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  • Featured in Physics
  • Open Access

Fast Sideband Control of a Multimode Cavity Memory with Weak Dispersive Coupling to a Transmon

Jordan Huang1,*, Thomas J. DiNapoli1, Gavin Rockwood1, Ming Yuan2, Prathyankara Narasimhan1, Eesh Gupta3, Mustafa Bal4, Francesco Crisa4, Sabrina Garattoni4 et al.

Yao Lu4, Liang Jiang2, and Srivatsan Chakram1,†

  • *Contact author: jah499@scarletmail.rutgers.edu
  • †Contact author: schakram@physics.rutgers.edu
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Phys. Rev. X 16, 011058 – Published 17 March, 2026

DOI: https://doi.org/10.1103/t4cv-y398

Abstract

Mitigating ancilla-mediated error channels is a critical challenge in controlling high-quality superconducting cavities using circuit quantum electrodynamics (cQED). We address this by weakening the dispersive coupling while demonstrating fast, high-fidelity multimode control through transmon-mediated sideband interactions. We implement transmon-cavity SWAP gates with speeds up to 30 times larger than the bare dispersive coupling. Combined with transmon rotations, this enables universal state preparation in a single mode, though achieving unitary gates and extending control to multiple modes remains a challenge. In this work, we overcome this limitation by introducing two control strategies: (i) a shelving technique that stores populations in sideband-transparent states, and (ii) a method that exploits the dispersive shift to implement photon-number-selective transmon-cavity SWAP gates. We use these protocols to prepare Fock and binomial code states across any of the ten modes of a multimode cavity with millisecond coherence times—serving as a multimode quantum memory. We demonstrate unitaries that encode and decode an arbitrary qubit state from the transmon into corresponding vacuum and Fock state superpositions, as well as entangled NOON states of cavity mode pairs—a scheme extendable to arbitrary multimode Fock encodings in the cavity modes. Furthermore, we implement a new binomial encoding gate that converts arbitrary transmon superpositions into binomial code states in any cavity mode at a rate exceeding the dispersive shifts in our system, achieving an average post-selected state fidelity of 96.3% in a 4  μs gate time. By using precalibrated transmon and sideband pulses, our work demonstrates multimode control with significantly reduced calibration overhead, enabling efficient unitary operations using sideband interactions in multimode cQED systems.

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synopsis

Refining Control of Quantum Memories

Published 17 March, 2026

A new technique efficiently and reliably manipulates information held in a quantum memory.

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Popular Summary

Controlling high-quality superconducting cavities without introducing ancilla-mediated errors is a central challenge in quantum computing. We address this challenge by weakening the dispersive coupling to a transmon while maintaining fast, high-fidelity control via charge-driven sideband interactions. We implement transmon-cavity SWAP gates up to 30 times faster than those enabled by the bare dispersive coupling, enabling universal state preparation across ten cavity modes with millisecond coherence times, as well as encoding gates that map qubits onto superposition and entangled states across the cavity-memory modes. This approach enables a binomial-code encoding gate in the weak-dispersive regime that achieves a postselected state fidelity of 96.3% with minimal calibration overhead. Our results establish a robust, scalable framework for sideband control in multimode circuit quantum electrodynamics, bridging the gap between gate speed and long-lived quantum memory.

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Phys. Rev. X 16, 011058– Published 17 March, 2026
  • Received 11 May 2025
  • Revised 16 October 2025
  • Accepted 22 January 2026
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