Superconductors, Quantum Dots, and Spin Entanglement

Mahn‐Soo Choi, Christoph Bruder, Daniel Loss

Jami Rönkkö

Entanglement is a valuable resource for quantum computing and information technologies. A promising way to obtain spin-entangled electrons is the splitting of Cooper pairs residing in superconductors. This is realized by a nanodevice called Cooper pair splitter. A Cooper pair splitter is realized by tunnel-coupling a superconductor to two quantum dots, which are each further tunnelcoupled to separate terminal leads. This setup enables the extraction and splitting of Cooper pairs from the superconductor into the terminals via the quantum dots. The quantum dots are commonly fabricated from semiconducting materials like InAs and InSb, that can manifest spin-orbit interaction. This thesis studies the effect of spin-orbit interaction on the spin state of the electron pair in the Cooper pair splitter. Activating the spin-orbit effects in a Cooper pair splitter requires a static magnetic field to be applied to the quantum dots. Together with the gate electrodes controlling the electrostatic energy of the quantum dots, the external magnetic field provides a handle for addressing different two-particle spin states. The spin-orbit interaction can be activated by various combinations of the gate voltage and Zeeman magnetic field, that create resonances between different states of the system. The most interesting coherent evolution, that can be invoked in this way, involves the simultaneous activation of the two spin-polarized triplet states. This gives a rise to an entangled Bell state, (|↑↑>+|↓↓>) / √2, that moreover exhibits spin blockade and thus stabilizes entanglement in the Cooper pair splitter. Secondly, the spin-orbit interaction can be used to bypass suppressed Cooper pair tunneling at a high superconductor-quantum dots detuning. Introducing a large Zeeman magnetic field allows Cooper pairs to tunnel in to the dots given that their spin state flips to the triplet |↓↓>. Although this regime lacks spin-entanglement, it is of interest for creating triplet Cooper pairs and spin-polarized superconducting currents.

Olivier Sauret, D. Feinberg, Thierry Martin

The operation of a source of entangled electron spins, based on a superconductor and two quantum dots in parallel, is described in detail with the help of quantum master equations. These are derived including the main parasitic processes in a fully consistent and nonperturbative way, starting from a microscopic Hamiltonian. The average current is calculated, including the contribution of entangled and nonentangled pairs. The constraints on the operation of the device are illustrated by a calculation of the various charge state probabilities.

Gordey B. Lesovik, Thierry Martin, G. Blatter

Gary J. Mooney, Gregory A. L. White, Charles D. Hill +1 more

Robert Hussein, Alessandro Braggio, Michele Governale

Abstract The control of nonlocal entanglement in solid state systems is a crucial ingredient of quantum technologies. We investigate a Cooper‐pair splitter based on a double quantum dot realized in a semiconducting nanowire. In the presence of interdot tunneling the system provides a simple mechanism to develop spatial entanglement even in absence of nonlocal coupling with the superconducting lead. We discuss the possibility to control the symmetry (singlet or triplet) of spatially separated, entangled electron pairs taking advantage of the spin‐orbit coupling of the nanowire. We also demonstrate that the spin‐orbit coupling does not impact over the entanglement purity of the nonlocal state generated in the double quantum dot system. Cooper‐pair splitter realized in a nanowire acting as double quantum dot which is coupled to one superconductor (S) and two normal leads (N). Electron singlets in the superconductor are split nonlocally into the different dots as schematically shown in the figure. The entangled electrons in the two quantum dots are then transferred by tunneling, with rate , to the normal leads.