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.
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.
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.
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.
Recent experiments have demonstrated superconducting transmon qubits with semiconductor nanowire Josephson junctions. These hybrid gatemon qubits utilize field effect tunability characteristic of semiconductors to allow complete qubit control using gate voltages, potentially a technological advantage over conventional flux-controlled transmons. Here, we present experiments with a two-qubit gatemon circuit. We characterize qubit coherence and stability and use randomized benchmarking to demonstrate single-qubit gate errors below 0.7% for all gates, including voltage-controlled Z rotations. We show coherent capacitive coupling between two gatemons and coherent swap operations. Finally, we perform a two-qubit controlled-phase gate with an estimated fidelity of 91%, demonstrating the potential of gatemon qubits for building scalable quantum processors.
Superconducting transmon qubits based on hybrid superconductor-semiconductor Josephson junctions (gatemons) offer gate tunability, but their relaxation times remain well below those of state-of-the-art transmons, and the origin of this discrepancy is not fully understood. Here, we co-fabricate gatemons and SIS-junction transmons with nominally identical circuit layouts, gate dielectrics, and control lines, so that the Josephson element is the only intentional distinction. Across multiple chips, transmons in this architecture reach relaxation times in the tens of microseconds, whereas gatemons saturate in the few-microsecond range. Using the transmons as on-chip references, we construct a loss budget including Purcell decay, spontaneous emission through the control line, and internal dielectric loss, and find that the corresponding T1 limits exceed all measured gatemon values by more than an order of magnitude. Temperature-dependent T1 measurements follow a common quasiparticle-activation model and yield similar superconducting gaps for S-Sm-S and SIS junctions, indicating that the reduced gatemon coherence is dominated by additional temperature-independent, junction-intrinsic dissipation.
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.
Gate-tunable transmon qubits are based on quantum conductors used as weak links within hybrid Josephson junctions. These gatemons have been implemented in just a handful of systems, all relying on extended conductors, namely epitaxial semiconductors or exfoliated graphene. Here we present the coherent control of a gatemon based on a single molecule, a one-dimensional carbon nanotube, which is integrated into a circuit quantum electrodynamics architecture. The measured qubit spectrum can be tuned with a gate voltage and reflects the quantum dot behavior of the nanotube. Our ultraclean integration, using a hexagonal boron nitride substrate, results in record coherence times of 200 ns for carbon nanotube-based qubits. Furthermore, we investigate its decoherence mechanisms, thus revealing a strong gate dependence and identifying charge noise as a limiting factor. On top of positioning carbon nanotubes as contenders for future quantum technologies, our work paves the way for studying microscopic fermionic processes in low-dimensional quantum conductors. Gatemons, or gate-tunable transmons, are superconducting qubits based on hybrid Josephson junctions, which typically use extended quantum conductors as weak links. Here the authors report a gatemon made with a carbon-nanotube-based junction, showing improved coherence time compared to graphene-based devices.