We have investigated the flux transfer characteristics of a switchable flux transformer comprising a superconducting loop and a DC superconducting quantum interference device (DC-SQUID). This system can be used to couple multiple flux qubits with a controllable coupling strength. Its characteristics were measured using a flux input coil and a DC-SQUID for readout coupled to the transformer loop in a dilution refrigerator. The observed characteristics are consistent with the calculation results. We have demonstrated the reversal of the slope of the characteristics and the complete switching off of the transformer, which are useful features for its application as a controllable coupler for flux qubits.
We propose a method for switchable coupling between superconducting qubits using double resonance. The inter-qubit coupling is achieved by applying near-resonant oscillating fields to the two qubits. The deviation from resonance relaxes the criterion of strong driving fields while still allowing for a fully entangling two-qubit gate. This method avoids some of the shortcomings of previous proposals for switchable coupling. We discuss the possible application of our proposal to a pair of inductively coupled flux qubits, and we consider the extension to phase qubits.
We report on a controlled method to fabricate in-situ a superconducting (SC) nanostructure at the apex of the standard W tip of a Scanning Tunnelling Microscope (STM) by pulsing the tunnelling voltage on a graphene covered metal surface. We have characterized the SC properties of the resulting tip as a function of temperature and magnetic field, obtaining a transition temperature of 3.3 K and a critical field well above 3T. The SC nanotip is robust, stable, and achieves atomic resolution. The non-SC tip can be easily recovered by controlled voltage pulsing on a clean metal surface. The present…
We realize a device allowing for tunable and switchable coupling between two superconducting resonators mediated by an artificial atom. For the latter, we utilize a persistent current flux qubit. We characterize the tunable and switchable coupling in frequency and time domain and find that the coupling between the relevant modes can be varied in a controlled way. Specifically, the coupling can be tuned by adjusting the flux through the qubit loop or by saturating the qubit. Our time domain measurements allow us to find parameter regimes for optimal switch performance with respect to qubit driv…
Diode is one of basic electronic components. It has a nonreciprocal current response, associated with a broken space/time reversal symmetry. Here we demonstrate prototypes of superconducting diodes operational at zero magnetic field. They are based on conventional niobium planar Josephson junctions, in which space/time symmetry is broken by a combination of self-field effect from nonuniform bias and stray fields from a trapped Abrikosov vortex. We demonstrate that nonreciprocity of critical current in such diodes can reach an order of magnitude and rectification efficiency can exceed 70$\%$. F…
We constructed a superconducting/ferromagnetic hybrid system in which the ordering of the pinning potential landscape for flux quanta can be manipulated. Flux pinning is induced by an array of magnetic nanodots in the magnetic vortex state, and controlled by the magnetic history. This allows switching on and off the collective pinning of the flux-lattice. In addition, we observed field-induced superconductivity that originates from the annihilation of flux quanta induced by the stray fields from the magnetic vortices.
Superconducting quantum circuits possess the ingredients for quantum information processing and for developing on-chip microwave quantum optics. From the initial manipulation of few-level superconducting systems (qubits) to their strong coupling to microwave resonators, the time has come to consider the generation and characterization of propagating quantum microwaves. In this paper, we design a key ingredient that will prove essential in the general frame: a swtichable coupling between qubit(s) and transmission line(s) that can work in the ultrastrong coupling regime, where the coupling stren…
We present the results from an experimental study of the magneto-transport of superconducting wires of amorphous Indium-Oxide, having widths in the range 40 - 120 nm. We find that, below the superconducting transition temperature, the wires exhibit clear, reproducible, oscillations in their resistance as a function of magnetic field. The oscillations are reminiscent of those which underlie the operation of a superconducting quantum interference device.
