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When a magnetic moment is embedded in a metal, it captures itinerant electrons to form the Kondo cloud1,2, which can spread out over a few micrometres3,4. For a metal with dense magnetic impurities such that Kondo clouds overlap with each other, correlated ground states are formed. When the impurities form a regular lattice, the result is a heavy fermion or anti-ferromagnetic order depending on the dominant interaction5,6. Even in the case of random impurities, overlapping Kondo clouds are expected to form a coherent ground state. Here, we examine this issue by performing electrical transport and high-precision tunnelling density-of-states (DOS) spectroscopy measurements in a highly P-doped crystalline silicon metal where disorder-induced localized magnetic moments exist7. We detect the Kondo effect in the resistivity of the Si metal below 2 K and an exotic pseudogap in the DOS with gap edge peaks at a Fermi energy below 100 mK. The DOS gap and peaks are tuned by applying an external magnetic field and transformed into a metallic Altshuler-Aronov gap8 in the paramagnetic disordered Fermi liquid (DFL) phase. We interpret this phenomenon as the Kondo condensation, the formation of a correlated ground state of overlapping Kondo clouds, and its transition to a DFL. The boundary between the Kondo condensation and DFL phases is identified by analysing distinct DOS spectra in the magnetic field-temperature plane. A detailed theoretical analysis using a holographic method 9 , 10 , 11 reproduces the unusual DOS spectra, 1, supporting our scenario. Our work demonstrates the observation of the magnetic version of Bardeen-Cooper-Shrieffer (BCS) pair condensation and will be useful for understanding complex Kondo systems., Comment: 34 pages,5+6 figures, accepted in nature physics

The remarkable advances of quantum computation technology with superconducting qubits based on circuit quantum electrodynamics (QED) architecture have been achieved by improving control, protection and measurement of the quantum states at the same time. At the heart of all these quantum operations, the significant enhancement of the qubit coherence time during the last decades was the key. Even after all these advances, the coherence and relaxation time of superconducting qubits still requires further improvements toward fault-tolerant quantum computation. Here, we report our observation of a strongly enhanced lifetime of an in-situ tunable superconducting transmon qubit on a silicon substrate that is embedded in a three-dimensional copper cavity. We measured a lifetime of the qubit of up to 84 µs, which is the best reported value of an in-situ tunable transmon on a silicon substrate. In our experiment, the in-situ frequency tunability over a broad range enabled the Purcell factor to be controlled continuously by detuning the qubit frequency against the resonator frequency in the strong dispersive regime. The silicon substrate has its own importance because the substrate should be fully compatible with the conventional semiconductor processes so that scalability and multi-chip module capability are guaranteed. In order to control the Purcell factor with another parameter, we displaced the qubit position in the cavity and observed a longer relaxation time with a smaller coupling coefficient. We believe that this systematic study and the control technique of the Purcell effect in the circuit QED design, together with minimizing the microwave photon loss through an improvement of the fabrication, will contribute to the realization of a practical large-scale quantum computer based on superconducting qubit technology.

Quantum computing devices are inevitably subject to errors. To leverage quantum technologies for computational benefits in practical applications, quantum algorithms and protocols must be implemented reliably under noise and imperfections. Since noise and imperfections limit the size of quantum circuits that can be realized on a quantum device, developing quantum error mitigation techniques that do not require extra qubits and gates is of critical importance. In this work, we present a deep learning-based protocol for reducing readout errors on quantum hardware. Our technique is based on training an artificial neural network with the measurement results obtained from experiments with simple quantum circuits consisting of singe-qubit gates only. With the neural network and deep learning, non-linear noise can be corrected, which is not possible with the existing linear inversion methods. The advantage of our method against the existing methods is demonstrated through quantum readout error mitigation experiments performed on IBM five-qubit quantum devices., 10 pages, 10 figures

We experimentally demonstrate that metal/oxide/floating-Schottky junction has multiple effective capacitances depending on the amount of electrons stored in the floating metal electrode. The oxide thin film covering the Schottky junction is used to trap electrons in the floating metal. The electron flow into and out of the floating metal is controlled by applying voltage pulses of opposite polarities onto the semiconductor substrate. With the amount of excess charges in the floating metal, the depletion capacitance of Schottky junction varies synchronously, which dominantly affects the effective capacitance of the whole junction. Interestingly, the capacitance of metal/oxide/floating-Schottky junction measured as a function of applied voltage pulse shows a hysteretic behavior, which supports its capacitive memory effect. It is expected that our metal/oxide/floating-Schottky junction can work as a memcapacitor capable of recording multiple switchable capacitance values and it can be readily fabricated with the current Si CMOS technology.

