Kryo 2013 Modern AC Josephson voltage standards at PTB J. Kohlmann, F. Müller, O. Kieler, Th. Scheller, R. Wendisch, B. Egeling, L. Palafox, J. Lee, and R. Behr Physikalisch-Technische Bundesanstalt
Φ 0 = h / 2e Josephson voltage standard U n = n Φ 0 f Quantum standard for voltage reference to physical constants Kryo 2013, Bad Herrenalb 1
Josephson voltage standards important application of Superconductor Electronics unique characteristics for precision measurements (due to superconductivity: magnetic flux quanta) niche market DC applications: commercial suppliers, up to 10 V AC applications: under development motivation: extension of high accuracy from DC to AC goals: 1 V, 10 V, pure waveforms (achieved in part) Requirements * large series arrays of Josephson junctions Kryo 2013, Bad Herrenalb 2 2 / 38
Outline Fundamentals: AC Josephson voltage standards Technology: Nb x Si 1-x Josephson junctions Josephson voltage standards for AC applications: * binary-divided JJ series arrays (70 GHz drive) * pulse-driven JJ series arrays Combing binary-divided array + pulse-driven array Conclusions and outlook Work supported in part by the EU within EMRP JRP SIB59 Q-WAVE Kryo 2013, Bad Herrenalb 3
AC Josephson voltage standards How to make an AC Josephson voltage standard? V n = n m Φ 0 f f / GHz 15 70 pulse-driven arrays V 1 / µv 30 145 No. JJ 330 000 70 000 binary-divided arrays (10 V) (m = number of junctions) Both versions need overdamped JJs: SNS junctions shunted SIS junctions SINIS junctions S-Sc-S junctions Robust & reliable technology required V 1 -V 1 Kryo 2013, Bad Herrenalb 4
Technological requirements overdamped JJs robust, reliable, reproducible (10 V array: 70,000 JJs) different operation ranges: V C 150 µv (70 GHz), j C 10 ka/cm 2 (binary-divided) V C 30 µv (15 GHz), j C 50 ka/cm 2 (pulse-driven) C C => high-resistance material for barrier required (binary metallic alloys not suitable): material near metal-insulator transition Nb x Si 1-x : electrical junction parameters tunable by Nb content x from SNS-like (x > 0.12) to SIS (x < 0.08) Kryo 2013, Bad Herrenalb 5
Co-sputtered Nb x Si 1-x barrier JJs adjustment of characteristic voltag ge V C / µv PJVS x = 0.10 x = 0.11 x = 0.12 x = 0.14 x = 0.16 x = 0.18 x = 0.20 Nb content decreases barrier thickness decreases JAWS Ar 100 j c and V c independently by x and d critical current density j C / ka/cm 2 Kryo 2013, Bad Herrenalb 6
Nb x Si 1-x double-stacked JJs Stacked junctions for higher integration 100 nm 30 nm 211 nm Nb 30 nm NbSi 61 nm Nb 28 nm NbSi 163 nm Nb P. Hinze et. al. (PTB) Barrier: homogeneous, amorphous (TEM) Kryo 2013, Bad Herrenalb 7
Programmable voltage standards (PJVS) V n = n m Φ 0 f overdamped Jos. junctions binary-divided series array 10 V computer controlled bias sources ~ ~ ~ ~ f rf > V 1 2V 1 4V 1 8V 1 x xx xxxx xxxxxxxx output voltage D/A converter with fundamental accuracy Load Kryo 2013, Bad Herrenalb 8
PJVS: 10 V SNS arrays design 69,632 JJs embedded in 128 parallel microstriplines (Z 0 = 16 Ω) min. 136 Josephson junctions (JJ) (size of a single JJ: 6 µm x 20 µm) MW array field AuPd ground bonding-pads for 19 array segments (16 bits) Nb ground (floating) max. 582 JJs per stripline chip size: 24 mm x 10 mm Kryo 2013, Bad Herrenalb 9