In this work we fabricate and characterize a functionalized superconducting (SC) Nb tip of a scanning tunnelling microscope (STM). The tip is functionalized with a Tetracyanoquinodimethane molecule (TCNQ) that accepts charge from the tip and develops a magnetic moment. As a consequence, in scanning tunnelling spectroscopy (STS), sharp, bias symmetric sub-gap states identified as Yu-Shiba-Rusinov (YSR) bound states appear against the featureless density of states of a metallic gr/Ir(111) sample. Although the coupling regime of the magnetic impurity with the SC tip depends on the initial absorpt…
A new type of a superconducting quantum interference device (SQUID) based on a single superconducting loop without Josephson junctions and with asymmetric link-up of current leads is proposed. This SQUID offers advantages in simplicity of fabrication and higher sensitivity of magnetic flux. Magnetic field dependence of the critical current in aluminium rings with asymmetric link-up of current leads has been measured in order to confirm the possibility of making this type of SQUID.
This letter presents fabrication and characterization of an optimized SQUIPT (superconducting quantum interference proximity transistor). The present device, characterized with reduced tunnel junction area and shortened normal-metal section, demonstrates no hysteresis at low temperatures as we increased the Josephson inductance of the weak link by decreasing its cross section. It has consequently almost an order of magnitude improved magnetic field responsivity as compared to the earlier design. The modulation of both the current and the voltage across the junction have been measured as a func…
Numerical study of momentum-dependent gap function is presented to make clear the origin of superconductivity in copper oxides. We claim that antinodal region with pronounced nesting feature of the Fermi contour gives rise to superconducting pairing with large momentum under screened Coulomb repulsion. Such a pairing results in both spatial checkerboard pattern of the superconducting state below Tc and a gapped state of incoherent pairs in a broad temperature range above Tc. We explain the momentum dependence of the coherent spectral weight detected in angle-resolved photoemission spectroscopy…
Magnetoresistance oscillations were observed on networks of superconducting ultrathin Nb nanowires presenting evidences of either thermal or quantum activated phase slips. The magnetic transport data, discussed in the framework of different scenarios, reveal that the system behaves coherently in the temperature range where the contribution of the fluctuations is important.
A new method of preparation of radio-frequency superconducting quantum interference devices on MgB2 thin films is presented. The variable-thickness bridge was prepared by a combination of optical lithography and of the scratching by an atomic force microscope. The critical current of the nanobridge was 0.35 uA at 4.2 K. Non-contact measurements of the current-phase characteristics and of the critical current vs. temperature have been investigated on our structures.
Theory of self induced resonances in asymmetric two-junction interferometer device is presented. In real devices it is impossible to have an ideal interferometer free of imperfections. Thus, we extended previous theoretical approaches introducing a model which contains several asymmetries: Josephson current $ε$, capacitances $χ$ and dissipation $ρ$ presented in an equivalent circuit. Moreover, non conventional symmetry of the order parameter in high temperature superconducting quantum interference devices forced us to include phase asymmetries. Therefore, the model has been extended to the cas…
The operational dc-SQUID based on intrinsic Josephson junctions in Bi$_2$Sr$_2$CaCu2O$_{8+δ}$ high-$T_c$ superconductor is fabricated and studied. The novel in-plane loop layout and the developed in-situ endpoint detection method allowed an accurate control of the number of junctions in the SQUID. A clear periodic modulation of the superconducting current as a function of magnetic flux through the SQUID loop is observed. This is an unambiguous evidence for the quantum interference phenomenon in Bi$_2$Sr$_2$CaCu2O$_{8+δ}$ single crystals.