A transmon qubit in the 3-dimensional microwave cavity is a versatile system for various circuit QED experiments. We have performed numerical calculations of the vacuum Rabi coupling $g$ in 3D transmon circuit QED system, and compared the values with the experimental measurements. We used COMSOL Multiphysics RF package with an appropriate model to efficiently calculate the coupling strength. The calculation agrees with the measurement within a few percents in simple cases. Depending on the position of the qubit in the cavity, we calculated the coupling strengths to different eigenmodes. We compared our result for the fundamental mode (TE101) and a higher mode (TE201) with the experiment by changing the qubit position, as shown in Fig. 2. This simulation and characterization enable efficient design methods for future 3D circuit QED experiments. We confirm that we can design and predict the qubit coupling to the cavity modes with enough accuracy in 3D circuit QED system.

Quantum information processing using superconducting qubit based on Josephson junctionhas become one of the most promising candidates for possible realization of a quantum computer. In the heart of the qubit circuits, the superconducting microwave resonator plays a key role in quantum operations and measurementswhich , enables single-photon level microwave quantum optics. During last decade, t he coherence time, or the lifetime of the quantum state, of the superconducting qubit has been dramatically improved. Among several technological innovations, improvement of superconducting microwave resonatorthe ’s quality has been the main driving force in the gettingqubit performance almost ready for elementary quantum computing architecture. In this paper, I will briefly review very recent progresses of the superconducting microwave resonators especially aimed for quantum device applications during the last decadeThe progresses have been driven by ingenious circuit d. gn, esimaterial improvement, and new measurement techniques. Even a rather radical idea of three-dimensional large resonators have been successfully implemented in a qubit circuit. All those efforts contributed our understanding of the qubit decoherence to mechanism and as a result to the improvement of qubit performance.

We report our development of a broadband shot noise measurement experiment at Korea Research Institute of Standards and Science for noise thermometry. A precision RF measurement setup in the frequency range near 1 GHz has been installed in a He-3 refrigerator. For noise thermometry, we fabricated an Al-AlOx-Al tunnel junction by the double-angle shadow evaporation technique as a source of noise. Temperature measurement was performed from 0.3 K to room temperature. The influence of each RF component was analyzed and tested to stabilize the measurement system. We compare two different diode detectors for RF power measurement. We describe the thermal anchoring scheme and the influence of tunnel junction imperfections.

Recently, sputter-deposited MoSi2 films have been successfully used as a barrier material in Nb/MoSi2/Nb Josephson junctions. In this report, we present our study on the deposition conditions of MoSi2 film for electronics application. We investigated the film stress and sheet resistance as a function of deposition conditions. We also studied the micro-structural change in the film according to the deposition conditions using electron microscopes. We suggest that the thermal strain need to be considered in order to get a reliable junction.

The voltage from a single, microwave-biased Josephson junction is a small quantity; thus useful voltages are generated only through series arrays of many thousands of junctions. Arrays of superconductor-normal metal-superconductor junctions have been fabricated and tested with as many as 16,500 junctions per array. The arrays are optimized for the highest voltage operation with the largest operating margins for the current bias. Measurements show that these arrays, driven with 20 GHz microwaves, generate a dc voltage greater than 680 mV per array with a dc bias margin over 1 mA. To increase the microwave uniformity across the array, the transmission line impedance has been tapered. By use of this technique, ac Josephson voltages over 110 mVrms per array have been generated, also with over 1 mA dc bias margin.

