While reimagining the hardware can unlock potential in tomorrow’s quantum computers, software can also play a role in improving their performance. The two honorable mention papers led by Ding explore this avenue, developing software solutions for blocking interference between qubits and efficiently recycling qubits — two strategies can help users get the most from the limited scale of current and near-future quantum machines.
“Crosstalk” is one of the most common issues with today’s superconducting quantum computers, occurring when unwanted interference happens between neighboring qubits, introducing errors during the running of a program. Often this crosstalk occurs when two qubits share resonant frequencies; not unlike when a radio receives competing signals from two stations using the same broadcast channel.
While hardware solutions have been used by manufacturers for this issue, Ding’s paper with Pranav Gokhale, Sophia Fuhui Lin, Richard Rines, Thomas Propson, and Chong puts the software compiler in charge of assigning frequencies so that this crosstalk doesn’t occur. The approach systematically tunes qubit frequencies according to input programs, minimizing the chance of errors and improving program success rate by more than 13 times. Ding’s research found that it could be immediately useful for some of the systems under development by the quantum computing industry.
“This work proves that a software solution, in this case, works surprisingly well,” Ding said. “Our compiler shows significant improvement in average gate fidelity, compared to IBM’s fixed frequency systems, and we allow the tunable qubits fixed coupler design, like Rigettii’s, to achieve similar fidelity of a more complex design with tunable couplers, such as Google’s systems. It is a great example where software design can greatly simplify hardware complexity required for scaling up quantum systems.”
A second paper, authored by Ding with Xin-Chuan Wu, Adam Holmes, Ash Wiseth, Diana Franklin, Margaret Martonosi, and Chong, updates the classical computing principle of “garbage collection” for the quantum world. In systems with limited memory, garbage collection algorithms find saved information that is no longer needed for a program to run and dumps it, freeing up resources for future steps. Just as this principle was important for the early days of computing in the mid-20th century, it’s critical for today’s quantum computer operating with only dozens of qubits.
Ding and his colleagues propose SQUARE (Strategic QUantum Ancilla REuse), the first system-level quantum memory management technique. The approach uses “uncomputation,” recycling a used qubit so that it can be redeployed elsewhere. As this process involves high operational costs, a user must find the right balance between performing uncomputation too frequently or too seldomly. As with the crosstalk strategy, the researchers optimized this decision-making by assigning it to the compiler, evaluating the best places in a program to perform this recycling task, even for the current generation of noisy intermediate-scale computers.
“We observe that garbage collection of scratch qubits is, surprisingly, often cheaper than moving new scratch qubits across a quantum chip,” Ding said. “This changes the way quantum systems allocate and manage qubits, greatly extending their practical use for a given machine size.”
IEEE Micro published its annual “Top Picks from the Computer Architecture Conferences” in its May/June 2021 issue. The awards recognize “significant and insightful papers that have the potential to influence the work of computer architects for years to come.” Chong’s group and EPiQC previously received two Top Picks honors in 2020, while a paper from the group of UChicago CS associate professor Hank Hoffmann received an honorable mention in 2019.