January in quantum physics

The beginning of the year was filled with great physics. Lots of interesting theory has been done, including by me (but more details on that later). But today I want to talk about three experiments that push our abilities to control matter using light into new regimes. In two of them, scientists were able to observe quantum effects in the motion of levitated objects for the first time. In the third one, the authors used their incredible control of single atoms to create a very thin and light mirror.

Levitated particles enter the quantum regime

Optical levitation
L. Magrini, Y. Coroli/University of Vienna

Observing quantum effects with large objects is difficult, but that does not stop researchers from trying
. It is a particular problem for optically levitated particles whose collisions with surrounding gas molecules quickly muddle any quantum effects. But scientists from the ETH in Zurich and the University of Vienna were able to push the limits of their experiments and see quantum effects in the motion of levitated particles.

When light is shone onto a particle, it can be scattered. Most often, nothing particularly interesting happens during this process. But sometimes, the energy (or frequency) of the photon changes. If it becomes higher, the photon took some energy from the motion of the particle (this energy is equal to one motional quantum or phonon). If it becomes lower, the photon deposited one phonon worth of energy into the motion. At the motional ground state, there are no phonons and photons thus cannot take more energy out of the particle.
Researchers from the ETH haven’t yet cooled the particle motion all the way to the ground state. But they got close enough to be able to see the asymmetry in the number of photons that created or absorbed a phonon. In classical physics, there’s no need for the kinetic energy of the particle to leave in discrete packets. The experiment thus shows that the motion of the particle obeys the laws of quantum physics.
Their colleagues from the University of Vienna went one step further and cooled the motion of their particle all the way down to the ground state. By creating an imbalance in the photon scattering, they created a situation where most photons take energy out of the particle motion. This enabled them to reach a state where the particle motion has less than one phonon of energy on average.

Building mirrors from individual atoms

Atomic mirror
Taken from arXiv:2001.00795

When talking about a mirror, most people think of a slab of glass with a layer of metal on one side. In physics laboratories, mirrors can take more exotic forms, such as repeating thin layers of various materials (which might be transparent when one thick slab is used). But all these mirrors are usually bulky. Now, researchers from the Max Planck Institute for Quantum Optics and University of California, Berkeley, created a mirror from individual atoms in a single layer.

When light impinges on an atom and has the same frequency as the transition from the atom’s ground to its excited state, it will be reflected. A single atom can thus serve as a mirror, albeit a very small one. A layer of atoms can thus act as a mirror, but only if the atoms are sufficiently close together. The team around Immanuel Bloch is the first to achieve this by trapping the atoms in an optical lattice. Ensuring that the atoms interact with each other (making their response to incoming light stronger), scientists created one large mirror instead of an array of small ones.
This structure is not going to replace your regular bathroom mirror (although it could have its advantages—such as no fogging while you’re in the shower!). But it pushes our capabilities to control large numbers of atoms one step further. It also creates an interesting device—a mirror which reflects only light in a narrow range of frequencies (around the interal transition of the atoms). This can be used to study new effects in interaction between light and matter.

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