Building chips for quantum computing
During the last decade it has been experimentally proven that superconducting circuits can serve as quantum mechanical two-level systems, qubits, for quantum information processing. Besides experiments with individual qubits, entangled macroscopic quantum states have been created in two Josephson-junction qubits permanently coupled by a capacitor. The individual junction bias currents were used to control the interaction between the qubits by tuning their energy level spacing in and out of resonance. Nonetheless, in order to build a functional, scalable quantum computer, a network design is needed that would allow coupling of an arbitrary large number of qubits. In principle, coupling of only the nearest-neighbour qubits is sufficient to build a computationally universal set of gates. Urged by this need, the SQUBIT project partners investigated the design and functionality of a network of charge qubits - Single Cooper Pair Transistors (SCPT) - with loop-shaped electrodes. The variable inductive qubit coupling was achieved by letting the charge-qubit loops intersect at a non-linear oscillator, share the coupling Josephson tunnelling junction and by selectively applying bias currents. Such a current-biased coupling scheme does not need local magnetic fields to control the coupling, fields which would produce unwanted parasitic long-range interactions. Another important feature is the possibility of operating at the charge degeneracy point of each qubit, where the quantum decoherence effect is minimised and gate operations are very simple. Neighbouring qubits in an arbitrarily long chain were coupled, and several independent two-qubit gates were performed simultaneously by means of this current-biased coupling mechanism. Most importantly, entangling two-qubit gates are control-phase gates, which together with single-qubit gates can provide a universal set of operations, offering new opportunities for the experimental implementation of elementary quantum information processing.
