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From single qubit to many-body readout in Circuit Quantum Electrodynamics platforms

Alessandro Chessari

Published Nov 19, 2025• 0 citations
Publisher

Abstract

Robust manipulation and control of quantum systems require a fast and efficient way to read out their state. After cavity quantum electrodynamics (cavity QED) pioneered the manipulation of neutral atoms and ions with light, circuit quantum electrodynamics (circuit QED) became the dominant readout paradigm for artificial atoms such as superconducting circuits, with dispersive readout widely adopted for its stability and efficiency.Recently, longitudinal readout has emerged as a potentially faster readout protocol, but relies on sophisticated parametric modulation, whose underlying mechanisms have remained unclear so far.Longitudinal interactions have been demonstrated across platforms ranging from superconducting qubits to quantum dots, and rely on driving the qubit at the resonator frequency. In different regimes, the coupling emerges parametrically in the drive strength, growing linearly as the drive increases.In this manuscript, we experimentally demonstrate the breakdown of this parametric regime in a two-level quantum dot charge qubit and devise a unifying Floquet theory to establish a universal connection between AC Stark shift, longitudinal coupling and dispersive readout in cQED.We derive analytical results supported by exact numerical simulations that we apply to superconducting and spin-hybrid cQED systems, showcasing the flexibility of faster-than-dispersive longitudinal readout.Inspired by cQED’s readout capabilities in single qubit measurements, we address many-body state readout in quantum simulators.Since the dynamical structure factor (DSF) encodes key information about density excitations in quantum systems, we propose Many-Body Admittance Spectroscopy (MADS) as a weakly invasive method to probe the DSF by analyzing the transmission coefficients of microwave signals coupled to the quantum simulator.We demonstrate the approach on a one-dimensional spinless fermionic chain, representative of quantum dot implementations of the Heisenberg model, and we discuss, supported by numerical simulations obtained using exact diagonalization and density matrix renormalization group (DMRG), the possibility to measure the qualitative modifications of the DSF caused by interactions. Our results highlight the potential of MADS for characterizing the dynamical properties in highly tunable solid-state quantum simulators, extending the toolbox for the readout of quantum simulators.

