Capacitive Tunable Coupler for Cryogenic Quantum Signal Switching
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Solution Overview
Problem
Conventional microwave mechanical, electro-mechanical, and electronic switches are not compatible with on-chip integration and cryogenic operation of superconducting electronic circuits, and tunable filters using active components or ferroelectric materials are difficult to control with single flux quantum technologies, leading to issues like high return loss and limited bandwidth.
Innovation Solution
A capacitively-driven tunable coupler system using a coupling capacitor and RF SQUID with a Josephson element, allowing for variable inductance and control of coupling between quantum objects, enabling signal passing or blocking through flux injection, which operates at cryogenic temperatures and is compatible with single flux quantum signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional microwave mechanical, electro-mechanical, and electronic switches are used for coupling, then switching function is achieved, but compatibility with on-chip integration and cryogenic operation is lost
Solution Approach 1:
The patent replaces mechanical switches with a superconducting quantum interference device (SQUID) based coupler that uses magnetic flux control instead of mechanical movement. The SQUID loop with Josephson junctions provides switching functionality through quantum mechanical effects, eliminating mechanical components and enabling cryogenic operation and on-chip integration.
Solution Approach 2:
The patent changes the control parameter from mechanical position or electrical voltage to magnetic flux. By applying external magnetic flux to the SQUID loop, the coupling strength between resonators is tuned continuously from strong coupling to complete decoupling. This parameter change enables compatibility with superconducting circuits operating at cryogenic temperatures.
2Ease of operation
If tunable filters using active components or ferroelectric materials are used, then filter functionality is achieved, but control with single flux quantum technologies becomes difficult
Solution Approach 1:
The patent replaces active electronic components and ferroelectric materials with a SQUID-based superconducting device. The SQUID is naturally compatible with single flux quantum (SFQ) control technology, as it responds directly to magnetic flux changes. This substitution simplifies the control mechanism by using native superconducting control signals rather than requiring complex interfacing with non-superconducting materials.
3Reliability
If superconducting microwave filters are used for switching applications, then superconducting operation is achieved, but return loss, bandwidth, and out-of-band isolation deteriorate
Solution Approach 1:
The patent implements a dynamic coupling mechanism where the SQUID's inductance can be continuously adjusted by applying magnetic flux. This allows the system to dynamically switch between strong coupling (for signal transmission) and complete decoupling (for isolation). The dynamic control enables excellent out-of-band isolation when decoupled while maintaining broad bandwidth when coupled, overcoming the limitations of fixed superconducting filters.
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
The system provides flexible and efficient tunable coupling at any point along a transmission-line resonator, enabling couplings not possible with inductive couplers, with low power dissipation and SFQ compatibility, allowing for bidirectional signal exchange between quantum objects.
Implementation Method 1
the RF SQUID comprising a Josephson element connected between the first connecting node and the second connecting node
Implementation Method 2
at least one flux injection element configured to bias the Josephson element to variably weaken the strength of coupling between the first and second quantum objects by altering the inductance of the Josephson element
Implementation Method 3
a coupling capacitor connected between the open end of the first quantum object and a first connecting node
Implementation Method 4
Injected flux can uncouple the objects and thereby isolate the objects from exchanging signals between them. In the absence of injected flux, the objects are coupled together to pass signals between them
Data Source
Figure 1
Figure 2A~2B
Figure 2C~2D
AI summary
A capacitively-driven tunable coupler includes a coupling capacitor (110) connecting an open end of a quantum object (104) (i.e., an end of the object that cannot have a DC path to a low-voltage rail, such as a ground node, without breaking the functionality of the object) to an RF SQUID (108) having a Josephson element capable of providing variable inductance and therefore variable coupling to another quantum object (106).