Dissipation is ubiquitous in quantum systems, and its interplay with fluctuations is critical to maintaining quantum coherence. We experimentally investigate the dissipation dynamics in single-walled carbon nanotubes coupled to superconductors. The voltage-current characteristics display gate-tunable hysteresis, with sizes that perfectly correlate with the normal state resistance RN, indicating the junction undergoes a periodic modulation between underdamped and overdamped regimes. Surprisingly, when a device's Fermi-level is tuned through a local conductance minimum, we observe a gate-controlled transition from superconducting-like to insulating-like states, with a "critical" R_N value of about 8-20 kohm.
We investigate electronic transport in Josephson junctions formed by single-walled carbon nanotubes coupled to superconducting electrodes. We observe enhanced zero-bias conductance (up to 10e^2/h) and pronounced sub-harmonic gap structures in differential conductance, which arise from the multiple Andreev reflections at superconductor/nanotube interfaces. The voltage-current characteristics of these junctions display abrupt switching from the supercurrent branch to resistive branch, with a gate-tunable switching current ranging from 50 pA to 2.3 nA. The finite resistance observed on the superc…
We present the theory of the Josephson effect in nanotube dots where an SU(4) symmetry can be realized. We find a remarkably rich phase diagram that significantly differs from the SU(2) case. In particular, π-junction behavior is largely suppressed. We analytically obtain the Josephson current in various parameter regions: (i) in the Kondo regime, covering the full crossover from SU(4) to SU(2), (ii) for weak tunnel couplings, and (iii) for large BCS gap. The transition between these regions is studied numerically.
We investigate electronic transport in single layer graphene coupled to superconducting electrodes. In these Josephson transistors, we observe significant suppression in the critical current Ic and large variation in the product IcRn in comparison to theoretic predictions in the ballistic limit. We show that the depression of Ic can be explained by premature switching in underdamped Josephson junctions described within the resistively and capacitively shunted junction (RCSJ) model. By considering the effect of premature switching and dissipation, the calculated gate dependence of product IcRn …
Carbon nanotube (CNT) Josephson junctions in the open quantum dot limit exhibit superconducting switching currents which can be controlled with a gate electrode. Shapiro voltage steps can be observed under radiofrequency current excitations, with a damping of the phase dynamics that strongly depends on the gate voltage. These measurements are described by a standard RCSJ model showing that the switching currents from the superconducting to the normal state are close to the critical current of the junction. The effective dynamical capacitance of the nanotube junction is found to be strongly gat…
A supercurrent transistor is a superconductor-semiconductor hybrid device in which the Josephson supercurrent is switched on and off using a gate voltage. While such devices have been studied using DC transport, radio-frequency measurements allow for more sensitive and faster experiments. Here a supercurrent transistor made from a carbon nanotube is measured simultaneously via DC conductance and radio-frequency reflectometry. The radio-frequency measurement resolves all the main features of the conductance data across a wide range of bias and gate voltage, and many of these features are seen m…
We have investigated the Josephson inductance $L_J$ of a Superconducting Cooper Pair Transistor (SCPT). We traced the inductance using microwave reflection measurements on a tuned resonance circuit in which a SCPT was mounted in parallel to a $ \sim 200$ pH strip line inductance. When the inverse of the Josephson inductance, determined on the charge-phase bias plane for a SCPT with a Josephson to Coulomb energy ratio of $E_J/E_C =1.75$, is integrated over the phase, we obtain a current-phase relation, which is strongly non-sinusoidal near the charge degeneracy point.
We have performed the first experiments in a superconductor - normal metal - superconductor single electron transistor in which there is an extra superconducting strip partially overlapping the normal metal island in good metal-to-metal contact. Superconducting proximity effect gives rise to current peaks at voltages below the quasiparticle threshold. We interpret these peaks in terms of the Josephson-quasiparticle process and discuss their connection with the proximity induced energy gap in the normal metal island.
We report on the first realization of Nb-based \textit{all-metallic} gated Dayem nano-bridge field-effect transistors (Nb-FETs). These Josephson devices operate up to a temperature of $\sim 3$ K, and exhibit full suppression of the supercurrent thanks to the application of a control gate voltage. The dependence of the kinetic inductance and of the transconductance on gate voltage promises a performance already on par with so far realized metallic Josephson transistors, and let to foresee the implementation of a superconducting digital logic based on Nb-FETs. We conclude by showing the practica…
The Josephson quasiparticle (JQP) cycle in a voltage-biased superconducting single-electron transistor (SSET) combines coherent Cooper pair tunneling with incoherent quasiparticle decay. We have measured the influence of current flow through an independently-biased SSET on the JQP cycle when the two SSET's have a strong mutual capacitive coupling. We find, among other effects, that the JQP current in one SSET is enhanced by the presence of a quasiparticle current in the other SSET. A simplified model of the coupled-SSET system is presented which reproduces the enhancement effect.
