Junction Series Arrays Research Articles

Quantum sampling AC standard for electrical power metrology based on programmable Josephson junction series array

We report on the realization and preliminary validation of a quantum-based sampling power standard, based on a 1 V Programmable Josephson Voltage Standard (PJVS). The PJVS has been integrated into an existing sampling power standard along with a new synchronous multiplexer. Measurement scenarios for voltage and power measurements have been implemented and internally validated. Results of the measurement comparison against the Italian primary electrical power standard at 53Hz agree well within the respective uncertainties, and did not exceed 15μW/VA from 120V to 240V and 5A and any power factor. Likewise, measurements of alternating voltage up to a few kilohertz show good agreement when compared against AC/DC voltage transfer method. The article illustrates the quantum power standard and its main constituents, measurement scenario, comparison results and discussion, along with a preliminary estimation of the uncertainty budget.

SQUID current sensors with an integrated thermally actuated input current limiter

The development of superconducting quantum interference device (SQUID) current sensors with a thermally actuated input current limiter (TCL) integrated into the input circuit of the sensor is presented. The TCL is based on an unshunted Josephson Junction (JJ) series array, around which meanders a galvanically isolated, but tightly, thermally coupled, resistive heater element. By applying a current to the heater, the JJ critical currents can be reduced or completely suppressed, while the other on-chip SQUID circuit elements remain unaffected. The functional parameters of the TCL are determined by direct transport measurements and by static and dynamic flux coupling tests via the input circuit. In liquid helium, a heater power, which reliably suppresses the critical current of the JJ array to zero, of ∼1.5 mW was needed, with the TCL normal state resistance being at 500 Ω. In this configuration, typical TCL switching times of approximately 20 µs were observed in the direct transport measurements. The TCL can be used to temporarily or even permanently disable malfunctioning channels in multichannel SQUID magnetometer systems, when feedback into their otherwise superconducting input circuits is not possible. In doing so, significant signal distortions in neighbouring channels from screening currents in these input circuits are avoided. Furthermore, by completely suppressing and restoring the TCL critical current, dc offset currents in the superconducting input circuits can be prevented. This is relevant, for instance, in SQUID-based spin precession experiments on hyperpolarized noble gases, in which even dc currents of a few µA in the input circuit can lead to significant magnetic field distortions in the adjacent sample region contributing to the transverse spin-spin relaxation rate .

Flux focused series arrays of long Josephson junctions for high-dynamic range magnetic field sensing

Series arrays of closely spaced, planar long Josephson junctions were demonstrated to be transducers of magnetic flux featuring high-dynamic range, wide-bandwidth, and the capability to operate at cryogenic nitrogen temperatures. By tuning and scaling the geometry of these devices, it is possible to improve their sensitivity to an applied magnetic field and to generate higher voltage responses. Moreover, these devices feature linear voltage responses allowing for the potential of unlocked operation. Herein, we study the flux focusing effect in series arrays of planar Josephson junctions, which are well-suited to fabrication in thin films of the high-transition temperature superconductor YBa2Cu3O7−δ via helium focused ion beam irradiation. We present efforts to characterize the array geometry and properties for magnetic field sensing, with investigations of single Josephson junction behavior and demonstrations of small and large series arrays of Josephson junctions. Furthermore, two-tone spectroscopy is performed to quantify the practical linearity of the voltage response. In this work, a series array of 2640 long Josephson junctions is demonstrated, achieving a sensitivity of 1.7 mV/μT and a linear response over a region of 10.6 μT resulting in a dynamic range of 117 dB while operating at 40 K.

Evaluation of Self-Field Effects in Magnetometers Based on Meander-Shaped Arrays of Josephson Junctions or SQUIDs Connected in Series.

Arrays of superconducting quantum interference devices (SQUIDs) are highly sensitive magnetometers that can operate without a flux-locked loop, as opposed to single SQUID magnetometers. They have no source of ambiguity and benefit from a larger bandwidth. They can be used to measure absolute magnetic fields with a dynamic range scaling as the number of SQUIDs they contain. A very common arrangement for a series array of SQUIDs is with meanders as it uses the substrate area efficiently. As for most layouts with long arrays, this layout breaks the symmetry required for the elimination of adverse self-field effects. We investigate the scaling behavior of series arrays of SQUIDs, taking into account the self-field generated by the bias current flowing along the meander. We propose a design for the partial compensation of this self-field. In addition, we provide a comparison with the case of series arrays of long Josephson junctions, using the Fraunhofer pattern for applications in magnetometry. We find that compensation is required for arrays of the larger size and that, depending on the technology, arrays of long Josephson junctions may have better performance than arrays of SQUIDs.

