Superconducting Josephson Junctions Research Articles

A new behavioral-level model of superconducting Josephson junctions with Simulink

A new behavioral-level model of superconducting Josephson junctions with Simulink

AC metrology applications of the Josephson effect

The performance of programmable voltage signals that exploit the quantum behavior of superconducting Josephson junctions continues to improve and enhance measurements in metrology, communications, and quantum control. We review advances in pulse-driven digital synthesis techniques with Josephson-junction-based devices. Quantum-based synthesis of voltage waveforms has been demonstrated at frequencies up to 3 GHz and rms amplitudes up to 4 V. Josephson pulse generators have also been used to control and characterize superconducting qubits.

Vector substrate-based Josephson junctions

We present a way to fabricate bicrystal Josephson junctions of high-Tc cuprate superconductors that are not grown on bulk bicrystalline substrates. Based on vector substrate technology, this approach makes use of a few tens-of-nanometer-thick bicrystalline membranes transferred onto conventional substrates. We demonstrate 24° [001]-tilt YBa2Cu3O7−x Josephson junctions fabricated on sapphire single crystals by utilizing 10-nm-thick bicrystalline SrTiO3 membranes. This technique allows one to manufacture bicrystalline Josephson junctions of high-Tc superconductors on a large variety of bulk substrate materials, providing distinctive degrees of freedom in designing the junctions and their electronic properties. Furthermore, it offers the capability to replace the fabrication of bulk bicrystalline substrates with thin-film growth methods.

Josephson junction of nodal superconductors with a Rashba and Ising spin-orbit coupling

Josephson junction of nodal superconductors with a Rashba and Ising spin-orbit coupling

Signatures of Majorana bound states in the diffraction patterns of extended superconductor–topological insulator–superconductor Josephson junctions

Signatures of Majorana bound states in the diffraction patterns of extended superconductor–topological insulator–superconductor Josephson junctions

Evidence of superconducting Fermi arcs

An essential ingredient for the production of Majorana fermions for use in quantum computing is topological superconductivity1,2. As bulk topological superconductors remain elusive, the most promising approaches exploit proximity-induced superconductivity3, making systems fragile and difficult to realize4–7. Due to their intrinsic topology8, Weyl semimetals are also potential candidates1,2, but have always been connected with bulk superconductivity, leaving the possibility of intrinsic superconductivity of their topological surface states, the Fermi arcs, practically without attention, even from the theory side. Here, by means of angle-resolved photoemission spectroscopy and ab initio calculations, we identify topological Fermi arcs on two opposing surfaces of the non-centrosymmetric Weyl material trigonal PtBi2 (ref. 9). We show these states become superconducting at temperatures around 10 K. Remarkably, the corresponding coherence peaks appear as the strongest and sharpest excitations ever detected by photoemission from solids. Our findings indicate that superconductivity in PtBi2 can occur exclusively at the surface, rendering it a possible platform to host Majorana modes in intrinsically topological superconductor–normal metal–superconductor Josephson junctions.

Detection of colored noise correlation time with Josephson junctions

It is shown that the effect of an exponentially correlated noisy source on the escape times of a Josephson junction depends upon the noise correlation time. A careful analysis reveals that two effects are preeminent: the increase of the stability (as measured by the average escape time from the metastable state) and the increase of the extreme events in the tails of the escape time distribution. The changes occur if the noise correlation exceeds the characteristic time of the Josephson junction. As the superconducting device is very fast, with a characteristic time easily below the nanosecond, the device is responsive even to very short noise correlation. We speculate that it is thus possible to estimate in situ noise correlation with a superconducting Josephson junction through the measurements of the escapes, without a high-frequency sampling of the noise. Moreover, as the superconducting circuit can be cooled as much as necessary, the device offers the further advantage to be intrinsically very quiet.

Broadband multi-source excited meander embedding sector antenna for series superconducting Josephson junction THz mixer

Series Josephson junction (JJ) structure has been presented to enhance the impedance of a superconducting terahertz mixer to match the coupled antenna. And the impedance matching between a single antenna and low-impedance JJs embedded in the antenna has been a research hotspot recently. However, the reported antennas all work in resonant modes, resulting in a strict requirement for the positions and the number of the JJs in series. In this paper, we propose a meander embedding sector antenna operated in a traveling-wave mode for series JJs. Multi-source excitation in an antenna is applied to the numerical simulation. The experiment shows that up to seven JJs in series have been successfully inserted into the antenna, and the antenna still maintains good performance. Furthermore, up to 48 % of the 10-dB relative bandwidth of this antenna is achieved, which has a great advantage for wideband superconducting series JJ devices.

