When designing a next-generation quantum computer, a surprisingly big problem is bridging the communication gap between the classical and quantum worlds. Such computers need specialized control and readout electronics to translate back and forth between the human operator and quantum computer languages – but existing systems are cumbersome and expensive.
However, a new system of control and readout electronics, known as the Quantum Instrumentation Control Kit, or QICK, developed by engineers at the US Department of Energy’s Fermi National Accelerator Laboratory, has been shown to significantly improve the performance of quantum computers while reducing the cost of control equipment. .
“The development of the Quantum Instrumentation Control Kit is an excellent example of U.S. investment in joint quantum technology research with industry-university-government partnerships to accelerate quantum research and development technologies. pre-competitive,” said Harriet Kung, DOE deputy director for science. programs for the Office of Science and Acting Associate Director of Science for High Energy Physics.
The faster, more cost-effective controls were developed by a team of Fermilab engineers led by Senior Principal Engineer Gustavo Cancelo in conjunction with the University of Chicago whose goal was to create and test an FPGA controller (field -programmable gate array-based) for quantum computing experiments. David Schuster, a physicist at the University of Chicago, led the university’s lab which helped with specifications and verification on real hardware.
“It’s exactly the type of project that combines the strengths of a national laboratory and a university,” Schuster said. “There is a clear need for an ecosystem of open-source control hardware, and it is rapidly being adopted by the quantum community.”
Engineers designing quantum computers take on the challenge of bridging the two seemingly incompatible worlds of quantum and classical computers. Quantum computers are based on the counterintuitive, probabilistic rules of quantum mechanics that govern the microscopic world, allowing them to perform calculations that ordinary computers cannot. Because people live in the macroscopic visible world where classical physics reigns, control and readout electronics serve as an interpreter connecting these two worlds.
The control electronics use signals from the classical world as instructions for the computer’s quantum bits, or qubits, while the readout electronics measure the states of the qubits and relay that information to the classical world.
A promising technology for quantum computers uses superconducting circuits as qubits. Currently, most control and readout systems for superconducting quantum computers use off-the-shelf commercial equipment not specialized for the task. As a result, researchers often have to string together a dozen or more expensive components. The cost can quickly reach tens of thousands of dollars per qubit, and the large size of these systems creates more problems.
Despite recent advances in technology, qubits still have a relatively short lifespan, typically a fraction of a millisecond, after which they generate errors. “When working with qubits, time is critical. Conventional electronics take time to respond to qubits, which limits computer performance,” Cancelo said.
Just as the efficiency of an interpreter depends on fast communication, the efficiency of a control and reading system depends on its execution time. And a large system made up of many modules means long lead times.
To solve this problem, Cancelo and his team at Fermilab designed a compact control and playback system. The team integrated the capabilities of an entire rack of equipment into a single circuit board slightly larger than a laptop computer. The new system is specialized, but versatile enough to be compatible with many superconducting qubit designs.
“We are designing a general instrument for a wide variety of qubits, hoping to cover those that will be designed in six months or a year,” Cancelo said. “With our control and readout electronics, you can achieve features and performance that are difficult or impossible to achieve with commercial equipment.”
Controlling and reading qubits depends on microwave pulses – radio waves at frequencies similar to the signals that transmit phone calls and heat up microwave dinners. The Fermilab team’s radio frequency (RF) board contains more than 200 elements: mixers to adjust frequencies; filters to remove unwanted 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 some qubit parameters. Combined with an FPGA board, or field-programmable gate array, which serves as the “brain” of the computer, the RF board provides everything scientists need to successfully communicate with the quantum world.
Both compact boards cost about 10 times less to produce than conventional systems. In their simplest configuration, they can control eight qubits. Integrating all RF components into a single board enables faster, more accurate operation as well as real-time feedback and error correction.
“You have to inject very, very fast and very, very short signals,” said Fermilab engineer Leandro Stefanazzi, a member of the team. “If you don’t control the frequency and duration of these signals very precisely, your qubit won’t behave the way you want it to.”
Designing the RF board and layout took about six months and presented considerable challenges: adjacent circuit elements had to be precisely matched for signals to propagate smoothly and without interference with each other. Additionally, engineers had to carefully avoid setups that would pick up stray radio waves from sources such as cell phones and WiFi. Along the way, they ran simulations to make sure they were on the right track.
The design is now ready for fabrication and assembly, with the goal of having functional RF boards this summer.
Throughout the process, Fermilab engineers tested their ideas with the University of Chicago. The new RF board is ideal for researchers like Schuster looking to make fundamental advances in quantum computing using a wide variety of quantum computing architectures and devices.
“I often joke that this board will potentially replace almost all the test equipment I have in my lab,” Schuster said. “Teaming up with people who can make electronics work at this level is incredibly rewarding for us.”
The new system is easily scalable. Frequency multiplexing qubit controls, analogous to sending multiple telephone conversations over the same cable, would allow a single RF card to control up to 80 qubits. Thanks to their small size, several dozen boards could be linked together and synchronized on the same clock as part of larger quantum computers. Cancelo and his colleagues described their new system in an article recently published in the AIP Examination of scientific instruments.
The Fermilab engineering team took advantage of a new commercial FPGA chip, the first to integrate digital-to-analog and analog-to-digital converters directly into the board. It dramatically speeds up the process of creating the interface between FPGA and RF boards, which would have taken months without it. To improve future versions of its control and playback system, the team began designing its own FPGA hardware.
The development of QICK was supported by QuantISED, the Quantum Science Center (QSC) and later by the Superconducting Quantum Materials and Systems Center (SQMS) hosted by Fermilab. QICK electronics are important for research at SQMS, where scientists are developing long-lived superconducting qubits. It is also of interest to a second National Quantum Center where Fermilab plays a key role, the QSC hosted by Oak Ridge National Laboratory.
A low-cost version of the hardware is now available only to universities for educational purposes. “Because of its low cost, it allows small institutions to have powerful quantum control without spending hundreds of thousands of dollars,” Cancelo said.
“From a scientific point of view, we are working on one of the hottest physics topics of the decade as an opportunity,” he added. “From a technical point of view, what I appreciate is that many areas of electronic engineering must come together to be able to carry out this project.”
The Fermi National Accelerator Laboratory is America’s premier national laboratory for particle physics and accelerator research. A laboratory of the United States Department of Energy’s Office of Science, Fermilab is located near Chicago, Illinois, and is operated under contract by the Fermi Research Alliance LLC, a partnership between the University of Chicago and the Universities Research Association, Inc. Visit?Fermilab website?and follow us on Twitter at?@Fermilab.