Superconducting circuits for storing quantum information
The integrated circuit components of classical computers are rapidly approaching the so-called "quantum limit", beyond which the current description of devices will not be valid. Instead of avoiding quantum mechanical effects, developers have the opportunity to exploit them when designing devices, as means for more effective computation. Quantum states could allow logic devices to have many different possible values simultaneously - a big advantage over conventional methods for representing information, confined to a single logical value at a given time. Within the SQUBIT project, a new type of superconducting qubit system based on manipulations of the Andreev bound state levels formed in an atomic-size quantum point contact (QPC) was proposed. Switching between the two persistent current states in the radio-frequency superconducting quantum interference device (rf-SQUID), in which the QPC was embedded, could be achieved by employing the Andreev levels' time evolution. Research work at the Chalmers University of Technology focused on theoretical aspects of the quantum dynamics of these superconducting qubits (SQUBIT), which differ from macroscopic flux qubits in several important respects. To derive an effective quantum Hamiltonian, which could describe coupled Andreev levels and intrinsic electromagnetic fluctuations, a path integral approach, commonly used in macroscopic quantum coherence (MQC) theory, was employed. The first essential steps in developing coupled Josephson tunnel-junction qubits with controllable macroscopic properties and long coherence time to test the quantum dynamics SQUBIT circuits have already been made. They represent probably the most realistic approach for a technology of scalable quantum information processors.