Stack growth of wafer-scale van der Waals superconductor heterostructures.

Two-dimensional (2D)van der Waals (vdW) heterostructures have attracted considerable attention in recent years1-5. The most widely used method of fabrication is to stack mechanically exfoliated micrometre-sized flakes6-18, but this process is not scalable for practical applications. Despite thousands of 2D materials being created, using various stacking combinations1-3,19-21, hardly any large 2D superconductors can be stacked intact into vdW heterostructures, greatly restricting the applications for such devices. Here we report a high-to-low temperature strategy for controllably growing stacks of multiple-layered vdW superconductor heterostructure (vdWSH) films at a wafer scale. The number of layers of 2D superconductors in the vdWSHs can be precisely controlled, and we have successfully grown 27double-block, 15 triple-block, 5 four-block and 3 five-block vdWSH films(where one block represents one 2D material). Morphological, spectroscopic and atomic-scale structural analyses reveal the presence of parallel, clean and atomicallysharp vdW interfaces on a large scale, with very little contamination between neighbouring layers. The intact vdW interfaces allow us to achieve proximity-induced superconductivity and superconducting Josephson junctions on a centimetre scale. Our process for making multiple-layered vdWSHs can easily be generalized to other situations involving 2D materials, potentially accelerating the design of next-generation functional devices and applications22-24.

Multi-source excited travelling-wave bowtie antenna based on a meander series of YBCO bicrystal Josephson junctions

Series superconducting Josephson junctions (JJs), as terahertz (THz) mixers, have attracted increasing attentions due to their advantages in solving the saturation problem of superconducting mixers, achieving higher mixing harmonics and higher sensitivity in THz band. However, the normal-state resistances of the series JJs are so low that there exists an impedance mismatch between the coupled antenna and the JJs. In this paper, a meander embedding travelling-wave bowtie antenna is proposed for a series of bicrystal JJs. With this antenna, not only the problem of impedance mismatch can be solved, but also more series JJs can be inserted into the antenna. Five and even seven series JJs in the meander match the proposed antenna well. Furthermore, combined current distribution, far-field radiation patterns and parameter study are investigated in the numerical simulation to analyze this antenna.

Energy efficient half-flux-quantum circuit aiming at milli-kelvin stage operation

Half-flux-quantum (HFQ) circuits are based on 0–π superconducting quantum interference devices (SQUIDs) and is one of the energy-efficient superconductor digital circuits. The bit energy is determined by the critical current I cn of 0–π SQUID, which can be easily tuned with the loop inductance and junction critical current. In this work, an alternative π–π–π SQUID is adopted to demonstrate HFQ circuits to simplify the fabrication process and enhance circuit energy efficiency. The properties of superconductor/ferromagnet/insulator/superconductor Josephson junctions (π-JJs) are measured with temperature dependence from 4.2 K down to 10 mK. HFQ toggle flip-flops (TFFs) are successfully demonstrated at frequencies of up to 6.7 GHz and 44.5 GHz at temperatures of 4.2 K and 10 mK, respectively. Comparing the HFQ TFF with its rapid single-flux quantum counterpart under the same fabrication process, it is anticipated that the HFQ TFF will exhibit approximately 70% reduction in both static and dynamic energy dissipation. This research establishes the foundation for developing cryogenic interface control and readout circuits for large-scale quantum computing in the future.

Terahertz Radiations and Switching Phenomena of Intrinsic Josephson Junctions in High-Temperature Superconductors: Josephson Phase Dynamics in Long- and Short-Ranged Interactions

Terahertz Radiations and Switching Phenomena of Intrinsic Josephson Junctions in High-Temperature Superconductors: Josephson Phase Dynamics in Long- and Short-Ranged Interactions

Sensitivity of a DC SQUID with a non-sinusoidal current-phase relation in its junctions

In ballistic superconductor–normal metal–superconductor Josephson junctions, such as those made from graphene or high mobility semiconductors, the current-phase relation may not have the common, sinusoidal form but can be skewed to have a peak supercurrent at a phase difference greater than π/2. Here, we use a numerical simulation that includes thermal noise to investigate the sensitivity of a DC superconducting quantum interference device (SQUID) with such junctions. The simulation uses a resistively and capacitively shunted junction model where the current-phase relation of each junction can be defined as an arbitrary function. The modulation, transfer function, noise, and sensitivity of a SQUID are calculated for different types of current-phase relation. For the examples considered here, we find that the flux sensitivity of the SQUID is always degraded by forward skewing of the current-phase relation, even in cases where the transfer function of the SQUID has been improved.

