R&D Information
Single-electron Ammeter Based on Bidirectional
Counting of Single ElectronsVol. 4 No. 8 Aug. 2006

Single-electronammeter
LCurrent
60R400PC300
40200
201000Counts (per second)Single-electrontransistor

Current (aA)NTT, in collaboration with the Tokyo Institute ofTechnology, the Japan Science and Technology
Agency, and Tohoku University, has successfullydemonstrated an extremely sensitive ammeter thatcan measure extremely small currents that cannot bemeasured with any existing meter. The single-electron ammeter can detect the flow of one electron ineither direction in real time, whereas a conventional
high-sensitivity ammeter requires millions of electrons in order to detect them as a current. It consistsof two quantum dots and a point contact integrated ina semiconductor device. A quantum dot is a small
conductive nanostructure that accommodates a small
number of electrons and a point contact is a small
point-like contact through which electrical current
flows between electrodes. An electron entering orescaping from one of the quantum dots influences thecurrent flowing through the point contact, and thisenables the change in electron occupation of a quantum dot to be monitored.
NTT fabricated this single-electron ammeter using
semiconductor nanofabrication techniques. Eachquantum dot has a diameter of about 100 nm and twoof them are placed about 200 nm from the point contact. When an electron travels through the two quantum dots, the current through the point contact cantake one of three different values depending on thelocation of the electron. Jumping between these values, for example, an electron entering the left dot,
moving to the right dot, and escaping via the rightlead, can be identified as an electron flow. In this way,a single-electron flow can be detected and the average current can be obtained by counting the net electron flow. NTT demonstrated the performance of thesingle-electron ammeter by connecting it to a singleelectron transistor and successfully obtained the current flowing through the single-electron transistor(Fig. 1). The correct path of an electron in the device
could also be traced.
Photon counters, in which individual photons canbe counted, have been widely used to detect faint0
Noise level: 3 aA
(20 electrons per second)

–160

–140

–120

Gate voltage (mV)Fig. 1. Demonstration of the single-electron ammeter.

light. Similarly, the present single-electron countershould be useful for detecting extremely small currents in various applications. It should be especially
useful for studies on nanoelectronics, which seek to
examine electron transport through nanostructures,
single modules, and biological cells, and often
require the measurement of extremely small currents.
Combining the ammeter with a device that converts
photons or electron-spins into electronic charges
could lead to the development of sensitive detectors
for light or magnetic fields. Furthermore, statisticalanalyses of current noise measured with the singleelectron ammeter would identify quantum entanglement, in which quantum information is shared
between two separated electrons.
For further information, please contact:
Planning Department
NTT Science and Core Technology Laboratory
GroupE. Tamechika
Tel: +81-46-240-5152
http://www.ntt.co.jp/sclab/contact_e/51