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arXiv:2603.29308

Change in bit-flip times of Kerr parametric oscillators caused by their interactions

Yuya Kano, Yohei Kawakami, Shumpei Masuda, Tomohiro Yamaji, Aiko Yamaguchi, Tetsuro Satoh, Ayuka Morioka, Kiyotaka Endo, Yuichi Igarashi, Masayuki Shirane, Tsuyoshi Yamamoto

quant-phSubmitted Mar 31, 2026• Announced Apr 1, 2026• 0 citations
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Abstract

We experimentally investigate how interactions between Kerr parametric oscillators (KPOs) degrade their bit-flip times, where a bit flip is defined as a transition between the two degenerate ground states of a KPO. Interactions between KPOs cause quantum states of KPOs to leak outside the computational subspace, leading to bit flips. Bit flips degrade fidelity and pose a significant problem for KPO-based quantum information processing. We performed an experiment in which a weak microwave signal is injected into one KPO to emulate photon injection from another KPO, and find that the bit-flip time decreases by an order of magnitude due to induced excitations, depending on the frequency and power of the injected signal. Methods to mitigate the decrease in bit-flip times caused by interactions between KPOs are discussed, including adjusting the pump frequencies, coherent-state amplitudes, and couplings between KPOs. These findings provide valuable insights for scaling up KPO-based quantum computers.

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We experimentally investigate how interactions between Kerr parametric oscillators (KPOs) degrade their bit-flip times, where a bit flip is defined as a transition between the two degenerate ground states of a KPO. Interactions between KPOs cause quantum states of KPOs to leak outside the computational subspace, leading to bit flips. Bit flips degrade fidelity and pose a significant problem for KPO-based quantum information processing. We performed an experiment in which a weak microwave signal is injected into one KPO to emulate photon injection from another KPO, and find that the bit-flip time decreases by an order of magnitude due to induced excitations, depending on the frequency and power of the injected signal. Methods to mitigate the decrease in bit-flip times caused by interactions between KPOs are discussed, including adjusting the pump frequencies, coherent-state amplitudes, and couplings between KPOs. These findings provide valuable insights for scaling up KPO-based quantum computers.
Kerr parametric oscillators (KPOs), which can be implemented with superconducting parametrons possessing large Kerr nonlinearity, have been attracting much attention in terms of their applications to quantum annealing, universal quantum computation, and studies of quantum many-body systems. It is of practical importance for these studies to realize fast and accurate tunable coupling between KPOs in a simple manner. We develop a simple scheme of fast tunable coupling of KPOs with high tunability in speed and amplitude using the fast transitionless rotation of a KPO in the phase space based on the shortcuts to adiabaticity. Our scheme enables rapid switching of the effective coupling between KPOs, and can be implemented with always-on linear coupling between KPOs, by controlling the phase of the pump field and the resonance frequency of the KPO without controlling the amplitude of the pump field nor using additional drive fields and couplers. We apply the coupling scheme to a two-qubit gate, and show that our scheme realizes high gate fidelity compared with a purely adiabatic one, by mitigating undesired nonadiabatic transitions.
Kerr parametric oscillators (KPOs) can stabilize the superpositions of coherent states, which can be utilized as qubits, and are promising candidates for realizing hardware-efficient quantum computers. Although elementary gates for universal quantum computation with KPO qubits have been proposed, these gates are usually based on adiabatic operations and thus need long gate times, which result in errors caused by photon loss in KPOs realized by, e.g., superconducting circuits. In this work we accelerate the elementary gates by experimentally feasible control methods, which are based on numerical optimization of pulse shapes for shortcuts to adiabaticity. By numerical simulations, we show that the proposed methods can achieve speedups compared to adiabatic ones by up to six times with high gate fidelities of 99.9%. These methods are thus expected to be useful for quantum computers with KPOs.
We study parametric oscillations of two capacitively coupled Kerr parametric oscillators (KPOs) with frequency-chirped two- and one-photon drives. The two-KPO system adiabatically evolves from the initial vacuum state to an oscillation state corresponding to a solution state in quantum-annealing applications. Frequency chirping dynamically changes the detuning between resonance and oscillation frequencies during parametric modulation and reduces unwanted population transfer to excited states caused by pure dephasing and photon loss. We observe that frequency chirping increases the success probability to obtain the solution state and that simulations taking into account pure dephasing reproduce experiments with and without frequency chirping. This study demonstrates the effectiveness and applicability of frequency chirping to a KPO-based quantum annealer.
Quantum computing crucially relies on maintaining quantum coherence for the duration of a calculation. Bosonic quantum error correction protects this coherence by encoding qubits into superpositions of noise-resilient oscillator states. In the case of the Kerr-cat qubit (KCQ), these states derive their stability from being the quasi-degenerate ground states of an engineered Hamiltonian in a driven nonlinear oscillator. KCQs are experimentally compatible with on-chip architectures and high-fidelity operations, making them promising candidates for a scalable bosonic quantum processor. However, their bit-flip time must increase further to fully leverage these advantages. Here, we present direct evidence that the bit-flip time in a KCQ is limited by leakage out of the qubit manifold and experimentally mitigate this process. We coherently control the leakage population and measure it to be > 9%, twelve times higher than in the undriven system. We then cool this population back into the KCQ manifold with engineered dissipation, identify conditions under which this suppresses bit-flips, and demonstrate increased bit-flip times up to 3.6 milliseconds. By employing both Hamiltonian confinement and engineered dissipation, our experiment combines two paradigms for Schrödinger-cat qubit stabilization. Our results elucidate the interplay between these stabilization processes and indicate a path towards fully realizing the potential of these qubits for quantum error correction.
Abstract A Kerr parametric oscillator (KPO) is a Kerr-nonlinear resonator driven parametrically, which can serve as a qubit in quantum information processing. Spectroscopy using a weak probe field provides a practical method for extracting system parameters. In this work, we theoretically investigate reflection and transmission measurements of two-photon and four-photon KPOs, introducing interference effects between inter-level transitions. Because KPOs typically exhibit energy-level degeneracy, a probe field can simultaneously resonate with multiple inter-level transitions. We extend the previous theory of reflection measurements by incorporating the interaction between inter-level transitions and off-diagonal elements of the density matrix which had previously been neglected. We demonstrate that interference among these transitions substantially modifies the spectrum. We identify the conditions for the interference, as well as those under which the off-diagonal elements of the density matrix affect the spectrum. The theory is also generalized to transmission measurements, and is applicable to a broad class of systems beyond KPOs.
Quantum Kerr parametric oscillators (KPOs) are systems out of equilibrium with a wide range of applications in quantum computing, quantum sensing, and fundamental research. They have been realized in superconducting circuits and photonic platforms. In this work, we explore the onset of ground-state and excited-state quantum phase transitions in KPOs, focusing on the role of the phase-space rotational symmetry when the driving frequency is $\ensuremath{\mu}$ times the oscillator's natural frequency, specifically for $\ensuremath{\mu}=1,2,3,4$. These cases are experimentally accessible in superconducting circuits, where the Floquet quasienergy spectrum can also be studied as a function of tunable control parameters. Using the classical Hamiltonian of the system, we identify the stationary points associated with quantum phase transitions and analyze the emergence of both real and avoided level crossings, examining their influence on the energy spectrum and tunneling dynamics. Our findings provide insights into the engineering of robust quantum states, quantum dynamics control, and onset of quantum phase transitions with implications for critical quantum sensing.