Current Challenges for Quantum Computing

We have known for over twenty years that quantum computers would have unique powers for solving certain classes of computational problems.  Throughout these twenty years, workers have striven to identify a physical setting in which high-quality qubits can be created and employed in a quantum computing system.  Very promising devices have been identified in several different areas of low-temperature electronics, namely in superconductor and in single-electron semiconductor structures (e.g., quantum dots).  Rudimentary efforts at scale-up are presently underway; even for modules of 10 qubits, the complexity of the classical electronic control system becomes one of the main barriers to further progress. The specifications of this control system are now well defined, and are daunting. In this talk I will touch on two aspects of this control problem.  First, I indicate the problems with unintended couplings between qubits in multi-qubit structures.  For superconducting qubit systems, I show our current methodology for accurately characterizing these couplings.  Second, I suggest solutions to the problem of miniaturizing the microwave circulator, using the quantum Hall effect; current circulators take up so much space in existing experiments that they limit the physical scale-up of the systems.