The ultrastrong photon-to-magnon coupling is reached with an on-chip hybrid system using superconducting phenomena.

In this work, ultrastrong photon-to-magnon coupling is demonstrated for on-chip multilayered superconductor-ferromagnet-insulator hybrid thin-film structures reaching a coupling strength above 6 GHz, coupling ratio about 0.6, single-spin coupling strength about 350 Hz, and cooperativity about ${10}^4$. Strong characteristics of coupling are achieved owing to a radical suppression of the photon phase velocity in an electromagnetic resonator. With achieved coupling, the spectrum reveals the inapplicability of the Dicke quantum model, and evidences a contribution of the diamagnetic $A^2$ interaction term in the Hamiltonian of the system, which satisfies the Thomas-Reiche-Kuhn sum rule. The contribution of the $A^2$ term denotes the validity of the Hopfield quantum model and manifests observation of a different hybrid polariton quasiparticle, namely the plasmon-magnon polariton.

Josephson vortices play an essential role in superconducting quantum electronics devices. Often seen as purely conceptual topological objects, 2π-phase singularities, their observation and manipulation are challenging. Here we show that in Superconductor—Normal metal—Superconductor lateral junctions Josephson vortices have a peculiar magnetic fingerprint that we reveal in Magnetic Force Microscopy (MFM) experiments. Based on this discovery, we demonstrate the possibility of the Josephson vortex generation and manipulation by the magnetic tip of a MFM, thus paving a way for the remote inspection and control of individual nano-components of superconducting quantum circuits.

In this work, we briefly overview various options for Josephson junctions, which should be scalable down to nanometer range for utilization in nanoscale digital superconducting technology. Such junctions should possess high values of critical current, $I_c$, and normal state resistance, $R_N$. Another requirement is the high reproducibility of the junction parameters across a wafer in a fabrication process. We argue that superconductor-normal metal-superconductor ($SN$-$N$-$NS$) Josephson junction of “variable thickness bridge” geometry is a promising choice to meet these requirements. Theoretical analysis of the $SN$-$N$-$NS$ junction is performed in the case where the distance between the $S$ electrodes is comparable to the coherence length of the $N$ material. The restriction on the junction geometrical parameters providing the existence of superconductivity in the $S$ electrodes is derived for the current flowing through the junction of an order of $I_c$. The junction heating, as well as available mechanisms for the heat removal, is analyzed. The obtained results show that a $SN$-$N$-$NS$ junction with a high (submillivolt) value of $I_c{R}_N$ product can be fabricated from a broadly utilized combination of materials like Nb/Cu using well-established technological processes. The junction area can be scaled down to that of semiconductor transistors fabricated in the frame of a 40-nm process.

In Josephson junctions, a supercurrent across a nonsuperconducting weak link is carried by electron-hole bound states. Because of the helical spin texture of nondegenerate topological surface states, gapless bound states are established in junctions with topological weak link. These have a characteristic 4π-periodic current phase relation (CΦR) that leads to twice the conventional Shapiro step separation voltage in radio frequency–dependent measurements. In this context, we identify an attenuated first Shapiro step in (Bi_0.06Sb_0.94)₂Te₃ (BST) Josephson junctions with AlO_x capping layer. We further investigate junctions on narrow, selectively deposited BST nanoribbons, where surface charges are confined to the perimeter of the nanoribbon. Within these junctions, previously identified signatures of gapless bound states are absent. Because of confinement, transverse momentum sub-bands are quantized and a topological gap opening is observed. Surface states within these quantized sub-bands are spin degenerate, which evokes bound states of conventional 2π-periodic CΦR within the BST nanoribbon weak link.