Superconducting qubit based on superconducting circuits consist of a superconducting capacitor and a Josephson junction with the transmon geometry is extensively used in advanced quantum processors, pursuing scalable quantum computing. The tuning of the qubit frequency of the transmon relies on magnetic flux-dependent interference between the supercurrents of two superconductor-insulator-superconductor (S-I-S) Josephson junctions in a superconducting loop. Josephson junction based on superconductor-semiconductor-superconductor (S-Sm-S) materials opens up a possibility to the gate-tunable transmon, referred to as the "gatemon", in which the qubit frequency can be tuned by electrostatic mean. Recent realizations of gatemons on III-V material platforms show impressive development on the alternative to the transmon, yet still leave a big question on the scalability. The silicon-germanium (SiGe) heterostructure is one of the potential platforms to host hybrid devices due to its high hole mobility and the low Schottky barrier at the Ge-metal interface. Additionally, the compatibility with the silicon-based semiconductor industry is a capable advantage for the scaling-up qubit platform.In this thesis, we develop gatemons based on the Al-Ge-Al Josephson junction in the SiGe heterostructure. Firstly, the robust fabrication recipe, found on a top-down approach, for Josephson Field Effect Transistors (JoFETs) is established. We perform measurements exhaustively on the JoFETs to study their properties as a function of the gate voltage, temperature, and magnetic field. The devices show gate-tunability of the critical current and the normal state resistance . The devices are estimated to have a high-transparency superconductor-semiconductor interface, as demonstrated by the high product on the SiGe heterostructure. In the finite-voltage regime, the features corresponded to multiple Andreev reflections (MARs) are observed. Then, we fabricate and characterize niobium nitride (NbN) superconducting resonators on SiGe heterostructure. We measure the resonators in the transmission mode and extract the resonant frequency , internal quality factor, and coupling quality factor from the transmission coefficient. Following that, we develop the fabrication process to integrate Al-Ge-Al junctions shunted with capacitors, or, in other words, gatemon, into the resonator scheme and perform the fabrication according to the design. We demonstrate the anticrossing feature in one of the fabricated gatemons. The resonant frequency of the gatemon is mapped using the two-tone spectroscopy technique and is found to be gate-tunable. The qubit has large spectral linewidth, implying a low coherence time. Additionally, we conduct the current-phase relationship (CPR) measurement on the junctions in the Superconducting QUantum Interference Device (SQUID) geometry. We can demonstrate that the junctions pose non-sinusoidal CPR. Further, the integer and half-integer Shapiro steps are observed in the current-voltage characteristic curves of irradiated junctions. This indicates that our junctions have the element, which can open up another possibility toward protected qubits.
Superconducting qubit based on superconducting circuits consist of a superconducting capacitor and a Josephson junction with the transmon geometry is extensively used in advanced quantum processors, pursuing scalable quantum computing. The tuning of the qubit frequency of the transmon relies on magnetic flux-dependent interference between the supercurrents of two superconductor-insulator-superconductor (S-I-S) Josephson junctions in a superconducting loop. Josephson junction based on superconductor-semiconductor-superconductor (S-Sm-S) materials opens up a possibility to the gate-tunable transmon, referred to as the "gatemon", in which the qubit frequency can be tuned by electrostatic mean. Recent realizations of gatemons on III-V material platforms show impressive development on the alternative to the transmon, yet still leave a big question on the scalability. The silicon-germanium (SiGe) heterostructure is one of the potential platforms to host hybrid devices due to its high hole mobility and the low Schottky barrier at the Ge-metal interface. Additionally, the compatibility with the silicon-based semiconductor industry is a capable advantage for the scaling-up qubit platform.In this thesis, we develop gatemons based on the Al-Ge-Al Josephson junction in the SiGe heterostructure. Firstly, the robust fabrication recipe, found on a top-down approach, for Josephson Field Effect Transistors (JoFETs) is established. We perform measurements exhaustively on the JoFETs to study their properties as a function of the gate voltage, temperature, and magnetic field. The devices show gate-tunability of the critical current and the normal state resistance . The devices are estimated to have a high-transparency superconductor-semiconductor interface, as demonstrated by the high product on the SiGe heterostructure. In the finite-voltage regime, the features corresponded to multiple Andreev reflections (MARs) are observed. Then, we fabricate and characterize niobium nitride (NbN) superconducting resonators on SiGe heterostructure. We measure the resonators in the transmission mode and extract the resonant frequency , internal quality factor, and coupling quality factor from the transmission coefficient. Following that, we develop the fabrication process to integrate Al-Ge-Al junctions shunted with capacitors, or, in other words, gatemon, into the resonator scheme and perform the fabrication according to the design. We demonstrate the anticrossing feature in one of the fabricated gatemons. The resonant frequency of the gatemon is mapped using the two-tone spectroscopy technique and is found to be gate-tunable. The qubit has large spectral linewidth, implying a low coherence time. Additionally, we conduct the current-phase relationship (CPR) measurement on the junctions in the Superconducting QUantum Interference Device (SQUID) geometry. We can demonstrate that the junctions pose non-sinusoidal CPR. Further, the integer and half-integer Shapiro steps are observed in the current-voltage characteristic curves of irradiated junctions. This indicates that our junctions have the element, which can open up another possibility toward protected qubits.
