We realize a Λ system in a superconducting circuit, with metastable states exhibiting lifetimes up to 8 ms. We exponentially suppress the tunneling matrix elements involved in spontaneous energy relaxation by creating a "heavy" fluxonium, realized by adding a capacitive shunt to the original circuit design. The device allows for both cavity-assisted and direct fluorescent readouts, as well as state preparation schemes akin to optical pumping. Since direct transitions between the metastable states are strongly suppressed, we utilize Raman transitions for coherent manipulation of the states.

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Detecting weak radio-frequency electromagnetic fields plays a crucial role in a wide range of fields, from radio astronomy to nuclear magnetic resonance imaging. In quantum optics, the ultimate limit of a weak field is a single photon. Detecting and manipulating single photons at megahertz frequencies presents a challenge because, even at cryogenic temperatures, thermal fluctuations are appreciable. Using a gigahertz superconducting qubit, we observed the quantization of a megahertz radio-frequency resonator, cooled it to the ground state, and stabilized Fock states. Releasing the resonator from our control, we observed its rethermalization with nanosecond resolution. Extending circuit quantum electrodynamics to the megahertz regime, we have enabled the exploration of thermodynamics at the quantum scale and allowed interfacing quantum circuits with megahertz systems such as spin systems or macroscopic mechanical oscillators.

We employ quantum optimal control theory to realize quantum gates for two protected superconducting circuits: the heavy-fluxonium qubit and the 0-π 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-π 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.

In the semiconductor industry, shrinking geometries and increasing process complexity have greatly increased the demand for TEM analysis of specific submicron regions. Until recently, samples of this nature have been difficult if not impossible to prepare. We have combined cross-sectional TEM sample preparation (XTEM) and the precise material sputtering of focussed ion beam milling (FIB) to thin samples to electron transparency. We call this sample preparation technique FIBXTEM.Three advantages of this technique are: 1) The area of interest can be analyzed in the scanning electron microscope before final thinning; 2) Any specific defect area becomes a candidate for TEM analysis, including failed sub-micron structures; and 3) Samples are generally artifact-free and of uniform thickness.Key elements of the FIBXTEM technique include precision planar polishing, unique holders for mounting and transferring samples between systems, and the FIB-induced deposition of a sacrificial protective layer over the area of interest during ion thinning.This technique extends the use of TEM analysis into new areas of semiconductor process development and failure analysis. Recent applications for materials problem solving and failure analysis are discussed.