Silicon quantum current standard
Our rigorous long term goal is to find a high-yield (100 pA) and metrologically accurate (10 ppb uncertainty) current source which could be used to redefine the unit of electric current, the ampere. This would revolutionize the International System of Units (SI) and allow for the possibility to compare the values of the Planck constant and electron charge in different quantum measurements with a stringent accuracy. In this so-called quantum metrological triangle experiment, the voltage is aquired from the ac Josephson effect, resistance from the quantum Hall effect, and current from the single charge pump. The Ohm’s law is then used to compare the fundamental quantities of nature.
In the past, we have studied the so-called sluice, i.e., a flux-assisted Cooper pair pump. We were able to pump as high current as 1 nA but with uncertainty down to a few percent, which is not enough for triangle measurement [Vartiainen et al., Appl. Phys. Lett. 90, 082102 (2007)]. We have also been working on SINIS-turnstiles [Nature Phys. 4, 120 (2008)] and silicon single-electron turnstiles [Appl. Phys. Lett. 92, 212103 (2011)]. However, the current experiments are done with silicon single-electron pumps, which are even more promising to be the new current standard.
Single-electron turnstile could be used for the quantum current standard. In a turnstile, one or more electron are allowed to tunnel from current source lead to a small island and then to further to a current drain lead. The benefit of employing a semiconductor such as silicon is that the tunneling rates can be adjusted more accurately than in the metallic structures. Also it allows higher charging energies, which prevents pumping errors. We have likely been able to reach error of a few per mille [Appl. Phys. Lett. 92, 212103 (2011)] (strictly speaking the uncertainty was 2%), but silicon quantum dots have lot of potential to be improved to a even to the uncertainty 10 ppb. The fundamental disadvantage of a turnstile is that it requires a bias voltage to operate and hence energy will always be dissipated in the vicinity of the device. This excess energy may be harmful for the accurate operation of the current source.
We are currently working on an electron pump that does not have the fundamental disadvantage of the dissipated power as in the turnstile. The new device is more accurate and more adjustable than the turnstile device. There are two major improvements compared to the turnstile: stronger capacitive coupling to the quantum dot and single-electron transistor. A single-electron transistor is used to monitor pumping errors of the pump which enables the observation of higher accuracy in current through the system.