CONVENER

Robert Shekhter (University of Gothenburg, Sweden)

CO-CONVENER

Hee Chul Park (Pukyong National University, Korea)

Sunghun Park (IBS, Korea)

MEMBERS

Amnon Aharony (Tel Aviv University, Israel)

Sang-Jun Choi (Weurtzburg University, Germany)

Debashree Chowdhury (IIT Roorkee, India)

Ora Entin-Wohlman (Tel Aviv University, Israel)

Leonid Gorelik (Chalmers University of Technology, Sweden)

Jae-Ho Han (IBS, Korea)

Mats Jonson (University of Gothenburg, Sweden)

Chulki Kim (KIST, Korea)

Nojoon Myoung (Chosun University, Korea)

Anton Parafilo (IBS, Korea)

Danko Radić (University of Zagreb, Croatia)

Junho Suh (KRISS, Korea)

Chang-Hwan Yi (IBS, Korea)

OVERVIEW

The Coulomb blockade for tunneling of Cooper pairs from one superconductor to another through a small grain can be removed electrostatically. This leads to an entanglement of the two superconducting condensates and quantum states, with a quantum-fluctuating charge on the grain [known as a Cooper-pair-box (CPB) states], are formed. In this way, a bit of quantum information – a qubit – is implemented. Time-dependent mechanical displacements of the grain allow for spatial transportation of the Cooper-pair box, hence transferring quantum information in space (so-called “flying qubit states” [1]). Furthermore, the entanglement of the qubit states with the nanomechanical oscillations of the grain [2] permits the transduction of quantum information into nanomechanical cat-states [3], providing a means to control quantum NEM networks.

In its ground state, the BCS wave function of a homogeneous superconductor describes Cooper-paired electron states. These states are time-versed with respect to each other and are eigenstates of an operator that projects the electronic spin on a certain axis. The nature of this superconducting pairing can change significantly in spin-inhomogeneous materials such as structures with paramagnetic impurities, conductors affected by spatially inhomogeneous magnetic fields, etc. The spin-active superconducting weak link is another example of such a spin inhomogeneity. Recently we have shown [4] that spin-dependent electron tunneling through a voltage-biased micro-constriction between two bulk superconductors creates a static magnetization near the constriction and an ac Josephson-like spin current. Spin-dependent tunneling generates quantum fluctuations of the electronic spin, which results in the formation of a coherent superposition of spin-singlet and spin-triplet Cooper pairs.

Quantum fluctuations of the electric charge and electronic spin, carried by Cooper pairs in a spin-active superconducting weak link, form a promising basis for implementing spin- and charge quantum bits with possible new functionalities. The present application, being an extension of the 2023 ASG program, aims to explore this possibility. Both the dynamics of the electronic charge and the electronic spin, set by superconducting quantum ordering of electrons, and mesoscopic phenomena set by nanometer-size electric weak links (such as Coulomb blockade, strong spin-orbit coupling, and coupling to nanomechanical degrees of freedom), will be in the focus of the suggested research.