Superconducting qubit based on superconducting circuits consist of a superconducting capacitor and a Josephson junction with the transmon geometry is extensively used in advanced quantum processors, pursuing scalable quantum computing. The tuning of the qubit frequency of the transmon relies on magnetic flux dependent interference between the supercurrents of two superconductor-insulator superconductor(S-I-S) Josephson junctions in a superconducting loop. Josephson junction based on superconductor-semiconductor-superconductor (S-Sm-S) materialsopens up a possibility to the gate-tunable transmon, referred to as the "gatemon", in which the qubit frequency can be tuned by electrostatic mean. Recent realizations of gatemons on III-V material platforms show impressive development on the alternative to the transmon, yet still leave a big question on the scalability. The silicon-germanium (SiGe) heterostructure is one of the potential platforms to host hybrid devices due to its high hole mobility and the low Schottky barrier at the Ge-metal interface. Additionally, the compatibility with the silicon-based semiconductor industry is a capable advantage for the scaling-up qubit platform.In this thesis, we develop gatemons based on the Al-Ge-Al Josephson junction in the SiGe heterostructure. Firstly, the robust fabrication recipe, found on a top down approach, for Josephson Field Effect Transistors (JoFETs) is established. We perform measurements exhaustively on the JoFETs to study their properties as a function of the gate voltage, temperature, and magnetic field. The devices show gate-tunability of the critical current (IC) and the normal state resistance (RN). The devices are estimated to have a high-transparency superconductor/semiconductor interface, as demonstrated by the high ICRN product on the SiGe heterostructure. In the finite-voltage regime, the features corresponded to multiple Andreev reflections (MARs) are observed. Then, we fabricate and characterize niobium nitride (NbN) superconducting resonators on SiGe heterostructure. We measure the resonators in the transmission mode and extract the resonant frequency (fr), internal quality factor (Qi), and coupling quality factor (Qc) from the transmission coefficient (S21). Following that, we develop the fabrication process to integrate Al-Ge-Al junctions shunted with capacitors, or, in other words,gatemon, into the resonator scheme and perform the fabrication according to the design. We demonstrate the anticrossing feature in one of the fabricated gatemons. The resonant frequency of the gatemon is mapped using the two-tone spectroscopy technique and is found to be gate-tunable. The qubit has large spectral linewidth, implying a low coherence time. Additionally, we conduct the current-phase relationship (CPR) measurement on the junctions in the Superconducting QUantum Interference Device (SQUID) geometry. We can demonstrate that the junctions pose non-sinusoidal CPR. Further, the integer and half-integer Shapiro steps are observed in the current-voltage characteristic curves of irradiated junctions. This indicates that our junctions have the cos 2φ element, which can open up another possibility toward protected qubits.
