A proposed new way of defining the standard unit of electrical resistance would do away with the need for strong magnetic fields when measuring it. The new technique is based on memristors, which are programmable resistors originally developed as building blocks for novel computing architectures, and its developers say it would considerably simplify the experimental apparatus required to measure a single quantum of resistance for some applications.
Electrical resistance is a physical quantity that represents how much a material opposes the flow of electrical current. It is measured in ohms (Ω), and since 2019, when the base units of the International System of Units (SI) were most recently revised, the ohm has been defined in terms of the von Klitzing constant h/e2, where h and e are the Planck constant and the charge on an electron, respectively.
To measure this resistance with high precision, scientists use the fact that the von Klitzing constant is related to the quantized change in the Hall resistance of a two-dimensional electron system (such as the one that forms in a semiconductor heterostructure) in the presence of a strong magnetic field. This quantized change in resistance is known as the quantum Hall effect (QHE), and in a material like GaAs or AlGaAs, it shows up at fields of around 10 Tesla. Generating such high fields typically requires a superconducting electromagnet, however.
A completely different approach
Researchers connected to a European project called MEMQuD are now advocating a completely different approach. Their idea is based on memristors, which are programmable resistors that “remember” their previous resistance state even after they have been switched off. This previous resistance state can be changed by applying a voltage or current.
In the new work, a team led by Gianluca Milano of Italy’s Istituto Nazionale di Ricerca Metrologia (INRiM); Vitor Cabral of the Instituto Português da Qualidade; and Ilia Valov of the Institute of Electrochemistry and Energy Systems at the Bulgarian Academy of Sciences studied a device based on memristive nanoionics cells made from conducting filaments of silver. When an electrical field is applied to these filaments, their conductance changes in distinct, quantized steps.
The MEMQuD team reports that the quantum conductance levels achieved in this set-up are precise enough to be exploited as intrinsic standard values. Indeed, a large inter-laboratory comparison confirmed that the values deviated by just -3.8% and 0.6% from the agreed SI values for the fundamental quantum of conductance, G0, and 2G0, respectively. The researchers attribute this precision to tight, atomic-level control over the morphology of the nanochannels responsible for quantum conductance effects, which they achieved by electrochemically polishing the silver filaments into the desired configuration.
A national metrology institute condensed into a microchip
The researchers say their results are building towards a concept known as an “NMI-in-a-chip” – that is, condensing the services of a national metrology institute into a microchip. “This could lead to measuring devices that have their resistance references built-in directly into the chip,” says Milano, “so doing away with complex measurements in laboratories and allowing for devices with zero-chain traceability – that is, those that do not require calibration since they have embedded intrinsic standards.”
Yuma Okazaki of Japan’s National Institute of Advanced Industrial Science and Technology (AIST), who was not involved in this work, says that the new technique could indeed allow end users to directly access a quantum resistance standard.
New candidate emerges for a universal quantum electrical standard
“Notably, this method can be demonstrated at room temperature and under ambient conditions, in contrast to conventional methods that require cryogenic and vacuum equipment, which is expensive and require a lot of electrical power,” Okazaki says. “If such a user-friendly quantum standard becomes more stable and its uncertainty is improved, it could lead to a new calibration scheme for ensuring the accuracy of electronics used in extreme environments, such as space or the deep ocean, where traditional quantum standards that rely on cryogenic and vacuum conditions cannot be readily used.”
The MEMQuD researchers, who report their work in Nature Nanotechnology, now plan to explore ways to further decrease deviations from the agreed SI values for G0 and 2G0. These include better material engineering, an improved measurement protocol, and strategies for topologically protecting the memristor’s resistance.
