Longitudinal (curvature) couplings of an N-level qudit to a superconducting resonator at the adiabatic limit and beyond

Abstract

Understanding how and to what magnitude solid-state qubits couple to metallic wires is crucial to the design of quantum systems such as quantum computers. Here, we investigate the coupling between a multilevel system, or qudit, and a superconducting (SC) resonator's electromagnetic field, focusing on the interaction involving both the transition and diagonal dipole moments of the qudit. Specifically, we explore the effective dynamical (time-dependent) longitudinal coupling that arises when a solid-state qudit is adiabatically modulated at small gate frequencies and amplitudes, in addition to a static dispersive interaction with the SC resonator. We derive Hamiltonians describing the longitudinal multilevel interactions in a general dispersive regime, encompassing both dynamical longitudinal and dispersive interactions. These Hamiltonians smoothly transition between their adiabatic values, where the couplings of the nth level are proportional to the level's energy curvature concerning a qudit gate voltage, and the substantially larger dispersive values, which occur due to a resonant form factor. We provide several examples illustrating the transition from adiabatic to dispersive coupling in different qubit systems, including the charge (1e double quantum dot) qubit, the transmon, the double-quantum-dot singlet-triplet qubit, and the triple-quantum-dot exchange-only qubit. In some of these qubits, higher-energy levels play a critical role, particularly when their qubit's dipole moment is minimal or zero. For an experimentally relevant scenario involving a spin-charge qubit with magnetic field gradient coupled capacitively to a SC resonator, we showcase the potential of the dynamical longitudinal coupling. It enables close-to-quantum-limited quantum nondemolition measurements and remote geometric phase gates, demonstrating its practical utility in quantum information processing.

Source-reported materials — not catalogue approval

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