Josephson Junctions Research Articles

Anomalous Fraunhofer-like patterns in quantum anomalous Hall Josephson junctions

The intriguing interplay between topology and superconductivity has attracted significant attention, given its potential for realizing topological superconductivity. In the quantum anomalous Hall insulators (QAHIs)-based junction, the supercurrents are carried by the chiral edge states, characterized by a 2Φ0 magnetic flux periodicity (Φ0=h/2e is the flux quantum, h the Planck constant, and e the electron charge). However, experimental observations indicate the presence of bulk carriers in QAHI samples due to magnetic dopants. In this study, we reveal a systematic transition from edge-state to bulk-state dominant supercurrents as the chemical potential varies from the bulk gap to the conduction band. This results in an evolution from a 2Φ0-periodic oscillation pattern to an asymmetric Fraunhofer pattern. Furthermore, a Fraunhoher-like pattern emerges due to the coexistence of chiral edge states and bulk states caused by magnetic domains, even when the chemical potential resides within the gap. These findings not only advance the theoretical understanding but also pave the way for the experimental discovery of the chiral Josephson effect based on QAHI doped with magnetic impurities. Published by the American Physical Society 2024

Time-dependent driving and topological protection in the fractional Josephson effect

Time-dependent driving and topological protection in the fractional Josephson effect

Beyond the standard model of topological Josephson junctions: From crystalline anisotropy to finite-size and diode effects

A planar Josephson junction is a versatile platform to realize topological superconductivity over a large parameter space and host Majorana bound states. With a change in the Zeeman field, this system undergoes a transition from trivial to topological superconductivity accompanied by a jump in the superconducting phase difference between the two superconductors. A standard model of these Josephson junctions, which can be fabricated to have a nearly perfect interfacial transparency, predicts a simple universal behavior. In that model, at the same value of Zeeman field for the topological transition, there is a π phase jump and a minimum in the critical superconducting current, while applying a controllable phase difference yields a diamond-shaped topological region as a function of that phase difference and a Zeeman field. In contrast, even for a perfect interfacial transparency, we find a much richer and nonuniversal behavior as the width of the superconductor is varied or the Dresselhaus spin–orbit coupling is considered. The Zeeman field for the phase jump, not necessarily π, is different from the value for the minimum critical current, while there is a strong deviation from the diamond-like topological region. These Josephson junctions show a striking example of a nonreciprocal transport and superconducting diode effect, revealing the importance of our findings not only for topological superconductivity and fault-tolerant quantum computing but also for superconducting spintronics.

Manipulation of Cooper-Pair Tunneling via Domain Structure in a van der Waals Ferromagnetic Josephson Junction.

Ferromagnetic Josephson junctions play a key role in understanding the interplay between superconductivity and ferromagnetism in condensed matter physics. The magnetic domain structures of the ferromagnet in such junctions can significantly affect the tunneling of the superconducting Cooper pairs due to the strong interactions between Cooper pairs and local magnetic moments in the ferromagnetic tunnel barrier. However, the underlying microscopic mechanism of relevant quasiparticle tunneling processes with magnetic domain structures remains largely unexplored. Here, the manipulation of Cooper-pair tunneling in the NbSe2/Cr2Ge2Te6/NbSe2 ferromagnetic Josephson junction is demonstrated by using a multidomain ferromagnetic barrier with anisotropic magnetic moments. The evolution of up-, down-magnetized domain and Bloch domain structures in Cr2Ge2Te6 barrier under external magnetic fields leads to the enhancement of the critical tunneling supercurrent and an unconventional dual-peak feature with two local maxima in the field-dependent critical current curve. The phenomenon of magnetic-field-modulated critical tunneling supercurrent can be well explained by the competition between the coherence length of tunneling Cooper pairs and the size of magnetic domain walls in Cr2Ge2Te6 barrier. This kind of ferromagnetic Josephson junction provides an intriguing material system for manipulating Cooper-pair tunneling by tuning the local magnetic moments within magnetic Josephson junction devices.