Investigation of Broadband Wilkinson Power Dividers for Pulse-Driven Josephson Voltage Standards

This paper describes recent developments for increasing the output voltage of the Josephson Arbitrary Waveform Synthesizer (JAWS) at PTB. For this purpose, we developed modified broadband Wilkinson power dividers, which enable the operation of two or four Josephson junction series arrays per RF channel of the pulse pattern generator and thus the generation of increased output voltages. In detail, we designed and investigated two different Wilkinson power dividers: the one-stage three-section Wilkinson power divider with extended bandwidth and the two-stage single-section Wilkinson divider with more outputs to combine four parallel series arrays. They were both integrated with triple-stacked Josephson junction series arrays. Additionally, a new modulator pulse amplifier was introduced into the JAWS set-up to provide sufficient power for all the junction arrays. Our measurement results show that spectrally pure sinusoidal waveforms were successfully generated with both types of power dividers. With the one-stage three-section Wilkinson power divider, we obtained RMS voltages of nearly 53 mV at a clock-frequency of 15 GHz combined with a test array of 3000 Josephson junctions. As for the two-stage single-section Wilkinson power divider combined with a test array of 6000 Josephson junctions, we synthesized RMS output voltages of about 105 mV. These test results show a promising prospect for quantum voltage standard applications.

Series arrays of planar long Josephson junctions for high dynamic range magnetic flux detection

We investigated series arrays of closely spaced, planar long Josephson junctions for magnetic field transduction in Earth’s field, with a linear response and high dynamic range. The devices were fabricated from thin film high-temperature superconductor YBa2Cu3O7−δ (YBCO) thin films, using focused helium ion beam irradiation to create the Josephson barriers. Four series arrays, each consisting of several hundreds of long junctions, were fabricated and electrically tested. From fits of the current-voltage characteristics, we estimate the standard deviation in critical current to be around 25%. Voltage-magnetic field measurements exhibit a transfer function of 42 mV/mT and a linear response over a range of 303 μT at 71 K, resulting in a dynamic range of 124 dB.

Localizing quantum phase slips in one-dimensional Josephson junction chains

We studied quantum phase-slip (QPS) phenomena in long one-dimensional Josephson junction series arrays with tunable Josephson coupling. These chains were fabricated with as many as 2888 junctions, where one sample had a separately tunable link in the middle of the chain. Measurements were made of the zero-bias resistance, R0, as well as current–voltage characteristics (IVC). The finite R0 is explained by QPS and shows an exponential dependence on with a distinct change in the exponent at R0 = RQ = h/4e2. When R0 > RQ, the IVC clearly shows a remnant of the Coulomb blockade, which evolves to a zero-current state with a sharp critical voltage as EJ is tuned to a smaller value. The zero-current state below the critical voltage is due to coherent QPSs and we show that these are enhanced when the central link is weaker than all other links. Above the critical voltage, a negative, differential resistance is observed, which nearly restores the zero-current state.

SNS Junctions for AC Josephson Voltage Standards

Abstract We developed and fabricated series arrays of overdamped SNS Josephson junctions for AC Josephson voltage standards (S: superconductor, N: normal conductor). The junctions are based on NbxSi1-x barriers allowing nearly independent tuning of critical current density and characteristic voltage by varying niobium content and thickness of the barrier. We optimised the junction barrier deposition for the co-sputtered NbxSi1-x so that we are able to fabricate Josephson junction series arrays with up to 69,632 junctions for output voltages up to 10 V under 70 GHz microwave irradiation. The width of the constant-voltage steps at 10 V exceeds 1 mA for a typical critical current of about 3 mA.