PJVS: 10 V SNS arrays fabrication PTB process similar to fundamental SNEP (7 levels) e-beam lithography until wiring level (DC circuit) ground and load are deposited on top of the circuit (1.5 µm SiO 2 as dielectric layer) JJs embedded in microstrip lines Kryo 2013, Bad Herrenalb 10
PJVS: 20 V SNS arrays measurement volta age V / V with microwaves: 70 GHz (80 mw) without microwaves V 20V / µv Nb x Si 1-x JJ for operation at 70 GHz: 139,624 SNS JJs (69,632 double stacks) I / ma I c = 3 ma I c R n = 130 µv current I / ma F. Mueller et al., IEEE Trans. Appl. Supercond. 23 (2013) 1101005 Kryo 2013, Bad Herrenalb 11
Pulse-driven Josephson arrays V n = n m Φ 0 f p arbitrary waveform computer pulse-patterngenerator (PPG) SNS JAWS chip @ 4,2 K spectrum analyzer Σ - modulation current pulses array output V n = n m Φ 0 f p quantized waveform Kryo 2013, Bad Herrenalb 12
Pulse-driven Josephson arrays Josephson Arbitrary Waveform Synthesiser (JAWS) 468.75 Hz, 132 mv RMS 123 dbc synthesis of arbitrary waveforms with pure spectra 375 mv PP Kryo 2013, Bad Herrenalb 13
Pulse-driven Josephson arrays Josephson Arbitrary Waveform Synthesiser (JAWS) Josephson array carrier Kryo 2013, Bad Herrenalb 14
Combination of PJVS and JAWS? pure waveforms and higher voltages? stepwise signal JAWS PJVS arbitrary waveforms higher harmonics pure spectra Kryo 2013, Bad Herrenalb 15
PJVS + JAWS: principle Goal: spectrally pure signals of 1 V (10 V) PJVS: stepwise signal, higher harmonics; transients JAWS: harmonics of PJVS signal are compensated => PJVS + JAWS = sine wave PJVS JAWS (idea: J. Kohlmann et al, IEEE Trans. Instrum. Meas. 56 (2007) 472) Kryo 2013, Bad Herrenalb 16
PJVS + JAWS: setup 50 Ω PJVS JAWS Kryo 2013, Bad Herrenalb 17
PJVS + JAWS: measurement Power / dbm power / dbm -20 PJVS: binary-divided 1 V array (8,192 JJs: 1.18 V) JAWS: pulse-driven array (4,795 JJs: 47 mv) (compensation of about 4,000 harmonics up to 1 MHz) -40 PJVS -60-80 -100-120 -140-160 -180 Stufenförmiges Signal 1.18-V Amplitude 156.25 Hz 256 samples (14 bit) 156.25 Hz sine wave -180 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Frequency / khz frequency / khz frequeny / khz 256 samples (14 bit) 156.25 Hz, 838.4 mv RMS frequency frequeny / khz R. Behr et al, IEEE Trans. Instrum. Meas. 62 (2013) 1634 power / dbm power / dbm -122 dbc PJVS + JAWS Kryo 2013, Bad Herrenalb 18
Conclusions and outlook Binary-divided arrays Pulse-driven arrays U n (t) = n m Φ 0 f 70 GHz 15 GHz computer controlled bias sources m(t) f p (t) pulse drive output voltage number of Joseph. junctions + 1 V and 10 V (also 20 V) + high accuracy (sampling mode) - transients limit accuracy - spectra: many higher harmonics pulse repetition frequency + pure spectra + high accuracy - output voltage < 1 V The PTB is a member of FLUXONICS e.v. (www.fluxonics.org) Kryo 2013, Bad Herrenalb 19
Conclusions and outlook Binary-divided arrays Pulse-driven arrays U n (t) = n m Φ 0 f 70 GHz 15 GHz computer controlled bias sources output voltage Combination: m(t) f p (t) PJVS + JAWS pure spectrum at 1 V pulse drive number of Joseph. junctions + 1 V and 10 V (also 20 V) + high accuracy (sampling mode) - transients limit accuracy - spectra: many higher harmonics pulse repetition frequency + pure spectra + high accuracy - output voltage < 1 V The PTB is a member of FLUXONICS e.v. (www.fluxonics.org) Kryo 2013, Bad Herrenalb 20
Kryo 2013, Bad Herrenalb 21