We investigate the role of gap characteristics such as anisotropy and inequality of the gaps in the quasiparticle interferences of iron pnictides using a five-orbital tight-binding model. We examine how the difference in the sensitivities exhibited by the sign-changing and -preserving $s$-wave superconductivity in an annular region around ($π, 0$), which can be used to determine the sign change of the superconducting gap, gets affected when the gaps are unequal on the electron and hole pocket. In addition, we also discuss how robust these differentiating features are on changing the quasiparti…
Point contact junctions made from two pieces of MgB2 can be adjusted to exhibit either superconductor-insulator-superconductor (SIS) or superconductor-normal metal-superconductor (SNS) current-voltage characteristics. The SIS characteristics are in good agreement with the standard tunneling model for s-wave superconductors, and yield an energy gap of (2.02 +/- 0.08) meV. The SNS characteristics are in good agreement with the predictions of the resistively-shunted junction model. DC Superconducting QUantum Interference Devices made from two SNS junctions yield magnetic field noise as low as 35 …
We demonstrate the role of proximity effect in the thermal hysteresis of superconducting constrictions. From the analysis of successive thermal instabilities in the transport characteristics of micron-size superconducting quantum interference devices with a well-controlled geometry, we obtain a complete picture of the different thermal regimes. These determine whether the junctions are hysteretic or not. Below the superconductor critical temperature, the critical current switches from a classical weak-link behavior to one driven by the proximity effect. The associated small amplitude of the cr…
We propose the implementation of a switch of particle statistics with an embedding quantum simulator. By encoding both Bose-Einstein and Fermi-Dirac statistics into an enlarged Hilbert space, the statistics of quantum particles may be changed in situ during the time evolution, from bosons to fermions and from fermions to bosons, as many times as desired. We illustrate our proposal with few-qubit examples, although the protocol is straightforwardly extendable to larger numbers of particles. This proposal can be implemented on different quantum platforms such as trapped ions, quantum photonics, …
It is a well known phenomenon in optics that spectroscopic resolution of a diffraction grating is much better compared to an interference device having just two slits, as in the Young's famous double-slit experiment. On the other hand, it is well known that a classical superconducting quantum interference device (SQUID) is analogous to the optical double-slit experiment. Here we report experiments and present a model describing a superconducting analogue to the diffraction grating, namely an array of superconducting islands positioned on a topological insulator (TI) film Bi0.8Sb1.2Te3. In the …
We present an experimental study of two-dimensional superconducting quantum interference filters (2D-SQIFs) in the presence of a magnetic field B. The dependences of the dc voltage on the applied magnetic field are characterized by a unique delta-like dip at B=0, which depends on the distribution of the areas of the individual loops, and on the bias current. The voltage span of the dip scales proportional to the number of rows simultaneously operating at the same working point. In addition, the voltage response of the 2D-SQIF is sensitive to a field gradient generated by a control line and sup…
We describe the performance of amplifiers in the 4 GHz--8 GHz range using Direct Current Superconducting Quantum Interference Devices(DC SQUIDs) in a lumped element configuration. We have used external impedance transformers to couple power into and out of the DC SQUIDs. By choosing appropriate values for coupling capacitors, resonator lengths and output component values, we have demonstrated useful gains in several frequency ranges with different bandwidths, showing over 27 GHz of power gain-bandwidth product. In this work, we describe our design for the 4 GHz--8 GHz range and present data de…
Micron size superconducting quantum interference devices ($μ$-SQUID) of lead (Pb), for probing nano-magnetism, were fabricated and characterized. In order to get continuous Pb films with small grain size, Pb was thermally evaporated on a liquid nitrogen cooled Si substrate. Pb was sandwiched between two thin Cr layers for improved adhesion and protection. The SQUID pattern was made by e-beam lithography with Pb lift-off after deposition. The current-voltage characteristics of these devices show a critical current, which exhibits the expected SQUID oscillations with magnetic field, and two re-t…
We report the fabrication of a directly coupled superconducting quantum interference device (SQUID) magnetometer in MgB2 using a focused ion beam (FIB) to create Josephson junctions in a 70 nm thick film of MgB2. The SQUID shows a voltage modulation (deltaV) of 175 mV at a temperature of 10 K and operates over a temperature range from 10 K to 24 K. We find excellent agreement between the measured maximum transfer functions and those predicted by theory. We have measured the magnetic flux noise at 20 K to be as low as 14 micro Phi-0 per root Hz.
We successfully exploit the parabolic shape of the dc voltage output dip around B=0 of a Superconducting Quantum Interference Filter (SQIF) to mix weak external rf signals. The two tone response of weak time harmonic electromagnetic fields has been detected on the spectral voltage output of the SQIF at frequency f_0 = f_1 - f_2, for various frequencies f_1 and f_2 ranging from few MHz up to 20 GHz. The two tone response is a characteristic function of static magnetic field B and of bias current I_b, related to the second derivative of the dc voltage output.
