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arXiv:2407.08869 · npj Quantum Information

Ultra-dispersive resonator readout of a quantum-dot qubit using longitudinal coupling

Benjamin Harpt, J. Corrigan, Nathan Holman, Piotr Marciniec, Danna Rosenberg, D. Yost, Rabindra Nath Das, Rusko Ruskov, Charles Tahan, William D. Oliver, Robert McDermott, Mark G. Friesen, M. A. Eriksson

Published Jan 17, 2025• 8 citations
arXivHTMLPDF

Abstract

We perform readout of a quantum-dot hybrid qubit coupled to a superconducting resonator through a parametric, longitudinal interaction mechanism. Our experiments are performed with the qubit and resonator frequencies detuned by ~10 GHz, demonstrating that longitudinal coupling can facilitate semiconductor qubit operation in the ‘ultra-dispersive’ regime of circuit quantum electrodynamics.

DOI
10.1038/s41534-025-00962-w

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We perform readout of a quantum-dot hybrid qubit coupled to a superconducting resonator through a parametric, longitudinal interaction mechanism. Our experiments are performed with the qubit and resonator frequencies detuned by ~10 GHz, demonstrating that longitudinal coupling can facilitate semiconductor qubit operation in the ‘ultra-dispersive’ regime of circuit quantum electrodynamics.
In this study, we provide a full experimental characterization of the parametric longitudinal coupling between a complementary metal oxide semiconductor (CMOS) charge qubit and an off-chip rf resonator. Following Corrigan et al., Phys. Rev. Appl. 20, 064005 (2023), we activate parametric longitudinal coupling by driving the charge qubit at the resonator frequency. Managing the crosstalk between the drive applied to the qubit and the resonator allows for the systematic study of the dependence of the longitudinal and dispersive charge-photon couplings on the qubit-resonator detuning and the applied drive. Our experimental estimations of the charge-photon couplings are perfectly reproduced by theoretical simple formulas, without relying on any fitting parameter. We go further by showing a parametric displacement of the resonator's steady state, conditional on the qubit state, and the insensitivity of the longitudinal coupling constant to the photon population of the resonator. Our results pave the way for the exploration of the photon-mediated longitudinal readout and coupling of multiple and distant spins, with long coherent times, in hybrid CMOS circuit quantum electrodynamics architectures.
Semiconductor quantum dots in silicon demonstrate exceptionally long spin lifetimes as qubits and are therefore promising candidates for quantum information processing. However, control and readout techniques for these devices have thus far employed low frequency electrons, in contrast to high speed temperature readout techniques used in other qubit architectures, and coupling between multiple quantum dot qubits has not been satisfactorily addressed.This dissertation presents the design and characterization of a semiconductor charge qubit based on double quantum dot in silicon with an integrated microwave resonator for control and readout. The 6 GHz resonator is designed to achieve strong coupling with the quantum dot qubit, allowing the use of circuit QED control and readout techniques which have not previously been applicable to semiconductor qubits. To achieve this coupling, this document demonstrates successful operation of a novel silicon double quantum dot design with a single active metallic layer and a coplanar stripline resonator with a bias tee for dc excitation.Experiments presented here demonstrate quantum localization and measurement of both electrons on the quantum dot and photons in the resonator. Further, it is shown that the resonator-qubit coupling in these devices is sufficient to reach the strong coupling regime of circuit QED. The details of a measurement setup capable of performing simultaneous low noise measurements of the resonator and quantum dot structure are also presented here.The ultimate aim of this research is to integrate the long coherence times observed in electron spins in silicon with the sophisticated readout architectures available in circuit QED based quantum information systems. This would allow superconducting qubits to be coupled directly to semiconductor qubits to create hybrid quantum systems with separate quantum memory and processing components.
Coupling qubits to a superconducting resonator provides a mechanism to enable long-distance entangling operations in a quantum computer based on spins in semiconducting materials. Here, we demonstrate a controllable spin-photon coupling based on a longitudinal interaction between a spin qubit and a resonator. We show that coupling a singlet-triplet qubit to a high-impedance superconducting resonator can produce the desired longitudinal coupling when the qubit is driven near the resonator's frequency. We measure the energy splitting of the qubit as a function of the drive amplitude and frequency of a microwave signal applied near the resonator antinode, revealing pronounced effects close to the resonator frequency due to longitudinal coupling. By tuning the amplitude of the drive, we reach a regime with longitudinal coupling exceeding 1 MHz. This mechanism for qubit-resonator coupling represents a stepping stone towards producing high-fidelity two-qubit gates mediated by a superconducting resonator.
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.
Understanding the coupling of superconducting (SC) wires to solid state qubits is vital to the design of quantum computers. To date, there has not been a complete theory applicable to all parameter regimes, from the traditional dispersive regime of circuit QED to the resonance regime to the far-off resonance situation in the adiabatic regime. This work presents that theory in the context of modulated longitudinal coupling, which promises to enhance the readout and coupling speed and fidelity through a resonator of solid-state qubit operations.
Superconducting resonators coupled to semiconductor quantum dots provide a powerful platform to investigate light-matter interaction and decoherence mechanisms in solid-state quantum systems. Here we study a hybrid circuit quantum electrodynamics architecture consisting of a GaAs double-quantum-dot charge qubit capacitively coupled to a high-impedance, frequency-tunable SQUID-array resonator. By tuning the qubit transition frequency over the range ωq/2π∼3ω_\mathrm{q}/2π\sim 3ωq​/2π∼3-666 GHz, we perform frequency-resolved decoherence spectroscopy of the charge qubit across a broad energy window. Time-resolved measurements enable us to disentangle relaxation and pure dephasing processes and to identify distinct decoherence regimes as a function of qubit frequency. We find that at lower frequencies (≤4.5\leq 4.5≤4.5 GHz) dephasing dominates the qubit linewidth, whereas at higher frequencies energy relaxation becomes the leading contribution. The measured frequency dependence of the relaxation rate exhibits a cubic scaling, consistent with charge-qubit decay dominated by coupling to a piezoelectric phonon bath and providing frequency-resolved access to the corresponding phonon-induced spectral density. Our results show that hybrid semiconductor--superconducting circuits can serve as sensitive spectroscopic tools to probe microscopic decoherence mechanisms relevant for a wide range of hybrid quantum devices.