Josephson Ring Modulator Qubit-Resonator With Drive-Activated Coupling
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Solution Overview
Problem
In superconducting quantum mechanical devices, the coupling between the qubit and readout resonator is always ON, leading to qubit dephasing due to noise photons, which requires complex hardware and software solutions to mitigate, including bulky and expensive cryogenic isolators and circulators, and increases the qubit's susceptibility to electromagnetic noise.
Innovation Solution
A superconducting quantum mechanical device with a bridge circuit comprising four Josephson junctions and two resonator sections, where the qubit and resonator modes are orthogonal, allowing coupling only through a third mode when a control drive is applied, reducing the need for cryogenic isolators and circulators by filtering at a unique frequency band.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the coupling between qubit and readout resonator is always ON, then the qubit state can be measured by detecting changes in resonator frequency, but noise photons in the resonator cause qubit dephasing and loss of phase coherence
Solution Approach 1:
The patent implements dynamic coupling between the qubit and resonator using a time-dependent coupling mechanism. The coupling strength is modulated in time, allowing the system to switch between coupled and uncoupled states. This enables the qubit to be isolated from noise photons during idle periods while maintaining coupling capability during measurement operations, thus resolving the contradiction between measurement access and noise protection.
Solution Approach 2:
The patent employs periodic modulation of the coupling interaction between qubit and resonator. By applying periodic drive pulses at specific frequencies, the coupling is activated only during designated measurement windows and deactivated otherwise. This periodic on/off coupling pattern allows the system to achieve measurements when needed while protecting the qubit from continuous noise exposure, balancing measurement precision with phase coherence preservation.
2Loss of time
If the bandwidth of the resonator is increased to decrease measurement time, then the qubit can be measured faster, but the lifetime of the qubit decreases due to the Purcell effect
Solution Approach 1:
The patent applies local quality modification by introducing a Purcell filter with frequency-selective properties into the resonator circuit. The filter is designed to have high transmission only at the specific qubit frequency while providing strong attenuation at other frequencies. This localized frequency-domain filtering allows the resonator to maintain narrow bandwidth (protecting qubit lifetime) while still enabling efficient coupling and measurement at the qubit's resonant frequency, thus resolving the time-lifetime tradeoff.
Solution Approach 2:
The patent utilizes parameter changes by dynamically adjusting the effective coupling strength and resonator bandwidth through external control parameters. By modifying the coupling Hamiltonian parameters and applying drive frequencies that match the qubit-resonator interaction, the system can optimize the measurement rate without permanently increasing the resonator bandwidth. This parametric control allows fast measurements when needed while maintaining long qubit coherence times during idle periods.
3Duration of action of stationary object
If a Purcell filter is added to the readout resonator to combat the Purcell effect, then the qubit lifetime is protected, but the hardware complexity increases
Solution Approach 1:
The patent merges the Purcell filter functionality directly into the readout resonator structure itself, rather than adding it as a separate external component. The filter elements are integrated into the resonator's geometric design, creating a unified structure that performs both resonance and frequency-selective filtering functions. This integration reduces the number of discrete components, simplifies the overall architecture, and lowers fabrication complexity while maintaining the qubit lifetime protection benefits.
Solution Approach 2:
The patent designs the resonator structure to serve multiple functions simultaneously: it acts as both the readout resonator for measuring qubit states and as the Purcell filter for protecting qubit lifetime. The same physical structure provides both the resonant coupling necessary for measurement and the frequency-selective attenuation necessary for Purcell effect suppression. This multi-functionality eliminates the need for separate dedicated filter components, reducing hardware complexity while maintaining both measurement capability and qubit protection.
4Object-affected harmful factors
If cryogenic isolators and circulators are added to protect against electromagnetic noise, then the qubit is protected from noise, but the device becomes bulky, expensive, and harder to thermalize
Solution Approach 1:
The patent extracts the noise filtering function from the physical domain (requiring bulky cryogenic isolators and circulators) and implements it in the frequency domain through circuit design. By using Josephson junctions and nonlinear circuit elements, the system achieves noise rejection through frequency-selective coupling and nonlinear filtering mechanisms that are inherently integrated into the quantum circuit. This extraction eliminates the need for large external noise isolation hardware, reducing the device footprint and simplifying thermalization while maintaining noise protection.
Solution Approach 2:
The patent replaces mechanical noise isolation components (isolators and circulators) with superconducting circuit-based filtering mechanisms. Instead of using physical waveguide components with magnetic materials that require complex thermal management, the system uses Josephson junctions and nonlinear circuit elements to achieve frequency-selective noise rejection. This substitution transitions from mechanical/electromagnetic isolation to quantum-circuit-based filtering, dramatically reducing the hardware footprint and thermalization complexity while maintaining effective noise protection.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design protects the qubit from dephasing due to photon noise, decreases the number of cryogenic isolators needed, and allows for faster and more accurate qubit state measurement by filtering at a unique frequency, enhancing the stability and efficiency of quantum processor architectures.
Implementation Method 1
a first Josephson junction, a second Josephson junction electrically connected to the first Josephson junction, a third Josephson junction electrically connected to the second Josephson junction and a fourth Josephson junction electrically connected to the third Josephson junction and the first Josephson junction
Implementation Method 2
a first resonator section electrically connected to the first and second Josephson junctions at a node therebetween, and a second resonator section electrically connected to the third and fourth Josephson junctions at a node therebetween such that the first resonator section, the second resonator section and the bridge circuit form a resonator having a resonance frequency corresponding to the second resonance eigenmode
Data Source
AI summary
A superconducting quantum mechanical device includes first, second, third and fourth Josephson junctions connected in a bridge circuit having first, second and third resonance eigenmodes. The device also includes first and second capacitor pads. The first and second capacitor pads and the bridge circuit form a superconducting qubit having a resonance frequency corresponding to the first resonance eigenmode. The device further includes first and second resonator sections. The first and second resonator sections and the bridge circuit form a resonator having a resonance frequency corresponding to the second resonance eigenmode. The device also includes a source of magnetic flux arranged proximate the bridge circuit. The source of magnetic flux is configured to provide, during operation, a magnetic flux through the bridge circuit to cause coupling between the first, second and third resonance eigenmodes when the third resonance eigenmode is excited.