Floris R. Braakman

This paper introduces a novel integration method of localized metallic back-gates into fully-depleted silicon-on-insulator (FDSOI) multi-gate FETs, enabling robust front-to-back electrostatic coupling from room temperature to cryogenic conditions, without the need for substrate implantation. The fabrication process, termed the Nanomole process, utilizes nanometric vapor-phase etching of the buried oxide or silicon substrate with vapor-HF and XeF2 gases. This is followed by atomic layer deposition (ALD) of a dielectric material and Pt, with precise patterning achieved through inductively coupled plasma etching. Detailed analysis of the process demonstrates controllable etching rates based on device geometry, providing calibrated guidelines for scalable manufacturing. Symmetric mid-k dual-gating is reported in devices featuring a Si-film thickness of 24 nm, with a top and bottom gate oxide equivalent thickness (EOT) of 6.5 nm. Electrical characterization of multi-gate FDSOI SETs, operated as FETs, confirms effective threshold voltage tuning through dual-gate operation, with consistent performance from room temperature to millikelvin regimes. Additionally, quantum mechanical simulations based on the effective mass approximation at 4 K offer insights into the electrostatic behavior of dual-gated SOI quantum dot devices in both planar and nanowire geometries. This scalable and versatile technological solution opens new possibilities for advanced quantum devices, such as charge and spin qubits, by enabling in situ control over volume inversion, electron valley splitting, and spin-orbit interaction.

Focused-electron-beam-induced deposition is a promising technique for patterning nanomagnets in a single step. We fabricate cobalt nanomagnets in such a process and characterize their content, saturation magnetization, and stray magnetic field profiles by using a combination of transmission electron microscopy and scanning nitrogen-vacancy (NV) magnetometry. We find agreement between the measured stray field profiles and saturation magnetization with micromagnetic simulations. We further characterize magnetic domains and grainy stray magnetic fields in the nanomagnets and their halo side-deposits. This work may aid in the evaluation of Co nanomagnets produced through focused electron-beam-induced deposition for applications in spin qubits, magnetic field sensing, and magnetic logic.

Focused-electron-beam-induced deposition is a promising technique for patterning nanomagnets for spin qubit control in a single step. We fabricate cobalt nanomagnets in such a process, obtaining cobalt contents and saturation magnetizations comparable to or higher than those typically obtained using electron-beam lithography. We characterize the nanomagnets using transmission electron microscopy and image their stray magnetic field using scanning NV magnetometry, finding good agreement with micromagnetic simulations. The magnetometry reveals the presence of magnetic domains and halo side-deposits, which are common for this fabrication technique. Finally, we estimate dephasing times for electron spin qubits in the presence of disordered stray fields due to these side-deposits.

Fast feedback from cryogenic electrical characterization measurements is key for the development of scalable quantum computing technology. At room temperature, high-throughput device testing is accomplished with a probe-based solution, where electrical probes are repeatedly positioned onto devices for acquiring statistical data. In this work, we present a probe station that can be operated from room temperature down to below 2 K. Its small size makes it compatible with standard cryogenic measurement setups with a magnet. A large variety of electronic devices can be tested. Here, we demonstrate the performance of the prober by characterizing silicon fin field-effect transistors as a host for quantum dot spin qubits. Such a tool can massively accelerate the design–fabrication–measurement cycle and provide important feedback for process optimization toward building scalable quantum circuits.

We developed a process to fabricate nanoscale metallic gate electrodes on scanning probe cantilevers, including on the irregular surface of protruding cantilever tips. The process includes a floating-layer technique to coat the cantilevers in an electron-beam resist. We demonstrate gate definition through a lift-off process and through an etching process. The cantilevers maintain a high force sensitivity after undergoing the patterning process. Our method allows the patterning of nanoscale devices on fragile scanning probes, extending their functionality as sensors.

We describe an apparatus for the implementation of hybrid optomechanical systems at 4 K. The platform is based on a high-finesse, micrometer-scale fiber Fabry–Perot cavity, which can be widely tuned using piezoelectric positioners. A mechanical resonator can be positioned within the cavity in the object-in-the-middle configuration by a second set of positioners. A high level of stability is achieved without sacrificing either performance or tunability, through the combination of a stiff mechanical design, passive vibration isolation, and an active Pound–Drever–Hall feedback lock incorporating a reconfigurable digital filter. The stability of the cavity length is demonstrated to be better than a few picometers over many hours both at room temperature and at 4 K.

A key challenge in quantum computation is the implementation of fast and local qubit control while simultaneously maintaining coherence. Qubits based on hole spins offer, through their strong spin-orbit interaction, a way to implement fast quantum gates. Strikingly, for hole spins in one-dimensional germanium and silicon devices, the spin-orbit interaction has been predicted to be exceptionally strong yet highly tunable with gate voltages. Such electrical control would make it possible to switch on demand between qubit idling and manipulation modes. Here, we demonstrate ultrafast and universal quantum control of a hole spin qubit in a germanium/silicon core/shell nanowire, with Rabi frequencies of several hundreds of megahertz, corresponding to spin-flipping times as short as ~1 ns - a new record for a single-spin qubit. Next, we show a large degree of electrical control over the Rabi frequency, Zeeman energy, and coherence time - thus implementing a switch toggling from a rapid qubit manipulation mode to a more coherent idling mode. We identify an exceptionally strong but gate-tunable spin-orbit interaction as the underlying mechanism, with a short associated spin-orbit length that can be tuned over a large range down to 3 nm for holes of heavy-hole mass. Our work demonstrates a spin-orbit qubit switch and establishes hole spin qubits defined in one-dimensional germanium/silicon nanostructures as a fast and highly tunable platform for quantum computation.