An integrated microwave-to-optics interface for scalable quantum computing.

Microwave-to-optics transduction is emerging as a vital technology for scaling quantum computers and quantum networks. To establish useful entanglement links between qubit processing units, several key conditions must be simultaneously met: the transducer must add less than a single quantum of input-referred noise and operate with high efficiency, as well as large bandwidth and high repetition rate. Here we present a design for an integrated transducer based on a planar superconducting resonator coupled to a silicon photonic cavity through a mechanical oscillator made of lithium niobate on silicon. We experimentally demonstrate its performance with a transduction efficiency of 0.9% with 1 µW of continuous optical power and a spectral bandwidth of 14.8 MHz. With short optical pulses, we measure the added noise that is limited to a few photons, with a repetition rate of up to 100 kHz. Our device directly couples to a 50 Ω transmission line and can be scaled to a large number of transducers on a single chip, laying the foundations for distributed quantum computing.

Subgap spectroscopy along hybrid nanowires by nm-thick tunnel barriers.

Tunneling spectroscopy is widely used to examine the subgap spectra in semiconductor-superconductor nanostructures when searching for Majorana zero modes (MZMs). Typically, semiconductor sections controlled by local gates at the ends of hybrids serve as tunnel barriers. Besides detecting states only at the hybrid ends, such gate-defined tunnel probes can cause the formation of non-topological subgap states that mimic MZMs. Here, we develop an alternative type of tunnel probes to overcome these limitations. After the growth of an InSb-Al hybrid nanowire, a precisely controlled in-situ oxidation of the Al shell is performed to yield a nm-thick AlOx layer. In such thin isolating layer, tunnel probes can be arbitrarily defined at any position along the hybrid nanowire by shadow-wall angle-deposition of metallic leads. In this work, we make multiple tunnel probes along single nanowire hybrids and successfully identify Andreev bound states (ABSs) of various spatial extension residing along the hybrids.

Electrostatic control of the proximity effect in the bulk of semiconductor-superconductor hybrids.

The proximity effect in semiconductor-superconductor nanowires is expected to generate an induced gap in the semiconductor. The magnitude of this induced gap, together with the semiconductor properties like spin-orbit coupling and g-factor, depends on the coupling between the materials. It is predicted that this coupling can be adjusted through the use of electric fields. We study this phenomenon in InSb/Al/Pt hybrids using nonlocal spectroscopy. We show that these hybrids can be tuned such that the semiconductor and superconductor are strongly coupled. In this case, the induced gap is similar to the superconducting gap in the Al/Pt shell and closes only at high magnetic fields. In contrast, the coupling can be suppressed which leads to a strong reduction of the induced gap and critical magnetic field. At the crossover between the strong-coupling and weak-coupling regimes, we observe the closing and reopening of the induced gap in the bulk of a nanowire. Contrary to expectations, it is not accompanied by the formation of zero-bias peaks in the local conductance spectra. As a result, this cannot be attributed conclusively to the anticipated topological phase transition and we discuss possible alternative explanations.

In situ electrochemistry inside a TEM with controlled mass transport.

The field of electrochemistry promises solutions for the future energy crisis and environmental deterioration by developing optimized batteries, fuel-cells and catalysts. Combined with in situ transmission electron microscopy (TEM), it can reveal functional and structural changes. A drawback of this relatively young field is lack of reproducibility in controlling the liquid environment while retaining the imaging and analytical capabilities. Here, a platform for in situ electrochemical studies inside a TEM with a pressure-driven flow is presented, with the capability to control the flow direction and to ensure the liquid will always pass through the region of interest. As a result, the system offers the opportunity to define the mass transport and control the electric potential, giving access to the full kinetics of the redox reaction. In order to show the benefits of the system, copper dendrites are electrodeposited and show reliable electric potential control. Next, their morphology is changed by tuning the mass transport conditions. Finally, at a liquid thickness of approximately 100 nm, the diffraction pattern revealed the 1,1,1 planes of the copper crystals, indicating an atomic resolution down to 2.15 Å. Such control of the liquid thickness enabled elemental mapping, allowing us to distinguish the spatial distribution of different elements in liquid.