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Parametrically modulated longitudinal coupling



Recent years have seen remarkable progress in the fabrication of superconducting devices obeying quantum laws that interact with single microwave photons. In circuit quantum electrodynamics (cQED), a superconducting quantum bit (qubit) is strongly coupled to the electromagnetic field of a high-quality microwave resonator. cQED is widely recognized as one of the most promising architectures for realizing a quantum computer.

To achieve this goal, it is absolutely necessary that the reading of the state of a qubit be fast, of very high fidelity and ideally non-destructive. In order to make a quantum processor, two-qubit logic gates must also be made with great speed and a very high degree of fidelity.


Our two inventions form a new architecture for the future quantum processor.

By modulating the longitudinal coupling of a qubit to a microwave cavity at the frequency of the cavity, this makes it possible to make an ideal reading of the state of the qubit with very great rapidity. The formalism of the principle of measurement is general whereas only one harmonic oscillator (the cavity) is considered in interaction with one or more quantum systems at two levels (the qubit). The interaction must be longitudinal and modulate over time while the measurement is performed by detecting the phase of the output signal from the cavity.

By having two qubits coupled in this way with the same cavity, it is possible to generate a very fast and high-fidelity two-qubit logic gate by modulating the coupling of the qubits simultaneously at a frequency far from resonance with the cavity. . Here, the cavity plays the role of a quantum bus providing an effective interaction between the qubits. The logic gate is made by controlling the amplitude, duration and phase of the signals modulating the interaction of the qubits with the cavity.



  • New architecture for a future quantum processor simultaneously realizing the conditions allowing a fast measurement of the coherent control of quantum information.

    • Meet the demanding constraints to be fulfilled in order to perform quantum computing.

  • Quickly measure the state of one or more qubits with a high level of fidelity.

    • Invention allowing fast, high fidelity and non-destructive reading.

  • Invention providing fast, very high fidelity two-qubit logic gates.


  • Technology offering competitive advantages by enabling major gains.

    • Huge interest in the scientific community towards quantum processors and this type of measurement and interaction.

    • Researchers from several major universities have expressed interest in implementing the longitudinal parametric qubit measurement device.


  • Architecture applicable to all kinds of physical systems that can relate to the basic model (cavity + two-level system + parametric longitudinal coupling).

    • New architecture allowing a multitude of new applications.

  • Reading quantum information with increased speed and efficiency.

  • Quantum gates allowing more efficient and precise control of information.

  • Without disturbance to incident noise signals in the network.

  • Flexibility of possible configurations.



  • TRL-2


  • US patent no. 20190005403A1, published on January 3, 2019.

  • US patent application no. 2017/0262765A1, granted July 3, 2018.

  • US patent application no. 10013657B2, granted July 3, 2018.

  • Canadian patent application no. CA3010686A1, filed July 5, 2018.


  • Collaborations with research partners to deepen the development and extend to other types of circuits and fields of application.

  • Licensing with existing companies.

Project Director: François Nadeau

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