Electric Dipole Coupling of a Bilayer Graphene Quantum Dot to a High-Impedance Microwave Resonator
Max Josef Ruckriegel, Lisa Maria Gächter, David Kealhofer, Mohsen Bahrami Panah, Chuyao Tong, Christoph Adam, Michele Masseroni, Hadrien Duprez, Rebekka Garreis, Kenji Watanabe, Takashi Taniguchi, Andreas J. Wallraff, Thomas Ihn, K. Ensslin, W. Huang
High Resolution Image Download MS PowerPoint Slide We implement circuit quantum electrodynamics (cQED) with quantum dots in bilayer graphene, a maturing material platform that can host long-lived spin and valley states. Our device combines a high-impedance ( Z r ≈ 1 kΩ) superconducting microwave resonator with a double quantum dot electrostatically defined in a graphene-based van der Waals heterostructure. Electric dipole coupling between the subsystems allows the resonator to sense the electric susceptibility of the double quantum dot from which we reconstruct its charge stability diagram. We achieve sensitive and fast detection of the interdot transition with a signal-to-noise ratio of 3.5 within 1 μs integration time. The charge–photon interaction is quantified in the dispersive and resonant regimes by comparing the resonator response to input–output theory, yielding a coupling strength of g /2π = 49.7 MHz. Our results introduce cQED as a probe for quantum dots in van der Waals materials and indicate a path toward coherent charge–photon coupling with bilayer graphene quantum dots.
High Resolution Image Download MS PowerPoint Slide We implement circuit quantum electrodynamics (cQED) with quantum dots in bilayer graphene, a maturing material platform that can host long-lived spin and valley states. Our device combines a high-impedance ( Z r ≈ 1 kΩ) superconducting microwave resonator with a double quantum dot electrostatically defined in a graphene-based van der Waals heterostructure. Electric dipole coupling between the subsystems allows the resonator to sense the electric susceptibility of the double quantum dot from which we reconstruct its charge stability diagram. We achieve sensitive and fast detection of the interdot transition with a signal-to-noise ratio of 3.5 within 1 μs integration time. The charge–photon interaction is quantified in the dispersive and resonant regimes by comparing the resonator response to input–output theory, yielding a coupling strength of g /2π = 49.7 MHz. Our results introduce cQED as a probe for quantum dots in van der Waals materials and indicate a path toward coherent charge–photon coupling with bilayer graphene quantum dots.
Bilayer graphene is a maturing material platform for gate-defined quantum dots that hosts long-lived spin and valley states. Implementing solid-state qubits in bilayer graphene requires a fundamental understanding of such confined electronic systems. In particular, states of two and three carriers, for which the exchange interaction between particles plays a crucial role, are a cornerstone for qubit readout and manipulation. Here we report on the spectroscopy of few-carrier states in bilayer graphene quantum dots, using circuit quantum electrodynamics (cQED) techniques that offer substantially improved energy resolution compared to standard transport techniques. Measurements using a superconducting high-impedance resonator capacitively coupled to the double quantum dot reveal dispersive features of two and three electron states, enabling the detection of Pauli spin and valley blockade and the characterization of the spin-orbit gap at zero magnetic field. The results deepen our understanding of few-carrier spin and valley states in bilayer graphene quantum dots and demonstrate that cQED techniques are a powerful state-selective probe for semiconductor nanostructures.
Semiconductor quantum dots in which electrons or holes are isolated via electrostatic potentials generated by surface gates are promising building blocks for semiconductor-based quantum technology.Here, we investigate double-quantum-dot (DQD) charge qubits in GaAs capacitively coupled to highimpedance superconducting quantum interference device array and Josephson-junction array resonators.We tune the strength of the electric-dipole interaction between the qubit and the resonator in situ using surface gates.We characterize the qubit-resonator coupling strength, the qubit decoherence, and the detuning noise affecting the charge qubit for different electrostatic DQD configurations.We find all quantities to be systematically tunable over more than one order of magnitude, resulting in reproducible decoherence rates Γ 2 =2π < 5 MHz in the limit of high interdot capacitance.In the opposite limit, by reducing the interdot capacitance, we increase the DQD electric-dipole strength and, therefore, its coupling to the resonator.Employing a Josephson-junction array resonator with an impedance of approximately 4 kΩ and a resonance frequency of ω r =2π ∼ 5.6 GHz, we observe a coupling strength of g=2π ∼ 630 MHz, demonstrating the possibility to operate electrons hosted in a semiconductor DQD in the ultrastrong-coupling regime (USC).The presented results are essential for further increasing the coherence of quantum-dot-based qubits and investigating USC physics in semiconducting QDs.
