To make an atom-sized machine, you need quantum mechanics

Think about how your computer or laptop has a lot of things inside that get hot. Today you cool that down with a fan that blows air. In nanomachines or quantum computers, small cooling devices could be useful, ”says Dario Poletti of the Singapore University of Technology and Design (SUTD).

This work offers a new vision of the mechanics of these devices. The work is a collaboration between researchers from the Center for Quantum Technologies (CQT) and the Department of Physics at the National University of Singapore (NUS), SUTD and the University of Augsburg in Germany. The results were published in the peer-reviewed journal. npj Quantum Information May 1st.

Engines and refrigerators are machines described by thermodynamics, a branch of science that tells us how energy moves within a system and how we can extract useful work. A classic motor converts energy into useful work. A refrigerator works to transfer heat, lowering the local temperature. They are, in some ways, opposite.

People have made small heat engines before using a single atom, a single molecule, and flaws in diamond. A key difference about this device is that it shows quantum in its action. “We want to understand how we can build thermodynamic devices with only a few atoms. Physics is not well understood, so our work is important to know what is possible, ”says Manas Mukherjee, principal investigator for CQT, NUS, who led the experimental work.

The researchers studied the thermodynamics of a single barium atom. They designed a scheme in which lasers move one of the electrons in the atom between two energy levels as part of a cycle, causing some of the energy to be pushed into the vibrations of the atom. Just like a car’s engine consumes gasoline to move the pistons and charge its battery, the atom uses the energy of lasers as fuel to increase its vibratory motion. The vibrations of the atom act like a battery, storing energy that can be extracted later. Rearrange the cycle and the atom acts as a refrigerator, removing energy from vibrations.

In any mode of operation, quantum effects appear in correlations between electronic states and vibrations of the atom. “At this scale, the energy transfer between the motor and the load is a bit confusing. It is no longer possible to just work on the load, you are forced to transfer some heat, “says Poletti. He worked the theory with collaborators Jiangbin Gong at NUS Physics and Peter Hänggi in Augsburg. The confusion makes the process less efficient, but experimenters could still make it work.

Mukherjee and colleagues Noah Van Horne, Dahyun Yum, and Tarun Dutta used a barium atom from which one electron (a negative charge) is removed. This causes the atom to charge positively, so it can be more easily held within a metal chamber by electric fields. All the rest of the air is removed around you. The atom is laser removed to move it through a four-stage cycle.

The researchers measured the vibration of the atom after applying 2 to 15 cycles. They repeated a set number of cycles up to 150 times, measuring on average how much vibrational energy was present at the end. They could see that the vibrating energy increased when the atom was removed with a motor cycle, and decreased when the changes followed the cycle of the refrigerator.

Understanding the atom-sized machine involved complicated calculations and observations. The team needed to track two thermodynamic quantities known as ergotropy, which is energy that can be converted into useful work, and entropy, which is related to disorder in the system. Both ergotropy and entropy increase as the atomic machine works. There is still a simple way to look at it, says first author and Ph.D. student Van Horne, “Generally speaking, we have designed a small machine that creates entropy as it fills up with free energy, just like children when they are given too much sugar.”

More information:
Noah Van Horne et al, single-atom energy conversion device with a quantum charge, npj Quantum Information (2020). DOI: 10.1038 / s41534-020-0264-6