We are implementing a new standard for 60 Hz power measurements based on precision sinusoidal reference voltages from two independent programmable Josephson voltage standards (PJVS): one for voltage and one for current. The National Institute of Standards and Technology PJVS systems use series arrays of Josephson junctions to produce accurate quantum-based DC voltages. Using stepwise-approximation synthesis, the PJVS systems produce sinewaves with precisely calculable RMS voltage and spectral content. We present measurements and calculations that elucidate the sources of error in the RMS voltage that are intrinsic to the digital-synthesis technique and that are due to the finite rise times and transients that occur when switching between the discrete voltages. Our goal is to reduce all error sources and uncertainty contributions from the PJVS synthesized waveforms to a few parts in 10 7 so that the overall uncertainty in the AC-power standard is a few parts in 106

We have developed a method of bonding superconductive integrated-circuit chips to interchangeable, microwave-compatible, flexible cryo-packages. This "flip-chip on flex" technology will greatly improve the service life and reliability of our Josephson systems because the present press-contacts to the chip are replaced with directly soldered connections. The new method eliminates the most common failure mode for our Josephson chips, which has been the degradation and variation of the contact resistances of the chip pads due to mechanical wear upon repeated thermal cycles from 4/spl deg/K to room temperature. The superior microwave properties of this packaging provide improved operating margins for our devices. We have demonstrated the reliability of the bonds with repeated thermal cycling for 100% operational chips with 40 connections (67 410 Josephson junctions).

Superconductor-normal metal-superconductor (SNS) Josephson junctions have proven to be a critical technology for voltage standards. NIST has used SNS junctions for both dc and ac programmable voltage standards. Previous devices have used primarily PdAu as a normal-metal barrier material. In this paper we present measurements of circuits having MoSi/sub 2/ barriers. Stacking enables the junctions to be packed more densely, thus increasing design flexibility and margins for the microwave circuits. In this work, measurements are presented from two- and three-junction stacks for application to ac Josephson voltage standards, which show output voltage and distortion near our best previously published results.

The authors have made tall, uniform stacked Josephson junction arrays by developing an etch process using an inductively coupled plasma etcher. This process produces vertical profiles in thin-film multilayers of alternating superconducting and normal metals. Such a vertical etch is necessary to obtain uniform properties in high-density arrays in order to achieve operating margins for voltage-standard applications. The authors use dry-etchable MoSi/sub 2/ for the normal metal barrier of our niobium Josephson junctions and obtain working series arrays with stacks up to 10 junctions tall. The authors present dc and microwave electrical characteristics of distributed and lumped arrays and discuss the technological improvements needed to fabricate lumped arrays for voltage-standard applications.

Using a new circuit design and double-stacked junction technology we have demonstrated fully functional high-resolution programmable voltage standard chips with 67 410 junctions that operate up to a maximum output voltage of 2.6 V. The circuit uses double-stacked junctions, where two junctions are fabricated in each stack, in order to increase the output voltage. We have also improved the voltage resolution at fixed frequencies 16-fold by using a circuit optimized for three voltage states and 128-fold below 2 V by oppositely biasing two large arrays whose junction counts differ by a single stack. These circuits operate over a frequency range from 14 GHz to 19 GHz with a 2-mA maximum operating current range.

We have developed a terahertz time-domain spectroscopy (THz-TDS) system for transmission and reflection measurements of metallic thin films. Using our THz-TDS system, we studied the conventional superconductor niobium (Nb) in the normal state in the spectral range from 5 to 50 cm -1 . Both the real and imaginary parts of the conductivity are acquired without Kramers-Kronig analysis. Nb exhibits a nearly frequency independent real conductivity spectrum in the terahertz range, with a very small imaginary part.

NIST is using and developing superconductor-normal metal-superconductor (SNS) Josephson arrays for both programmable DC and AC voltage standards. Increasing the output voltage is difficult because the output voltage per junction is small; hence series arrays with large numbers of junctions are needed. The best way to generate higher voltages and achieve the best operating margins for the broadband drive signals is by densely packing the junctions into shorter arrays. NIST has been working on stacked SNS junctions to achieve this goal. By stacking junctions in the array, more junctions may be placed per length, while preserving a lumped microwave element. In this paper we introduce our results on stacked SNS junctions using MoSi/sub 2/ and Ti as barrier materials. These barriers were chosen because they can be reactive-ion etched (RIE) in contrast to our standard PdAu barriers, which must be wet etched. Using RIE, alternating layers of barrier material and Nb may be etched in a single step. We indirectly quantify the junction uniformity in the arrays by measuring the current range of the constant-voltage steps when the arrays are biased with a microwave drive.

