Electron detection

An ammeter for nanoelectronic measurement has been developed in Japan that is so sensitive it can detect a single electron flowing in either direction in real time.

Nippon Telegraph and Telephone Corporation (NTT) collaborated with the Tokyo Institute of Technology (TokyoTech), the Japan Science and Technology Agency (JST), and Tohoku University to demonstrate the device. The single-electron ammeter can be used to measure extremely small currents undetectable to previous meters and to analyse the motion of an individual electron. It has potential applications in nanoelectronics and developing quantum information technologies.

A conventional sensitive ammeter requires the flow of millions of electrons in order to detect current. NTT’s single-electron ammeter integrates two quantum dots and a point contact in a semiconductor device. A quantum dot is a conductive nanostructure that accommodates a small number of electrons, in this case set so the number of electrons can change by only one.

An electron entering or escaping from one of the quantum dots influences the current flowing through the point contact, allowing the change in the electron occupation in a quantum dot to be monitored. Two quantum dots are required in order to identify the direction in which the electron is moving.

The researchers made the single-electron ammeter using semiconductor nanofabrication techniques. Each quantum dot has a diameter of about 100 nanometres and is placed about 200 nanometres from the point contact, a tiny contact through which electrical current flows between two electrodes.

When an electron travels through the two quantum dots, the current through the point contact shows three different values depending on the location of the electron. Jumping between these values can be understood as, for example, an electron entering the left dot, moving to the right dot, and exiting via the right lead. In this way, a single-electron flow can be detected precisely and average current can be obtained by counting the net electron flow.

The single-electron counter could have an important role in nanoelectronics, examining electron transport through nanostructures, single molecules, and biological cells, all of which require measurement of extremely small currents.