Kavli Affiliate: Oskar Painter
| First 5 Authors: Harald Putterman, Kyungjoo Noh, Rishi N. Patel, Gregory A. Peairs, Gregory S. MacCabe
| Summary:
Cat qubits, a type of bosonic qubit encoded in a harmonic oscillator, can
exhibit an exponential noise bias against bit-flip errors with increasing mean
photon number. Here, we focus on cat qubits stabilized by two-photon
dissipation, where pairs of photons are added and removed from a harmonic
oscillator by an auxiliary, lossy buffer mode. This process requires a large
loss rate and strong nonlinearities of the buffer mode that must not degrade
the coherence and linearity of the oscillator. In this work, we show how to
overcome this challenge by coloring the loss environment of the buffer mode
with a multi-pole filter and optimizing the circuit to take into account
additional inductances in the buffer mode. Using these techniques, we achieve
near-ideal enhancement of cat-qubit bit-flip times with increasing photon
number, reaching over $0.1$ seconds with a mean photon number of only $4$.
Concurrently, our cat qubit remains highly phase coherent, with phase-flip
times corresponding to an effective lifetime of $T_{1,text{eff}} simeq 70$
$mu$s, comparable with the bare oscillator lifetime. We achieve this
performance even in the presence of an ancilla transmon, used for reading out
the cat qubit states, by engineering a tunable oscillator-ancilla dispersive
coupling. Furthermore, the low nonlinearity of the harmonic oscillator mode
allows us to perform pulsed cat-qubit stabilization, an important control
primitive, where the stabilization can remain off for a significant fraction
(e.g., two thirds) of a $3~mathrm{mu s}$ cycle without degrading bit-flip
times. These advances are important for the realization of scalable
error-correction with cat qubits, where large noise bias and low phase-flip
error rate enable the use of hardware-efficient outer error-correcting codes.
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