SHOT NOISE SUPPRESSION IN INDIVIDUAL AND SERIES ARRAYS OF MAGNETIC TUNNEL JUNCTIONS

Charge-current shot noise is investigated in single magnetic tunnel junctions and devices having multiple junctions that are connected in series. The ratio of the measured shot noise in single junctions to the expected Poisson value, namely the Fano factor, F, is observed to vary from 1 to well below 0.5. Deviations from F = 1 are attributed to localized states (defects) located in the tunnel barrier or at the interfaces with the magnetic electrodes. For series arrays of junctions, the Fano factor scales inversely with the number (1 ≤ N ≤ 30) of junctions in series, even for junctions exhibiting sub-Poissonian (F < 1) shot noise. The 1/N scaling is consistent with the incoherent tunneling of electrons across junctions and indicates that each junction behaves as an individual noise source. The advantages of incorporating series arrays of magnetic tunnel junctions into devices for magnetic field sensing are discussed.

Millimeter-wavelength radiation from arrays of discrete high-temperature superconductorJosephson junctions

Josephson junction arrays could be competitive candidates for radiation sources in thefrequency range from 0.5 to 1.5 THz. This is the range where compact solid-statecontinuous wave sources are most lacking. We investigated the millimeter-wave radiationfrom arrays of high-temperature superconductor bicrystal Josephson junctions embedded ina quasi-optical resonator. The novel approach of utilizing the intrinsic resonance modes ofthe substrate to improve the impedance coupling between the series array of discreteJosephson junctions and the quasi-optical resonator was investigated by experiments andelectromagnetic field simulations. Radiation from the arrays containing up to 536 junctionswas detected at about 80 GHz and liquid nitrogen temperature using a low-noisereceiver. The detected maximum radiation power of the arrays was about 50 pW andthe peak voltage agreed exactly with the Josephson current–voltage relation.

Self-radiation from arrays of niobium Josephson junctions embedded in the open resonator

This paper focuses on self-radiation from arrays of Josephson junctions embedded in a quasi-optical resonator. The mechanism of coupling this radiation to the principal mode of the open resonator is illustrated using experiments and simulations with CST microwave studio software. Comparing the microstrips and dielectric resonators used as the antennas of the series arrays of discrete Josephson junctions, we demonstrate that the dielectric resonator antennas are more effective than microstrips.

Collective cavity mode excitations in arrays of Josephson junctions

We report on oscillations of a Josephson-junction system revealing a close analogy with fundamental effects known in laser physics. The experiments are performed on series arrays of junctions whose $IV$ curves show evidence of a mode in which all the junctions oscillate in synchronism on resonances appearing, in zero external magnetic field, at multiples of the first cavity mode. Evidence is provided that the mode is generated by collective oscillations which develop spontaneously because the frequencies of the modes are equal within a 1% uncertainty. Numerical simulations are employed to elucidate the reasons leading to the synchronous collective excitation and to explain why it is characterized by a low emission of electromagnetic radiation.

Sub-$\mu{\rm m}$ SNS Josephson Junction Arrays for the Josephson Arbitrary Waveform Synthesizer

The experimental set-up of a Josephson arbitrary waveform synthesizer (JAWS) has been further optimized to generate AC voltages with fundamental accuracy. Josephson junction series arrays for the JAWS, consisting of sub-mum SNS type junctions (superconductor-normal conductor-superdonductor), are fabricated using a technology which combines electron-beam lithography and chemical-mechanical polishing. Bipolar pulses are realized by the superposition of unipolar pulses from a pulse pattern generator and a continuous sinusoidal microwave provided by a signal generator. A second-order delta-sigma modulator is optimized to generate a high-quality pulse code.

Development and Investigation of SNS Josephson Arrays for the Josephson Arbitrary Waveform Synthesizer

Superconductor-normal metal-superconductor(SNS) Josephson junction series arrays are developed and investigated for applications in the Josephson arbitrary waveform synthesizer (JAWS). The series arrays contain up to about 10 000 junctions arranged in a meanderlike design. AC voltages and arbitrary waveforms are synthesized by operation of the Josephson junctions by short current pulses. Higher harmonics are at least 110 dB below the fundamental. To verify the generated waveforms, a spectrum analyzer that has initially been calibrated using a binary-divided programmable Josephson voltage standard is used.

