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APS March Meeting Abstracts

Cross-correlation noise measurements of a graphene-based SQUID magnetometer

Jonathan R. Prance, Michael D. Thompson, R. P. Haley, Yu. A. Pashkin, M. Ben Shalom, Vladimir I. Fal’ko, Harriet van der Vliet, Anthony J. Matthews, Ziad Melhem

Published Jan 1, 2019• 0 citations

Abstract

Lateral superconductor/graphene structures can be used to make Josephson junctions with low contact resistances and gate-tuneable critical currents. These junctions have the potential to provide new functionality for superconducting devices. For most devices, e.g. transmon qubits and SQUID sensors, it is important to quantify the intrinsic noise of the junctions. The voltage noise of low resistance junctions is typically below the noise floor of room temperature amplifiers. By cross-correlating the signals from two parallel amplifiers, we can detect signals down to 100 pV/√Hz, well below the noise floor of each amplifier. Using this technique, we characterise the voltage noise of a NbTi DC SQUID with graphene junctions in a frequency range from Hz to kHz. Combined with measurements of the SQUID's gain, we map its sensitivity across a range of operating conditions and find that the best-case sensitivity of the device is similar to traditional low temperature SQUIDs with oxide tunnel junctions. This project has received funding from the European Union's Horizon 2020 research and innovation programme under Grant agreement No 785219.

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We present measurements of DC superconducting quantum interference devices based on Nb/graphene/Nb Josephson junctions. The superconducting proximity effect in graphene can be used to build Josephson junctions whose critical current can be controlled by field-effect gates. These junctions combine the tunability of semiconductor Josephson junctions with the high critical currents and low contact resistances of metal SNS junctions. By using local gates, the SQUID junction critical currents can be modified individually and this allows the sensitivity and symmetry of the SQUID to be controlled in-situ. We compare the critical current of the SQUID with simulations that include a non-sinusoidal current phase relation in the junctions, as expected for ballistic graphene junctions. We also investigate the transfer function of the device in both symmetric and asymmetric configurations and find a highest transfer function of 300 μV/Φ0. Graphene Josephson junctions have the potential to add functionality to existing technologies; for example, to make SQUID magnetometers with tunable sensitivity or superconducting qubits with fast electrical control.

Bulletin of the American Physical Society•Mar 17, 2017•0 citations

An ultra-low-noise tunnel junction dc SQUID

M. B. Ketchen, Richard F. Voss

The design, fabrication, and noise properties of an ultra-low-noise tunnel junction dc SQUID (superconducting quantum interference device) are presented. The two resistively shunted Pb-alloy tunnel junctions are each 3.6 μm diam. The SQUID has an inductance L of ≈11 pH, a critical current per junction, I0 of ≈100 μA, and a resistance per shunt R of ≈2 Ω. At 2.3 K, eI0R≈kBT, and simple theory predicts that the minimum intrinsic energy sensitivity should be ≈2h. The actual minimum value of the measured high-frequency (70 kHz) flux noise at 4.2 K is φn=1.3×10−7φ0 Hz−1/2. This corresponds to an intrinsic energy sensitivity, φ2n/2L, of 3.2×10−33 J Hz−1 or 5h. Lowering the bath temperature to 1.8 K gave no improvement in performance.

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Noise performance of superconductive magnetometers based on long Josephson tunnel junctions

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The low-current fluctuations at cryogenic temperatures together with the low dynamical resistance in the resonant states of Josephson tunnel junctions allow for the realization of superconducting oscillators up to the THz range with ultra-low spectral linewidth. By virtue of the Josephson frequency-voltage relationship, we show that the same properties can be exploited for the practical realization of magnetic flux-to-voltage transducers based on the flux–flow in long Josephson tunnel junctions whose intrinsic low-frequency voltage fluctuations at amount to few , that is, too small to be measured by any present semiconductor electronics. Nevertheless, by using a double transformer SQUID amplifier we demonstrate that the (amplitude) voltage spectral density, , of an all-niobium sensor does not exceed the level of and is not affected by excess noise at least down to few hertz. Such ultra-low white noise, corresponding to a magnetic field noise , together with a highly linear and broadband voltage responsivity over a wide magnetic flux range, makes the flux–flow magnetometers potentially competitive with SQUID-based devices.

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Applied Physics Letters•May 29, 2017•17 citations

All-Nb low-noise dc SQUID with 1-μm tunnel junctions

Richard F. Voss, Robert B. Laibowitz, Stanley I. Raider +1 more

Tunnel-junction dc SQUIDs are being fabricated from Nb films using electron-beam lithography to define linewidths as small as 0.5 μm. The barriers in the Nb-NbOx-Nb junctions are grown in an oxygen plasma discharge, and the Pd2Si resistive shunts are formed by annealing Pd on the Si substrates. These devices have excellent storage and recycling properties. At a measurement frequency of 40 kHz, a SQUID with an inductance of about 1 nH, a critical current of about 4 μA, and a parallel shunt resistance of about 15 Ω had an intrinsic energy resolution of 2.5×10−32 J Hz−1 at 4.2 K and 1.1×10−32 J Hz−1 at 1.6 K, in good agreement with a model of the dc SQUID.