Hybrid circuit quantum electrodynamics (cQED) aims at coupling various quantum degrees of freedom, among which are spin and charge degrees of freedom in gate defined quantum dots, phonons or magnons... with quantized electromagnetic fields in superconducting microwave cavities to investigate fundamental physics questions or for quantum computation and simulation. However, low microwave losses, key for many hybrid cQED experiments, are challenging to achieve given the often exotic and/or complex material stacks (e.g. semiconducting material, ferromagnets, or piezoelectric materials) required to host the various quantum degrees of freedom. In this work, we present a 3D-integration process to overcome this challenge for semi-industrial silicon MOS spin qubits. The process is based on dense indium bump interconnects at a pitch of 10 μm and superconducting thin films of Niobium Nitride (NbN). First, we report on DC and RF interconnect properties that demonstrate a high galvanic interconnection yield and internal quality factors above 105 in the single photon regime for NbN resonators interrupted by a single indium bump interconnect. Eventually, we fabricated a 3D-integrated hybrid circuit quantum electrodynamics (cQED) device based on a semi-industrial MOS hole double quantum dot and a high impedance NbN resonator. For this device, we report a cavity internal quality factor above 10000 and demonstrate record sensitivity for gate-based dispersive readout of the charge degree of freedom with an SNR of 100 in 300 ns. Finally, we demonstrate strong spin-photon coupling of gs/2π = 75 MHz, which highlights the viability of 3D-integration for quantum dot based hybrid spin circuit quantum electrodynamics and opens to high-fidelity spin readout and microwave photon-based remote spin qubit entanglement.
Abstract Scaling up spin qubits in silicon-based quantum dots is one of the pivotal challenges in achieving large-scale semiconductor quantum computation. To satisfy the connectivity requirements and reduce the lithographic complexity, utilizing the qubit array structure and the circuit quantum electrodynamics (cQED) architecture together is expected to be a feasible scaling scheme. A triple-quantum dot (TQD) coupled with a superconducting resonator is regarded as a basic cell to demonstrate this extension scheme. In this article, we investigate a system consisting of a silicon TQD and a high-impedance TiN coplanar waveguide (CPW) resonator. The TQD can couple to the resonator via the right double-quantum dot (RDQD), which reaches the strong coupling regime with a charge–photon coupling strength of g 0/(2π) = 175 MHz. Moreover, we illustrate the high tunability of the TQD through the characterization of stability diagrams, quadruple points (QPs), and the quantum cellular automata (QCA) process. Our results contribute to fostering the exploration of silicon-based qubit integration.
Cavity quantum electrodynamics (cQED) provides strong light-matter interactions that can be used for manipulating and detecting quantum states. The interaction can be enhanced by increasing the resonator’s impedance, while approaching the quantum impedance (h/e2) remains challenging. Edge plasmons emergent as chiral bosonic modes in the quantum Hall channels provide high quantized impedance of h/νe2 that can exceed 10 kΩ for the Landau-level filling factor ν ≤ 2, well beyond the impedance of free space. Here, we apply such a high-impedance plasmon mode in a quantum-Hall plasmon resonator to demonstrate dispersive detection of a nearby charge qubit formed in a double quantum dot. The phase shift in microwave transmission through the plasmon resonator follows the dispersive shift associated with the qubit state, in agreement with the cQED theory. The high impedance allows us to perform dispersive detection of qubit spectroscopy with a plasmon resonator having a broad bandwidth. Leveraging these topological edge modes, our results establish two-dimensional topological insulators as a new platform of cQED. Recent work reported plasmon-assisted tunneling in a double quantum dot coupled to a plasmon mode in a quantum Hall plasmon resonator. Here, the authors leverage this system to achieve broadband dispersive readout of a semiconductor charge qubit formed by a double quantum dot.
This thesis investigates hybrid circuit quantum electrodynamics (cQED) as a versatile platform that unites superconducting resonators with semiconductor quantum dots. The central motivation is to harness strong light-matter coupling at the single-photon level, which enables coherent photon-qubit interactions and paves the way for quantum technologies in computation, communication, and sensing. The first part of this work combines hybrid cQED with holes in planar Ge/SiGe, a material system that has only recently become accessible for quantum devices. Planar Ge offers unique advantages for this purpose and has attracted considerable attention in the context of quantum computing. In contrast, its potential for hybrid cQED has remained largely unexplored, making the present work one of the first demonstrations of strong light-matter interaction in this platform. Within this framework, we demonstrate strong charge-photon coupling in planar Ge by observing vacuum Rabi splitting between a hole charge qubit based on a double quantum dot (DQD) and a microwave resonator. We employ a frequency-tunable high-impedance resonator to perform qubit spectroscopy, enabling systematic exploration of coupling regimes and confirming device controllability. In addition, we use this setup to carry out excited-state spectroscopy of strongly correlated states in the DQD, revealing the formation of Wigner molecules and illustrating how the hybrid architecture can be harnessed to access many-body physics. While these experiments in Ge demonstrate how strong coupling can be harnessed to explore qubit physics and correlated states, they also exemplify the wider potential of hybrid cQED. A particularly pressing application is in quantum optics, where efficient single-photon detectors are still lacking in the microwave regime. Such detectors are essential for enabling experiments in quantum communication and microwave quantum optics. In the second part of this thesis, we address this challenge by realizing a photodetector based on a GaAs DQD coupled to a superconducting resonator. The detector operates via photon-to-charge conversion and achieves efficiencies approaching unity. Strong coupling plays a central role by ensuring that incoming photons are absorbed before being lost to the environment, which is key to the high performance of the detector. We further exploit the frequency tunability of the resonator to investigate device performance and identify the parameters that optimize detection efficiency. The broader impact of this work lies in advancing quantum technologies that rely on coherent microwave photons. Applications include spin qubit readout, long-distance photon-mediated coupling, quantum communication, and microwave quantum optics. Looking forward, planar Ge is identified as a highly competitive platform for hybrid cQED. Future directions include the development of single-photon sources, quantum- limited detectors, and analog quantum simulators based on engineered resonator-qubit arrays.