Improved design of superconductor/normal conductor/superconductor Josephson junctionseries arrays for an ac Josephson voltage standard

Superconductor/normal conductor/superconductor (SNS) Josephson junction series arraysare the subject of research for the realization of a Josephson arbitrary waveform synthesizer(JAWS) based on pulse-driven Josephson junctions. Lumped arrays of SNS Josephsonjunctions are investigated for this application at the Physikalisch-Technische Bundesanstalt(PTB). The series arrays consisting of Nb/HfTi/Nb SNS junctions are embedded in thecenter line of a coplanar waveguide transmission line. To increase the ac outputvoltage, 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 theJAWS. The arrays are investigated by dc measurements and for test purposes undercontinuous microwave irradiation at different frequencies. Flat Shapiro steps weremeasured for meander-like series arrays with 21 parallel lines containing up to10 752 junctions. A sine wave with a frequency of 2.5 kHz and a peak voltage of214 mVPeak was synthesized under pulse mode operation.

Linking Primary Power Standards and Programmable Josephson Arrays

Recently, precision waveforms with accuracies better than 1 part in 107 at power frequencies have been available from series arrays of shunted Josephson junctions operated as digital to analog conv...

SNS Josephson Junction Series Arrays for the Josephson Arbitrary Waveform Synthesizer

The Josephson arbitrary waveform synthesizer (JAWS) allows AC voltages to be generated with fundamental accuracy. Lumped arrays of superconductor/normal conductor/superconductor (SNS) Josephson junctions were developed and investigated for this application. Combining electron-beam lithography and chemical-mechanical polishing, series arrays of sub-micrometer SNS junctions consisting of Nb/HfTi/Nb were fabricated. The junction size varied from 0.04 mum2 to 4 mum2 in series arrays up to 2560 junctions in different microwave designs. Stable Shapiro steps are generated in a frequency range from 5 GHz to 20 GHz using continuous microwave irradiation. Using a simple pulse pattern at different pulse repetition frequencies, the arrays show current-voltage characteristics with flat Shapiro steps. Sinusoidal AC voltages were synthesized using a commercial pulse pattern generator. Peak-to-zero voltages of some mV for frequencies below 1 kHz were achieved. Higher harmonics were measured to be more than 80 dB below the fundamental tone. The behavior of the arrays was investigated in more detail by measuring the amplitude-frequency spectra.

Macroscopic quantum tunneling in globally coupled series arrays of Josephson junctions

We present a quantitative analysis of an escape rate for switching from the superconducting state to a resistive one in series arrays of globally coupled Josephson junctions. A global coupling is provided by an external shunting impedance. Such an impedance can strongly suppress both the crossover temperature from the thermal fluctuation to quantum regimes, and the macroscopic quantum tunneling (MQT) in short Josephson junction series arrays. However, in large series arrays we obtain an enhancement of the crossover temperature, and a giant increase of the MQT escape rate. The effect is explained by excitation of a spatial-temporal charge instanton distributed over a whole structure. The model gives a possible explanation of recently published experimental results on an enhancement of the MQT in single crystals of high-${T}_{c}$ superconductors.

Development of Josephson junction series arrays for synthesis of AC voltages and arbitrary waveforms

Josephson junctions operated by short pulses make the synthesis of AC and arbitrary waveforms possible. Series arrays of intrinsically shunted SINIS and SNS Josephson junctions were developed and characterized for applications in this Josephson arbitrary waveform synthesizer (S: Superconductor, I: Insulator, N: Normal conductor). First measurements under pulse irradiation are presented.

Enhanced Macroscopic Quantum Tunneling inBi2Sr2CaCu2O8+δIntrinsic Josephson-Junction Stacks

We have investigated macroscopic quantum tunneling in Bi(2)Sr(2)CaCu(2)O(8 + delta) intrinsic Josephson junctions at millikelvin temperatures using microwave irradiation. Measurements show that the escape rate for uniformly switching stacks of Nu junctions is about Nu(2) times higher than that of a single junction having the same plasma frequency. We argue that this gigantic enhancement of the macroscopic quantum tunneling rate in stacks is boosted by current fluctuations which occur in the series array of junctions loaded by the impedance of the environment.