University of Massachusetts Associate Professor Joseph Bardin of the Electrical and Computer Engineering Department is working with Google to create a cryogenic quantum controller that operates in extreme cold and consumes less than 2 milliwatts of power — 1,000 times less than Google’s current control electronics. As an article in VentureBeat explains, “Google says it has made significant progress toward an efficient, reliable, and scalable means of controlling quantum-systems electronics — systems it hopes will someday solve computationally complex problems beyond the reach of classical machines.”
At the International Solid State Circuits Conference in San Francisco this week, Bardin unveiled a groundbreaking cryogenic controller fabricated using CMOS technology and designed in partnership with the Google AI Quantum team.
“Building a quantum computer that can solve practical problems that would otherwise be classically intractable due to the computation complexity, cost, energy consumption, or time to solution is the longstanding goal of the AI Quantum team,” write Bardin and Erik Lucero, staff research scientist and hardware lead on the Google AI Quantum Team, in a newly posted blog.
However, they note that significant innovation is required to transition from today’s experimental quantum computing systems to a full-fledged quantum computer. “Current thresholds suggest a first-generation, error-corrected, quantum computer will require on the order of 1 million physical qubits, which is more than four orders of magnitude more qubits than exist in Bristlecone, our 72-qubit quantum processor.”
A qubit is basically the quantum version of a binary digit, and Bristlecone is currently one of the world’s most advanced quantum processors.
Bardin and Lucero add that “Increasing the number of physical qubits needed for a fault-tolerant quantum computer while maintaining high quality control of each qubit are intertwined and exciting technological challenges that will require inventions beyond simply copying and pasting our current control architecture.”
The VentureBeat article explains that Google currently runs programs on its prototypical Bristlecone quantum processor by applying gigahertz-frequency analog signals produced by digital-to-analog waveform generators packaged in server-room racks. Google estimates that even cooling the 150 waveform generators required to run Bristlecone would overwhelm its cooling system by 1,500 times.
As the two researchers say, one other critical challenge is reducing the number of input/output control lines per qubit by relocating the room-temperature, analog-control electronics to the 3-kelvin stage (about -454.27 degrees Fahrenheit) in the cryostat, while maintaining high-quality qubit control.
To explain the need for such frigid temperatures, the two researchers observe that “Quantum computers operate in extreme cold in order to limit the amount of energy introduced into the system, and to minimize the chances a qubit…inadvertently flips between quantum states.”
As a step towards solving all these challenges, this week the team presented their first-generation, cryogenic-CMOS, single-qubit controller at the San Francisco conference. Using commercial CMOS technology, the 1-millimeter-by-1.6-millimeter controller — which provides an instruction set for single-qubit operations — runs at between room temperature and 3 degrees Kelvin and consumes less than 2 milliwatts of power.
Ultimately, Bardin and Lucero set out to develop custom integrated circuits to control qubits from within the cryostat to reduce the physical I/O connections to and from future quantum processors. These integrated circuits would be designed to operate in the ultra-cold environment, specifically 3 kelvin, and turn digital instructions into analog control pulses for qubits. A key research objective was first to design a custom integrated circuit with low power requirements in order to prevent warming up the cryostat.
“We designed our [integrated circuit] to dissipate no more than 2 milliwatts of power at 3 kelvin, which can be challenging, as most physical CMOS models assume operation closer to 300 kelvin,” as Bardin and Lucero write. “After design and fabrication of the [integrated circuit] with the low-power design constraints in mind, we verified that the cryogenic-CMOS qubit controller worked at room temperature. We then mounted it in our cryostat at 3 kelvin and connected it to a qubit (mounted at 10 millikelvin in the same cryostat). We carried out a series of experiments to establish that the cryogenic-CMOS qubit controller worked as designed, and most importantly, that we hadn't just installed a heater inside our cryostat.”
Baseline experiments for their new quantum control hardware, including T1, Rabi oscillations, and single qubit gates, show similar performance compared to standard room-temperature qubit control electronics. Meanwhile, qubit coherence time was virtually unchanged, and high-visibility Rabi oscillations were observed by varying the amplitude of the pulses out of the cryogenic-CMOS qubit controller—a signature response of a driven qubit. (February 2019)