Preserving phase coherence and linearity in cat qubits with exponential bit-flip suppression

• URL: http://arxiv.org/abs/2409.17556v1
• Date: Thu, 26 Sep 2024 05:57:51 GMT
• Title: Preserving phase coherence and linearity in cat qubits with exponential bit-flip suppression
• Authors: Harald Putterman, Kyungjoo Noh, Rishi N. Patel, Gregory A. Peairs, Gregory S. MacCabe, Menyoung Lee, Shahriar Aghaeimeibodi, Connor T. Hann, Ignace Jarrige, Guillaume Marcaud, Yuan He, Hesam Moradinejad, John Clai Owens, Thomas Scaffidi, Patricio Arrangoiz-Arriola, Joe Iverson, Harry Levine, Fernando G. S. L. Brandão, Matthew H. Matheny, Oskar Painter,
• Abstract summary: Cat qubits can exhibit an exponential noise bias against bit-flip errors with increasing mean photon number. We show how to overcome this challenge by coloring the loss environment of the buffer mode with a multi-pole filter. We achieve near-ideal enhancement of cat-qubit bit-flip times with increasing photon number.
• Score: 29.57902297308655
• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• Abstract: 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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