Journal of Applied Physics•Apr 1, 1980•31 citations

Lateral superconductor/graphene structures can be used to make Josephson junctions with low contact resistances and gate-tuneable critical currents. These junctions have the potential to provide new functionality for superconducting devices. For most devices, e.g. transmon qubits and SQUID sensors, it is important to quantify the intrinsic noise of the junctions. The voltage noise of low resistance junctions is typically below the noise floor of room temperature amplifiers. By cross-correlating the signals from two parallel amplifiers, we can detect signals down to 100 pV/√Hz, well below the noise floor of each amplifier. Using this technique, we characterise the voltage noise of a NbTi DC SQUID with graphene junctions in a frequency range from Hz to kHz. Combined with measurements of the SQUID's gain, we map its sensitivity across a range of operating conditions and find that the best-case sensitivity of the device is similar to traditional low temperature SQUIDs with oxide tunnel junctions. This project has received funding from the European Union's Horizon 2020 research and innovation programme under Grant agreement No 785219.

We present measurements of DC superconducting quantum interference devices based on Nb/graphene/Nb Josephson junctions. The superconducting proximity effect in graphene can be used to build Josephson junctions whose critical current can be controlled by field-effect gates. These junctions combine the tunability of semiconductor Josephson junctions with the high critical currents and low contact resistances of metal SNS junctions. By using local gates, the SQUID junction critical currents can be modified individually and this allows the sensitivity and symmetry of the SQUID to be controlled in-situ. We compare the critical current of the SQUID with simulations that include a non-sinusoidal current phase relation in the junctions, as expected for ballistic graphene junctions. We also investigate the transfer function of the device in both symmetric and asymmetric configurations and find a highest transfer function of 300 μV/Φ0. Graphene Josephson junctions have the potential to add functionality to existing technologies; for example, to make SQUID magnetometers with tunable sensitivity or superconducting qubits with fast electrical control.

The design, fabrication, and noise properties of an ultra-low-noise tunnel junction dc SQUID (superconducting quantum interference device) are presented. The two resistively shunted Pb-alloy tunnel junctions are each 3.6 μm diam. The SQUID has an inductance L of ≈11 pH, a critical current per junction, I0 of ≈100 μA, and a resistance per shunt R of ≈2 Ω. At 2.3 K, eI0R≈kBT, and simple theory predicts that the minimum intrinsic energy sensitivity should be ≈2h. The actual minimum value of the measured high-frequency (70 kHz) flux noise at 4.2 K is φn=1.3×10−7φ0 Hz−1/2. This corresponds to an intrinsic energy sensitivity, φ2n/2L, of 3.2×10−33 J Hz−1 or 5h. Lowering the bath temperature to 1.8 K gave no improvement in performance.

The low-current fluctuations at cryogenic temperatures together with the low dynamical resistance in the resonant states of Josephson tunnel junctions allow for the realization of superconducting oscillators up to the THz range with ultra-low spectral linewidth. By virtue of the Josephson frequency-voltage relationship, we show that the same properties can be exploited for the practical realization of magnetic flux-to-voltage transducers based on the flux–flow in long Josephson tunnel junctions whose intrinsic low-frequency voltage fluctuations at amount to few , that is, too small to be measured by any present semiconductor electronics. Nevertheless, by using a double transformer SQUID amplifier we demonstrate that the (amplitude) voltage spectral density, , of an all-niobium sensor does not exceed the level of and is not affected by excess noise at least down to few hertz. Such ultra-low white noise, corresponding to a magnetic field noise , together with a highly linear and broadband voltage responsivity over a wide magnetic flux range, makes the flux–flow magnetometers potentially competitive with SQUID-based devices.

Electron-beam lithography has been used to produce ultra low noise Nb dc SQUIDs (Superconducting QUantum Interference Devices) with Josephson elements consisting of either small area tunnel junctions or very narrow variable thickness bridges (nanobridges). Detailed voltage noise and transfer function measurements have been made as a function of temperature and flux and current bias. These measurements allow a computation of the intrinsic energy resolution ε vs bias parameters as well as comparison with models of intrinsic SQUID noise. The best ε obtained was of order Planck's constant h. Moreover, the detailed characteristics of the SQUID voltage noise are in agreement with a small signal analysis that depends only on the current noise in each of the Josephson elements and measurable parameters.

Integrated NbN direct current superconducting quantum interference device (DC-SQUID) magnetometers were developed based on high-quality epitaxial NbN/AlN/NbN Josephson junctions for SQUID applications operating at high temperatures. We report the current–voltage and voltage–flux characteristics and the noise performance of the NbN DC-SQUIDs for temperatures ranging from 4.2 to 9 K. The critical current and voltage swing of the DC-SQUIDs decreased by 15% and 25%, respectively, as the temperature was increased from 4.2 to 9 K. The white flux noise of the DC-SQUID magnetometer at 1 kHz increased from 3.9 μΦ0/Hz1/2 at 4.2 K to 4.8 μΦ0/Hz1/2 at 9 K with 23% increase, corresponding to the magnetic field noise of 6.6 and 8.1 fT/Hz1/2, respectively. The results show that NbN DC-SQUIDs improve the tolerance of the operating temperatures and temperature fluctuations in SQUID applications.

Tunnel-junction dc SQUIDs are being fabricated from Nb films using electron-beam lithography to define linewidths as small as 0.5 μm. The barriers in the Nb-NbOx-Nb junctions are grown in an oxygen plasma discharge, and the Pd2Si resistive shunts are formed by annealing Pd on the Si substrates. These devices have excellent storage and recycling properties. At a measurement frequency of 40 kHz, a SQUID with an inductance of about 1 nH, a critical current of about 4 μA, and a parallel shunt resistance of about 15 Ω had an intrinsic energy resolution of 2.5×10−32 J Hz−1 at 4.2 K and 1.1×10−32 J Hz−1 at 1.6 K, in good agreement with a model of the dc SQUID.