A superconductor-normal metal-superconductor (SNS) transistor based on superconducting microcoolers is presented. The proposed 4-terminal device consists of a long SNS Josephson junction whose N region is in addition symmetrically connected to superconducting reservoirs through tunnel barriers (I). Biasing the SINIS line allows to modify the quasiparticle temperature in the weak link, thus controlling the Josephson current. We show that, in suitable voltage and temperature regimes, large supercurrent enhancements can be achieved with respect to equilibrium, due to electron ``cooling'' generate…
We have contacted single wall carbon nanotubes grown by chemical vapor deposition to superconducting Ti/Al/Ti electrodes. The device, we here report on is in the Kondo regime exhibiting a four-fold shell structure, where a clear signature of the superconducting electrodes is observed below the critical temperature. Multiple Andreev reflections are revealed by sub-gap structure and a narrow peak in the differential conductance around zero bias is seen depending on the shell filling. We interpret the peak as a proximity induced supercurrent and examine its interplay with Kondo resonances.
At a superconductor (S)-normal metal (N) junction pairing correlations can "leak-out" into the N region. This proximity effect [1, 2] modifies the system transport properties and can lead to supercurrent flow in SNS junctions [3]. Recent experimental works showed the potential of semiconductor nanowires (NWs) as building blocks for nanometre-scale devices [4-7], also in combination with superconducting elements [8-12]. Here, we demonstrate an InAs NW Josephson transistor where supercurrent is controlled by hot-quasiparticle injection from normal-metal electrodes. Operational principle is based…
We performed numerical simulations and experiments on Josephson vortex flow transistors based on parallel arrays of YBa2Cu3O(7-x) grain boundary junctions with a cross gate-line allowing to operate the same devices in two different modes named Josephson fluxon transistor (JFT) and Josephson fluxon-antifluxon transistor (JFAT). The simulations yield a general expression for the current gain vs. number of junctions and normalized loop inductance and predict higher current gain for the JFAT. The experiments are in good agreement with simulations and show improved coupling between gate line and ju…
Josephson junction field effect transistors (JJ-FET) share design similarities with metal-oxide-semiconductor field effect transistors, except for the source/drain contacts being replaced by superconductors. Similarly, the super current due to proximity effect is tunable by the gate voltage. In this study, we examine the feasibility of JJ-FET-based Boolean logic and memory elements for cryogenic computing, in light of recent advances in novel materials and fabrication techniques. Using a two-dimensional ballistic transport JJ-FET model, we implement circuit level simulations for JJ-FET logic g…
We demonstrate the first \textit{all-metallic} mesoscopic superconductor-normal metal-superconductor (SNS) field-effect controlled Josephson transistors (SNS-FETs) and show their full characterization from the critical temperature $T_c$ down to 50 mK in the presence of both electric and magnetic field. The ability of a static electric field -applied by mean of a lateral gate electrode- to suppress the critical current $I_s$ in a proximity-induced superconductor is proven for both positive and negative gate voltage values. $I_s$ suppression reached typically about one third of its initial value…
The charge-phase Josephson qubit based on a superconducting single charge transistor inserted in a low-inductance superconducting loop is considered. The loop is inductively coupled to a radio-frequency driven tank circuit enabling the readout of the qubit states by measuring the effective Josephson inductance of the transistor. The effect of qubit dephasing and relaxation due to electric and magnetic control lines as well as the measuring system is evaluated. Recommendations for operation of the qubit in magic points producing minimum decoherence are given.
