Effects of the integration of digital and quantum coherent superconducting electronics

The growing interest in the power of a fully functional quantum computer has motivated the scientific community to investigate a diverse array of quantum phenomenon in search of a candidate with which to construct such a machine. The success of integrated circuit technology for digital computers provides encouragement that such a scheme may prove successful for the scaling of a quantum computer. The radio frequency superconducting quantum interference device (rf-SQUID) has been predicted to display two-state quantum mechanical behavior, and can be easily integrated on a superconducting IC for use as a quantum bit. However, a solid state environment, such as an IC, contains many sources of noise which, when coupled to the quantum bit, can destroy the rf-SQUID's coherent quantum state.; The ultimate goal of this work is to observe coherent oscillations of flux in a rf-SQUID by using superconducting, high-speed, classical circuitry called rapid single flux quantum (RSFQ) logic to manipulate and measure the state of the rf-SQUID. In order to observe macroscopic quantum coherence in the rf-SQUID it must first be established that coherence will last sufficiently long to make a measurement. To this end simulations have been performed modelling the quantum evolution of a rf-SQUID in an environment with active integrated circuitry. It has been determined from simulation that the variance of Gaussian flux noise coupled to the qubit must be maintained below 10μΦ₀ in order to achieve a qubit coherence time longer than 30ns. It was also found that reducing the bandwidth of the white noise below the splitting frequency of the two-state rf-SQUID qubit dramatically increases the coherence time of the device.; In support of the simulations experiments were conducted to measure the ambient magnetic flux noise created by active RSFQ circuitry. The experiments determined that, with typical superconducting integrated circuit fabrication technology, a quantum coherent rf-SQUID qubit must be isolated from classical circuitry at a distance of 100–300μm in order for the flux noise coupled to the qubit to be maintained below the threshold determined by the simulations.