Scalability in the fabrication and operation of quantum computers is key to move beyond the NISQ era. So far, superconducting transmon qubits based on aluminum Josephson tunnel junctions have demonstrated the most advanced results, though this technology is difficult to implement with large-scale facilities. An alternative "gatemon" qubit has recently appeared, which uses hybrid superconducting/semiconducting (S/Sm) devices as gate-tuned Josephson junctions. Current implementations of these use nanowires however, of which the large-scale fabrication has not yet matured either. A scalable gatemon design could be made with CMOS Josephson Field-Effect Transistors as tunable weak link, where an ideal device has leads with a large superconducting gap that contact a short channel through high-transparency interfaces. High transparency, or low contact resistance, is achieved in the microelectronics industry with silicides, of which some turn out to be superconducting. The first part of the experimental work in this thesis covers material studies on two such materials: V3Si and PtSi, which are interesting for their high Tc, and mature integration, respectively. The second part covers experimental results on 50 nm gate length PtSi transistors, where the transparency of the S/Sm interfaces is modulated by the gate voltage. At low voltages, the transport shows no conductance at low energy, and well-defined features at the superconducting gap. The barrier height at the S/Sm interface is reduced by increasing the gate voltage, until a zero-bias peak appears around zero drain voltage, which reveals the appearance of an Andreev current. The successful gate modulation of Andreev current in a silicon-based transistor represents a step towards fully CMOS-integrated superconducting quantum computers.
Gate-tunable transmons (gatemons) employing semiconductor Josephson junctions have recently emerged as building blocks for hybrid quantum circuits. In this study, we present a gatemon fabricated in planar Germanium. We induce superconductivity in a two-dimensional hole gas by evaporating aluminum atop a thin spacer, which separates the superconductor from the Ge quantum well. The Josephson junction is then integrated into an Xmon circuit and capacitively coupled to a transmission line resonator. We showcase the qubit tunability in a broad frequency range with resonator and two-tone spectroscopy. Time-domain characterizations reveal energy relaxation and coherence times up to 75 ns. Our results, combined with the recent advances in the spin qubit field, pave the way towards novel hybrid and protected qubits in a group IV, CMOS-compatible material.
Gatemons are superconducting qubits resembling transmons, with a gate-tunable semiconducting weak link as the Josephson element. Here, we report a gatemon device featuring an aluminum microwave circuit on a Ge/SiGe heterostructure embedding a Ge quantum well. Owing to the superconducting proximity effect, the high-mobility two-dimensional hole gas confined in this well provides a gate-tunable superconducting weak link between two Al contacts. We perform Rabi oscillation and Ramsey interference measurements, demonstrate the gate-voltage dependence of the qubit frequency, and measure the qubit anharmonicity. We find relaxation times T 1 up to 119 ns, and Ramsey coherence times T 2 * up to 70 ns, and a qubit frequency gate-tunable over 3.5 GHz. The reported proof-of-concept reproduces the results of a very recent work [Sagi et al. Nat. Commun. 2024, 15, 6400] using similar Ge/SiGe heterostructures, thereby validating a novel platform for the development of gatemons and parity-protected cos(2ϕ) qubits.
The performance and coherence of superconducting quantum systems are fundamentally tied to the quality of the materials from which they are built. In superconducting transmon qubits, many loss mechanisms and sources of decoherence originate from material imperfections or unwanted oxide formation. Likewise, semiconductor–superconductor hybrid systems depend on high-quality interfaces to ensure strong coupling between electronic states in the two materials. In such hybrid devices, the accessible operating regime is often dictated by the intrinsic material properties of either the semiconductor or the superconductor. Advancing quantum device performance therefore requires exploring new material systems and optimizing their physical characteristics. In this thesis, we present the development of several superconducting material platforms for quantum technologies, including superconducting qubits, single-photon detectors, and semiconductor superconductor hybrid devices. We focus on the growth and characterization of superconducting films and heterostructures, and we establish clear connections between their material properties and the resulting device performance. First, we will present a novel semiconductor-superconductor hybrid material based on in-situ deposited epitaxial layers of Nb and Al on a shallow InAs quantum well. In this structure, the Al interlayer provides an epitaxial connection between an in-situ magnetron sputtered Nb or NbTi thin film and a shallow InAs two dimensional electron gas. This metal-to-metal epitaxy is achieved by optimization of the material stack and results in an induced superconducting gap of approximately 1 meV, determined from transport measurements of superconductor semiconductor Josephson junctions. This induced gap is approximately five times larger than the values reported for Al-based hybrid