A Qubit Based on SINIS Josephson Tunnel Junction
Forming controlled super positions of quantum states lies at the heart of quantum computing and devices that perform this function have been termed "qubits". The device chosen here to realize a qubit is based on a double-barrier SINIS Josephson junction (where S is a superconductor, I is an insulator, and N is a normal metal). The basic strategy involves the controlled manipulation of the occupations in a 2-level system comprised of the two lowest (the ground and the first excited) Andreev bound states (ABS) that can form within an SINIS junction. In fact the parameters of the SINIS-based qubit can chosen such that only the above two ABS levels are present; this conclusion, which independently follows from the theory, is in agreement with preliminary experimental data taken on Nb/AlOxAl/AlOx/Nb double-barrier junctions.
The required control of the two Andreev states is achieved by applying appropriate bias voltages and transport currents to a device fabricated in a three-terminal geometry. The control parameters turn out to be the voltage across one of the barriers, and the transport current across one of the super conducting layers; these quantities play the role of the dynamic magnetic fields, which enter the associated qubit Hamiltonian.
The primary goal of this proposal is to achieve the controlled manipulation of the Andreev bound states and, in parallel, advance the understanding of the underlying physical mechanisms. This will include: i) extending the present technology for preparing two-terminal SINIS devices to 3-terminal devices; ii) performing measurements on the ABS characteristics, including the observation of Rabi oscillations between the two ABS levels, iii) further developing the theory of localized ABS inside double barrier junctions to allow for arbitrary barrier transparency and arbitrary impurity concentrations in all three electrodes, iv) performing detailed numerical simulations of the switching dynamics; v) studying the recombination and decoherence effects that relate to the performance of these devices, and lastly vi) examining strategies for connecting qubits to form simple logic circuits.
