Cryogenic Hardware at Bleximo: Superconducting Quantum Processor Packaging and Shielding
By Ziggy Kotchetkov and Jack Tooley
Quantum devices are poised to expand the boundaries of modern computation. Bleximo is focused on application-specific quantum computers using superconducting circuits, and this approach presents a number of complex challenges — often equaling the difficulty, expense, and complexity of advanced aerospace equipment. To meet these challenges, our Cryo Team designs, builds, and deploys cryogenic hardware to cool circuits down to a temperature near absolute zero while insulating them from electromagnetic noise and other naturally occurring conditions that degrade the performance of this intricate technology.
Qubits are the heart of a quantum processor. Superconducting qubits live on a surface of silicon or sapphire chips. The smallest chips are typically no larger than a fingernail, and in the near future, the processor’s size will grow to ~50–100 cm2. The Cryo Team treats each chip as an ultra-sensitive object, which must be cooled to temperatures below 10mK (that is, -273.14° C or -459.65° F) and protected from a wide spectrum of electromagnetic radiation, as well as other potential sources of contamination. These sources include thermal radiation, static and alternating electromagnetic fields, cosmic rays, heat conducted through mechanical connections, surface contamination during chip preparation, complications during the mounting process in the refrigerator, and stresses from the cooling process.
The printed circuit board (PCB) and chip box are the first layer of protection and communication between the chip and the outside world. Together, they create a “light-tight” container roughly the size of a hockey puck that ensures the only electromagnetic radiation the qubits receive comes from the control lines on the PCB. The Cryo Team works closely with the Processor Team to ensure the radio-frequency cables and DC signal paths coming in and out of the chip box are well-insulated from one another and preserve the weak signals they carry, which control and measure the quantum states of the qubits.
The chip box is then tightly bolted to a series of structures that act as large heatsinks, thermally connecting it to the mixing chamber — the coldest part of the dilution refrigerator. A strong thermal connection between the chip and the mixing chamber ensures cold, low-noise, coherent qubits, which are required to tackle increasingly complex quantum computations. To best support the performance of this connection, the heatsink components are often made from ultra-high purity and high-conductivity copper. Additionally, many parts of the system can be plated with high-purity gold to fill the microscopic gaps between bolted joints, resulting in further decreased thermal resistance as heat flows from part to part.
Low resistance thermal connections are also required by the signal cables, various radio-frequency elements, and electromagnetic shielding — including superconducting shields with coatings of infrared absorption compounds and layers of “mu-metal” magnetic shielding. The Cryo Team ensures each of these is well connected to the source of cooling and are all efficiently thermalized along with the chip packaging. CAD and finite-element analysis software are used to ensure that all components are lightweight enough to minimize cooldown times, are free of geometric bottlenecks that choke the flow of heat, and have a low number of bolted joints. Deliberate design decisions are paired with good assembly practices — components are thoroughly cleaned and polished before assembly and are then clamped as tightly together as possible to create strong thermal connections.
In addition to presenting thermal constraints, dilution refrigerators are quite space-constrained. Structural components, heat sinks, shielding, cabling, and signal devices must all fit within the refrigerator and must remain accessible when a new setup is required. Every component is thus designed with strict tolerancing and modularity requirements. To further ensure this precise fit, every part is 3D printed before it is manufactured out of metal; this way the entire assembly can be fit-tested before the final product is ever constructed.
At all stages of the design and manufacturing process, the Cryo Team employs a customized cloud-based relational database tool to carefully track the construction and performance of our hardware. This tool allows for efficient troubleshooting when unknown noise sources and design issues crop up, saving time and freeing the team up to focus on high-value design work for our partners and clients.
Bleximo’s packaging and shielding solution is experimentally proven to increase qubit coherence by several times in A/B testing and is fully compatible with BlueFors LD and XLD refrigerators as well as Oxford Instruments ProteoxMX and ProteoxLX refrigerators. If you’re interested in learning more, reach out to us!
Ziggy Kotchetkov, email@example.com
Jack Tooley, firstname.lastname@example.org