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Bulletin of the American Physical Society

Qubit Feedback on a Five-Qubit Transmon Device

Amy Greene, Bharath Kannan, Morten Kjærgaard, Mollie E. Schwartz, Danna Rosenberg, Jonilyn Yoder, David K. Kim, T. W. Larsen, Philip Krantz, Simon Gustavsson, William D. Oliver

Published Mar 7, 2018• 0 citations
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The presence of non-equilibrium quasiparticles in superconducting resonators and qubits operating at millikelvin temperature has been known for decades. One metric for the number of quasiparticles affecting qubits is the rate of singleelectron change in charge on the qubit island (i.e. the charge parity rate). Here, we have utilized a Ramsey-like pulse sequence to monitor changes in the parity states of five transmon qubits. The five qubits have shunting capacitors with two different geometries and fabricated from both Al and Ta. The charge parity rate differs by a factor of two for the two transmon designs studied here but does not depend on the material of the shunting capacitor. The underlying mechanism of the source of parity switching is further investigated in one of the qubit devices by increasing the quasiparticle trapping rate using induced vortices in the electrodes of the device. The charge parity rate exhibited a weak dependence on the quasiparticle trapping rate, indicating that the main source of charge parity events is from the production of quasiparticles across the Josephson junction. To estimate this source of quasiparticle production, we simulate and estimate pair-breaking photon absorption rates for our two qubit geometries and find a similar factor of two in the absorption rate for a background blackbody radiation temperature of T ∗ ∼ 350 mK.
Superconducting transmon qubits are a leading technology for quantum information processing, yet their reliable operation rests on meticulous calibration and characterization routines. These processes have been fine-tuned and are relatively well understood by the quantum computing community. Nevertheless, it is often challenging for newcomers to compile all the available information into a practical experimental flow. In this tutorial, we present a comprehensive walkthrough for the characterization and optimization of tunable transmon qubits, demonstrated on a commercial five-qubit processor. Moving beyond theoretical description, we detail in a straightforward manner the complete workflow, from cryogenic setup and wiring to parametric amplifier optimum operation, flux sweet-spot identification, pulse calibration, and readout optimization. We also demonstrate the characterization of qubit-qubit coupling, covering all steps before multiqubit operations. This guide serves as a reference for experimentalists seeking to efficiently bring up transmon-based quantum devices.
Quantum systems based on superconducting circuits with qubits with quantized energy levels serve as an efficient physical background for constructing different quantum devices, for instance, quantum sensors. The important class of such engineering devices belongs to the systems with quantum tunneling effects, like Josephson junctions, where the set of energy levels is influenced by the external fields. These fields change the phase properties of the measuring qubits, and, in this way, they can be detected. The weakly anharmonic oscillator is often referred as a transmon qubit. Such qubits are constructed with superconducting capacitor structures connected by Josephson junctions, they can be controlled by microwave pulses of a few nanoseconds. The quantum sensing mechanism studied here is based on coupling the measuring transmon superconducting qubits with magnetic materials. The sensing protocol of Ramsey Fringes interferometry covers several stages and demands high-quality control over the sensor coherence. Although the weakly nonlinear properties of transmons prevent the sensing procedure from the strong influence of external perturbations, nevertheless, the extension of the coherence time interval and minimization of the dephasing effects are still a matter of great importance. Here we discuss the improvement of the quantum sensor performance by the application of feedback control in the form of gradient descent algorithm over the dephasing factor-function.