Learning-based Calibration of Flux Crosstalk in Transmon Qubit Arrays

Superconducting quantum processors comprising flux-tunable data and coupler qubits are a promising platform for quantum computation. However, magnetic flux crosstalk between the flux-control lines and the constituent qubits impedes precision control of qubit frequencies, presenting a challenge to scaling this platform. In order to implement high-fidelity digital and analog quantum operations, one must characterize the flux crosstalk and compensate for it. In this work, we introduce a learning-based calibration protocol and demonstrate its experimental performance by calibrating an array of 16 flux-tunable transmon qubits. To demonstrate the extensibility of our protocol, we simulate the crosstalk matrix learning procedure for larger arrays of transmon qubits. We observe an empirically linear scaling with system size, while maintaining a median qubit frequency error below $300$ kHz

Quantum transport and localization in 1d and 2d tight-binding lattices

AbstractParticle transport and localization phenomena in condensed-matter systems can be modeled using a tight-binding lattice Hamiltonian. The ideal experimental emulation of such a model utilizes simultaneous, high-fidelity control and readout of each lattice site in a highly coherent quantum system. Here, we experimentally study quantum transport in one-dimensional and two-dimensional tight-binding lattices, emulated by a fully controllable 3 × 3 array of superconducting qubits. We probe the propagation of entanglement throughout the lattice and extract the degree of localization in the Anderson and Wannier-Stark regimes in the presence of site-tunable disorder strengths and gradients. Our results are in quantitative agreement with numerical simulations and match theoretical predictions based on the tight-binding model. The demonstrated level of experimental control and accuracy in extracting the system observables of interest will enable the exploration of larger, interacting lattices where numerical simulations become intractable.</jats:p

Composition-dependent structural transition in epitaxial Bi_{1−x}Sb_{x} thin films on Si(111)

Bismuth-Antimony alloys (Bi1-xSbx) are topological insulators between 7-22% Sb in bulk crystals, with an unusually high conductivity suitable for spin-orbit torque applications. Reducing the thickness of epitaxial Bi1-xSbx films is expected to increase the maximum bandgap through quantum confinement, which may improve isolation of topological surface state transport. Like Bi(001) on Si(111), Bi1-xSbx has been predicted to form a black phosphorus-like allotrope with unique electronic properties in nanoscale films; however, the impact of Sb alloying on both the bulk-like and nanoscale crystal structures on Si(111) is currently unknown. Here we demonstrate that the allotropic transition in ultrathin epitaxial Bi1-xSbx films on Si(111) is suppressed above 8-9% Sb, resulting in an unexpected (012) orientation within the topologically insulating regime. The metallic temperature-dependent conductivity associated with surface states in Bi(001) was not observed in the Bi1-xSbx(012) films, suggesting that the (012) orientation may significantly reduce surface state transport. Growth on a Bi(001) buffer layer may prevent this orientation transition. Finally, we demonstrate that Sb alloying improves the continuity and quality of nanoscale Bi1-xSbx(012) films in the thickness regime expected for the black phosphorus allotrope, suggesting a promising route to large-area growth of puckered-layer 2-D Bi1-xSbx, which will be necessary to harness its unique electronic properties in practical applications.This work was primarily supported by both the Texas Instruments Semiconductor Research Corporation Graduate Fellowship Program and the National Science Foundation through the Center for Dynamics and Control of Materials; an NSF MRSEC under Cooperative Agreement No. DMR-1720595.Center for Dynamics and Control of Material

Hexagonal boron nitride as a low-loss dielectric for superconducting quantum circuits and qubits

Dielectrics with low loss at microwave frequencies are imperative for high-coherence solid-state quantum computing platforms. We study the dielectric loss of hexagonal boron nitride (hBN) thin films in the microwave regime by measuring the quality factor of parallel-plate capacitors (PPCs) made of NbSe$_{2}$-hBN-NbSe$_{2}$ heterostructures integrated into superconducting circuits. The extracted microwave loss tangent of hBN is bounded to be at most in the mid-10$^{-6}$ range in the low temperature, single-photon regime. We integrate hBN PPCs with aluminum Josephson junctions to realize transmon qubits with coherence times reaching 25 $\mu$s, consistent with the hBN loss tangent inferred from resonator measurements. The hBN PPC reduces the qubit feature size by approximately two-orders of magnitude compared to conventional all-aluminum coplanar transmons. Our results establish hBN as a promising dielectric for building high-coherence quantum circuits with substantially reduced footprint and, with a high energy participation that helps to reduce unwanted qubit cross-talk