Quantum mechanics has been thought to be relevant mostly for microscopic systems, but the collective variables of larger objects, such as the currents and voltages of an electrical circuit sometimes also require to be described quantum-mechanically. In our team, we investigate the quantum properties of superconducting electrical circuits including Josephson junctions in order to study single-Cooper-pair electronics, develop electrical metrology, perform fundamental quantum physics experiments and, in the long-term, build a prototype quantum processor.
The elementary block of our experiments is a circuit that behaves as an artificial two-level system or in the language of quantum information a qubit. Our qubit design is a simple circuit made of a superconducting loop with 2 Josephson junctions, shunted by a capacitor. It is an evolution of the Quantronium qubit studied previously in the group, nicknamed the “transmon”.One interesting property of this circuit is that it can be strongly coupled to the electromagnetic field of a superconducting resonator, giving rise to effects similar to those studied in the field of Cavity Quantum Electrodynamics with real atoms coupled to single photons. The resonator can serve as an interface between the qubit and the external world, acting as a protecting filter during quantum state manipulation and as a high-fidelity detector when needed. It can also mediate inter-qubit couplings. Based on these ideas, we have designed a quantum processor architecture that could in principle be scalable to a small number of qubits, and we currently perform experiments in order to demonstrate its operation.