Spin-singlet and spin-triplet superconducting correlations in Josephson junctions

Anatoliy Lotkov, Maria Spethmann, Daniel Loss

Hybrid devices integrating quantum dots with Josephson junctions are gaining interest because they combine spin-based quantum computing with circuit quantum electrodynamics (circuit QED) methods. In particular, Andreev spin qubits have shown significant experimental progress including strong two-qubit coupling, and are predicted to exhibit all-to-all connectivity. Here we propose superconducting singlet-triplet (SST) qubits that rely on parallel-aligned double quantum dots in Josephson junctions. While Andreev spin qubits require spin-orbit interaction to unlock the spin degree-of-freedom, SST qubits do not require spin-orbit interaction, making the advantages of hybrid devices available to a wider range of materials. Similar to Andreev spin qubits, the qubit states couple to the superconducting phase across the junction, which allows for control and readout using circuit QED, and supports all-to-all connectivity. Only $N$ flux lines are required to perform any single- and two-qubit gate among $N$ qubits, and thus the overhead of control lines is small. Finally, linear protection from charge or flux noise makes these qubits interesting candidates for a future quantum processor.

Luis Antonio Pérez, J. Samuel Millán, Chumín Wang

Andreas Moor, Anatoly Fjodorovich Volkov, K. B. Efetov

We show that the fully polarized triplet $s$-wave component is characterized not only by the spin direction, but also by chirality. Interaction of a polarized triplet component and a singlet one results in creation of triplet Cooper pairs with opposite spin direction or of different chiralities. Such spin transformation leads to interesting phenomena in multiterminal magnetic Josephson junctions. We calculate the dc Josephson current ${I}_{\text{J}}$ in a multiterminal Josephson contact of the ${S}_{\text{m}}/n/{S}_{\text{m}}^{\ensuremath{'}}$ type with ``magnetic'' superconductors ${S}_{\text{m}}$ that generate fully polarized triplet components. The superconductors ${S}_{\text{m}}$ are attached to magnetic insulators (filters) which let pass electrons with a fixed spin direction only. The filter axes are assumed to be oriented antiparallel to each other. The Josephson current is zero in two-terminal Josephson junction, i.e., in $S/n/{S}_{\text{m}}$ or in ${S}_{\text{m}}/n/{S}_{\text{m}}^{\ensuremath{'}}$ contact. But in the three-terminal Josephson junction, with another $S$ superconductor attached to the normal wire, the finite current ${I}_{\text{J}}$ appears flowing from the $S$ superconductor to ${S}_{\text{m}}$ superconductors. The currents through the right (left) superconductors ${S}_{\text{m}}$ are opposite in sign, ${I}_{\text{R}}\ensuremath{\equiv}{I}_{\text{J}}={I}_{\text{c}}sin({\ensuremath{\chi}}_{\text{R}}+{\ensuremath{\chi}}_{\text{L}}\ensuremath{-}2\ensuremath{\chi})=\ensuremath{-}{I}_{\text{L}}$, where ${\ensuremath{\chi}}_{\text{L/R}}$ and $\ensuremath{\chi}$ are the phases of superconductors ${S}_{\text{m}},{S}_{\text{m}}^{\ensuremath{'}}$, and $S$, respectively. We discuss possibilities of experimental observation of the effect.

Mohammad Alidoust, Klaus Halterman, Jacob Linder

Ashok Das, J. Gamboa, Fernando Méndez +1 more

Eric C. Gingrich

The last several decades have seen an explosion in the use and size of computers for scientific applications. The US Department of Energy has set an ExaScale computing goal for high performance computing that is projected to be unattainable by current CMOS computing designs [1]. This has led to a renewed interest in superconducting computing as a means of beating these projections. One of the primary requirements of this thrust is the development of an efficient cryogenic memory. Estimates of power consumption of early Rapid Single Flux Quantum (RSFQ) memory designs are on the order of MW, far too steep for any real application [1]. Therefore, other memory concepts are required. S/F/S Josephson Junctions, a class of device in which two superconductors (S) are separated by one or more ferromagnetic layers (F) has shown promise as a memory element. Several different systems have been proposed utilizing either the spin-singlet or spin-triplet superconducting states [2]. This talk will discuss the concepts underpinning these devices, and the recent work done to demonstrate their feasibility. [1] Energy-Efficient Superconducting Computing Power Budgets and Requirements, D. Scott Holmes, Andrew L. Ripple, Marc A. Manheimer, IEEE Trans. Appl. Supercon., Vol 23, No. 3, June 2013 [2] B. M. Niedzielski et al., “Use of Pd-Fe and Ni-Fe-Nb as soft magnetic layers in ferromagnetic Josephson junctions for nonvolatile cryogenic memory,” IEEE Trans. Appl. Supercond., vol. 24, no. 4, Aug. 2014, Art. ID. 1800307.