Mintu Mondal

Phase Fluctuations in a Strongly Disordered s -Wave NbN Superconductor Close to the Metal-Insulator Transition

We explore the role of phase fluctuations in a three-dimensional s-wave superconductor, NbN, as we approach the critical disorder for destruction of the superconducting state. Close to critical disorder, we observe a finite gap in the electronic spectrum which persists at temperatures well above T(c). The superfluid density is strongly suppressed at low temperatures and evolves towards a linear-T variation at higher temperatures. These observations provide strong evidence that phase fluctuations play a central role in the formation of a pseudogap state in a disordered s-wave superconductor.

Digital Quantum Simulation of Spin Models with Circuit Quantum Electrodynamics

Systems of interacting quantum spins show a rich spectrum of quantum phases and display interesting many-body dynamics. Computing characteristics of even small systems on conventional computers poses significant challenges. A quantum simulator has the potential to outperform standard computers in calculating the evolution of complex quantum systems. Here, we perform a digital quantum simulation of the paradigmatic Heisenberg and Ising interacting spin models using a two transmon-qubit circuit quantum electrodynamics setup. We make use of the exchange interaction naturally present in the simulator to construct a digital decomposition of the model-specific evolution and extract its full dynamics. This approach is universal and efficient, employing only resources which are polynomial in the number of spins and indicates a path towards the controlled simulation of general spin dynamics in superconducting qubit platforms.

Measurement of magnetic penetration depth and superconducting energy gap in very thin epitaxial NbN films

We investigate evolution of the magnetic penetration depth and superconducting energy gap in epitaxial NbN films using a low frequency mutual inductance technique and tunneling spectroscopy using a low temperature scanning tunneling microscope (STM). The superconducting transition temperature (Tc) for films grown under optimal growth conditions decreases monotonically from 15.87K to 9.16K as the film thickness is decreased from 50nm to 3nm. With decrease in film thickness delta(0) monotonically decreases, whereas lambda(0) monotonically increases. We observe that Tc, lambda(o) and delta(0) are well described by Bardeen-Cooper-Schrieffer (BCS) theory in all films other than the two thinnest ones where we see evidence of the Kosterlitz-Thouless-Berezinski (KTB) transition close to Tc.

Andreev bound state and multiple energy gaps in the noncentrosymmetric superconductor BiPd

We report directional point contact Andreev reflection (PCAR) measurements on high-quality single crystals of the non-centrosymmetric superconductor, BiPd. The PCAR spectra measured on different crystallographic faces of the single crystal clearly show the presence of multiple superconducting energy gaps. For point contacts with low resistance, in addition to the superconducting gap feature, a pronounced zero bias conductance peak is observed. These observations provide strong evidence of the presence of unconventional order parameter in this material.

Role of the vortex-core energy on the Berezinskii-Kosterlitz-Thouless transition in thin films of NbN.

We analyze the occurrence of the Berezinskii-Kosterlitz-Thouless (BKT) transition in thin films of NbN at various film thickness, by probing the effect of vortex fluctuations on the temperature dependence of the superfluid density below T(BKT) and of the resistivity above T(BKT). By direct comparison between the experimental data and the theory, we show the crucial role played by the vortex-core energy in determining the characteristic signatures of the BKT physics, and we estimate its dependence on the disorder level. Our work provides a paradigmatic example of BKT physics in a quasi-two-dimensional superconductor.

Enhancement of the finite-frequency superfluid response in the pseudogap regime of strongly disordered superconducting films.

We measure the frequency dependence of the complex ac conductivity of NbN films with different levels of disorder in frequency range 0.4-20 GHz. Films with low disorder exhibit a narrow dynamic fluctuation regime above T_c as expected for a conventional superconductor. However, for strongly disordered samples, the fluctuation regime extends well above T_c, with a strongly frequency-dependent superfluid stiffness which disappears only at a temperature T* close to the pseudogap temperature obtained from scanning tunneling measurements. Such a finite-frequency response is associated to a marked slowing down of the superconducting fluctuations already below T*. The corresponding large length-scale fluctuations suggest a scenario of thermal phase fluctuations between superconducting domains in a strongly disordered s-wave superconductor.The persistence of a soft gap in the density of states above the superconducting transition temperature Tc, the pseudogap, has long been thought to be a hallmark of unconventional high-temperature superconductors. However, in the last few years this paradigm has been strongly revised by increasing experimental evidence for the emergence of a pseudogap state in strongly-disordered conventional superconductors. Nonetheless, the nature of this state, probed primarily through scanning tunneling spectroscopy (STS) measurements, remains partly elusive. Here we show that the dynamic response above Tc, obtained from the complex ac conductivity, is highly modified in the pseudogap regime of strongly disordered NbN films. Below the pseudogap temperature, T*, the superfluid stiffness acquires a strong frequency dependence associated with a marked slowing down of critical fluctuations. When translated into the length-scale of fluctuations, our results suggest a scenario of thermal phase fluctuations between superconducting domains in a strongly disordered s-wave superconductor.

Rapid High-Fidelity Single-Shot Dispersive Readout of Superconducting Qubits

The speed of quantum gates and measurements is a decisive factor for the overall fidelity of quantum protocols when performed on physical qubits with finite coherence time. Reducing the time required to distinguish qubit states with high fidelity is therefore a critical goal in quantum information science. The state-of-the-art readout of superconducting qubits is based on the dispersive interaction with a readout resonator. Here, we bring this technique to its current limit and demonstrate how the careful design of system parameters leads to fast and high-fidelity measurements without affecting qubit coherence. We achieve this result by increasing the dispersive interaction strength, by choosing an optimal linewidth of the readout resonator, by employing a Purcell filter, and by utilizing phase-sensitive parametric amplification. In our experiment, we measure 98.25% readout fidelity in only 48 ns, when minimizing read-out time, and 99.2% in 88 ns, when maximizing the fidelity, limited predominantly by the qubit lifetime of 7.6 us. The presented scheme is also expected to be suitable for integration into a multiplexed readout architecture.

Correlated conductance fluctuations close to the Berezinskii-Kosterlitz-Thouless transition in ultrathin NbN films.

We probe the presence of long-range correlations in phase fluctuations by analyzing the higherorder spectrum of resistance fluctuations in ultra-thin NbN superconducting films. The nonGaussian component of resistance fluctuations is found to be sensitive to film thickness close to the transition, which allows us to distinguish between mean field and Berezinskii-Kosterlitz-Thouless (BKT) type superconducting transitions. The extent of non-Gaussianity was found to be bounded by the BKT and mean field transition temperatures and depend strongly on the roughness and structural inhomogeneity of the superconducting films. Our experiment outlines a novel fluctuation-based kinetic probe in detecting the nature of superconductivity in disordered low-dimensional materials.

Phase Diagram and Upper Critical Field of Homogeneously Disordered Epitaxial 3-Dimensional NbN Films

We report the evolution of superconducting properties with disorder, in 3-dimensional homogeneously disordered epitaxial NbN thin films. The effective disorder in NbN is controlled from moderately clean limit down to Anderson metal–insulator transition by changing the deposition conditions. We propose a phase diagram for NbN in temperature-disorder plane. With increasing disorder, we observe that as kFl→1 the superconducting transition temperature (Tc) and normal state conductivity in the limit T→0 (σ0) go to zero. The phase diagram shows that in homogeneously disordered 3-D NbN films, the metal–insulator transition and the superconductor–insulator transition occur at a single quantum critical point, kFl∼1.

Temperature dependence of resistivity and Hall coefficient in strongly disordered NbN thin films

We report the temperature dependence of resistivity