Quantum AC Voltage Standards

Standards based on the Josephson effect are being increasingly utilized for generation of quantum-accurate ac waveforms. These standards fall into two broad categories: those whose output is generated as a stepwise-approximated sine wave via programmable Josephson arrays and those with a true sinusoidal output generated by pulses supplied to Josephson junctions. This paper presents a survey of the present state of quantum ac sources and discusses future implementation of these systems for improved ac metrology. We focus mainly on developments at the National Institute of Standards and Technology but also summarize relevant work elsewhere.