David W. Abraham

Implementing a strand of a scalable fault-tolerant quantum computing fabric

With favourable error thresholds and requiring only nearest-neighbour interactions on a lattice, the surface code is an error-correcting code that has garnered considerable attention. At the heart of this code is the ability to perform a low-weight parity measurement of local code qubits. Here we demonstrate high-fidelity parity detection of two code qubits via measurement of a third syndrome qubit. With high-fidelity gates, we generate entanglement distributed across three superconducting qubits in a lattice where each code qubit is coupled to two bus resonators. Via high-fidelity measurement of the syndrome qubit, we deterministically entangle the code qubits in either an even or odd parity Bell state, conditioned on the syndrome qubit state. Finally, to fully characterize this parity readout, we develop a measurement tomography protocol. The lattice presented naturally extends to larger networks of qubits, outlining a path towards fault-tolerant quantum computing.

Improved superconducting qubit coherence using titanium nitride

We demonstrate enhanced relaxation and dephasing times of transmon qubits, up to ∼60u2009μs, by fabricating the interdigitated shunting capacitors using titanium nitride (TiN). Compared to qubits made with lift-off aluminum deposited simultaneously with the Josephson junction, this represents as much as a six-fold improvement and provides evidence that surface losses from two-level system (TLS) defects residing at or near interfaces contribute to decoherence. Concurrently, we observe an anomalous temperature dependent frequency shift of TiN resonators, which is inconsistent with the predicted TLS model.

Two‐dimensional, remote micropositioner for a scanning tunneling microscope

A simple walker is described that is suitable as the coarse positioner in a scanning tunneling microscope. The walker is fabricated from a single piece of piezoelectric ceramic and has feet coated with thin metal and insulating films. The walker can move in either of two orthogonal, horizontal directions on an insulated, metallized quartz substrate in steps that can be varied from 25 to 400 nm.

Direct imaging of Au and Ag clusters by scanning tunneling microscopy

Clusters of Au and Ag deposited on the surface of highly oriented pyrolitic graphite have been imaged in air using a scanning tunneling microscope. An image is shown of a 350‐A silver cluster obtained in the constant‐current (topographic) mode. In the variable‐current mode, clusters of 6–20 atoms have been observed with atomic resolution. The motion and growth of clusters on the support are observed.

Bulk and surface loss in superconducting transmon qubits

Decoherence of superconducting transmon qubits is purported to be consistent with surface loss from two-level systems on the substrate surface. Here, we present a study of surface loss in transmon devices, explicitly designed to have varying sensitivities to different surface loss contributors. Our experiments also encompass two particular different sapphire substrates, which reveal the onset of a yet unknown additional loss mechanism outside of surface loss for one of the substrates. Tests across different wafers and devices demonstrate substantial variation, and we emphasize the importance of testing large numbers of devices for disentangling different sources of decoherence.