Understanding the Complex RF Interconnect Requirements for Quantum Computing Technologies

By Dave Slack, Director of Engineering

Originally Published with Microwave Engineering Europe

Quantum computing technology represents the next frontier of computation based on the laws of quantum mechanics. The primary use cases today involve scenarios consisting of a complex problem with many thousands of inputs for applications, including cybersecurity, economic and financial modeling, thermodynamic and aerodynamic modeling, cosmology simulation, and more.

In quantum computing, information is processed using quantum bits, or qubits, instead of classical bits. Unlike the bits that power a classical computing machine, which can only exist in either 0 or 1 state, qubits can exist in multiple states simultaneously, making quantum computers much faster at certain types of computations. This enables quantum computers to efficiently solve complex problems that classical computers cannot solve, saving years of development time and substantial money in engineering design.

Qubits and microwave resonant circuits act similarly; they can be driven from a zero to one state with a microwave signal. Under this condition, the possibility of existing in a one or a zero state varies sinusoidally with time. Like other signals, qubits have a magnitude and phase relationship. In quantum computing, one of the limiting factors is that when the resonator is under this driven condition, it can only be predictable and controlled for a particular time period.

This is due to effects that cause it to lose energy and stop resonating. The limiters are known as the correlation of the qubit. When qubits become de-correlated and are no longer predictable and controlled, it creates computational issues analogous to bit errors in data.

Microwave hardware can feed these resonators and minimize contaminations. In fact, one of the prime limitations is ultra-low noise driving signals for qubits, especially low-phase noise, so a great deal of work has been done in the applications of ultra-low phase noise oscillators and similar technologies. When multiple bits are present, they can be coupled and controlled by driving a signal at a microwave frequency. It can be amplitude and phase modulated to give it specific properties. Aside from the low noise sources of these driving signals in precise modulation schemes, the hardware minimizes that contamination.

The Complexities of Quantum Computing Require Specialized RF and Microwave Cable Assemblies

Quantum states are fragile and must be maintained to prevent qubits from de-correlating. As a result, the correlation and control of qubits are fundamental issues to address in the technology’s development. It boils down to the fact that RF “noise” is introduced from thermal, magnetic, and mechanical sources.

High-performance coaxial cable assemblies (which include both the cable and connectors) are crucial in delivering precise, high-frequency microwave signals to the qubits to change their state and perform operations. Using coaxial cable assemblies in quantum computing also helps mitigate the effects of environmental noise and other sources of signal degradation, allowing for more reliable and precise quantum operations.

These assemblies must meet several requirements to deliver precise and stable signals for controlling qubits, including:

Low Loss: Low loss is necessary to minimize signal degradation and ensure high-quality signals reach the qubits.
Low noise: “Noise” means qubits lose their property of superposition, which prevents the quantum computer from working. Therefore, cable assemblies must be shielded to prevent interference and avoid disrupting the quantum information stored in the qubits.
Stability in Extreme Temperatures: The cable assemblies must maintain stable electrical properties over a wide temperature range to ensure consistent performance in challenging environments.
High-Frequency Range: The cable assemblies must be capable of operating at high frequencies, typically in the range of several GHz, to enable fast and accurate control of the qubits.
Immunity to Magnetic Interference: The systems must be immune to electromagnetic interference from other sources to prevent errors in quantum computation.

Low Loss, Low Noise

Successfully achieving low loss and noise comes down to material choices and construction methods. These options will be detailed in the following sections.

Extreme Temperature Conditions

One of the essential requirements for preventing interference is to operate in very low temperatures. Quantum computers must be exceptionally cold to be stable—typically colder than the vacuum of space—with temperatures down to -460 degrees Fahrenheit.

RF cables constructed using a silicon dioxide (SiO2) dielectric are used throughout the microelectronics industry for their exceptional insulating properties and to offer semi-rigid, highly temperature- and radiation-resistant cable solutions. Using the development techniques for creating SiO2 cables and harsh environment aspects as a launch point, the next logical step is to apply materials that are more advantageous to quantum computing applications.

RF cable assemblies are used throughout the entire system, starting at the bottom level where the quantum processor (QPU) is located through different temperature levels and out of the chamber to the control electronics. Two material types fit into this space well and are used at different temperature levels: a 4K down to 4mK NbTi construction, which is a superconducting cable designed to interface directly with the QPU, and a room temp to ~4K CuNi-based semi-rigid construction. Room temperature cables also play a vital role in this operation because phase stability is essential. These cables generally have varying lengths associated with them and cannot introduce any errors in the systems.

Smaller Spaces, Higher Frequencies

Technological advances across quantum computing are leading to more complicated requirements for RF systems that must accommodate higher frequencies inside devices that are constantly getting smaller.

For example, quantum computing requires signal access points close to processors.

Cabling is a challenge because in tight space configurations, traditional semi-rigid solutions have limitations; in very small sizes, they become fragile, making installation difficult as these assemblies are more prone to breakage. Using flexible cables specifically designed to optimize space, bending around tight corners, and connecting to various ports without wasted cable length is quickly becoming the preferred option. Durability and material selection are additional considerations as these cables are twisted around in tiny spaces in the challenging environments of quantum computers.

Reducing Magnetic Interference

Quantum computers transmit RF signals within a magnetic field. Non-magnetic coaxial cables are therefore required in critical areas of the signal path as the presence of any magnetic material in these components may cause interference.

As a result, a non-magnetic or non-metal coaxial cable or connector must be “invisible” to the magnetic field, which requires very low susceptibility and no field distortion. A class of hermetically sealed custom coaxial cabling assemblies addresses this by utilizing advanced manufacturing techniques that ensure zero electric field distortion.

Conclusion

Quantum technology is ushering in an era of significant improvements in computing performance. However, the technology is both incredibly promising and highly complex. To help quantum computers perform to their highest potential, unique RF interconnect solutions will be required. As this technology is in its infancy and rapidly developing, technologists should partner with an interconnect specialist with a wealth of experience in creating solutions that have powered the most innovative products to date in the most extreme conditions possible and can innovate as the technology evolves.