The transport properties of a YBa$_2$Cu$_3$O$_{7-δ}$ superconducting quantum interference device (SQUID) based on grooved Dayem bridge weak links are studied as a function of temperature: at high temperatures ($60~$K$<T<T_\mathrm{c}=89$~K) the weak links show properties similar to SNS junctions, while at temperatures below 60~K the weak links behave like short Dayem bridges. Using these devices, we have fabricated SQUID magnetometers with galvanically coupled in-plane pick-up loops: at $T=77$~K, magnetic field white noise levels as low as $63$~fT/$\sqrt{\mathrm{Hz}}$ have been achieved.
A microscopic theory of the magnetic-field modulation of critical currents is developed for plane Josephson junctions with anharmonic current-phase relations. The results obtained allow examining temperature-dependent deviations of the modulation from the conventional interference pattern. For tunneling through localized states in symmetric short junctions with a pronounced anharmonic behavior, the deviations are obtained and shown to depend on distribution of channel transparencies. For constant transparency the deviations vanish not only near Tc, but also at T=0. If Dorokhov bimodal distribu…
Superconducting Quantum Interference Devices (SQUIDs) can have excellent spin sensitivity depending on their magnetic flux noise, pick-up loop diameter, and distance from the sample. We report a family of scanning SQUID susceptometers with terraced tips that position the pick-up loops 300 nm from the sample. The 600 nm - 2 um pickup loops, defined by focused ion beam, are integrated into a 12-layer optical lithography process allowing flux-locked feedback, in situ background subtraction and optimized flux noise. These features enable a sensitivity of ~70 electron spins per root Hertz at 4K.
The authors demonstrate quadratic mixing of weak time harmonic electromagnetic fields applied to Superconducting Quantum Interference Filters, manufactured from high-$T_{\mathrm{c}}$ grain boundary Josephson junctions and operated in active microcooler. The authors use the parabolic shape of the dip in the dc-voltage output around B=0 to mix \emph{quadratically} two external rf-signals, at frequencies $f_{\mathrm{1}}$ and $f_{\mathrm{2}}$ well below the Josephson frequency $f_{\mathrm{J}}$, and detect the corresponding mixing signal at $| {f_{1}-f_{2}}| $. Quadratic mixing takes also place whe…
We show that quasiparticle interference (QPI) due to omnipresent weak impurities and probed by Fourier transform scanning tunneling microscopy and spectroscopy acts as a direct experimental probe of bulk odd-frequency superconducting pairing. Taking the example of a conventional $s$-wave superconductor under applied magnetic field, we show that the nature of the QPI peaks can only be characterized by including the odd-frequency pairing correlations generated in this system. In particular, we identify that the defining feature of odd-frequency pairing gives rise to a bias asymmetry in the QPI, …
We present measurements of an amplifier based on a dc superconducting quantum interference device (SQUID) with submicron Al-AlOx-Al Josephson junctions. The small junction size reduces their self-capacitance and allows for the use of relatively large resistive shunts while maintaining nonhysteretic operation. This leads to an enhancement of the SQUID transfer function compared to SQUIDs with micron-scale junctions. The device layout is modified from that of a conventional SQUID to allow for coupling signals into the amplifier with a substantial mutual inductance for a relatively short microstr…
Using both two orbital and five orbital models, we investigate the quasiparticle interference (QPI) patterns in the superconducting (SC) state of iron-based superconductors. We compare the results for nonmagnetic and magnetic impurities in sign-changed s-wave $\cos(k_x)\cdot\cos(k_y)$ and sign-unchanged $|\cos(k_x)\cdot\cos(k_y)|$ SC states. While the patterns strongly depend on the chosen band structures, the sensitivity of peaks around $(\pmπ,0)$ and $(0,\pmπ)$ wavevectors on magnetic or non-magnetic impurity, and sign change or sign unchanged SC orders is common in two models. Our results s…
The combination of superconducting and magnetic materials to create novel superconducting devices has been motivated by the discovery of Josephson critical current (Ics) oscillations as a function of magnetic layer thickness and the demonstration of devices with switchable critical currents. However, none of the hybrid devices have shown any spintronic effects, such as spin-transfer torque, which are currently used in room-temperature magnetic devices, including spin-transfer torque random-access memory and spin-torque nano-oscillators. We have developed nanopillar Josephson junctions with a m…
