Low Anharmonicity Transmons in Parametrically Driven Quantum Modules

Transmon qubits operate in a fairly narrow range of anharmonicity bounded from above by increasing sensitivity to charge dispersion, and from below by the need to control the system quickly with high fidelity. For many two-qubit gates, where the qubit nonlinearities are a main driver of the interaction strength, larger anharmonicity is desired. However, for parametric gates actuated with driven couplers such as a SNAIL, it is the SNAIL third-order nonlinearity which sets the scale of the interaction. Because of this, we’re free to optimize such gates with transmons designed with small anharmonicity which may have a number of benefits. In particular, these qubits should be far less sensitive to offset charge and more closely approximate a harmonic oscillator where it is common to find that T₂≈2T₁. Additionally, the anharmonicity strongly influences unwanted processes such as `ionization’, bifurcation, and transitions to chaos, and so these qubits may have interestingly different response to strong drives. In this talk, we discuss the design and characterization of transmons with 70 MHz anharmonicity and a g-e transition frequency around 4 GHz. We also explore the limitations on speed and fidelity of single- and mulit-qubit parametric gates and how well these can be overcome using pulse-engineering.