arXiv subjects

Ilya A. Simakov

Publications and source records attributed to Ilya A. Simakov.

10 recordsLinked to original sources

arXiv2026

Native CCZ Gate with Fluxonium Qubits and a Microwave-Driven Coupler

Native multi-qubit gates could reduce the overhead associated with decompositions into single- and two-qubit operations, but whether they can simultaneously provide high fidelity, simple control and robustness against parasitic interactions in scalable architectures remains unclear. Here we experimentally realize a 65-ns native controlled-controlled-phase operation, locally equivalent to the Toffoli gate, with a fidelity of 99.39(5)% in a three-qubit processor unit based on fluxonium qubits coupled via a microwave-driven transmon coupler. The implemented operation would require CZ fidelities of approximately 99.94% if realized through a conventional decomposition. The gate is implemented with a single control pulse, that relies on a simple calibration procedure yielding coherence-limited performance. This processor unit naturally extends to scalable two-dimensional layouts with low parasitic interactions. Altogether, these results establish native multi-qubit gates as a viable hardware-efficient primitive for scalable superconducting quantum processors.

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arXiv2026

Interleaved Benchmarking with Single-Qubit References

Cross-entropy benchmarking (XEB) with single-qubit reference sequences is widely used to characterize multi-qubit gates in large-scale quantum processors, despite the lack of a rigorous theoretical justification. Here we show that the commonly employed additive single-qubit errors approximation underlying this approach breaks down and leads to a systematic overestimation of gate fidelities. We derive an analytical expression for the joint decay of simultaneous single-qubit reference sequences, identify when interleaved circuits generate sufficient randomization for the standard depolarizing approximation, and obtain a refined expression for the interleaved gate fidelity estimation. Experiments on a superconducting quantum processor validate the theory and demonstrate that fidelities obtained using XEB with single-qubit references agree with those extracted from standard interleaved randomized benchmarking (IRB), while achieving higher precision due to reduced reference-sequence errors. Our results establish theoretical foundation for interleaved benchmarking protocols employing single-qubit reference sequences and show that, with appropriate post-processing, they provide a simple and accurate alternative to multi-qubit Clifford reference circuits.

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arXiv2025

Interaction-Resilient Scalable Fluxonium Architecture with All-Microwave Gates

Fluxonium qubits demonstrate exceptional potential for quantum processing; yet, realizing scalable architectures using them remains challenging. We propose a fluxonium-based square-grid design with fast $\sim63$~ns controlled-Z (CZ) gates, achieving coherent errors below $10^{-4}$, activated via microwave-driven transmon couplers. A central difficulty in such large-scale systems with all-microwave gates and, therefore, strong static couplings, is suppressing parasitic interactions that extend beyond nearest neighbors to include next-nearest elements. We address this issue by introducing several design strategies: the frequency allocation of both qubits and couplers, the localization of coupler wavefunctions, and a differential oscillator that suppresses residual long-range interactions. In addition, the architecture natively supports fast $\sim70$~ns CZZ gates -- three-qubit operations composed of two CZ gates sharing a common qubit -- which reduce the incoherent error by $\sim 35\%$ compared to performing the corresponding CZs sequentially. Together, these advances establish an interaction-resilient platform for large-scale fluxonium processors and can be adapted to a variety of fluxonium layouts.

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arXiv2025

Realization of a Quantum Error Detection Code with a Dynamically Reassigned Ancillary Qubit

Quantum error correction (QEC) is essential for achieving fault-tolerant quantum computing. While superconducting qubits are among the most promising candidates for scalable QEC, their limited nearest-neighbor connectivity presents significant challenges for implementing a wide range of error correction codes. In this work, we experimentally demonstrate a quantum error detection scheme that employs a dynamically reassigned ancillary qubit on a chain of three linearly connected transmon qubits. We show that this scheme achieves performance comparable to conventional static-ancilla circuits. Additionally, the approach facilitates efficient quantum state preparation, which we demonstrate with tomography of arbitrary logical states. Our results provide a flexible method for implementing QEC codes under connectivity constraints and highlight a promising path toward scalable quantum architectures.

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arXiv2025

Three-mode tunable coupler for superconducting two-qubit gates

Building a scalable universal high-performance quantum processor is a formidable challenge. In particular, the problem of realizing fast high-perfomance two-qubit gates of high-fidelity remains needful. Here we propose a building block for a scalable quantum processor consisting of two transmons and a tunable three-mode coupler allowing for a ZZ interaction control. We experimentally demonstrate the native CZ gate with the pulse duration of 60 ns achieving the two-qubit gate fidelity above 98%, limited mostly by qubit coherence time. Numerical simulations show that by optimizing the gate duration the fidelity can be pushed over 99.97%.

