Realization of a $CQ_3$ Qubit: energy spectroscopy and coherence

TL;DR

This paper introduces a new triple quantum dot charge qubit with a third order sweet spot, analyzes its coupling to photons, and investigates its decoherence properties near the proposed operating point.

Contribution

It proposes a novel $CQ_3$ qubit with a third order sweet spot and studies its energy spectroscopy, photon coupling, and decoherence mechanisms.

Findings

Strong coupling to single photons achieved.

Linewidth exhibits a local maximum near the operating point.

Quadrupolar noise significantly affects decoherence, even at the sweet spot.

Abstract

The energy landscape of a single electron in a triple quantum dot can be tuned such that the energy separation between ground and excited states becomes a flat function of the relevant gate voltages. These so-called sweet spots are beneficial for charge coherence, since the decoherence effects caused by small fluctuations of gate voltages or surrounding charge fluctuators are minimized. We propose a new operation point for a triple quantum dot charge qubit, a so-called $CQ_3$-qubit, having a third order sweet spot. We show strong coupling of the qubit to single photons in a frequency tunable high-impedance SQUID-array resonator. In the dispersive regime we investigate the qubit linewidth in the vicinity of the proposed operating point. In contrast to the expectation for a higher order sweet spot, we there find a local maximum of the linewidth. We find that this is due to a non-negligible contribution of noise on the quadrupolar detuning axis not being in a sweet spot at the proposed operating point. While the original motivation to realize a low-decoherence charge qubit was not fulfilled, our analysis provides insights into charge decoherence mechanisms relevant also for other qubits.