Quantum Institute: Visitor Schedule

The Quantum Lunch is regularly held on Thursdays in the Theoretical Division Conference Room, TA-3, Building 123, Room 121.
For more information, contact Diego Dalvit.

October 19 , 2006
12:30 PM

Mark W. Keller,
Quantum Electrical Metrology Division, NIST/Boulder

Examination of the Charge Quantum in a Single-Electron Pump

Abstract

In contrast to ordinary metallic circuits where charge flow is continuous, charge in single-electron tunneling (SET) circuits moves in discrete quanta. This allows charge motion to be controlled with impressive accuracy: a 7-junction SET pump can transfer charge at a rate ~ 10 MHz with an error ~ 10-8 per cycle. It is generally assumed that each cycle transfers a charge of exactly e, the free electron charge. To the extent that this is true, SET devices have an important role to play in fundamental metrology by providing a solid-state current source that is directly linked to a fundamental constant of nature. Such links are especially important in light of a recent proposal that the International System of Units (SI) be redefined in 2011 by assigning an exact value to e (as well as to the Planck constant, h, and other constants) as was done for the speed of light in 1983.

Is the SET charge quantum in fact exactly e? Theory provides surprisingly little guidance here, but I will discuss why this is not a trivial question and then present an experimental answer to the question: by placing a known number of SET charge quanta onto a known capacitor, and by measuring the resulting voltage across the capacitor using a Josephson voltage standard, one can compare the SET charge quantum to e. We find that the SET charge quantum is equal to e within a relative standard uncertainty of 1 part in 106, which puts a limit on possible corrections that is ~ 100 times smaller than the best previous result. This measurement is expected to reach an uncertainty ~ 3 parts in 107 in the near future, at which point it will also give useful information on possible corrections to the Josephson constant, KJ = 2e/h.