We demonstrate a sensitive method of charge detection based on radio-frequency readout of the Josephson inductance of a superconducting single-electron transistor. Charge sensitivity $1.4 \times 10^{-4}e/\sqrt{Hz}$, limited by preamplifier, is achieved in an operation mode which takes advantage of the nonlinearity of the Josephson potential. Owing to reactive readout, our setup has more than two orders of magnitude lower dissipation than the existing method of radio-frequency electrometry. With an optimized sample, we expect uncoupled energy sensitivity below $\hbar$ in the same experimental s…
The magnetic state of a quantum dot attached to superconducting leads is experimentally shown to be controlled by the superconducting phase difference across the dot. This is done by probing the relation between the Josephson current and the superconducting phase difference of a carbon nanotube junction whose Kondo energy and superconducting gap are of comparable size. It exhibits distinctively anharmonic behavior, revealing a phase mediated singlet to doublet transition. We obtain an excellent quantitative agreement with numerically exact quantum Monte Carlo calculations. This provides strong…
Josephson junction transistors and 50-junction arrays with linear junction dimensions from 200 nm down to 70 nm were fabricated from standard Nb/AlOx/Nb trilayers. The fabrication process includes electron beam lithography, dry etching, anodization, and planarization by chemical-mechanical polishing. The samples were characterized at temperatures down to 25 mK. In general, all junctions are of high quality and their I-U characteristics show low leakage currents and high superconducting energy gap values of 1.35 meV. The characteristics of the transistors and arrays exhibit some features in the…
The design for an inductive superconducting quantum interference proximity transistor with enhanced performance, the L-SQUIPT, is presented and analyzed. The interferometer is based on a double-loop structure, where each ring comprises a superconductor-normal metal-superconductor mesoscopic Josephson weak-link and the read-out electrode is implemented in the form of a superconducting tunnel probe. Our design allows both to improve the coupling of the transistor to the external magnetic field and to increase the characteristic magnetic flux transfer functions, thereby leading to an improved ult…
We have fabricated the Bloch transistor shunted on-chip by a small-sized Cr resistor with Rs about 1 kOhm. The Bloch transistor normally consists of two small Josephson junctions connected in series, which in our case have been replaced by two superconducting interferometer loops, each with two junctions in parallel. A capacitively coupled gate is supplied to control the induced charge of the small intermediate electrode (island) of the transistor. The measured I-V curves show no hysteresis and correspond to the operation of a effective Josephson junction at the high-damping and strong-noise l…
A niobium titanite nitride-based superconducting nanodevice in which the Josephson critical current can be modulated by a gate voltage - a Cooper-pair transistor - has proven a remarkably long parity lifetime exceeding one minute at temperatures close to absolute zero.
We study diffusive magnetic Josephson junctions with four superconducting terminals in the weak proximity limit where the leads are arranged in cross form. Employing the linearized Keldysh-Usadel technique, the anomalous Green's function and Josephson current are analytically obtained based on a quasiclassical theory using the Fourier series method. The derived results may be reduced to non-magnetic junctions by setting the exchange field equal to zero. We find that increments of the magnetic barrier thickness may cause a reversal of the supercurrent direction flowing into some of the leads, w…
Electronic transport through nanostructures is greatly affected by the presence of superconducting leads. If the interface between the nanostructure and the superconductors is sufficiently transparent, a dissipationless current (supercurrent) can flow through the device due to the Josephson effect. A Josephson coupling, as measured via the zero-resistance supercurrent, has been obtained via tunnel barriers, superconducting constrictions, normal metals, and semiconductors. The coupling mechanisms vary from tunneling to Andreev reflection. The latter process has always occurred via a normal-type…
We discuss how to control dc Josephson current by influencing the structure and nonequilibrium population of Andreev levels via external electrostatic gates, external current injection and electromagnetic radiation. In particular we will consider the "giant" Josephson current in "long" SIS tunnel junctions and the regular and anomalous nonequilibrium Josephson currents in three terminal SNS junctions. We will briefly discuss applications to the Josephson field effect transistor (JOFET) and to the newly invented Josephson interference transistor (JOINT).
A theoretical model of high-T_c Josephson Field Effect Transistors (JoFETs) based on a Ginzburg-Landau free energy expression whose parameters are field- and spatially- dependent is developed. This model is used to explain experimental data on JoFETs made by the hole-overdoped Ca-SBCO bicrystal junctions (three terminal devices). The measurements showed a large modulation of the critical current as a function of the applied voltage due to charge modulation in the bicrystal junction. The experimental data agree with the solutions of the theoretical model. This provides an explanation of the lar…
Scalability in the fabrication and operation of quantum computers is key to move beyond the NISQ era. So far, superconducting transmon qubits based on aluminum Josephson tunnel junctions have demonstrated the most advanced results, though this technology is difficult to implement with large-scale facilities. An alternative "gatemon" qubit has recently appeared, which uses hybrid superconducting/semiconducting (S/Sm) devices as gate-tuned Josephson junctions. Current implementations of these use nanowires however, of which the large-scale fabrication has not yet matured either. A scalable gatem…
We study theoretically the current-voltage characteristics (IVCs) of the Josephson field effect transistor - a ballistic SNINS junction with superconducting (S) electrodes confining a planar normal metal region (N), which is controlled by the gate induced potential barrier (I). The calculations were performed using the computation technique developed earlier for long single-channel junctions in the coherent multiple Andreev reflections (MAR) regime. We find significant difference of the subgap current structure in these junctions compared to the subharmonic gap structure in tunnel junctions an…
We consider problems of one dimensional interacting fermions confined to a finite size, multichannel geometry. Concentrating on Luttinger liquids and carbon nanotubes, we use nontrivial boundary conditions to represent the effect of external leads, and apply our framework to transport problems in a Josephson junction setup. We present an exact computation of all correlation functions, including finite-size and temperature effects, for two sets of solvable boundary conditions. In all cases, we compute physical quantities like the Josephson current and the pairing order parameter profile.