DC superconducting quantum interference devices (dc-SQUIDs) were fabricated in Co-doped BaFe2As2 epitaxial films on (La, Sr)(Al, Ta)O3 bicrystal substrates with 30deg misorientation angles. The 18 x 8 micro-meter^2 SQUID loop with an estimated inductance of 13 pH contained two 3 micro-meter wide grain boundary junctions. The voltage-flux characteristics clearly exhibited periodic modulations with deltaV = 1.4 micro-volt at 14 K, while the intrinsic flux noise of dc-SQUIDs was 7.8 x 10^-5 fai0/Hz^1/2 above 20 Hz. The rather high flux noise is mainly attributed to the small voltage modulation de…
We propose a transistor-like circuit including two serially connected segments of a narrow superconducting nanowire joint by a wider segment with a capacitively coupled gate in between. This circuit is made of amorphous NbSi film and embedded in a network of on-chip Cr microresistors ensuring a sufficiently high external electromagnetic impedance. Assuming a virtual regime of quantum phase slips (QPS)in two narrow segments of the wire, leading to quantum interference of voltages on these segments, this circuit is dual to the dc SQUID. Our samples demonstrated appreciable Coulomb blockade volta…
Superconducting Quantum Interference Filters (SQIFs) are arrays of superconducting loops of different sizes including Josephson Junctions (JJ). For a random distribution of sizes, they present a non-periodic response to an applied magnetic field, with an extended linear regime and a sizable field sensitivity. Such properties make SQIFs interesting devices to detect the magnetic component of electromagnetic waves at microwave frequencies. We have used the highly scalable technique of ion irradiation to make High Tc SQUIDs and SQIFs based on commercial YBa2Cu3O7 films, and studied their properti…
Quasiparticle interference (QPI) by means of scanning tunneling microscopy/spectroscopy (STM/STS), angle resolved photoemission spectroscopy (ARPES), and multi-orbital tight bind- ing calculations are used to investigate the band structure and superconducting order parameter of LiFeAs. Using this combination we identify intra- and interband scattering vectors between the hole (h) and electron (e) bands in the QPI maps. Discrepancies in the band dispersions inferred from previous ARPES and STM/STS are reconciled by recognizing a difference in the $k_z$ sensitivity for the two probes. The observ…
In the usual perception, surface superconductivity is associated with the surface nucleation of a superconducting condensate above the upper critical field in type-II superconductors or with a rearrangement of phonon properties and the electron-phonon coupling near surfaces/interfaces. Recently, it has been found that there is another example when the surface superconducting temperature is increased up to 20-25% as compared to the bulk one due to constructive interference of superconducting pair states. In the present work, we demonstrate that in fact, such an interferenceinduced enhancement c…
Geometric frustration emerges when local interaction energies in an ordered lattice structure cannot be simultaneously minimized, resulting in a large number of degenerate states. The numerous degenerate configurations may lead to practical applications in microelectronics, such as data storage, memory and logic. However, it is difficult to achieve extensive degeneracy, especially in a two-dimensional system. Here, we showcase in-situ controllable geometric frustration with massive degeneracy in a two-dimensional flux quantum system. We create this in a superconducting thin film placed underne…
We use low-temperature scanning electron microscopy combined with SQUID detection of magnetic flux to image vortices and to investigate low-frequency flux noise in YBa$_2$Cu$_3$O$_7$ thin film SQUIDs. The low-frequency flux noise shows a nonlinear increase with magnetic cooling field up to 60 $μ$T. This effect is explained by the surface potential barrier at the SQUID hole. By correlating flux noise data with the spatial distribution of vortices, we obtain information on spatial fluctuations of vortices on a microscopic scale, e.g. an average vortex hopping length of approximately 10 nm.