Fermion-parity qubit in a proximitized double quantum dot

Bound states in quantum dots coupled to superconductors can be in a coherent superposition of states with different electron number but with the same fermion parity. Electrostatic gating can tune this superposition to a sweet spot, where the quantum dot has the same mean electric charge independent of its electron-number parity. Here, we propose to encode quantum information in the local fermion parity of two tunnel-coupled quantum dots embedded in a Josephson junction. At the sweet spot, the qubit states have zero charge dipole moment. This protects the qubit from dephasing due to charge noise acting on the potential of each dot, as well as fluctuations of the (weak) interdot tunneling. At weak interdot tunneling, relaxation is suppressed because of disjoint qubit states. However, for strong interdot tunneling the system is protected against noise affecting each quantum dot separately (energy-level noise, dot-superconductor tunneling fluctuations, and hyperfine interactions). Finally, we describe initialization and readout as well as single-qubit and two-qubit gates by pulsing gate voltages. Published by the American Physical Society 2024

Kerr nonlinearity and parametric amplification with an Al-InAs superconductor–semiconductor Josephson junction

Kerr nonlinearity and parametric amplification with an Al-InAs superconductor–semiconductor Josephson junction

Identification of odd-frequency superconducting pairing in Josephson junctions

Identification of odd-frequency superconducting pairing in Josephson junctions

Demonstration of 0−π transition in Josephson junctions containing unbalanced synthetic antiferromagnets

Josephson junctions containing ferromagnetic (F) materials have been the subject of intense study over the past two decades. The ground state of such junctions oscillates between 0 and π as the thickness of the ferromagnetic layer increases. For some applications, it might be beneficial to replace a very thin F layer with an unbalanced synthetic antiferromagnet (SAF) consisting of two F layers of different thicknesses whose magnetizations are coupled antiparallel to each other. According to theory, such a system should behave similarly to a single F layer whose thickness is equal to the difference of the two F-layer thicknesses in the SAF. We test that theoretical prediction with Josephson junctions containing unbalanced Ni/Ru/Ni SAFs, keeping the thickness of one layer fixed at 2.0 nm and varying the thickness of the other layer between 2.0 and 5.0 nm. We observe the first 0−π transition at a thickness difference of 0.86 nm, which closely matches the position of the transition observed previously using single Ni layers.

Anomalous supercurrent and diode effect in locally perturbed topological Josephson junctions

The simultaneous breaking of time-reversal and inversion symmetry can lead to peculiar effects in Josephson junctions, such as the anomalous Josephson effect or supercurrent rectification, which is a dissipationless analog of the diode effect. Due to their impact in new quantum technologies, it is important to find robust platforms and external means to manipulate the above-mentioned effects in a controlled way. Here, we theoretically consider a Josephson junction based on a quantum spin Hall system as the normal channel, subjected to a magnetic field in the direction defined by spin-momentum locking, and in the presence of a local tip in close proximity to one of the metallic edges in the normal region. We consider different local perturbations, model normal and magnetic tips, and study how they affect the Josephson response of the device. In particular, we argue that magnetic tips are a useful tool that allows for tunability of both ϕ0 response and supercurrent rectification.

Coherent Control of a Few-Channel Hole Type Gatemon Qubit.

Gatemon qubits are the electrically tunable cousins of superconducting transmon qubits. In this work, we demonstrate the full coherent control of a gatemon qubit based on hole carriers in a Ge/Si core/shell nanowire, with the longest coherence times in group IV material gatemons to date. The key to these results is a high-quality Josephson junction obtained using a straightforward and reproducible annealing technique. We demonstrate that the transport through the narrow junction is dominated by only two quantum channels, with transparencies up to unity. This novel qubit platform holds great promise for quantum information applications, not only because it incorporates technologically relevant materials, but also because it provides new opportunities, like an ultrastrong spin-orbit coupling in the few-channel regime of Josephson junctions.

Observation of Momentum Space Josephson Effects in Weakly Coupled Bose-Einstein Condensates.

The momentum space Josephson effect describes the supercurrent flow between weakly coupled Bose-Einstein condensates (BECs) at two discrete momentum states. Here, we experimentally observe this exotic phenomenon using a BEC with Raman-induced spin-orbit coupling, where the tunneling between two local band minima is implemented by the momentum kick of an additional optical lattice. A sudden quench of the Raman detuning induces coherent spin-momentum oscillations of the BEC, which is analogous to the ac Josephson effect. We observe both plasma and regular Josephson oscillations in different parameter regimes. The experimental results agree well with the theoretical model and numerical simulation and showcase the important role of nonlinear interactions. We also show that the measurement of the Josephson plasma frequency gives the Bogoliubov zero quasimomentum gap, which determines the mass of the corresponding pseudo-Goldstone mode, a long-sought phenomenon in particle physics. The observation of momentum space Josephson physics offers an exciting platform for quantum simulation and sensing utilizing momentum states as a synthetic degree.

