Long-lived quantum states enable quantum computing
Both optical and solid-state entanglement offer potential routes to quantum computing and secure communications. Sharing the very same physics and concepts with cavity quantum electrodynamics, in the solid-state architecture called circuit quantum electrodynamics, artificial atoms are made of Josephson junctions that are coupled to on-chip superconducting resonators. Resonant devices provide a controlled electromagnetic environment protecting qubits from energy relaxation. Despite the fact that circuit quantum electrodynamics has made spectacular progress over the years, preserving coherence in superconducting qubits is often challenging because these systems have strong coupling to electromagnetic fields. Circuits need to be non-dissipative – for example, the metallic parts involved should have zero resistance – so that signals can be carried from one part of the circuit to another without energy loss and hence decoherence. Within the EU-funded project CQ3D (3D circuit quantum electrodynamics with flux qubits), scientists developed new ways to control, couple and measure superconducting qubits in a near-perfect electromagnetic environment with minimal additional circuitry so as to prevent decoherence. Different experimental systems were conducted to study the processing of superconducting qubits and resonators. First, the team set up an experiment where transmon qubits made of aluminium were coupled to a 3D resonator. The aim was to further elucidate the dominant energy relaxation mechanisms, including quasiparticle tunnelling effects. For spin-based physical implementation of qubits, scientists exploited a special kind of defect in diamonds, nitrogen-vacancy centres, coupling their electron spins to resonators. By incorporating features from machined 3D cavities such as deeply etched substrates and large capacitive gaps in planar (2D) resonators, the performance gap of the two classes of devices was narrowed. Ultimately, the team realised flux qubits and characterised decoherence of these superconducting bits coupled to the resonator. Maintaining long coherence of superconducting bits is necessary for practical computation, otherwise all quantum magic disappears. The strong coupling of a superconducting qubit to a 3D resonator opens new possibilities for quantum computing, enabling two orders of magnitude longer coherence times. Project results were disseminated in four publications.
Keywords
Quantum computing, superconducting qubits, coherence times, CQ3D, flux qubits
