arXiv subjects

Daniel Kruti

Publications and source records attributed to Daniel Kruti.

4 recordsLinked to original sources

arXiv2026

Flux noise without flux tunability in superconducting qubits

Flux noise is unanimously recognised as a leading dephasing mechanism for flux-tunable superconducting qubits. However, our microscopic understanding remains incomplete, and basic effects like Faraday's law of induction have only very recently come into focus. Based on a quantum geometric description of the Faraday effect, we provide an in-depth derivation of the coupling of generic magnetic sources to thin film superconducting structures, under appropriate consideration of the device geometry. We apply the resulting framework to time-varying magnetic dipoles, describing surface or substrate spins, as well as current-carrying flux lines. We show that flux noise not only affects dephasing, but also provides a fundamental limit for the qubit quality factor - notably, even when the qubit contains no loops and is thus nominally not flux-tunable. Assuming surface spins as the origin for universal flux noise, we expect that this quality factor limit might be reached in the near term. For flux lines, we formulate a minimal safety distance to conserve the qubit performance, potentially constraining the scale-up of quantum hardware. This distance is boosted in the presence of large capacitor wings typical for transmons, due to a lensing of the electromotive field which is largely independent of Meissner screening.

cond-mat.mes-hall↗

arXiv2024

Interplay between evanescent scattering modes and finite dispersion in superconducting junctions

Superconducting junctions are essential building blocks for quantum hardware, and their fundamental behaviour remains a highly active research field. The behaviour of generic junctions is conveniently described by Beenakker's determinant formula, linking the subgap energy spectrum to the scattering matrix characterising the junction. In particular, the gap closing between bound and continuum states in short junctions follows from unitarity of the scattering matrix, and thus, from probability conservation. In this work, we critically reassess two assumptions: that scattering in short junctions is approximately energy-independent and dominated by planar channels. We argue that strongly energy-dependant scattering follows from finite dispersion of the conductor electrons even when they spend little time within the scattering region, and show that evanescent modes play a central role when cross-channel scattering is important. By generalising Beenakker's equation and performing a mapping to an effective Hamiltonian, we show that the gap closing is linked to a chiral symmetry. While finite energy dependence in the scattering breaks the chiral symmetry, we show two distinct mechanisms preserving the gap closing, each connected to new types of constraints on energy-dependant scattering matrices beyond unitarity. If the dispersive mode is planar, the gap closing is still preserved through a time-dependant probability conservation analysis of the scattering process. If the dispersive channel is evanescent, we derive a constraint which, notably, cannot follow from probability conservation. We thus demonstrate that Andreev physics reveal a much wider variety of properties of normal-metal scattering than commonly expected. We expect that our findings will have an impact on the dissipative behaviour of driven junctions, and offer a new perspective on fundamental properties of scattering matrices.

cond-mat.mes-hall↗

arXiv2024

On-demand analog space-time in superconducting networks: grey holes, dynamical instability and exceptional points

There has been considerable effort to mimic analog black holes and wormholes in solid state systems. Lattice realizations in particular present specific challenges. One of those is that event horizons in general have both white and black hole (grey hole) character, a feature guaranteed by the Nielsen-Ninomiya theorem. We here explore and extend the capability of superconducting circuit hardware to implement on-demand spacetime geometries on lattices, combining nonreciprocity of gyrators with the nonlinearity of Josephson junctions. We demonstrate the possibility of the metric sharply changing within a single lattice point, thus entering a regime where the modulation of system parameters is "trans-Planckian", and the Hawking temperature ill-defined. Instead of regular Hawking radiation, we find an instability in the form of an exponential burst of charge and phase quantum fluctuations over short time scales - a robust signature even in the presence of an environment. Moreover, we present a loop-hole for the typical black/white hole ambiguity in lattice simulations: exceptional points in the dispersion relation allow for the creation of pure black (or white) hole horizons, at the expense of a radical change in the dynamics of the wormhole interior.

cond-mat.mes-hall↗

arXiv2021

Fractionalized quantum criticality in spin-orbital liquids from field theory beyond the leading order

Two-dimensional spin-orbital magnets with strong exchange frustration have recently been predicted to facilitate the realization of a quantum critical point in the Gross-Neveu-SO(3) universality class. In contrast to previously known Gross-Neveu-type universality classes, this quantum critical point separates a Dirac semimetal and a long-range-ordered phase, in which the fermion spectrum is only partially gapped out. Here, we characterize the quantum critical behavior of the Gross-Neveu-SO(3) universality class by employing three complementary field-theoretical techniques beyond their leading orders. We compute the correlation-length exponent $ν$, the order-parameter anomalous dimension $η_ϕ$, and the fermion anomalous dimension $η_ψ$ using a three-loop $ε$ expansion around the upper critical space-time dimension of four, a second-order large-$N$ expansion (with the fermion anomalous dimension obtained even at the third order), as well as a functional renormalization group approach in the improved local potential approximation. For the physically relevant case of $N=3$ flavors of two-component Dirac fermions in 2+1 space-time dimensions, we obtain the estimates $1/ν= 1.03(15)$, $η_ϕ= 0.42(7)$, and $η_ψ= 0.180(10)$ from averaging over the results of the different techniques, with the displayed uncertainty representing the degree of consistency among the three methods.

cond-mat.str-el↗