Superconducting Qubits: Charge and Transmon Qubits

David Schuster

Diego Ristè, Cornelis Christiaan Bultink, M. J. Tiggelman +3 more

The tunnelling of quasiparticles across Josephson junctions in superconducting quantum circuits is an intrinsic decoherence mechanism for qubit degrees of freedom. Understanding the limits imposed by quasiparticle tunnelling on qubit relaxation and dephasing is of theoretical and experimental interest, particularly as improved understanding of extrinsic mechanisms has allowed crossing the 100 microsecond mark in transmon-type charge qubits. Here, by integrating recent developments in high-fidelity qubit readout and feedback control in circuit quantum electrodynamics, we transform a state-of-the-art transmon into its own real-time charge-parity detector. We directly measure the tunnelling of quasiparticles across the single junction and isolate the contribution of this tunnelling to qubit relaxation and dephasing, without reliance on theory. The millisecond timescales measured demonstrate that quasiparticle tunnelling does not presently bottleneck transmon qubit coherence, leaving room for yet another order of magnitude increase. Superconducting circuits are promising for quantum computing, but quasiparticle tunnelling across Josephson junctions introduces qubit decoherence. Ristè et al. convert a transmon qubit into its own real-time quasiparticle tunnelling detector and accurately measure induced decoherence in the millisecond range.

Wills, James Fox

Superconducting circuits are well established as a viable candidate for the realisation of quantum computers. Circuits based on the transmon qubit are now ubiquitous, owing to its simple design and reduced control wiring overhead. An issue with transmon qubit-based architectures is the always-on unwanted interactions that impose limits on gate speeds and introduce errors into their operation. In addition, understanding sources of noise and decoherence is essential to the low-error operation quantum computers. This thesis describes the implementation of a multi-mode superconducting qubit in a coaxial circuit QED architecture. Constructed from three superconducting islands, connected via two Josephson junctions, the device possesses two transmon-like modes with orthogonal field symmetries. The unique polarisation of each mode allows for engineering dissipation and coupling in the system, extending functionality beyond the single-mode transmon. Experimental results on the unit-cell of the two-mode coaxial transmon are presented, demonstrating coherent control and simultaneous dispersive readout of the modes of the device. A predictive theory of charge sensitivity in a multi-mode superconducting qubit is presented, and experimental results in agreement of this theory are shown, observing sensitivity to four charge-parity configurations and two independent gate-charge offsets. The utility of a multi-mode qubit as a charge detector in spatially tracking local-charge drift of ≃ 100 µm length scales is also shown, demonstrating the use of these devices as tools in understanding charge noise in superconducting circuits. Finally, a system of a pair of coupled two-mode coaxial transmons is introduced, demonstrating a highly mode-selective coupling architecture. A suppressed quantum crosstalk of 2 kHz between protected modes of the devices is measured, along with equal single qubit gate fidelities when operated both individually and simultaneously. A first characterisation of a microwave activated conditional phase interaction between computational modes driven via ancillary transitions (AT-MAP) is presented. Whilst not shown in this work, this state-dependent two-qubit interaction can be used to generate entanglement. Combined with the low crosstalk demonstrated, this shows the multi-mode qubit architecture is a promising candidate for the construction of larger scale quantum processors with fast gates and low crosstalk-related errors.

Gianluigi Catelani

Eran Ginossar, Eytan Grosfeld

is coupled to the zero energy parity states that arise from Majorana quasi-particles. We investigate the level structure and properties of the transmon qubit in this regime where even a small coupling causes hybridization of dierent transmon-parity states without compromising the suppression of charge dispersion. We show that the microwave photon-qubit coupling is sensitive to the gate bias and all the energy scales of the Hamiltonian. As well as a probe for topologicalsuperconductor excitations, we propose that this type of device could be used to realise a high coherence tunable four-level system in the superconducting circuits architecture.

Eddy Collin, Grégoire Ithier, P. Joyez +2 more