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Linear Quantum Coupler for Clean Bosonic Control

Aniket Maiti1,2,*, John W.O. Garmon1,2, Yao Lu1,2,†, Alessandro Miano1,2, Luigi Frunzio1,2, and Robert J. Schoelkopf1,2,‡

  • 1Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06511, USA
  • 2Yale Quantum Institute, Yale University, New Haven, Connecticut 06511, USA

  • *Contact author: aniket.maiti@yale.edu
  • †Present address: Superconducting Quantum Materials and Systems Center, Fermi National Accelerator Laboratory (FNAL), Batavia, Illinois 60510, USA.
  • ‡Contact author: robert.schoelkopf@yale.edu
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PRX Quantum 6, 040326 – Published 6 November, 2025

DOI: https://doi.org/10.1103/7rnm-rxhh

Abstract

Quantum computing with superconducting circuits relies on high-fidelity driven nonlinear processes. An ideal quantum nonlinearity would selectively activate desired coherent processes at high strength, without activating parasitic mixing products or introducing additional decoherence. The wide bandwidth of the Josephson nonlinearity makes this difficult, with undesired drive-induced transitions and decoherence limiting qubit readout, gates, couplers, and amplifiers. Significant strides have recently been made into building better “quantum mixers,” with promise being shown by Kerr-free three-wave mixers that suppress driven frequency shifts, and balanced quantum mixers that explicitly forbid a significant fraction of parasitic processes. We propose a novel mixer that combines both these strengths, with engineered selection rules that make it essentially linear (not just Kerr-free) when idle, and activate clean parametric processes even when driven at high strength. Furthermore, its ideal Hamiltonian is simple to analyze analytically, and we show that this ideal behavior is first-order insensitive to dominant experimental imperfections. We expect this mixer to allow significant advances in high-Q control, readout, and amplification.

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Popular Summary

Central to the building a useful quantum computer is the ability to entangle its qubits on demand—it is precisely this rate of entanglement compared to the natural lifetime of its qubits that sets its quality of operations. A common approach to achieve such on-demand entanglement is to put explicit “coupler” elements between the qubits. These couplers must default to an “off” state and turn on strong interactions only when desired, all while maintaining quantum coherence.

In superconducting qubits, one way to realize such couplers is to use nonlinear circuits that activate resonant interactions when driven with microwave drives. However, this gives rise to a fundamental trade-off: the nonlinearity of the coupler sets the strength of the interactions (and hence the speed of entanglement) when it is driven, but it also sets the strength of idle interactions when it is not driven. Our work breaks this fundamental trade-off by separating the coupler into two parts: a linear part that is active when idle and a nonlinear part that only activates when driven. The nonlinear part has carefully constructed symmetries that prevent the coupler from turning on spurious interactions even when strongly driven, allowing clean, fast processes.

Our “linear inductive coupler” combines these advantages into a simple circuit that functions as a drop-in replacement in current quantum architectures. In particular, we believe that it will allow significant strides in controlling “continuous-variable” modes of microwave light, which is a rich field of quantum research primarily limited by spurious nonlinear interactions.

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PRX Quantum 6, 040326– Published 6 November, 2025
  • Received 10 February 2025
  • Revised 16 May 2025
  • Accepted 17 June 2025
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