Longitudinal coupling used for resonator readout of quantum dot qubits

Superconducting resonators coupled to quantum dot qubits are expected to form key components in scalable quantum computing architectures [1]. Longitudinal coupling mechanisms in these systems can enable quantum-nondemolition readout even when the qubit and resonator frequencies are far apart [2,3]. This type of coupling has only recently been measured in quantum dot qubit devices [4], and its full utility remains undemonstrated. Here, we show resonator readout of a quantum dot hybrid qubit using dynamic longitudinal coupling, which is turned on by applying an ac modulation to the qubit. Good visibility is achieved at qubit-cavity detuning over 10 GHz. This allows measurement of qubit energy splittings, which are confirmed using reservoir tunneling spectroscopy. We assess the readout signal-to-noise ratio and discuss how to maximize its value for qubit operation by tuning the longitudinal coupling strength. Tunability and frequency flexibility are appealing features of dynamic longitudinal coupling, and our work demonstrates its potential as a powerful new technique for semiconductor qubit readout.

[1] N. Holman et al. npj Quantum Inf 7, 137 (2021).

[2] A. J. Kerman. New J Phys 15, 123011 (2013).

[3] R. Ruskov and C. Tahan. Phys Rev B 99, 245306 (2019).

[4] C. G. L. Bøttcher et al. Nat Commun 13, 4773 (2022).