Abstract : This technical report demonstrates nanofabrication technology for Niobium heterostructures and nanoscale quantum devicessuitable for implementation in novel qubit/quantum memory. In particular electron beam lithography and atomic layer depositionare used. These processes integrate layouts suitable for high frequency microwave/RF excitation of the qubits/quantum memory aswell as optically transparent gates in the area of the tunnel junction to enable combined spectroscopy and transport measurementswhile tuning the energy band at the tunnel junction. These devices are suitable for characterization in a cryo-magneto-opticalprobe station to access the superconducting regime of operation and that could be used for control and sending of qubit/quantummemory states to remote locations.
Abstract : This technical report demonstrates nanofabrication technology for Niobium heterostructures and nanoscale quantum devicessuitable for implementation in novel qubit/quantum memory. In particular electron beam lithography and atomic layer depositionare used. These processes integrate layouts suitable for high frequency microwave/RF excitation of the qubits/quantum memory aswell as optically transparent gates in the area of the tunnel junction to enable combined spectroscopy and transport measurementswhile tuning the energy band at the tunnel junction. These devices are suitable for characterization in a cryo-magneto-opticalprobe station to access the superconducting regime of operation and that could be used for control and sending of qubit/quantummemory states to remote locations.
Niobium offers the benefit of increased operating temperatures and frequencies for Josephson junctions, which are the core component of superconducting devices. However, existing niobium processes are limited by more complicated fabrication methods and higher losses than now-standard aluminum junctions. Combining recent trilayer fabrication advancements, methods to remove lossy dielectrics and modern superconducting qubit design, we revisit niobium trilayer junctions and fabricate all-niobium transmons using only optical lithography. We characterize devices in the microwave domain, measuring coherence times up to $62\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}\mathrm{s}$ and an average qubit quality factor above 105: much closer to state-of-the-art aluminum-junction devices. We find the higher superconducting gap energy also results in reduced quasiparticle sensitivity above 0.16 K, where aluminum-junction performance deteriorates. Our low-loss junction process is readily applied to standard optical-based foundry processes, opening alternative avenues for direct integration and scalability, and paves the way for higher-temperature and higher-frequency quantum devices.
This PhD Thesis is focused on the realization of superconducting quantum devices based on overdamped Nb/Al-AlOx/Nb SNIS (Superconductor - Normal metal - Insulator - Superconductor) tunneling Josephson junctions, interesting for several application fields (voltage metrology, digital electronics, radiation sensors, nanoSQUID, etc.). The challenges faced by quantum electronics and metrology are directing the new generations of devices towards smaller dimensions and higher levels of integration. Taking into account this requirement, the fabrication of SNIS-based devices has been addressed on downscaling the junction dimensions form the micro to the nanoscale. In particular, the effective area of junctions has been reduced exploiting three different lithographic techniques: the optical lithography to realize SNISs for a micrometer resolution, the Electron Beam Lithography (EBL) at the subµm and, finally, the Focused Ion Beam (FIB) sculpting method to achieve nanometer sizes. Specifically, prototypes have been realized exploiting the thin film technology, to guarantee a good control of electrical parameters of junctions, a higher reproducibility of their current-voltage I-V characteristics, and an accurate dimension control. Reliable and simpler fabrication processes have been implemented and validated, and the device downscaling was pursued without affecting the fundamental properties of SNIS junctions such as the non hysteretic I-V response and the skill on generating quantised voltage steps under radiofrequency irradiation. This work has thereby led to the definition and validation of a new generation of devices and processes down to the nanometer scale, and these approaches represent precious experiences of nanofabrication valuable for new research activities and projects
Abstract : Rare-earth ions are useful for the implementation of quantum memory. In particular, the energy levels and magnetic/spin properties of select ions may enable storage of quantum states. Also, the optical activity may enable optical read-out techniques and optical transmission of qubit states. In addition, the energy levels of ions can be used for designing superconducting qubits where the ionic configuration tunes the Hamiltonian for quantum computing. In the more near term, ions embedded in superconductor heterostructure can be used to implement cryogenic memory for superconducting digital computing. Our concept involves embedding rare-earth ions in superconductor devices to provide a low dissipation environment and access to the systems Hamiltonian. In this report we show results and success in integrating rare-earth neodymium by ion implantation in thin films of niobium and niobium-based heterostructure devices. We model the ion implantation process and confirm the concentration of the neodymium in niobium by energy dispersive X-ray (EDX) spectroscopy in a scanning electron microscope (SEM). We study the effect of optical illumination and magnetic stimulus on the transport of the devices at room temperature, as it is important to understand the films and devices so they can properly designed and optimized for utility as quantum memory. We find that the magnetic field has a strong effect on the response. We also find that the selectivity in the optical effect when using a broadband halogen lamp in comparison to a light emitting diode (LED) with selectivity in the excitation wavelength, reinforces the role of the magnetic and optically active Nd rare-earth ions on the device response.