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arXiv2025

Impact of qubit anharmonicity on near-resonant Rabi oscillations

Precise quantum control relies on a deep understanding of the dynamics of quantum systems under external drives. This study investigates the impact of anharmonicity on qubit dynamics under conditions typical for two-qubit entangling gates activated by weak near-resonant microwave drives. We measure the Rabi oscillation frequency as a function of drive amplitude and detuning. Our results reveal a linear dependence of the squared Rabi frequency on the squared drive amplitude, which relates to the ratio of detuning to anharmonicity, demonstrating strong agreement between experimental data and analytical predictions. Additionally, we analyze the leakage and phase errors arising from inaccurate Rabi frequency adjustments in the CZ gate implementation on fluxonium qubits driven by a microwave signal applied to the coupler.

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arXiv2024

High-fidelity transmon coupler activated CCZ gate on fluxonium qubits

The Toffoli gate takes a special place in the quantum information theory. It opens up a path for efficient implementation of complex quantum algorithms. Despite tremendous progress of the quantum processors based on the superconducting qubits, realization of a high-fidelity three-qubit operation is still a challenging problem. Here, we propose a novel way to perform a high-fidelity CCZ gate on fluxoniums capacitively connected via a transmon qubit, activated by a microwave pulse on the coupler. The main advantages of the approach are relative quickness, simplicity of calibration and significant suppression of the unwanted longitudinal ZZ interaction. We provide numerical simulation of 95-ns long gate of higher than 99.99% fidelity with realistic circuit parameters in the noiseless model and estimate an error of about 0.25% under the conventional decoherence rates.

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arXiv2024

Demonstration of a parity-time symmetry breaking phase transition using superconducting and trapped-ion qutrits

Scalable quantum computers hold the promise to solve hard computational problems, such as prime factorization, combinatorial optimization, simulation of many-body physics, and quantum chemistry. While being key to understanding many real-world phenomena, simulation of non-conservative quantum dynamics presents a challenge for unitary quantum computation. In this work, we focus on simulating non-unitary parity-time symmetric systems, which exhibit a distinctive symmetry-breaking phase transition as well as other unique features that have no counterpart in closed systems. We show that a qutrit, a three-level quantum system, is capable of realizing this non-equilibrium phase transition. By using two physical platforms -- an array of trapped ions and a superconducting transmon -- and by controlling their three energy levels in a digital manner, we experimentally simulate the parity-time symmetry-breaking phase transition. Our results indicate the potential advantage of multi-level (qudit) processors in simulating physical effects, where additional accessible levels can play the role of a controlled environment.

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arXiv2023

Coupler microwave-activated controlled phase gate on fluxonium qubits

Tunable couplers have recently become one of the most powerful tools for implementing two-qubit gates between superconducting qubits. A tunable coupler typically includes a nonlinear element, such as a SQUID, which is used to tune the resonance frequency of an LC circuit connecting two qubits. Here we propose a complimentary approach where instead of tuning the resonance frequency of the tunable coupler by applying a quasistatic control signal, we excite by microwave the degree of freedom associated with the coupler itself. Due to strong effective longitudinal coupling between the coupler and the qubits, the frequency of this transition strongly depends on the computational state, leading to different phase accumulations in different states. Using this method, we experimentally demonstrate a CZ gate of 44 ns duration on a fluxonium-based quantum processor, obtaining a fidelity of $97.6\pm 0.4 \%$ characterized by cross-entropy benchmarking.

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arXiv2022

High fidelity two-qubit gates on fluxoniums using a tunable coupler

Superconducting fluxonium qubits provide a promising alternative to transmons on the path toward large-scale superconductor-based quantum computing due to their better coherence and larger anharmonicity. A major challenge for multi-qubit fluxonium devices is the experimental demonstration of a scalable crosstalk-free multi-qubit architecture with high fidelity single-qubit and two-qubit gates, single-shot readout and state initialization. Here, we present a two-qubit fluxonium-based quantum processor with a tunable coupler element following our theoretical proposal [DOI: 10.1063/5.0064800]. We experimentally demonstrate fSim-type and controlled-Z gates with $99.55\%$ and $99.23\%$ fidelities, respectively. The residual ZZ interaction is suppressed down to the few kHz level. Using a galvanically coupled flux control line, we implement high fidelity single-qubit gates and ground state initialization with a single arbitrary waveform generator channel per qubit.

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