Understanding the Complex RF Interconnect Requirements to Ensure the Highest Performance for Rapidly Advancing Quantum Computing Technologies

By Dave Slack, Director of Engineering

Originally published in Microwave Journal

Quantum computing technology, the next frontier of computation, has been in development for several decades, but it’s starting to heat up as technology continues to advance rapidly. Representing a significant shift in computing performance capabilities, quantum computing will save years of development time and a substantial amount of money in engineering design as it becomes more prevalent.

Leading manufacturers already utilize some form of quantum computing to tackle incredibly complicated operations. The primary use cases involve scenarios consisting of a complex problem with many thousands of inputs for applications, including cybersecurity, financial and economic modeling, aerodynamic and thermodynamic modeling, cosmology simulation, and more. To use aerodynamic and thermodynamic modeling as an example, certain developments in hypersonic aircraft are stretching the limits of the known aerodynamic and aerothermodynamic design principles. Computer simulations of these phenomena using the best supercomputers available can take weeks to perform. Speeds and velocities still need to be well known, so much physical testing is also being done today to understand these properties. A quantum computer running those models instead would be substantially faster and involve much less physical testing.

Quantum Computing and Microwave Engineering

The primary element for computations is called a quantum bit or qubit. However, unlike the bits that power a classical computing machine representing data as a one or a zero, quantum data can simultaneously be a one and a zero. This mechanism enables a quantum computer to process information significantly faster and more efficiently than a classical computer.

Qubits act similarly to a microwave resonant circuit; they can be driven from a zero state to one state by moving them with a microwave signal. Under this driven condition, the probability of being a one or a zero state varies sinusoidally with time. Like other signals, the qubits have a magnitude and a phase relationship. One of the limiting factors in quantum computing is that when the resonator is under this driven condition, it can only be predictable and controlled for a certain period of time. This is because, like with any resonator, there are effects that will cause it to lose energy and stop resonating.

The limiters are called the correlation of the qubit. When qubits become de-correlated, and they are no longer predictable and controlled; it’s analogous to bit errors in data and creates computational issues. As a result, the correlation and control of qubits are one of the fundamental driving issues behind the technology development. It boils down to the fact that “noise” is introduced from thermal, magnetic, and mechanical sources.

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 is going on 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.

Quantum Computing Requires Specialized RF and Microwave Cable Assemblies

 The unique complexities of quantum computing require robust RF interconnects and cable assemblies to reliably transport qubit information to and from the quantum processor. This includes cables that can operate in extremely low temperatures, space-constricted environments, and options that will not interfere with applied magnetic fields.

Building on the Silicon Dioxide Cable Construction to Create New Solutions for Extreme Temperature Conditions

 One of the essential requirements to maintain low noise is to operate in very low temperatures. Quantum computers need to 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 excellent insulating properties and to offer semi-rigid cable solutions that are highly temperature and radiation resistant. These rugged, low-loss, and phase-stable coaxial assemblies were initially developed to support spaceflight missions, where the requirements of being vacuum sealed and able to withstand extremely low temperatures are essential. Silicon dioxide cable assemblies represent a significant advancement in coaxial cable technology, providing exceptionally low hysteresis with phase and loss values returning to the same values at a particular temperature even after being in extreme environments. This type of cable works at temperatures ranging from just above absolute zero to 1,000°C.

The silicon dioxide coaxial cable construction begins with a solid oxygen-free copper center conductor, a silicon dioxide insulating dielectric, and a stainless-steel jacket with copper cladding to act as the outer conductor.  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. There are two material types that fit into this space well and are applied at different temperature levels: room temp to ~4K CuNi based semi rigid construction and a 4K down to 4mK NbTi construction.  The NbTi is a superconducting cable that is designed to interface with the quantum processor directly.

Figure 1: A typical quantum computing system, with the main chamber housing the various levels to maintain temperature. At the bottom level is where the QCPU is located and held at superconductive temperatures. 

RF cable assemblies are used throughout the entire system starting at the QCPU through the different temperature levels and out of the chamber to the control electronics.  Room temperature cables play a vital role in this operation as well because phase stability is important. These cables generally have varying lengths associated with them and cannot introduce and errors in the systems.

Higher Frequencies, Smaller Spaces

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

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

Cabling for these systems is a challenge because in tight space configurations, traditional semi-rigid solutions have limitations; in very small sizes, they become too fragile, making installation difficult as these assemblies are more prone to breakage. Using flexible cables specially designed to optimize space, bending around tight corners, and connecting to various ports without wasted cable length is emerging as a 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 computing also requires non-magnetic coaxial cables to eliminate potential interference with applied magnetic fields. Non-magnetic coaxial cables and connectors are primarily used in applications that transmit RF signals within a magnetic field, including quantum computers. Quantum computers use non-magnetic components in critical areas of the signal path to eliminate potential interference with applied magnetic fields. The presence of any magnetic material in these components may interfere with the magnetic field.

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