Engineering the microwave to infrared noise photon flux for superconducting quantum systems

Electromagnetic filtering is essential for the coherent control, operation and readout of superconducting quantum circuits at milliKelvin temperatures. The suppression of spurious modes around transition frequencies of a few GHz is well understood and mainly achieved by on-chip and package considerations. Noise photons of higher frequencies – beyond the pair-breaking energies – cause decoherence and require spectral engineering before reaching the packaged quantum chip. The external wires that pass into the refrigerator and go down to the quantum circuit provide a direct path for these photons. This article contains quantitative analysis and experimental data for the noise photon flux through coaxial, filtered wiring. The attenuation of the coaxial cable at room temperature and the noise photon flux estimates for typical wiring configurations are provided. Compact cryogenic microwave low-pass filters with CR-110 and Esorb-230 absorptive dielectric fillings are presented along with experimental data at room and cryogenic temperatures up to 70 GHz. Filter cut-off frequencies between 1 to 10 GHz are set by the filter length, and the roll-off is material dependent. The relative dielectric permittivity and magnetic permeability for the Esorb-230 material in the pair-breaking frequency range of 75 to 110 GHz are measured, and the filter properties in this frequency range are calculated. The estimated dramatic suppression of the noise photon flux due to the filter proves its usefulness for experiments with superconducting quantum systems.

First-order phase transition in 97,98Mo isotopes

Experimental evidences of a first-order phase transition from a superfluid to a non-interacting Fermi gas system are studied for [Formula: see text]Mo isotopes. The experimental observations are compared with the semi-empirical macroscopic model and superfluid formalism. We have shown that the entropy excess ratio introduced in our previous publications within the superfluid model can describe the first-order phase transition due to pair-breaking in atomic nuclei.

Inhomogeneous Superconductivity and the ‘Pseudogap’ State of Novel Superconductors

This chapter discusses the high-Tc oxides, which display many unusual properties above Tc, especially for the underdoped compounds. One can observe some features typical for the superconducting state, such as the energy gap, anomalous diamagnetism, and the isotope effect; they coexist with finite resistance. These features are caused by an intrinsic inhomogeneity of the compound. Various energy scales (Tc, Tc*, T*) can be introduced. The system contains a set of superconducting ‘islands’ embedded in a normal metallic matrix. The inhomogeneity is caused by the statistical nature of doping and the pair-breaking effect. The formation of a macroscopic superconducting phase (at T = Tc) corresponds to the transition, which is of a percolative nature. The resistive and Meissner transitions are split. The granular superconductors are inhomogeneous and their properties are similar to those of doped systems. The ordered doping should lead to an increase in the value of the critical temperature.

Excitonic density wave and spin-valley superfluid in bilayer transition metal dichalcogenide

Artificial moiré superlattices in 2d van der Waals heterostructures are a new venue for realizing and controlling correlated electronic phenomena. Recently, twisted bilayer WSe2 emerged as a new robust moiré system hosting a correlated insulator at moiré half-filling over a range of twist angle. In this work, we present a theory of this insulating state as an excitonic density wave due to intervalley electron–hole pairing. We show that exciton condensation is strongly enhanced by a van Hove singularity near the Fermi level. Our theory explains the remarkable sensitivity of the insulating gap to the vertical electric field. In contrast, the gap is weakly reduced by a perpendicular magnetic field, with quadratic dependence at low field. The different responses to electric and magnetic field can be understood in terms of pair-breaking versus non-pair-breaking effects in a BCS analog of the system. We further predict superfluid spin transport in this electrical insulator, which can be detected by optical spin injection and spatial-temporal imaging.

Parity equilibration in nuclear level densities of 159–166Dy

Excitation-energy dependent parity ratios in the level densities of [Formula: see text] isotopes are calculated within a microscopic approach. Introducing a parity equilibration parameter, energy dependence of the transition from where a single parity dominates to a parity equilibrated state is compared among [Formula: see text] isotopes and its relation to the pairing effect is investigated. A correlation between the pair-breaking and the equilibration of parity distributions is observed for the considered isotopes.

Fundamental dissipation due to bound fermions in the zero-temperature limit

The ground state of a fermionic condensate is well protected against perturbations in the presence of an isotropic gap. Regions of gap suppression, surfaces and vortex cores which host Andreev-bound states, seemingly lift that strict protection. Here we show that in superfluid 3He the role of bound states is more subtle: when a macroscopic object moves in the superfluid at velocities exceeding the Landau critical velocity, little to no bulk pair breaking takes place, while the damping observed originates from the bound states covering the moving object. We identify two separate timescales that govern the bound state dynamics, one of them much longer than theoretically anticipated, and show that the bound states do not interact with bulk excitations.