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Quantum based spectrum analysis


Quantum defined AC voltages from a Josephson Arbitrary Waveform Synthesizer measure precisely the fundamental tone and harmonics at the output of a commercial signal generator.


In a differential measurement, the Josephson waveform is tuned until not only the fundamental component output by the commercial signal generator is nulled, but also the harmonic components present. This application of a recurring principle in metrology, a null measurement relative to a known reference, eliminates non-idealities of the spectrum analyzer.

Periodic waveforms are often characterised using Fourier spectral analysis that describes the waveform as the sum of a fundamental tone of frequency f and harmonics at frequencies 2f, 3f, etc. with different phase angles relative to the fundamental. The Josephson Arbitrary Waveform Synthesizer (JAWS) offers a nearly freely programmable frequency spectrum with quantum accurate amplitudes for the tones used (figure). The quantum based Josephson spectrum analyser employs this programmable spectrum to compensate the fundamental and harmonics of the waveform being measured. The principle was first tested by cancelling a JAWS sinewave with a second sinewave, also generated with JAWS. These first measurements at a frequency of 1 kHz and an amplitude of 100 mV showed that a resolution of the order of 1 nV could be achieved within a measuring time of approximately 45 seconds [1]. The complete preliminary adjustments for the measurement took only 30 minutes.

This method has now been applied to measure a commercial signal generator. In this case, the resolution is limited by the amplitude stability of the generator. The fundamental was cancelled to a rest-amplitude of -106 dB relative to the 100 mV amplitude. The measurement time was shortened to only 10 seconds [2]. A second iteration cancelled the largest harmonic tone at 3f to the level of the noise floor, 150 dB below the amplitude of the fundamental.
This novel method allows high resolution and fast measurements for the characterisation of commercial signal generators.

Frequency spectrum of the JAWS generating a 100 mV sine wave at 950 Hz

Figure: Frequency spectrum of the JAWS generating a 100 mV sine wave at 950 Hz.



[1]    Luis Palafox, Ralf Behr, Jonas Herick, Oliver Kieler, and Marco Schubert, “Josephson based spectrum analysis,” Conference Conf. Digest CPEM 2018, Paris, France, 2018.
[2]    Luis Palafox, Jonas Herick, Stephan Bauer, Marco Kraus, Oliver Kieler and Ralf Behr, “Applications of the Josephson based spectrum analyzer,” Conference Conf. Digest CPEM 2020, Denver, USA, 2020.





Department 2.6 “Electrical Quantum Metrology"