Squid Qubits for Quantum Computers

Our aim was to develop the concepts and perform the experiments to establish a clear path towards useful quantum coherent computation using superconducting flux-based qubits. We have concentrated upon using superconducting single flux quantum SFQ circuitry integrated on the qubit chip to act as the interface between a quantum circuit and laboratory electronics. We have developed SFQ circuits for initialization and for read-out of the quantum state of a qubit for precise timing of duration and exact biasing all with picosecond-scale precision. We developed a set of techniques to design such qubit control circuitry while preserving proper isolation not significantly contributing to decoherence. We also designed an SFQ experiment to investigate quantum behavior using foundry-fabricated chips at higher temperatures. We acquired and custom set-up a He3 refrigerator, and demonstrated SFQ circuit functionality at 300 mK for the first time. We have investigated a new method for quantum control both analytically and numerically, using isolated SFQ pulses instead of resonant excitation, and shown that the minimum operation time can be dramatically decreased ii some cases. However, the complete lack of custom fabrication has prevented us from making progress towards our experimental goals.