Overcoming the limitations of current nanofabrication techniques to achieve nanoscale feature sizes is essential for achieving new regimes of light-matter interactions at extreme frequencies and length scales. Here, we demonstrate a scalable nanofabrication platform capable of producing in-plane feature sizes down to 1.75 nm, pushing the boundaries of current top-down nanofabrication techniques. Using precise thickness control of atomic layer deposition (ALD) and employing widely spaced oxide nanofins, we transform conventional ALD into a surface structuring method that produces nanolaminates with sub-10 nm periodicities over large areas. The resulting nanostructures can be used as a one-dimensional gate array to control charge carriers in two-dimensional materials. As an initial demonstration, we integrate the platform with graphene and perform electron transport measurements. In the presence of the gate array enabled by the nanolaminate, we observe satellite Dirac peaks consistent with band-structure modulation, suggestive of quantum-confinement effects. Our platform paves the way for exploring previously inaccessible regimes of nanoscale light-matter interactions, holding significant promise for applications in short wavelength optics, electronics, and polaritonics.
Researches on the fabrication of ~ 0.1 × 0.1 μ m 2 superconductor–insulator–superconductor (SIS) Josephson junctions are reviewed. Today, a typical dimension is 1–10 μm for Josephson junctions in superconducting integrated circuits. These Josephson junctions are defined by well-established photolithographic technology with reactive ion etching (RIE), and for the superconductor, Nb is almost always used. The merits of Nb include the facts that the superconducting transition temperature Tc of Nb (9.2 K ) is higher than the boiling point of He (4.2 K ), and that Nb has excellent stability against thermal cycling between room temperature and liquid- He temperature. For the fabrication of ~ 0.1 × 0.1 μ m 2 junctions, on the other hand, there is a standard process with electron-beam lithography, shadow evaporation, and lift-off. This process works well for Al (Tc = 1.2 K ), however, it is not ideal for Nb . The scope of this brief review is the nanoscale junction with Nb electrodes. We will look at the efforts of optimizing the standard lift-off process for Nb , electron-beam-lithographic versions of the Nb Josephson-junction technology, focused-ion-beam (FIB) etching as a convenient alternative to electron-beam lithography and RIE, etc. In order to characterize nanoscale tunnel junctions, the single-charge transistor has been often fabricated. Therefore, a summary of its theoretical transport properties is also included.
The high kinetic inductance of niobium nitride (NbN) thin films can be used for an implementation of compact on-chip inductances in cryoelectronic circuits. Here, for the first time, we demonstrate the implementation of a hybrid superconducting technology that includes the fabrication of standard aluminum submicron Josephson junctions and the NbN atomic layer deposition process. As an example, we fabricated and characterized a single and array of Al Josephson junctions together with NbN interconnections. The main Al Josephson junction parameters as well as NbN superconducting properties are in a good agreement with the values obtained by our standard fabrication process. The combination of technological processes for the NbN layers with Al Josephson junction allows implementing a new generation of innovative superconducting devices for different applications.