Cryogenic quantum computing depends on maintaining highly controlled vacuum and low-temperature environments while delivering electrical power, control signals, readout signals, data, and, in some systems, optical signals to the equipment inside. This creates a significant interconnect challenge: every required connection must cross one or more physical boundaries without compromising vacuum integrity, introducing an avoidable leak path, or occupying more limited port space than necessary.

Quantum computing systems are typically built in layers. At the outer atmosphere-to-vacuum boundary, an application may require a vacuum feedthrough without direct exposure to the system’s lowest temperatures. Additional internal boundaries progressively transition toward cryogenic and low-Kelvin conditions. The quantum processor itself may operate at millikelvin temperatures, but this does not mean that every feedthrough in the system is located at or qualified for the millikelvin stage.

This distinction is important when specifying feedthroughs for quantum computing infrastructure. Douglas Electrical Components has documented wire-feedthrough performance through approximately 3 to 4 K, including liquid-helium temperature ranges. Available customer-shared data has not demonstrated performance degradation at a particular cryogenic temperature. However, applications in which a feedthrough must directly serve a millikelvin stage should be reviewed and validated as custom engineering requirements rather than treated as established catalog performance.

As quantum systems scale, the number and variety of connections can become increasingly difficult to manage. A cryostat or vacuum chamber has a limited number of ports, and each port has defined physical dimensions. Higher signal counts can therefore make connection density a system-level design consideration. The challenge is not simply passing signals through a boundary, but doing so while maintaining leak integrity, accommodating thermal cycling and material expansion differences, and supporting the required combination of electrical and optical interfaces.

Douglas Electrical Components offers two product approaches that can help address these requirements: custom hermetic flange feedthroughs and CircuitSeal™ hermetic printed circuit board feedthroughs.

Custom flange feedthroughs allow electrical conductors, coaxial connections, circuit boards, and optical fibers to pass through commonly used vacuum interfaces. Douglas can develop solutions for ConFlat, ISO, KF and other flange configurations, allowing the feedthrough to integrate with the interface already designed into a cryostat, vacuum chamber, or related piece of equipment. ConFlat flanges are often selected when especially high-quality vacuum sealing is required, while KF and ISO formats may suit other system architectures and servicing needs.

Within a custom flange, Douglas can seal different conductor and connection types, including wire, power conductors, coaxial interfaces such as SMA and BNC, printed circuit boards, and optical fiber. Where the electrical and mechanical requirements permit, multiple connection types may be incorporated into one assembly. This flexibility can help designers use available port space more effectively and potentially reduce the number of separate penetrations and leak paths within a system.

Optical fiber is also relevant to quantum computing architectures. Some systems use fiber-based connections, while others combine optical and electrical technologies. Fiber can help address certain electromagnetic-interference concerns, although it introduces its own design and integration considerations. Douglas can seal optical fiber independently or incorporate it into a custom feedthrough with other connection types when the application permits.

For designs requiring a large number of signal paths through a small opening, CircuitSeal offers a circuit-level alternative to conventional discrete wire or connector-pin arrangements. CircuitSeal hermetically seals a rigid printed circuit board, flexible circuit, hybrid circuit, or flat-flex cable directly into an assembly so that the circuit itself becomes the pass-through.

Because a printed circuit can carry many traces within a compact geometry, CircuitSeal can support higher connection density through a single sealed penetration. The circuit may terminate to connector receptacles, interface boards, or other application-specific connections on either side of the boundary. This approach gives equipment designers greater freedom to organize signal paths while addressing space and weight constraints.

A custom design may also combine a standard flange interface with CircuitSeal technology. In this concept, a rigid or flexible circuit is sealed into a flange housing selected to match the existing vacuum equipment. The ambient side and cold side can then be configured around the system’s connection requirements. Coaxial, power, wire, or fiber connections may also be considered for the same flange where technically viable.

Douglas feedthroughs are built around a proprietary, low-outgassing epoxy that has been evaluated to NASA outgassing standards. Low outgassing is especially important in vacuum systems where contamination can affect sensitive processes or equipment.

The epoxy is also tailored to cryogenic applications. As temperature decreases, the epoxy contracts more than the surrounding metal housing. This difference in thermal contraction can strengthen the epoxy’s mechanical engagement with the housing and copper conductors. Proper material selection and assembly design remain essential because actual performance depends on the feedthrough geometry, conductor materials, circuit construction, temperature stage, thermal cycling requirements, and other application-specific conditions.

Leak integrity is another essential consideration. Douglas commonly tests hermetic feedthroughs to a helium leak-rate standard of 1 × 10?? cc He/sec and can address lower leakage requirements when an application calls for them. Every Douglas part is electrically and mechanically tested. Because the company’s solutions are built to order rather than stocked as standard catalog parts, customers can work with Douglas engineers to define the required conductors, flange style, connection density, voltage, current, temperature range, mechanical configuration, and sealing performance.

Cryogenic quantum computing is not a one-size-fits-all application. The correct feedthrough depends on where the boundary is located within the system, the temperature that the assembly will experience, the required vacuum level, the types and number of signals involved, and the available port geometry. Addressing these factors early can help teams avoid connection-density limitations, unnecessary penetrations, and redesigns as quantum computing platforms move from laboratory development toward larger and more integrated systems.

Read the full blog to learn how custom hermetic flange feedthroughs and CircuitSeal hermetic PCB feedthroughs can help solve vacuum, cryogenic, and signal-density challenges in quantum computing systems.