Fig 1: Demonstration of quantum speedup with an electrical quantum processor running the Grover algorithm on 2 qubits.
Although quantum mechanics is usually considered as relevant only for microscopic systems, larger objects such as cleverly designed electrical circuits can also behave quantum-mechanically as a whole. 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 (see Fig. 2 below) analogue to quantum optics or atomic physics, and in the long-term, build a prototype quantum processor (see Fig. 1 above).
Fig. 2: Violation of the Legett-Garg inequality by continuous drive and dispersive measurement of a transmon qubit. The orange curve proves taht the our world is non macrorealistic, which means that observing an object does change its future state.
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. This qubit is a cooper pair box (CPB), i.e. a simple superconducting loop with 2 Josephson junctions, shunted by a capacitor coupled to an electric field. The last qubit version used by our group is the transmon developped at Yale university. 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 (see Fig. 3). 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.
Fig. 3: High fidelity single-shot readout of a transmon qubit. Readout resonator (top left) made non linear with a Josephson junction (bottom left), and frequency tunable transmon (bottom-middle). The graph shows single-shot measured Rabi oscillations with 93% contrast.