Circuit quantum electrodynamics with qubits based on semiconductor quantum dots can enable long-range two-qubit gates between distant spins, or improve gate-based charge sensing for readout. Compared to conventional resonators, high-impedance resonators improve the coupling to the qubits, but their losses to the quantum dot's leads are also larger. This study implements on-chip filtering using high-kinetic-inductance nanowires and thin-film capacitors to mitigate this source of losses. These filters are very compact and easily extended to a large number of leads, all while preserving state-of-the-art resonator quality factors.
Circuit quantum electrodynamics with qubits based on semiconductor quantum dots can enable long-range two-qubit gates between distant spins, or improve gate-based charge sensing for readout. Compared to conventional resonators, high-impedance resonators improve the coupling to the qubits, but their losses to the quantum dot's leads are also larger. This study implements on-chip filtering using high-kinetic-inductance nanowires and thin-film capacitors to mitigate this source of losses. These filters are very compact and easily extended to a large number of leads, all while preserving state-of-the-art resonator quality factors.
Circuit quantum electrodynamics (QED) employs superconducting microwave resonators as quantum buses. In circuit QED with semiconductor quantum-dot-based qubits, increasing the resonator impedance is desirable as it enhances the coupling to the typically small charge dipole moment of these qubits. However, the gate electrodes necessary to form quantum dots in the vicinity of a resonator inadvertently lead to a parasitic port through which microwave photons can leak, thereby reducing the quality factor of the resonator. This is particularly the case for high-impedance resonators, as the ratio of their total capacitance over the parasitic port capacitance is smaller, leading to larger microwave leakage than for 50-x resonators. Here, we introduce an implementation of on-chip filters to suppress the microwave leakage. The filters comprise a high-kinetic-inductance nanowire inductor and a thin-film capacitor. The filter has a small footprint and can be placed close to the resonator, confining microwaves to a small area of the chip. The inductance and capacitance of the filter elements can be varied over a wider range of values than their typical spiral inductor and interdigitated capacitor counterparts. We demonstrate that the total linewidth of a 6.4 GHz and approximately 3-kx resonator can be improved down to 540 kHz using these filters.
Spin-photon coupling presents an enticing opportunity for the long-range coupling of spin qubits. The spin-photon coupling rate, gs, is proportional to the charge-photon coupling rate, gc. To move deeper into the strong-coupling regime, gc can be enhanced by fabricating high-impedance cavities using high-kinetic-inductance films. Here, we report dc transport and microwave response investigations of niobium nitride (NbN) films of different thicknesses. The kinetic inductance increases rapidly as the film thickness is reduced below 50 nm and for 15-nm NbN films we measure a sheet kinetic inductance Lk,S=41.2pH/◻. As an application of the high-kinetic-inductance films, we fabricate compact LC filters that are commonly used to reduce microwave leakage in circuit quantum electrodynamics (cQED) devices. These filters feature up to 60 dB of attenuation near typical cavity resonance frequencies of fc=8 GHz.
We develop an engineered microwave environment for coupling high Q superconducting resonators to quantum dots using a multilayer fabrication stack for dot control wiring. Analytical and numerical models are presented, which show that high resonator quality factors can be attained by either minimizing the parasitic coupling capacitance to the leads or creating a low effective environmental impedance at the cavity frequency. We implement the later approach by fabricating low characteristic impedance (Zg≈10 Ω) microstrips on-chip for the dot bias wiring and show resonator quality factors of 8140 that can be attained without the addition of explicit filtering. Using this approach, we demonstrate single electron occupation in double and triple dots detected via dipole or quadrupole coupling to a superconducting resonator. Additionally, by using multilayer fabrication, we are able to improve ground plane integrity and keep microwave crosstalk below −20 dB out to 18 GHz while maintaining high wire density, which will be necessary for future circuit quantum electrodynamics quantum dot processors.
Circuit quantum electrodynamics (cQED) architecture enables long-range qubit coupling and nondemolition readout, with a key requirement of a small resonator photon leakage rate, κ. Several types of low-pass on-chip LC filter designs have been proposed to mitigate κ. However, the non-flat frequency response in the passband introduces waveform distortion, which may degrade qubit performance. In this work, we develop an on-chip compact Chebyshev filter for semiconductor quantum dots in cQED architecture. It exhibits a flat passband up to a 2 GHz cutoff frequency with microwave attenuation <−30 dB at 7 GHz, without observing the sign of damping oscillations induced by waveform distortion. We also analyze the integration of the filter with the cQED architecture and demonstrate that microwave attenuation meets the need for integration. The developed Chebyshev filter paves the way for the precise control of semiconductor quantum dots in cQED architecture with a low photon leakage rate.
Spin qubits in semiconductor quantum dots constitute one of the leading platforms for quantum information processing, combining long coherence times, high-fidelity gate operations, and compatibility with semiconductor industry technologies. In particular, holes confined in planar Ge/SiGe heterostructures have recently emerged as promising qubit candidates owing to their low effective mass, high mobility, low percolation density, absence of valley states, and strong spin-orbit interaction, which enables fast all-electrical spin control. At the same time, spin-orbit interaction also gives rise to a highly anisotropic and electrically tunable gyromagnetic response. Understanding and controlling this response is therefore essential for high-fidelity single- and multi-qubit operations. The realization of scalable quantum processors also requires efficient qubit readout and long-range connectivity. Circuit quantum electrodynamics (cQED), based on the coupling between artificial atoms and microwave photons confined in superconducting resonators, provides a natural framework to address these challenges. However, the implementation of hybrid semiconductor-superconductor devices in planar Ge has so far been hindered by large microwave losses originating from the heterostructure. In this thesis, we demonstrate, for the first time in planar Ge, strong coupling between a hole charge qubit in a double quantum dot and a high-impedance microwave resonator, reaching a cooperativity of C â 100. In addition, we leverage the frequency tunability of the resonator to perform detailed spectroscopy of a multi-level energy spectrum of low-lying states arising from strong Coulomb-correlation effects. To further improve the quality of the hole-photon interface, we investigate the source of microwave losses in different Ge/SiGe heterostructures, identifying defects at the lattice-mismatched Si/Ge interface as the dominant source of resonator losses in reverse-graded heterostructures. To mitigate this limitation, a deep tapered-etching approach is introduced, enabling the fabrication of superconducting resonators directly on etched heterostructures. This method yields internal quality factors exceeding Qint > 10 000 for high-impedance Josephson junction array resonators and Qint > 100 000 for 50 â Nb resonators. Finally, this thesis explores the spin properties of multi-hole quantum dots beyond the conventional single-orbital picture. By reconstructing and electrically tuning the g tensors of holes confined in different orbital states, we show that confinement can substantially modify both their magnitude and orientation. These results open new opportunities for engineering qubit properties through orbital occupation. Furthermore, highly coherent singlet-triplet oscillations are demonstrated in the seven-hole regime, yielding a quality factor of approximately 330 at in-plane magnetic fields as low as 2.5 mT. Together, these results establish a foundation for high-fidelity hole-spin-qubit operation based on g-tensor engineering and for the integration of planar Ge quantum dots with superconducting quantum circuits.
Circuit quantum electrodynamics is a powerful approach for scaling up the number of qubits in quantum processors, but the standard planar superconducting microwave resonators are poorly compatible with electron-spin qubits. The authors produce resonators that address the two main hurdles impeding the adoption of such a scaling mechanism for spin qubits: They strongly suppress unwanted vortex generation in magnetic fields up to 6 T, and they are projected to yield ten times the coupling of conventional devices.