We measured the superconducting gap of MgB2 thin films and pellets by utilizing point contact Andreev reflection method with a gold tip. From more than 100 measurements, we obtained the distribution of the superconducting gap of the material. This distribution shows two peaks, one at 2–3 meV and the other around 6–8 meV while BCS weak coupling limit predicts 5.9 meV. From temperature dependence measurements, we observed energy gap to nearly follow BCS-like behavior. These results also seem to agree well with the recent anisotropic energy gap estimation based on the ab initio calculation from the band structure of MgB2.

We demonstrate that the superconducting critical temperature (Tc) of thin niobium films can be electrically modulated in a liquid-gated geometry device. Tc can be suppressed and enhanced by applying positive and negative gate voltage, respectively, in a reversible manner within a range of about 0.1 K. At a fixed temperature below Tc, we observed that the superconducting critical current can be modulated by gate voltage. This result suggests a possibility of an electrically-controlled switching device operating at or above liquid helium temperature, where superconductivity can be turned on or off solely by the applied gate voltage., 7 pages, 5 figures

The determination of bit error rates in single flux quantum logic circuits operating at temperatures well above the temperature of liquid helium is essential since the question of stability against thermal noise arises. We determined experimentally the static and for the first time the dynamic error rate at temperatures of about 40 K with the help of simple test circuits. The static error rate has been investigated using two different circuits with bicrystal junctions in either single-layer or multilayer configuration. In both cases the internal state of a storage loop was observed by a dc-SQUID. A ring oscillator based on a Josephson transmission line allowed us to measure the dynamic error rate of a Josephson comparator at high frequencies. This circuit has been fabricated using junctions by focused-electron-beam irradiation. It is specially suited for the detection of seldom occurring switching errors.

The magnetic shielding performance (SP) of a moderate-size shielding cabinet (SC) was examined using the finite element method (FEM) for the frequency range from DC to 1 kHz. The calculated data, as well as the experimental data, reveal that the conductive Al plate is crucial for frequency above 1 Hz, whereas the magnetic mumetal is indispensable below 1 Hz. The complex behavior of the mumetal in SP originates from a competition between eddy current losses and magnetic permeability evidenced by field distribution inside the SC. Demagnetization after assembly of the mumetal is important for the improvement in SP. The analysis of the experimental results in terms of a linear combination of the calculated individual components and the origin for the deviation between simulated and measured shielding factor (SF) are discussed in detail.

We report our development on design, fabrication and characterization of various superconducting resonators for quantum device applications. The resonators are developed mainly for superconducting qubit implementation in circuit QED structure, but future application will include photon detectors such as microwave kinetic inductance detectors. First we designed and fabricated half-wavelength coplanar waveguide resonators with niobium thin film on a high resistive silicon wafer, with the fundamental frequencies in the range of 1 to 8 GHz. The loaded quality factors are controlled by the coupling capacitors, and the lower bound of the intrinsic quality factor was measured down to 7 mK in the dilution refrigerator. We have also designed quarter-wavelength resonators and lumped LC resonators coupled to a transmission line, and characterized multiple resonators in a single transmission line. For the 3D transmon qubit application, we designed and fabricated a rectangular box resonator with superconducting aluminum. The intrinsic quality factor of the box resonator exceeds three million at 7 mK. We will also present basic results on the integration of a transmon qubit in the superconducting resonator.

