always exit from the same side. Predictions were confirmed by observation.

Perhaps the idea of ​​building linear quantum computing systems based solely on phonons is not on the agenda because implementation using photons is more advanced and efficient, particularly for miniaturization of elements. However, this “half-mirror” of phonons opens the door for other hybrid quantum computing devices or for the development of acoustic communication networks. In Wonderland, imagination has no limits!