Scalable quantum processors
The serious limitations that current computers display in their speed of processing data have led researchers to search for faster alternatives. With the advent of quantum mechanics that offer almost unrestricted states, quantum computing technology has gained a universal popularity in the research world. Low capacitance superconducting tunnel junctions have shown increased potentialities for up-scaling processors and efficiently controlling qubits, as opposed to bits, of information. Urged by this, the SQUBIT project focused on the fabrication of quantum processors by exploiting Josephson junction, single-electron and superconducting quantum interference devices (SQUID) technologies. The latter are very sensitive magnetometers for measuring very small magnetic fields. The project developed superconducting qubits (squbits) and studied the control of the dynamics and de-coherence phenomena for achieving initiation, processing and read-out of squbit information. The Josephson junction circuitry is a non-linear dynamic system and quite sensitive to small disturbances, particularly at a bifurcation point. Exploitation of the sensitivity at this point offers differentiation of two quantum states if the phase space of the circuit is suitable for separating the final states. Through the analysis of the phase space of a Josephson junction circuit with frequency dependent damping, a switching current detector is optimised. Using a pulse and hold technique, an initial current pulse brings the junction closer to its bifurcation point and the subsequent hold provides sufficient time to distinguish between the two states. The generation of the pulse and hold waveform is performed with the aid of a new technique that provides a voltage step resulting in a linear voltage rise applied to a bias capacitor. A specific type of bias pulse-and-hold provided fast detection of switching, even under severe limitations of the bandwidth of junction voltage, and/or under low levels of switching current. For further information click at: http://fy.chalmers.se/~wendin/SQUBIT-2/(opens in new window)