Nb/MoSi2/Nb stacked superconductor–normal metal–superconductor (SNS) Josephson junctions has proven to be a good candidate for high-density series arrays for Josephson voltage-standard applications. As the junction density increases, self-heating becomes an issue because the high power density per junction (1 W/cm2) generates significant power dissipation under typical operating conditions. In this letter, we analyze the heating effect of these sandwich-type SNS junctions using a model to quantitatively estimate and predict thermal-transport properties of the stacked structures. We describe several strategies that reduce heating and demonstrate improved properties of stacked-junction arrays with enhanced cooling capacity.

Dense, vertically stacked Josephson junction arrays are being developed for voltage metrology applications. We present measurements of the uniformity and reproducibility of Nb/(MoSi2/Nb)N vertically stacked junctions that clarify the superconducting properties of the middle Nb superconducting electrode. Middle electrode thicknesses down to 20 nm have shown minimal suppression of the superconducting order parameter as measured through the critical current density. Even with a middle electrode thickness of 5 nm, excellent junction uniformity has been observed as demonstrated by the measurement of large Shapiro steps when the arrays are biased with microwaves. We also discuss the role of the superconducting coherence length in these arrays of high-density junctions.

Progress in the development of an evaluation system for thermal-transfer standard based on a Josephson waveform synthesizer at KRISS. The synthesizer adopting a Josephson junction array, which can generate arbitrary stepwise waveforms with a number of the quantum-voltage steps up to 2.5-V level amplitude. With the synthesizer, we have built a sampling voltmeter that measures the differential voltages between a sinusoidal waveform produced by a semiconductor-based ac source and the Josephson waveforms. We will present in the conference that the developed sampling voltmeter can be utilized as a reference for ac-dc difference measurements of a thermal transfer standard.

The pulse-driven ac Josephson voltage standard (ACJVS) system has been operated stably in Korea Research Institute of Standards and Science(KRISS). We have improved the system operation so that the distortion has been reduced compared to our previous report [1]. Since this system can generate arbitrary waveform with quantum-mechanically precise voltages, it can be applied to precision ac electrical measurements. We will present single-tone waveform synthesis results at different amplitudes and frequencies for ac metrology purpose, and will describe our preliminary result on ac-dc difference measurement using ACJVS and a commercial thermal transfer standard.

We fabricated an Al-AlO x -Al tunnel junction by the double-angle evaporation technique as a source of shot noise. A precision RF-power-measurement setup in the frequency range near 1 GHz was installed in a He-3 refrigerator. We measured temperature from 0.3 K to room temperature using shot noise thermometer and compared the temperature with that of conventional temperature sensors.

We report on the fabrication and measurements of a superconducting junction of a single-crystalline Au nanowire, connected to Al electrodes. Current-Voltage characteristic curve shows clear supercurrent branch below the superconducting transition temperature of Al and quantized voltage plateaus on application of microwave radiation, as expected from Josephson relations. Highly transparent (0.95) contacts very close to an ideal limit of 1 are formed at the interface between the normal metal (Au) and the superconductor (Al). The very high transparency is ascribed to the single crystallinity of a Au nanowire and the formation of an oxide-free contact between Au and Al. The sub-gap structures of the differential conductance are well explained by coherent multiple Andreev reflections (MAR), the hallmark of mesoscopic Josephson junctions. These observations demonstrate that single crystalline Au nanowires can be employed to develop novel quantum devices utilizing coherent electrical transport., 12 pages, 4 figures, to appear in ACS Nano

A waveform synthesizer adopting a superconductor-normal metal-superconductor junction array has been developed, which can generate arbitrary stepwise waveforms with a number of the quantum-voltage steps up to 1-V level amplitude. As an application of the synthesizer, we have built a sampling voltmeter which measures the differential voltages between a sinusoidal waveform produced by a semiconductor-based ac source and the Josephson waveforms. We carried out extensive sampling measurements for a 50-Hz sine wave with 1-V amplitude, applying various sampling apertures.

We report our experimental setup for the shot noise thermometry system at KRISS. Precision RF measurement setup in the frequency range of about 1 GHz has been installed in a He-3 refrigerator, which can be cooled down to 300 mK. The shot noise of Al-AlOx-Al tunnel junction, fabricated by two angle- evaporation technique, was measured and compared with theoretical curve. The inferred temperature from the noise curve agrees well with the measured temperature.