Measuring the Entropy of a Double Quantum Dot
Determining the entropy of a system typically entails measuring its heat capacity. That approach is harder to apply when the system consists of a very small number of electrons corralled in a tiny device. Fortunately, the entropy of such systems can be derived from the chemical potential, which is the energy needed to pack in an additional electron. This technique enabled the 2018 measurement of the entropy of a single quantum dot, a nanostructure that can trap electrons. Now David Kealhofer of the Swiss Federal Institute of Technology (ETH) Zurich and his collaborators have applied the approach to a double quantum dot [1]. Their work provides a stepping stone to future measurements of multiple coupled dots that possess topological or highly entangled states. It could also allow researchers to determine quantities that are even harder to measure, such as exotic quasiparticle couplings.
Kealhofer and his collaborators made their double quantum dot by attaching electrodes to a layered structure of gallium arsenide and aluminum gallium arsenide. The device could be tuned to behave as two isolated dots or as a coupled pair. As the dots were switched from one electron-occupation state to another, the change in entropy could be deduced from the gate voltage at which the two states were equally probable. When an isolated dot was measured, the entropy change for adding a single electron to an empty level, kB log 2, matched both theory and the 2018 measurement. But in the double-quantum-dot regime, when the number of electrons switched from 0 to 1, the researchers found the unexpected value of kB log 3. As more electrons were added or removed from the dot, richer behavior appeared, raising the researchers’ hopes that their technique could detect Majorana zero modes and other exotic states.
–Charles Day
Charles Day is a Senior Editor for Physics Magazine.
References
- D. Kealhofer et al., “Entropy of a double quantum dot,” Phys. Rev. Lett. 135, 206303 (2025).