High density fabrication process for single flux quantum circuits

We implemented, optimized, and fully tested a superconducting Josephson junction fabrication process over multiple runs tailored for integrated digital circuits that are used for control and readout of superconducting qubits operating at millikelvin temperatures. This process was optimized for highly energy efficient rapid single flux quantum (ERSFQ) circuits with critical currents reduced by a factor of ∼10 as compared to those operated at 4.2 K. Specifically, it implemented Josephson junctions with 10 μA unit critical current fabricated with 10 μA/μm2 critical current density. In order to circumvent the substantial size increase in the SFQ circuit inductors, we employed an NbN high kinetic inductance layer with 8.5 pH/sq sheet inductance. Similarly, to maintain the small size of junction resistive shunts, we used a non-superconducting PdAu alloy with 4.0 Ω/sq sheet resistance. For integration with quantum circuits in a multi-chip module, 5 and 10 μm height bump processes were also optimized. To keep the fabrication process in check, we developed and thoroughly tested a comprehensive process control monitor chip set.

Developing TSV wet cleaning chemistry for quantum computing application

New 3D-integration routes for superconducting Josephson junction qubits include the development of an interposer die with through‑silicon vias (TSV) for vertical and compact interconnections. Since fluoropolymer dielectric residues remaining in the vias may be detrimental for subsequent processes or qubit functionality, they must be removed without damaging the structure or other materials. Therefore, we developed a new O2 plasma-free, selective and non-toxic wet cleaning solution that can remove both the fluoropolymer from the vias and the photoresist used for TSV patterning from the surface. Key formulation physico-chemical properties are discussed based on cleaning results obtained by scanning electron microscopy (SEM) and electron dispersive spectroscopy (EDS). Also discussed is evidence of the dissolution and removal of fluoropolymer formed during Bosch etching. Finally, SEM and time-of-flight secondary ion mass spectrometry (ToF-SIMS) analyses confirm complete removal of fluoropolymer residue from the vias. These encouraging results confirm a viable, manufacturable one-step wet cleaning process for TSVs designed for 3D packaging of qubits and other applications.

Superconductor digital circuits with π junctions alone

We adopt superconductor/ferromagnet/insulator/superconductor (SFIS) Josephson junctions (JJs) as both switching JJs and intrinsic π phase shifters in superconductor digital circuits. The critical current density (Jc) and characteristic voltage (Vc) of the SFIS junctions are about 22 A/cm2 and 22 μV, respectively. The intrinsic π phase shift is confirmed by measuring the suppressed nominal critical current Icn and half-period-shifted modulation pattern of a π–π–π superconducting quantum interference device (SQUID) that contains three π-JJs in a superconducting loop. A single-flux-quantum (SFQ) circuit composed of a DC/SFQ, Josephson transmission line (JTL), and SFQ/DC converter based on SFIS JJs alone is demonstrated at 4.2 K. The energy dissipation of the SFQ/DC converter decreases by 80% because some JJs are self-biased by the π phase shifter. The intrinsic circulating currents induced by the π phase shifters lead to a narrow bias margin (±5%) and even error function, which can be solved by parameters optimization or circuit initialization in the future. The half-modulation period (Φ0/2) of a half-flux-quantum (HFQ) SQUID (a partial HFQ JTL) exhibits propagation of HFQ between π–π–π SQUIDs, indicating that more complex HFQ circuits can be developed with π-JJs alone in the future.

Implementation of SNS thermometers into molecular devices for cryogenic thermoelectric experiments

Thermocurrent flowing through a single-molecule device contains valuable information about the quantum properties of the molecular structure and, in particular, on its electronic and phononic excitation spectra and entropy. Furthermore, accessing the thermoelectric heat-to-charge conversion efficiency experimentally can help to select suitable molecules for future energy conversion devices, which—predicted by theoretical studies—could reach unprecedented efficiencies. However, one of the major challenges in quantifying thermocurrents in nanoscale devices is to determine the exact temperature bias applied to the junction. In this work, we have incorporated a superconductor–normal metal–superconductor Josephson junction thermometer into a single-molecule device. The critical current of the Josephson junction depends accurately on minute changes in the electronic temperature in a wide temperature range from 100 mK to 1.6 K. Thus, we present a device architecture which can enable thermoelectric experiments on single molecules down to millikelvin temperatures with high precision.

