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Conference on Precision Electromagnetic Measurements

A Lumped Array Josephson Pulse Quantizer for Quantum-Based Arbitrary Waveform Synthesizers

M. Schubert, G. Wende, T. May, H.‐G. Meyer

Published Jun 1, 2004• 4 citations
Publisher

Abstract

Central to quantum-based arbitrary waveform synthesizers is the Josephson pulse quantizer. Its linear dimensions should be small compared to the line wavelength in order to generate an ac voltage with quantum accuracy. Therefore, we use coplanar strip transmission lines to integrate a large number of Josephson junctions into a short array

DOI
10.1109/cpem.2004.305512

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Central to quantum-based arbitrary waveform synthesizers is the Josephson pulse quantizer. Its linear dimensions should be small compared to the line wavelength in order to generate an ac voltage with quantum accuracy. Therefore, we use coplanar strip transmission lines to integrate a large number of Josephson junctions into a short array
Central to quantum-based arbitrary waveform synthesizers is a Josephson junction array driven by short pulses. Its linear dimensions should be small compared to the line wavelength in order to generate an ac voltage with quantum accuracy. Therefore, we use coplanar strip transmission lines to integrate a large number of cross-type SNS Josephson junctions into multiple parallel arrays. First measurements show that a circuit with 1,760 junctions exhibits Shapiro steps over a large frequency range. By using pulse driving the chip generates unipolar voltages up to 6.4 mV at a clock frequency of 3.5 GHz, the maximum frequency of the available pulse pattern generator.
The paper is about development of the lumped Josephson junction arrays (JJAs) for the generation of accurate quantum-based ac voltages. By using semiconductor pattern generators, it was possible to synthesize unipolar and bipolar waveforms with amplitudes of almost 10 mV and frequencies of up to 122 kHz. Higher harmonics and other unwanted noise signals were suppressed by -90 dBc. Because of the unique layout of our JJA, it was possible to connect the circuit directly to the ground, thereby eliminating common-mode signals
The DC voltage across a Josephson junction is exactly determined by the number of single flux quanta Φ0passing the junction per second. By locking this flux train to an external sinusoidal microwave with the frequency f or a pulse train with the pulse rate f in such a way that one flux quantum is transferred per microwave period or per pulse respectively, the junction DC voltage becomes V=Φ0f in the simplest case. The precision of V is only determined by the uncertainty of the frequency control. As the output voltage is relatively small (V=140μV for f=70GHz), large series arrays of n Josephson junctions must be used to increase the voltage to practical levels V=nΦ0f. With a sinusoidal 70 GHz microwave drive and series arrays of up to 14000 SIS tunnel junctions modern DC voltage standards were developed in the past which may reach a reproducibility of a few parts in 10-10. These standards are commercially available and form the basis of the legal voltage metrology of most of the national laboratories worldwide. Because the reference voltages of these standards are widely overlapping it is difficult to rapidly adjust the desired voltage. It was therefore impossible to use these quantum voltmeters for the precise analysis of AC voltages. For this purpose the arrays had to be modified in many respects. The direct way to generate arbitrary Josephson waveforms V(t) is to rapidly alter the frequency f(t) with respect to the desired waveform V(t)=nf(t)Φ0. It requires a precise flux transfer control over a very wide frequency range.
This article presents a superconducting voltage source that generates modulated microwave waveforms with quantum-based stability. The voltage source—an RF Josephson Arbitrary Waveform Synthesizer (RF-JAWS)—uses a superconducting IC with an array of 4500 series-connected Josephson junctions (JJs) that are embedded in a coplanar waveguide (CPW) transmission line. The JJs are driven with 100-ps wide current pulses that force each JJ to generate voltage pulses with a quantized integrated area. A pulse sequence is created using a delta-sigma ($\Delta –––\Sigma )algorithmtogeneratea101−tonewaveformwitha40−MHzinstantaneousbandwidtharound1005MHzwithaflatpowerdistributionof−53dBmandaSchroederphasedistribution.TheJJ’squantum−basednonlinearityproducesanamplitudestabilitywithrespecttovariousdriveandbiasparametersof±0.005dBandadetrendedphasestabilityof±0.05°.Thestabilityofthesourceisverifiedwithouttheneedforcalibratedmeasurements.Another) algorithm to generate a 101-tone waveform with a 40-MHz instantaneous bandwidth around 1005 MHz with a flat power distribution of −53 dBm and a Schroeder phase distribution. The JJ’s quantum-based nonlinearity produces an amplitude stability with respect to various drive and bias parameters of ±0.005 dB and a detrended phase stability of ±0.05°. The stability of the source is verified without the need for calibrated measurements. Another)algorithmtogeneratea101−tonewaveformwitha40−MHzinstantaneousbandwidtharound1005MHzwithaflatpowerdistributionof−53dBmandaSchroederphasedistribution.TheJJ’squantum−basednonlinearityproducesanamplitudestabilitywithrespecttovariousdriveandbiasparametersof±0.005dBandadetrendedphasestabilityof±0.05°.Thestabilityofthesourceisverifiedwithouttheneedforcalibratedmeasurements.Another\Delta –––\Sigma $pulse sequence generates a 10-MHz quadrature-phase shift-keying (QPSK) waveform on a 1005-MHz carrier that has the same stability and the mean error vector magnitude (EVM) of −48 dB. This work is a step toward creating a quantum-based reference source for power and cross-frequency phase calibrations, as well as a reference-modulated signal source for calibrating telecommunication links.
Superconductor/normal conductor/superconductor (SNS) Josephson junction series arrays are the subject of research for the realization of a Josephson arbitrary waveform synthesizer (JAWS) based on pulse-driven Josephson junctions. Lumped arrays of SNS Josephson junctions are investigated for this application at the Physikalisch-Technische Bundesanstalt (PTB). The series arrays consisting of Nb/HfTi/Nb SNS junctions are embedded in the center line of a coplanar waveguide transmission line. To increase the ac output voltage, the Josephson junction series array is designed as a meander-like structure. A broadband response of the arrays is required for the complex pulse operation of the JAWS. The arrays are investigated by dc measurements and for test purposes under continuous microwave irradiation at different frequencies. Flat Shapiro steps were measured for meander-like series arrays with 21 parallel lines containing up to 10 752 junctions. A sine wave with a frequency of 2.5 kHz and a peak voltage of 214 mV Peak was synthesized under pulse mode operation.