Yen-Hsiang Lin

Protecting a superconducting qubit from energy decay by selection rule engineering

Quantum control of atomic systems is largely enabled by the rich structure of selection rules in the spectra of most real atoms. Their macroscopic superconducting counterparts have been lacking this feature, being limited to a single transition type with a large dipole. Here we report a superconducting artificial atom with tunable transition dipoles, designed such that its forbidden (qubit) transition can dispersively interact with microwave photons due to the virtual excitations of allowed transitions. Owing to this effect, we have demonstrated an in-situ tuning of qubit’s energy decay lifetime by over two orders of magnitude, exceeding a value of 2 ms, while keeping the transition frequency fixed around 3.5 GHzLong-lived transitions occur naturally in atomic systems due to the abundance of selection rules inhibiting spontaneous emission. By contrast, transitions of superconducting artificial atoms typically have large dipoles, and hence their lifetimes are determined by the dissipative environment of a macroscopic electrical circuit. We designed a multilevel fluxonium artificial atom such that the qubits transition dipole can be exponentially suppressed by flux tuning, while it continues to dispersively interact with a cavity mode by virtual transitions to the noncomputational states. Remarkably, energy decay time T_{1} grew by 2 orders of magnitude, proportionally to the inverse square of the transition dipole, and exceeded the benchmark value of T_{1}>2  ms (quality factor Q_{1}>4×10^{7}) without showing signs of saturation. The dephasing time was limited by the first-order coupling to flux noise to about 4  μs. Our circuit validated the general principle of hardware-level protection against bit-flip errors and can be upgraded to the 0-π circuit [P. Brooks, A. Kitaev, and J. Preskill, Phys. Rev. A 87, 052306 (2013)PLRAAN1050-294710.1103/PhysRevA.87.052306], adding protection against dephasing and certain gate errors.

Influence of GaAs surface termination on GaSb/GaAs quantum dot structure and band offsets

We have investigated the influence of GaAs surface termination on the nanoscale structure and band offsets of GaSb/GaAs quantum dots (QDs) grown by molecular-beam epitaxy. Transmission electron microscopy reveals both coherent and semi-coherent clusters, as well as misfit dislocations, independent of surface termination. Cross-sectional scanning tunneling microscopy and spectroscopy reveal clustered GaSb QDs with type I band offsets at the GaSb/GaAs interfaces. We discuss the relative influences of strain and QD clustering on the band offsets at GaSb/GaAs interfaces.

Quantifying the local Seebeck coefficient with scanning thermoelectric microscopy

We quantify the local Seebeck coefficient with scanning thermoelectric microscopy, using a direct approach to convert temperature gradient-induced voltages (V) to Seebeck coefficients (S). We use a quasi-3D conversion matrix that considers both the sample geometry and the temperature profile. For a GaAs p-n junction, the resulting S-profile is consistent with that computed using the free carrier concentration profile. This combined computational-experimental approach is expected to enable nanoscale measurements of S across a wide variety of heterostructure interfaces.

Influence of Sb incorporation on InGaAs(Sb)N/GaAs band alignment

We have investigated the influence of Sb incorporation on the effective band gaps and band offsets at InGaAs(Sb)N/GaAs interfaces grown by metalorganic vapor phase epitaxy. Cross-sectional scanning tunneling microscopy and spectroscopy reveal 1.2 eV (1.1 eV) effective band gaps of InGaAs(Sb)N alloys. At the InGaAsN/GaAs (InGaAsSbN/GaAs) interfaces, type II (type I) band offsets are observed. We discuss the relative influences of strain-induced splitting of the valence band and the incorporation of Sb on the band gaps and band offsets at InGaAsN/GaAs and InGaAsSbN/GaAs interfaces.

Ordered horizontal Sb{sub 2}Te{sub 3} nanowires induced by femtosecond lasers

Nanowires are of intense interest on account of their ability to confine electronic and phononic excitations in narrow channels, leading to unique vibronic and optoelectronic properties. Most systems reported to date exhibit nanowire axes perpendicular to the substrate surface, while for many applications (e.g., photodetectors and sensors), a parallel orientation may be advantageous. Here, we report the formation of in-plane Sb 2Te3 nanowires using femtosecond laser irradiation. High-resolution scanning transmission electron microscopy imaging and element mapping reveal that an interesting laser-driven anion exchange mechanism is responsible for the nanowire formation. This development points the way to the scalable production of a distinct class of nanowire materials with in-plane geometry.

Ordered horizontal Sb2Te3 nanowires induced by femtosecond lasers

Nanowires are of intense interest on account of their ability to confine electronic and phononic excitations in narrow channels, leading to unique vibronic and optoelectronic properties. Most systems reported to date exhibit nanowire axes perpendicular to the substrate surface, while for many applications (e.g., photodetectors and sensors), a parallel orientation may be advantageous. Here, we report the formation of in-plane Sb 2Te3 nanowires using femtosecond laser irradiation. High-resolution scanning transmission electron microscopy imaging and element mapping reveal that an interesting laser-driven anion exchange mechanism is responsible for the nanowire formation. This development points the way to the scalable production of a distinct class of nanowire materials with in-plane geometry.