Superconducting Diode Effect in a Constricted Nanowire

AbstractDue to isotropic superconducting properties and the lack of breaking of inversion symmetry for conventional s‐wave superconductors, a nonreciprocal superconducting diode effect is absent. Recently, a series of superconducting structures, including superconducting superlattice, and quantum‐material‐based superconducting Josephson junction, have exhibited a superconducting diode effect in terms of polarity‐dependent critical current. However, due to complex structures, these composite systems are not able to construct large‐scale integrated superconducting circuits. Here, it is demonstrated that the minimal superconducting electric component‐superconducting nanowire‐based diode with a nonreciprocal transport effect under a perpendicular magnetic field, in which the superconducting to normal metallic phase transition relies on the polarity and amplitude of the bias current. These nanowire diodes can be reliably operated near at all temperatures below the critical temperature, and the rectification efficiency at 2 K can be more than 24%. Moreover, the superconducting nanowire diode is able to rectify both square wave and sine wave signals. Combining the superconducting nanowire‐based diodes and transistors, superconducting nanowires hold the possibility to construct novel low‐dissipation superconducting integrated circuits.

Stabilizing persistent currents in an atomtronic Josephson junction necklace

Arrays of Josephson junctions are at the forefront of research on quantum circuitry for quantum computing, simulation, and metrology. They provide a testing bed for exploring a variety of fundamental physical effects where macroscopic phase coherence, nonlinearities, and dissipative mechanisms compete. Here we realize finite-circulation states in an atomtronic Josephson junction necklace, consisting of a tunable array of tunneling links in a ring-shaped superfluid. We study the stability diagram of the atomic flow by tuning both the circulation and the number of junctions. We predict theoretically and demonstrate experimentally that the atomic circuit withstands higher circulations (corresponding to higher critical currents) by increasing the number of Josephson links. The increased stability contrasts with the trend of the superfluid fraction – quantified by Leggett’s criterion – which instead decreases with the number of junctions and the corresponding density depletion. Our results demonstrate atomic superfluids in mesoscopic structured ring potentials as excellent candidates for atomtronics applications, with prospects towards the observation of non-trivial macroscopic superpositions of current states.

Superfluid stiffness and Josephson quantum capacitance: Adiabatic approach and topological effects

We bring forward a unified framework for the study of the superfluid stiffness and the quantum capacitance of superconducting platforms exhibiting conventional spin-singlet pairing. We focus on systems which in their normal phase contain topological band touching points or crossings, while in their superconducting regime feature a fully gapped energy spectrum. Our unified description relies on viewing these two types of physical quantities as the charge current and density response coefficients obtained for slow spatiotemporal variations of the superconducting phase. Within our adiabatic formalism, the two coefficients are given in terms of Berry curvatures defined in synthetic spaces. Our paper lays the foundation for the systematic description of topological diagonal superfluid responses induced by singularities dictating the synthetic Berry curvatures. We exemplify our approach for concrete one- and two-dimensional models of superconducting topological (semi)metals. We discuss topological phenomena which arise in the superfluid stiffness of bulk systems and the quantum capacitance of Josephson junctions. We show that both coefficients become proportional to a topological invariant which counts the number of topological touchings or crossings of the normal phase band structure. These topological effects can be equivalently viewed as manifestations of chiral anomaly. Our predictions appear experimentally testable in topological semimetals with proximity-induced pairing, such as in graphene-superconductor hybrids at charge neutrality. Published by the American Physical Society 2024

Topological superfluid responses of superconducting Dirac semimetals

We demonstrate that topological constraints do not only dictate the geometric part of the superfluid stiffness, but can also govern the superfluid stiffness. By introducing a general adiabatic approach for superfluid responses, we showcase such a possibility by proving that the stiffness of a superconducting Dirac cone in two dimensions (2D) is proportional to its topological charge. By relying on the emergent Lorentz invariance of Dirac electrons, we unify the superfluid stiffness and quantum capacitance in these systems. Based on this connection, we further predict a topological origin for the quantum capacitance of a Josephson junction where 2D massless Dirac electrons are sandwiched between two conventional superconductors. We show that the topological responses persist upon effecting strain, are resilient against weak disorder, and can be experimentally controlled via a Zeeman field. Remarkably, the nonuniversal topological quantization of the two superfluid responses, yet implies the universal topological quantization of the admittance modulus of the superconducting Dirac system in units of conductance. The quantum admittance effect arises when embedding the superconducting Dirac system in an ac electrical circuit with a frequency tuned at the absorption edge. These findings are in principle experimentally observable in graphene-superconductor hybrids. Published by the American Physical Society 2024

Multipolar condensates and multipolar Josephson effects

When single-particle dynamics are suppressed in certain strongly correlated systems, dipoles arise as elementary carriers of quantum kinetics. These dipoles can further condense, providing physicists with a rich realm to study fracton phases of matter. Whereas recent theoretical discoveries have shown that an unconventional lattice model may host a dipole condensate as the ground state, we show that dipole condensates prevail in bosonic systems due to a self-proximity effect. Our findings allow experimentalists to manipulate the phase of a dipole condensate and deliver dipolar Josephson effects, where supercurrents of dipoles arise in the absence of particle flows. The self-proximity effects can also be utilized to produce a generic multipolar condensate. The kinetics of the n-th order multipoles unavoidably creates a condensate of the (n + 1)-th order multipoles, forming a hierarchy of multipolar condensates that will offer physicists a whole new class of macroscopic quantum phenomena.

