Josephson Ring Modulator Qubit-Resonator Coupling for Low Dephasing
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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 complicates the architecture with the need for bulky, expensive cryogenic isolators and circulators to mitigate photon population and maintain phase coherence.
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 enabling filtering without them.
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 continuously monitored, 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 readout resonator using a Josephson ring modulator (JRM). The coupling strength is modulated by applying a local oscillator (LO) signal at the difference frequency between qubit and resonator frequencies. This allows the coupling to be turned ON during measurement and OFF during idle periods, preventing noise-induced dephasing while enabling on-demand measurement capability.
Solution Approach 2:
The patent employs periodic modulation of the coupling interaction through the JRM by applying an LO signal at a specific frequency. This periodic action creates time-dependent coupling that alternates between strong and weak coupling regimes, enabling controlled information transfer from qubit to resonator while limiting continuous exposure to noise photons.
2Loss of time
If the bandwidth of the resonator is increased to decrease measurement time, then the qubit lifetime decreases due to the Purcell effect
Solution Approach 1:
The patent introduces a Purcell filter with frequency-selective properties placed between the qubit and readout resonator. The filter has high transmission at the qubit frequency while providing strong attenuation at the resonator frequency. This local frequency-selective filtering allows fast readout through enhanced coupling at the qubit frequency while suppressing Purcell decay channels at other frequencies, thus extending qubit lifetime.
Solution Approach 2:
The Purcell filter acts as an intermediary component between the qubit and the readout resonator. It mediates the interaction by allowing efficient energy transfer at the qubit frequency while blocking unwanted coupling channels. This intermediary structure enables fast measurement without directly increasing the resonator bandwidth, thereby avoiding the Purcell effect.
3Object-affected harmful factors
If cryogenic isolators and circulators are added to protect against noise, then the architecture becomes more complex and hardware complexity increases
Solution Approach 1:
The patent extracts and eliminates the need for bulky cryogenic isolators and circulators by implementing dynamic coupling through the JRM. The on-demand coupling capability inherently protects the qubit from noise photons when coupling is OFF, removing the requirement for additional noise protection hardware and simplifying the overall architecture.
Solution Approach 2:
The system achieves self-protection against noise through the dynamic coupling mechanism. When measurement is not actively performed, the coupling is automatically turned OFF, and the qubit is naturally protected from noise photons without requiring external isolators or circulators. The system serves its own protection needs through intelligent coupling control.
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 configuration protects the qubit from dephasing, simplifies the architecture by reducing the number of cryogenic components needed, and allows for faster and more accurate qubit state measurement by filtering photon noise at a unique frequency.
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 resonance eigenmode, a second resonance eigenmode and a third resonance eigenmode
Data Source
Figure 1
Figure 2
Figure 3A~3C
AI summary
A superconducting quantum mechanical device includes first, second, third and fourthJosephson 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.