  The Kerr-cat qubit is a bosonic qubit in which multi-photon Schrodinger cat
states are stabilized by applying a two-photon drive to an oscillator with a
Kerr nonlinearity. The suppressed bit-flip rate with increasing cat size makes
this qubit a promising candidate to implement quantum error correction codes
tailored for noise-biased qubits. However, achieving strong light-matter
interactions necessary for stabilizing and controlling this qubit has
traditionally required strong microwave drives that heat the qubit and degrade
its performance. In contrast, increasing the coupling to the drive port removes
the need for strong drives at the expense of large Purcell decay. By
integrating an effective band-block filter on-chip, we overcome this trade-off
and realize a Kerr-cat qubit in a scalable 2D superconducting circuit with high
coherence. This filter provides 30 dB of isolation at the qubit frequency with
negligible attenuation at the frequencies required for stabilization and
readout. We experimentally demonstrate quantum non-demolition readout fidelity
of 99.6% for a cat with 8 photons. Also, to have high-fidelity universal
control over this qubit, we combine fast Rabi oscillations with a new
demonstration of the X(90) gate through phase modulation of the stabilization
drive. Finally, the lifetime in this architecture is examined as a function of
the cat size of up to 10 photons in the oscillator achieving a bit-flip time
higher than 1 ms and only a linear decrease in the phase-flip time, in good
agreement with the theoretical analysis of the circuit. Our qubit shows promise
as a building block for fault-tolerant quantum processors with a small
footprint.
