Circulators are essential components in many high-frequency systems, providing signal flow and protection, such as from transmitters to receivers. Traditional coaxial circulators are being developed for the rise in millimeter-wave (mmWave) system applications through 110 GHz, but even higher-frequency requirements await them; for example, circulators for quantum-computing systems.
Rather than use conventional ferrite Y-junction circulators for such applications, researchers from Raytheon BBN Technologies (Cambridge, Mass.) and the National Institute of Standards and Technology (NIST) in Boulder, Colo., report on several nonreciprocal devices that function as more compact, quantum-limited superconducting circulators at cryogenic temperatures.
On-chip, semiconductor-based circulators and isolators can be designed as very small components and are able to provide the high isolation needed for many high-frequency applications. However, they don’t always function well at the cryogenic temperatures of quantum-computing systems.
For that reason, several other superconducting circuit approaches, typically based on multiple Josephson junctions (JJs) with almost no resistance or loss at cryogenic temperatures or on superconducting quantum interference devices (SQUIDs), have been developed for possible integration into compact quantum-limited preamplifier circuits. These nonreciprocal circuits often mimic the behavior of much larger, microwave Y-junction circuits, although without the magnetic fields that must be avoided because of their deleterious effects on quantum-computing systems.
Much of the circulator research reported at cryogenic temperatures for quantum-computing systems mirrors the work being done on the design of circulators and isolators for RF, microwave, and mmWave frequencies for traditional commercial communications, defense, and test-and-measurement applications. However, they’re used at much lower power levels and in much smaller package sizes (to better fit within a cryostat with additional superconducting circuitry). The researchers feel that a healthy exchange of design information between the two research groups can greatly aid in the pursuit of high-performance circulator solutions for both applications areas.
See “Circulators at the Quantum Limit,” IEEE Microwave Magazine, April 2019, pp. 112-122.