DOI
10.70675/88fa093fz851fz4bdbzaa3ez17a3f556a758

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We devise a Floquet theory of longitudinal and dispersive readout in circuit quantum electrodynamics (cQED). By studying qubits coupled to cavity photons and driven at the resonance frequency of the cavity ${\ensuremath{\omega}}_{\mathrm{r}}$, we establish a universal connection between the qubit ac Stark shift and the longitudinal and dispersive coupling to photons. We find that the longitudinal coupling ${g}_{\ensuremath{\parallel}}$ is controlled by the slope of the ac Stark shift as function of the driving strength Aq{A}_{\mathrm{q}}Aq​, while the dispersive shift $\ensuremath{\chi}$ depends on its curvature. The two quantities become proportional to each other in the weak drive limit (${A}_{\mathrm{q}}\ensuremath{\rightarrow}0$). Our approach unifies the adiabatic limit (${\ensuremath{\omega}}_{\mathrm{r}}\ensuremath{\rightarrow}0$)---where ${g}_{\ensuremath{\parallel}}$ is generated by the static spectrum curvature (or quantum capacitance)---with the diabatic limit, where ${\ensuremath{\omega}}_{\mathrm{r}}$ is large and the static spectrum plays no role. We derive analytical results supported by exact numerical simulations. We apply them to superconducting and spin-hybrid cQED systems, showcasing the flexibility of faster-than-dispersive longitudinal readout.
The need for fast readout to measure the state of a quantum dot device has important applications in the capabilities of using them as a means for quantum information processing. Traditional charge sensors used to readout the charge state of a quantum dot device are very robust and sensitive electrometers that have been in use since the late 1980s. They pose an issue to the future of quantum dots as a viable means of quantum computing, and that is in their slow acquisition times of the charge state of the quantum dot. This rate of measurement means that implementing any quantum error correction scheme becomes very difficult or even unfeasible as the coherence times of these semiconductor systems is short compared to charge sensor’s acquisition times. A possibility to speed up the measurement process of the charge state is possible thanks to the atomic molecular and optical (AMO) physics community.Cavity quantum electrodynamics (CQED) is the study of light-matter interactions when both the light and matter are taken to be quantized. A photon may interact with an atom via a dipole interaction to excite it from its ground state to its excited state, and the reverse can occur where the atom emits a photon and decays from its excited state ii to the ground state. This fundamental interaction which has been extensively studied in the AMO community since the early 1960s had been adopted by the superconducting qubit community to measure the state of their qubits. These superconducting qubits, representing atoms, which are coupled to a resonant structure on chip, which represents the photons that can interact with the artificial atom. Since these systems were not traditional trapped or neutral atoms interacting with a photon via a laser, but instead planar circuit structures, the superconducting qubit community re-coined the term cavity quantum electrodynamics to circuit quantum electrodynamics, which we will refer to as cQED.The quantum dot community adapted these methods of what is known as dispersive readout; a capacitive change of the atomic like system, results in a dispersive shift which then alters the characteristic frequency response of the resonant structure. This has allowed for a new kind of charge sensor by means of measuring the resonators response due to its coupling to a quantum dot system. This can be done with very high frequency microwave circuitry, allowing much faster readout than is capable via traditional charge sensors. Thus, allowing a pathway to implement quantum error correction schemes with reasonable times compared to any dephasing or decoherence times of the semiconductor system. Furthermore, such fast readout allows for feedback protocols to be implemented to the system to correct for certain undesirable effects due to instruments or the system itself. Our work focuses on a limit of this cQED interaction of the quantum dots to a resonant structure allowing us to do fast readout of the quantum dot charge states via a dispersive shift.The work I will demonstrate in this dissertation will be in the efforts of fast readout on the charge occupation of a double quantum dot system which is galvanically coupled to a superconducting microwave resonator. This was done in a Si/SiGe heterostructure system where I designed, simulated, and fabricated the device which was optimized to have a stronger dispersive shift signal due to an enhanced gate lever arm that couples to the resonator. I will discuss our results of attaining a minimum integration time of 34.54 ns for a unity signal to noise ratio. This demonstrates a fast readout system in the community iii which could be extended to allow for even better readout speeds with faster instruments than what were available to me for the experiment. I will further discuss our characterization of the hybrid system and how we extracted a charge noise due to charging events between the two dots in our system up to around 10 kHz. This work demonstrates a robust dispersive readout scheme which helps in the scalability of hybrid superconducting-semiconducting architectures for means of quantum information processing.
Circuit quantum electrodynamics (cQED), exemplified by a coupled system of a superconducting qubit and a microwave cavity, is a powerful framework for the quantum control and measurement of macroscopic quantum states in solid-state circuits. Quantum circuits based on superconducting qubits have emerged as one of the leading platforms in the pursuit of fault-tolerant quantum computing. Despite significant progress in extending qubit lifetimes, refining experimental protocols, and scaling up quantum processors, errors arising from the control and measurement of superconducting qubits continue to hinder the realization of a universal quantum computer. To investigate the error mechanisms for state-of-the-art qubits, we realize long-lived superconducting transmon qubits that exhibit fluctuating lifetimes, with an average of 0.2 ms and a maximum exceeding 0.4 ms. We then investigate their dominant error mechanism. By introducing novel time-resolved error measurements that are synchronized with the operation of the pulse tube cooler in a dilution refrigerator, we find that mechanical vibrations from the pulse tube induce nonequilibrium dynamics in highly coherent qubits, leading to their correlated bit-flip errors. Our findings not only deepen our understanding of qubit error mechanisms but also provide valuable insights into potential error-mitigation strategies for achieving fault tolerance by decoupling superconducting qubits from their mechanical environments. We also present a novel scheme for operating a cQED system in a regime where the cavity is largely displaced. We find that a qubit drive at the bare cavity frequency can effectively displace the cavity field while minimizing the cavity-drive-induced dephasing and the ac Stark shift. Furthermore, we demonstrate that standard cQED protocols, such as qubit control, readout, and cavity single-photon generation, remain compatible with largely displaced cavity fields, even beyond the critical photon number. These findings open new pathways for exploring cQED dynamics in large-photon-number regimes, including the study of nonlinear phenomena in microwave cavities, the saturation of two-level system baths for extending the coherence of microwave quantum states, and the acceleration of the dispersive readout. Finally, mechanical resonators have emerged as integral components in quantum computing for storing and transducing quantum information. Low-frequency mechanical resonators, with extremely low energy dissipation rates, are particularly interesting for extending the lifetimes of stored quantum information. However, integrating low-frequency mechanical resonators with qubits is challenging because their small energy gaps make coherent energy exchange and ground-state preparation difficult. In this thesis, we report the first steps towards the characterization of a hybrid quantum device, consisting of a Transmon qubit and a circuit optomechanical system, both coupled to a 3D microwave cavity.
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 μ\mathrmμμm 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 10510^{5}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/2πg_{s}/2πgs​/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.
We show how to realize fast and high-fidelity quantum nondemolition qubit readout using longitudinal qubit-oscillator interaction. This is accomplished by modulating the longitudinal coupling at the cavity frequency. The qubit-oscillator interaction then acts as a qubit-state dependent drive on the cavity, a situation that is fundamentally different from the standard dispersive case. Single-mode squeezing can be exploited to exponentially increase the signal-to-noise ratio of this readout protocol. We present an implementation of this longitudinal parametric readout in circuit quantum electrodynamics and a possible multiqubit architecture.