Spin interference in quantum rings manipulated with quantum point contacts

Carlo daCunha, Nobuyuki Aoki, David K. Ferry

Yusuke Nishida, Yoshiaki Takahashi, Yoshiro Takahashi

J. Lantz

In this thesis we present theoretical investigations of the effects of Andreev bound states on the current transport in superconducting interferometers. We also investigate the slow dynamics of the Andreev states in a superconducting point contact, and the possible application as a quantum bit. We consider superconductor-normal metal-superconductor (SNS) and normal metalsuperconductor (NS) interferometers, where the contact region is a Y-shaped normal metal wave guide, and the two connection points to the same superconducting electrode can have different phases. The electric current in the interferometer is calculated as a function of the applied voltage and the phase difference . Andreev reflection in SNS and NS interferometers incorporates two features: interference in the arms of the Y-shaped normal region, and interplay with Andreev resonances. The latter feature yields rich phase dependent current structures in the subgap voltage region. The interference effect leads to a suppression of the current structures at . We investigate the effects on the Josephson current in NS interferometers due to current injection from the normal electrode. The two main effects of the nonequilibrium situation are: nonequilibrium population of the Andreev levels, which can result in enhancement, suppression, or even sign reversal of the Josephson current, and an anomalous interference Josephson effect, which gives rise to a long range Josephson effect, increasing with the voltage up to the superconducting gap . The two Andreev states in a superconducting quantum point contact can be accessed for manipulation and measurement by embedding the point contact in a superconducting loop. We calculate an effective Hamiltonian for the slow dynamics of the Andreev two-level system in the ring. Furthermore, we discuss methods of manipulation of the Andreev levels, and coupling of qubits. The state of the Andreev two-level system can be read out by monitoring the macroscopic quantum tunneling in a current biased Josephson junction, which is embedded in the superconducting ring of the qubit. We discuss the effects on the qubit, the readout scheme and the signal-to-noise ratio. THESIS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Coherent processes in Superconducting quantum interferometers and qubits

Thomas Ihn, Andreas Fuhrer, K. Ensslin +2 more

Audrey Cottet

The manipulation of the electronic spin degree of freedom is an important goal in the context of the development of quantum information and spin electronics. Coherent conductors connected to ferromagnetic contacts are particularly interesting in this context. Indeed, they are subject to spin-dependent quantum interferences, which can be used to manipulate the spin degree of freedom. In this talk, I will focus on effective fields induced inside a coherent conductor by a ferromagnetic contact. I will first discuss the nature of these fields. Second, I will show that these effective fields can be used to design a spin quantum bit adapted for Circuit Quantum Electrodynamics experiments[1]. This setup is based on a double quantum dot connected to two non-collinear ferromagnetic contacts. This allows to obtain a strong spin/photon coupling, which can be switched on and off with a simple gate voltage. Third, I will show that the contact-induced effective fields can also be used to induce unconventional superconducting correlations inside a normal metal wire[2]. [1] A. Cottet and T. Kontos, Phys. Rev. Lett. 105, 160502 (2010) [2] A. Cottet, unpublished

Rosario Capozza, Domenico Giuliano, Procolo Lucignano +1 more

We calculate the oscillations of the dc conductance across a mesoscopic ring, simultaneously tuned by applied magnetic and electric fields orthogonal to the ring. The oscillations depend on the Aharonov-Bohm flux and of the spin-orbit coupling. They result from mixing of the dynamical phase, including the Zeeman spin splitting, and of geometric phases. By changing the applied fields, the geometric phase contribution to the conductance oscillations can be tuned from the adiabatic (Berry) to the nonadiabatic (Ahronov-Anandan) regime. To model a realistic device, we also include nonzero backscattering at the connection between ring and contacts, and a random phase for electron wave function, accounting for dephasing effects.