Parasitic-free gate: A protected switch between idle and entangled states

TL;DR

The paper introduces the parasitic-free (PF) gate, a protected two-qubit gate for superconducting qubits that switches between idle and entangled states while maintaining zero total $ZZ$ interaction. By combining tunable circuit parameters with cross-resonance driving, the PF gate achieves a driven $ZX$ interaction without residual $ZZ$ in either mode, using the concept of dynamic freedom to cancel parasitic couplings. The scheme is demonstrated across multiple circuit realizations, including tunable and weakly tunable couplers, and analyzes both idle-mode (GI and AI) and entangled-mode operation, along with detailed error mitigation during coupler-frequency switching and driving. Gate times near 100 ns with fidelities approaching 99.9% are reported, highlighting robustness to leakage and decoherence and broad applicability to different superconducting qubit platforms. Overall, the PF gate provides a scalable path to high-fidelity, error-resilient quantum gates by integrating circuit tunability with driving to suppress parasitic interactions throughout the idle-to-entangled cycle.

Abstract

We propose a gate to switch superconducting qubit pairs in and out of a two-body interaction. This gate uses cross resonance driving on a tunable circuit with adjusted parameters and without accumulating residual ZZ interaction for idle and interacting qubits. It is imperative that this gate does not spread errors through the quantum register. Our detailed theoretical results show that these error-free modes do not necessarily require largely tunable circuits, such as magnetic modulation of qubits or couplers. We obtain the operational gate on weakly tuneable circuits as well and show that switching between them is remarkably fast.

Figures (15)

• Figure 1: Schematic switching PF gate between idle and entangled modes. Net parasitic $ZZ(\Omega)$ interaction after activating CR($\Omega$) gate may vanish at certain coupling strengths, i.e. $J_{\textup{on}}$ points. The static $ZZ$ interaction, shown as $ZZ(\Omega=0)$, has zeros at genuine and affine points.
• Figure 2: (a) The PF gate at idle mode is active during PF/I operation box (green) and at entangled mode during PF/E operation box (pink). (b) Left: PF gate circuit; Right: PF gate timing and components. $t_g$ is the duration of the entangled mode with $\tau_0$ being the rise/fall time from idle to entangled modes
• Figure 3: Energy levels $E_{q_1,c,q_2}$ and microwave-driven transitions in a frame co-rotating with the microwave pulse. Double arrowed solid (dashed) lines show computational (noncomputational) transitions. Shaded area shows near $E_{101}$ zone.
• Figure 4: Circuits with two qubits interacting via a coupler for implementing PF gate, contains (a) wide frequency-tunable coupler (b) weakly tunable coupler, and (c) a tunable frequency qubit. Lower panels show how PF gate on each circuit switches between I and E modes.
• Figure 5: Numerical static $ZZ$ strength in the seven devices listed in Table \ref{['tab:device']} versus coupler frequencies.