Anomalous temperature dependence of multiple Andreev reflections in a topological insulator Josephson junction

As a promising platform for unconventional superconductivity, Josephson junctions (JJs) of tetradymite topological insulators (TIs) and s-wave superconductors have been investigated in recent years. This family of TI materials, however, often suffers from spurious bulk transport, which hampers the observation of the exotic physics of their topological surface states. Thus, disentangling the transport mechanism of bulk and surface contributions in TI JJs is of high importance when investigating proximity induced superconductivity in those crystals. In this work, we add to the insights regarding these contributions by studying the temperature-dependent behaviour of a Bi2Te3-based JJ with transparent interfaces. In electrical transport measurements, we investigate differential conductance spectra of multiple Andreev reflections (MARs) and find a qualitative temperature-dependent change from peak features at low temperatures to dip features at higher ones. The observation of both kind of MAR patterns in a single JJ suggests contributions of diffusive bulk and ballistic surface states and links to a similar finding in the temperature dependence of the critical current. Our work advances the research of induced superconductivity in TIs and offers new avenues to study the induced superconductivity in the topological surface states of these materials.

High-Tc Superconducting Josephson Junction Harmonic Mixers with Stub Tuners on Integrated Bowtie Antennas

Ordinary mixers can hardly meet the requirements in terahertz (THz) communications due to the low-power and expensive THz sources. Sensitive harmonic mixers have been widely studied to avoid this problem, owing to the fact that the higher the number of harmonics, the lower the local oscillator (LO) frequency, and the lower the cost. High-Tc superconducting (HTS) Josephson junction (JJ) mixers are performing candidates for THz receiver frontends because of the advantages of excellent sensitivity, wide bandwidth, high harmonic number and low LO power requirement. However, the normal-state resistance of HTS JJ is so low that traditional antennas are difficult to match it. In other words, it is quite a challenge to match the input impedance to a low input impedance for traditional antennas, especially for antennas fed by coplanar striplines (CPSs). In this work, based on the structure of bowtie, two types of stub tuners were integrated to decrease the impedance of the bowtie antenna so as to improve the coupling efficiency between the traditional bowtie antenna and the JJ. Furthermore, HTS YBa2Cu3O7-δ (YBCO) JJ harmonic mixers coupled with the proposed structures and fed by CPSs are fabricated and measured. The measurements show that the JJ mixer coupled with a pair of open-end stubs of the bowtie antenna achieves up to 88 harmonics, with a conversion efficiency of −69.6 dB. In contrast, the JJ mixer coupled with a pair of lumped-element stubs of the bowtie antenna only attains to 30 harmonics, with a conversion efficiency of −73.4 dB. Additional numerical simulations indicate that the coupling efficiency is enhanced when the complex impedance of the antenna is explicitly considered. Compared with other coupled traditional antennas, the JJ mixer with bowtie antenna has the largest harmonic number. This work paves the way for the future application of low-frequency and low-cost LO for THz communications.

Josephson junctions of two-dimensional time-reversal invariant superconductors: Signatures of the topological phase

We determine the current-phase relation (CPR) of two-terminal configurations of Josephson junctions containing two-dimensional (2D) time-reversal invariant topological superconductors (TRITOPS), including TRITOPS-TRITOPS, as well as junctions between topological and non-topological superconductors (TRITOPS-S). We focus on long junctions for which several channels intervene in the tunneling coupling through the junction. We present a description of the topological edge modes for different TRITOPS models including $p$-wave pairing and the combination of $s$-wave pairing with spin-orbit coupling. We derive effective low-energy Hamiltonians to describe the Josephson junction, which can be solved analytically to explain the contribution of the edge states to the Josephson current as a function of the phase bias. We find that edge-modes yield singular corrections to the CPR for both junction types. The primary effects occur for the response of the Majorana zero-modes at half-flux quantum phase $\phi\approx \pi$ in TRITOPS-TRITOPS junctions and for integer flux quantum phase $\phi \approx 0$ for TRITOPS-S junctions, respectively. The former effect is particularly strong two-component nematic superconductors. The latter effect leads to a spontaneously broken time-reversal symmetry in the TRITOPS-S junction and to a breakdown of the bulk-boundary correspondence.