14 research outputs found
Universal gates for protected superconducting qubits using optimal control
We employ quantum optimal control theory to realize quantum gates for two
protected superconducting circuits: the heavy-fluxonium qubit and the 0-
qubit. Utilizing automatic differentiation facilitates the simultaneous
inclusion of multiple optimization targets, allowing one to obtain
high-fidelity gates with realistic pulse shapes. For both qubits, disjoint
support of low-lying wave functions prevents direct population transfer between
the computational-basis states. Instead, optimal control favors dynamics
involving higher-lying levels, effectively lifting the protection for a
fraction of the gate duration. For the 0- qubit, offset-charge dependence
of matrix elements among higher levels poses an additional challenge for gate
protocols. To mitigate this issue, we randomize the offset charge during the
optimization process, steering the system towards pulse shapes insensitive to
charge variations. Closed-system fidelities obtained are 99% or higher, and
show slight reductions in open-system simulations.Comment: 12 pages, 6 figure
Bulk crystal growth and electronic characterization of the 3D Dirac Semimetal Na3Bi
High quality hexagon plate-like Na3Bi crystals with large (001) plane
surfaces were grown from a molten Na flux. The freshly cleaved crystals were
analyzed by low temperature scanning tunneling microscopy (STM) and
angle-resolved photoemission spectroscopy (ARPES), allowing for the
characterization of the three-dimensional (3D) Dirac semimetal (TDS) behavior
and the observation of the topological surface states. Landau levels (LL) were
observed, and the energy-momentum relations exhibited a linear dispersion
relationship, characteristic of the 3D TDS nature of Na3Bi. In transport
measurements on Na3Bi crystals the linear magnetoresistance and Shubnikov-de
Haas (SdH) quantum oscillations are observed for the first time.Comment: To be published in a special issue of APL Material
Grain size in low loss superconducting Ta thin films on c-axis sapphire
In recent years, the implementation of thin-film Ta has led to improved
coherence times in superconducting circuits. Efforts to further optimize this
materials set have become a focus of the subfield of materials for
superconducting quantum computing. It has been previously hypothesized that
grain size could be correlated with device performance. In this work, we
perform a comparative grain size experiment with -Ta on -axis
sapphire. Our evaluation methods include both room-temperature chemical and
structural characterization and cryogenic microwave measurements, and we report
no statistical difference in device performance between small- and
larger-grain-size devices with grain sizes of 924 nm and 1700 nm,
respectively. These findings suggest that grain size is not correlated with
loss in the parameter regime of interest for Ta grown on c-axis sapphire,
narrowing the parameter space for optimization of this materials set
Imaging electronic states on topological semimetals using scanning tunneling microscopy
Following the intense studies on topological insulators, significant efforts
have recently been devoted to the search for gapless topological systems. These
materials not only broaden the topological classification of matter but also
provide a condensed matter realization of various relativistic particles and
phenomena previously discussed mainly in high energy physics. Weyl semimetals
host massless, chiral, low-energy excitations in the bulk electronic band
structure, whereas a symmetry protected pair of Weyl fermions gives rise to
massless Dirac fermions. We employed scanning tunneling microscopy/spectroscopy
to explore the behavior of electronic states both on the surface and in the
bulk of topological semimetal phases. By mapping the quasiparticle interference
and emerging Landau levels at high magnetic field in Dirac semimetals
CdAs and NaBi, we observed extended Dirac-like bulk electronic
bands. Quasiparticle interference imaged on Weyl semimetal TaAs demonstrated
the predicted momentum dependent delocalization of Fermi arc surface states in
the vicinity of the surface-projected Weyl nodes