materials and indicates the formation of highly-transparent interfaces that are required in high-quality hybrid material platforms. Then, we discuss a different approach in the next chapter where we develop a hybrid material based on a molecular beam epitaxy grown InAsSb surface quantum well with an in-situ deposited Nb layer directly on top of the quantum well. Relative to conventional Al-InAs based systems, the InAsSb surface quantum well offers a lower effective mass and stronger spin-orbit interaction, while the Nb layer has a higher critical temperature and a larger critical magnetic field. The in-situ deposition of the Nb layer directly on top of the quantum well results in a high-quality interface that enables strong coupling to the InAsSb quantum well. Transport measurements on Josephson junctions reveal an induced superconducting gap of 1.3 meV and a critical current density of 3.75 × 10³A/cm². Furthermore, a planar asymmetric SQUID is realized, exhibiting gate-tunable superimposed oscillations originating from both the individual Josephson junction and the full SQUID loop. The large induced superconducting gap combined with strong spin-orbit interaction position this material as an attractive platform for experiments exploring gate-tunable superconductivity and topological superconducting devices. In the next chapter, we will focus on establishing a link between Nb film crystal properties and their performance in superconducting circuits. In this study, we investigate the microwave properties of niobium resonators with different crystalline properties and related surface topographies. We analyze a series of magnetron sputtered films in which the Nb crystal orientation and surface topography are changed by varying the substrate temperatures between room temperature and 975 K. The lowest-loss resonators that we measure have quality factors of over 10⁶ at single-photon powers, among the best ever recorded using the Nb on sapphire platform. We observe the highest quality factors in films grown at an intermediate temperature regime of the growth series (550 K) where the films display both preferential ordering of the crystal domains and low surface roughness. Furthermore, we analyze the temperature-dependent behavior of our resonators to learn about how the quasi-particle density in the Nb film is affected by the niobium crystal structure and the presence of grain boundaries. Then, we explore yet a different material platform by attempting to realize onchip NbTiN single photon detectors on LiNbO₃, an emerging photonic platform with high potential for scalable quantum information processing. We use DC reactive magnetron sputtering to grow high-quality NbTiN thin films and investigate the impact of substrate temperature during NbTiN growth. We achieve four nm thick superconducting films with a critical temperature (𝑇𝑐) of 12.3 K grown at a substrate temperature of 825 K. We demonstrate that the increased substrate temperature reduces the oxygen concentration in our films and improves the overall stoichiometry. In addition, we integrate these films with the LiNbO3 platform and investigate the obtained interface quality. Finally, we fabricate superconducting nanowire single photon detectors from the NbTiN film on LiNbO₃ and characterize the detector performance. Lastly, we report a gate-tunable Josephson diode effect in hybrid nanowire junctions consisting of a spin-orbit-coupled semiconductor core coated with epitaxial ferromagnetic insulator and superconductor shells. The wires display a hysteretic superconducting window as a function of axial magnetic field. In the superconducting regime, the devices exhibit nonreciprocal current transport, with the diode efficiency showing a strong dependence on back-gate voltage. The effect persists in a remanent magnetization state following a controlled demagnetization procedure, establishing zero-field operation. The combination of results presented in this thesis stress the importance of a material-focused approach in advancing the performance of quantum hardware. The novel hybrid material platforms developed in this thesis expand the operating regime of devices, which could enable future experiments involving gate-tunable superconductivity or topological properties. Similarly, we demonstrate that optimizing superconductor crystal structure benefits quantum devices, such as single-photon detectors and superconducting resonators.
The development of quantum circuits based on hybrid superconductor-semiconductor Josephson junctions holds promise for exploring their mesoscopic physics and for building innovative superconducting devices. The gate-tunable superconducting transmon qubit (gatemon) is the paradigmatic example of such a superconducting circuit; however, gatemons typically suffer from unstable and hysteretic qubit frequencies with respect to the applied gate voltage and reduced coherence times compared to conventional transmons fabricated from aluminum-oxide-based Josephson junctions. Here, we develop methods for characterizing these challenges in gatemons and deploy these methods to compare the impact of shunt-capacitor designs on gatemon performance. Our results indicate a strong frequency- and design-dependent behavior of the qubit stability, hysteresis, and dephasing times. Moreover, we achieve highly reliable tuning of the qubit frequency with 1-MHz precision over a range of several gigahertz, along with improved stability in grounded gatemons compared to gatemons with a floating capacitor design.