Electron counting in quantum dots

TL;DR

This paper demonstrates real-time single-electron counting in quantum dots using charge detection, revealing electron correlations, back-action effects, and enabling microwave photon detection at the single-photon level.

Contribution

It introduces a high-precision charge detection method for quantum dots, enabling electron counting, correlation analysis, and single-photon microwave detection.

Findings

Electrons exhibit anti-bunching due to Coulomb interactions.

The quantum point contact acts as both detector and source of back-action.

The device functions as a frequency-selective microwave single-photon detector.

Abstract

We use time-resolved charge detection techniques to investigate single-electron tunneling in semiconductor quantum dots. The ability to detect individual charges in real-time makes it possible to count electrons one-by-one as they pass through the structure. The setup can thus be used as a high-precision current meter for measuring ultra-low currents, with resolution several orders of magnitude better than that of conventional current meters. In addition to measuring the average current, the counting procedure also makes it possible to investigate correlations between charge carriers. In quantum dots, we find that the strong Coulomb interaction makes electrons try to avoid each other. This leads to electron anti-bunching, giving stronger correlations and reduced noise compared to a current carried by statistically independent electrons. The charge detector is implemented by monitoring changes in conductance in a near-by capacitively coupled quantum point contact. We find that the quantum point contact not only serves as a detector but also causes a back-action onto the measured device. Electron scattering in the quantum point contact leads to emission of microwave radiation. The radiation is found to induce an electronic transition between two quantum dots, similar to the absorption of light in real atoms and molecules. Using a charge detector to probe the electron transitions, we can relate a single-electron tunneling event to the absorption of a single photon. Moreover, since the energy levels of the double quantum dot can be tuned by external gate voltages, we use the device as a frequency-selective single-photon detector operating at microwave energies.