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Breakthrough: quantum-precise 1-volt alternating current obtained for the first time

Pulse-driven AC Josephson voltage standard with effective output voltages of up to 1 volt for generation of arbitrary waveforms of the highest quality

PTB-News 2.2016
Especially interesting for

metrology institutes

calibration laboratories

manufacturers of electrical precision measuring instruments

At PTB, the output voltage of a pulsedriven AC Josephson voltage standard has been significantly increased by using triple-stacked Josephson junctions, and by series connection of eight circuits with a total of 63 000 junctions; where previously obtained maximum output voltages were around an effective value of 300 mV, a standard value of 1 volt (important for metrological applications) has now been achieved for the first time. A precision comparison with an AC quantum voltmeter, at a frequency of 250 Hz, demonstrated an excellent agreement of (3.5 ± 11.7) nV/V. The increase obtained in the effective voltage opens up a range of new application possibilities.

Top left: chip with two JAWS circuits (with a total of around 18 000 integrated Josephson contacts). Center: scanning electron microscope image of a JAWS circuit. Top right and bottom left: detailed enlargement of the circuit.

Pulse-driven AC Josephson voltage standards make it possible to generate spectrally pure, arbitrary waveforms; for this reason, they are also referred to as Josephson Arbitrary Waveform Synthesizers (JAWS). They are based on series arrays of superconducting Josephson junctions of the kind manufactured in the Clean Room Center of PTB. In a complex multilayer thin-film process enhanced by PTB, a sequence of three layers (superconductor – normal conductor – superconductor) is used, of which the middle, very thin normal conductor layer of NbxSi1−x weakly connects the two superconductor Nb layers. The extremely stable and reproducible deposition conditions allow layers of well-defined thickness to be manufactured. In this way, stacks of three Josephson junctions each – i.e., four Nb layers and three NbxSi1−x layers in sequence – were manufactured with a high fabrication yield. On a single chip (10 mm · 10 mm), two circuits are integrated with a total of around 18 000 Josephson junctions (see image). The combination of eight JAWS circuits (i.e., four chips) resulted in a series connection of 63 000 Josephson junctions in total. By irradiating these JAWS circuits with a pulse signal in the GHz frequency range, sine waves are generated. The output voltage achieved was 1 volt and demonstrated outstanding spectral purity (signalto- noise ratio better than 120 dB).

An AC quantum voltmeter, also developed at PTB (see PTB News 2013/2), was used to check the accuracy of the sine waves generated in this way. This enabled a direct comparison of two different Josephson voltage standards, performed at 1 volt across the frequency range of 30 Hz to 1.5 kHz – the first such comparison in the world. The “quantum accuracy” of the new, pulse-driven 1-volt JAWS system was proven with an excellent agreement of (3.5 ± 11.7) nV/V at a frequency of 250 Hz.

The fact that an effective voltage of 1 volt can be generated opens up a wide range of new possibilities for applying JAWS in the field of electrical AC voltage metrology such as calibrating measuring instruments, analog-to-digital and digital- to-analog converters, AC/DC standards,


Oliver Kieler
Department 2.4 Quantum Electronics
+49 (0)531 592-1623

Scientific publication

R. Behr, O. Kieler, J. Lee, S. Bauer, L. Palafox, J. Kohlmann: Direct comparison of a 1 V Josephson arbitrary waveform synthesizer and an ac quantum voltmeter. Metrologia 52(4), 528–537 (2015)