Control of quantum computers has always required fast, precise coordination between a traditional computer and the quantum computer. Mostly, these are custom systems and not cheap (or easy) to build. Recently researchers from Fermilab and the University of Chicago reported developing new control system – the Quantum Instrumentation Control Kit – that’s both faster and tenfold less expensive than more commonly used control systems.

As described in an article on Fermilab website, “Currently, most control and readout systems for superconducting quantum computers use off-the-shelf commercial equipment not specialized to the task. As a result, researchers often must string together a dozen or more expensive components. The cost can quickly add up to tens of thousands of dollars per qubit, and the large size of these systems creates more problems.”

The faster and more cost-efficient controls were developed by a team of Fermilab engineers led by senior principal engineer Gustavo Cancelo whose goal was to create and test a field-programmable gate array-based (FPGA) controller for quantum computing experiments. David Schuster, a physicist at the University of Chicago, led the university’s lab that helped with the specifications and verification on real hardware.

“This is exactly the type of project that combines the strengths of a national laboratory and a university,” said Schuster. “There is a clear need for an open-source control hardware ecosystem, and it is being rapidly adopted by the quantum community.”

Designing a compact system was a major goal. “The team incorporated the capabilities of an entire rack of equipment in a single electronics board slightly larger than a laptop. The new system is specialized, yet it is versatile enough to be compatible with many designs of superconducting qubits,” according to the report. A paper on the work was published last week in AIP Review of Scientific Instruments.

Here’s the abstract:

We introduce a Xilinx RF System-on-Chip (RFSoC)-based qubit controller (called the Quantum Instrumentation Control Kit, or QICK for short), which supports the direct synthesis of control pulses with carrier frequencies of up to 6 GHz. The QICK can control multiple qubits or other quantum devices. The QICK consists of a digital board hosting an RFSoC field-programmable gate array, custom firmware, and software and an optional companion custom-designed analog front-end board. We characterize the analog performance of the system as well as its digital latency, important for quantum error correction and feedback protocols. We benchmark the controller by performing standard characterizations of a transmon qubit. We achieve an average gate fidelity of ℱ = 99.93%. All of the schematics, firmware, and software are open-source.

In the Fermilab article, Cancelo is quoted, “We are designing a general instrument for a large variety of qubits, hoping to cover those that will be designed six months or a year from now. With our control and readout electronics, you can achieve functionality and performance that is hard or impossible to do with commercial equipment.”

The control and readout of qubits depend on microwave pulses — radio waves at frequencies similar to the signals that carry mobile phone calls and heat up microwave dinners. The Fermilab team’s radio frequency (RF) board contains more than 200 elements: mixers to tweak the frequencies; filters to remove undesired frequencies; amplifiers and attenuators to adjust the amplitude of the signals; and switches to turn signals on and off. The board also contains a low-frequency control to tune certain qubit parameters. Together with a commercial FPGA, which serves as the “brains” of the computer, the RF board provides everything scientists need to communicate successfully with the quantum world.

The researchers reported the two compact boards of their system cost about 10 times less to produce than conventional systems. “In their simplest configuration, they can control eight qubits. Integrating all the RF components into one board allows for faster, more precise operation as well as real-time feedback and error correction,” according to the article.

“You need to inject signals that are very, very fast and very, very short,” said Fermilab engineer Leandro Stefanazzi, a member of the team. “If you don’t control both the frequency and duration of these signals very precisely, then your qubit won’t behave the way you want.”

Designing the RF board and layout took about six months and presented several challenges: adjacent circuit elements had to match precisely so that signals would travel smoothly without bouncing and interfering with each other and engineers had to carefully avoid layouts that would pick up stray radio waves from sources like cell phones and Wi-Fi. Along the way, they ran simulations to verify that they were on the right track.

The design is now ready for fabrication and assembly, with the goal of having working RF boards this summer. A low-cost version of the hardware is now available only for universities for educational purposes. “Due to its low cost, it allows smaller institutions to have powerful quantum control without spending hundreds of thousands of dollars,” said Cancelo.

Source: Fermilab

Link to paper, https://aip.scitation.org/doi/10.1063/5.0076249

Feature image, Figure 1 from the paper, “The Quantum Instrumentation Control Kit (QICK). The QICK consists of two pieces of hardware: the commercial ZCU111 RFSoC evaluation board (left), which connects to the QICK RF board (right), which can be used for additional signal up/downconversion, amplification, and filtering.”