Phase dynamics of tunnel Al-based ferromagnetic Josephson junctions

By measuring the current–voltage characteristics and the switching current distributions as a function of temperature, we have investigated the phase dynamics of Al tunnel ferromagnetic Josephson junctions (JJs), designed to fall in the typical range of parameters of state-of-the-art transmons, providing evidence of phase diffusion processes. The comparison with the experimental outcomes on non-magnetic JJs with nominally the same electrodynamical parameters demonstrates that the introduction of ferromagnetic barriers does not cause any sizeable detrimental effect and supports the notion of including tunnel ferromagnetic JJs in qubit architectures.

An Integrated Approach towards VLSI Implementation of SFQ Logic using Standard Cell Library and Commercial Tool Suite

Abstract The semiconductor industry seeks energy-efficient alternatives as Moore’s law nears its limits. The Single Flux Quantum (SFQ) integrated circuits (ICs) using thousands of niobium Josephson junctions (JJs) and operating at 4 K show great promise for digital computing circuits at high speed (>20 GHz) and low power (a few nW per junction). The leading logic families are Rapid Single Flux Quantum (RSFQ), and its energy-efficient variant (ERSFQ). IARPA’s SuperTools program aims to develop integrated design tools for superconductor electronics, targeting SFQ and Adiabatic Quantum-Flux-Parametron (AQFP) logic families. This paper presents a passive transmission line (PTL) based standard cell library for SFQ logic, designed with Synopsys Electronic Design Automation (EDA) software tools for MIT-LL 100μA/μm2 SFQ5ee fab node. The dual RSFQ/ERSFQ standard cell library facilitates seamless integration of SFQ RTL-to-GDS design flow with Synopsys Fusion Compiler, an automated design tool. The SFQ RTL-to-GDS flow entails logic synthesis, checking, placement, clock synthesis, and routing. Row-based placement for library cells and H-tree clock tree structures are employed. Fusion Compiler’s effectiveness is validated with Hypres designs such as finite impulse response (FIR) filters, scalable multiply-accumulate (MAC) units, and memory arrays, comparing single and dual clocking schemes. The synergy between Hypres and Synopsys achieves a milestone by demonstrating the design of a digital superconducting circuit with over 10 million JJs, facilitated by a fully automated design tool for the first time. Challenges in very large-scale SFQ scaling are also discussed.

Parity-spin superconducting qubit based on topological insulators

We propose to utilize two parity-protected qubits which are built based on superconductor/topological-insulator/superconductor (SC/TI/SC) Josephson junction to implement a parity-spin superconducting qubit. The SC/TI/SC Josephson junctions have identical Josephson potential, which is robust against fabrication variations and guarantees the reliable cos⁡2ϕ energy-phase relation for implementing a parity-protected qubit. By viewing the even and odd parity ground states of a single parity-protected qubit as spin- 12 states, we construct the logic qubit states using the total parity odd subspace of two parity-protected qubits, refereed to parity-spin qubit. This parity-spin qubit exhibits robustness against charge noise, similar to a singlet-triplet (S-T) qubit’s immunity to global magnetic field fluctuations. Meanwhile, the flux noise cannot directly couple two states with the same total parity and is significantly suppressed. Benefiting from the simultaneous protection from charge and flux noise, we demonstrate an enhancement of both T 1 and T 2 coherence times. Our work presents a TI-based approach to engineer symmetry-protected superconducting qubits.

Automatic Cell Placement for Josephson Transmission Lines in a Cell-Based Layout Design Environment for RSFQ Circuits

Abstract This study proposes a method for automatically tiling appropriate Josephson transmission line (JTL) cells for a given set of paths. Digital RSFQ circuits have been commonly designed in a cell-based design environment. The RSFQ cell library contains pre-designed fragments of JTLs as cells. Appropriate JTL cells are selected and tiled. This method formulates the problem as an instance of integer linear programming (ILP). Subsequently, instances are solved using a solver program. The method generates the tiling of cells according to the solver result. The number of Josephson junctions for JTLs, delay of paths, or weighted sum of such parameters is represented as a linear function and can be optimized. The proposed method performed JTL cell-tiling for several circuits, including a full adder. The runtime was within one second.