We investigated, at temperature $4.2\,\mathrm{K}$, electric transport, flux noise and resulting spin sensitivity of miniaturized Nb direct current superconducting quantum interference devices (SQUIDs) based on submicron Josephson junctions with HfTi barriers. The SQUIDs are either of the magnetometer-type or gradiometric in layout. In the white noise regime, for the best magnetometer we obtain a flux noise $S_Φ^{1/2}=250\,\mathrm{n}Φ_0/\mathrm{Hz}^{1/2}$, corresponding to a spin sensitivity $S^{1/2}_μ\,\ge\,29\,μ_B/\mathrm{Hz}^{1/2}$. For the gradiometer we find $S_Φ^{1/2}=300\,\mathrm{n}Φ_0/\…
The superconducting state is achieved by the condensation of Cooper pairs and is protected by the superconducting gap. The pairing interaction between the two electrons of a Cooper pair determines the superconducting gap function. Thus, it is very pivotal to detect the gap structure for understanding the mechanism of superconductivity. In cuprate superconductors, it has been well established that the superconducting gap may have a d-wave function Δ = Δ_0cos2θ. This gap function has an alternative sign change by every pi/2 in the momentum space when the in-plane azimuthal angle theta is scanned…
We present the design, realization and performance of a three-axis vector nano Superconducting QUantum Interference Device (nanoSQUID). It consists of three mutually orthogonal SQUID nanoloops that allow distinguishing the three components of the vector magnetic moment of individual nanoparticles placed at a specific position. The device is based on Nb/HfTi/Nb Josephson junctions and exhibits linewidths of $\sim 250$ nm and inner loop areas of $600 \times 90$ nm$^2$ and $500 \times 500$ nm$^2$. Operation at temperature $T=4.2$ K, under external magnetic fields up to $\sim 50$ mT is demonstrate…
A crucial step in revealing the nature of unconventional superconductivity is to investigate the symmetry of the superconducting order parameter. Scanning tunneling spectroscopy has proven a powerful technique to probe this symmetry by measuring the quasiparticle interference (QPI) which sensitively depends on the superconducting pairing mechanism. A particularly well suited material to apply this technique is the stoichiometric superconductor LiFeAs as it features clean, charge neutral cleaved surfaces without surface states and a relatively high Tc~18K. Our data reveal that in LiFeAs the qua…
By measuring the spatial distribution of differential conductance near impurities on Fe sites, we have obtained the quasi-particle interference (QPI) patterns in the (Li$_{1-x}$Fe$_x$)OHFe$_{1-y}$Zn$_y$Se superconductor with only electron Fermi surfaces. By taking the Fourier transform on these patterns, we investigate the scattering features between the two circles of electron pockets formed by folding or hybridization. We treat the data by using the recent theoretical approach [arXiv:1710.09089] which is specially designed for the impurity induced bound states. It is found that the supercond…
We investigate the design and functionality of a network of loop-shaped charge qubits with switchable nearest-neighbor coupling. The qubit coupling is achieved by placing large Josephson junctions at the intersections of the qubit loops and selectively applying bias currents. The network is scalable and makes it possible to perform a universal set of quantum gates. The coupling scheme allows gate operation at the charge degeneracy point of each qubit, and also applies to charge-phase qubits. Additional Josephson junctions included in the qubit loops for qubit readout can also be employed for q…
Nano-scale superconducting quantum interference devices (nano-SQUIDS) where the weak-links are made from nano-bridges --- i.e., nano-bridge--SQUIDs (NBSs) --- are one of the most sensitive magnetometers for nano-scale magnetometry. Because of very strong non-linearity in the nano-bridge--electrode joints, the applied magnetic flux ($Φ_{a}$) -- critical current ($I_{c}$) characteristics of NBSs differ very significantly from conventional tunnel-junction-SQUIDs, especially when nano-bridges are long and/or the screening parameter is large. However, in most of the theoretical descriptions, NBSs h…
In this paper, we propose a scheme to realize three-qubit quantum phase gate of one qubit simultaneously controlling two target qubits using four-level superconducting quantum interference devices (SQUIDs) coupled to a superconducting resonator. The two lowest levels |0> and |1> of each SQUID are used to represent logical states while the higher energy levels |2> and |3> are utilized for gate realization. Our scheme does not require adiabatic passage, second order detuning, and the adjustment of the level spacing during gate operation which reduce the gate time significantly. The scheme is gen…
