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The Journey To Scalable Qubit Control

The Journey To Scalable Qubit Control
27 March, 2023

At the heart of every quantum computer is a control system that manipulates the quantum states of qubits to store information and perform calculations.  Historically, labs used homegrown control systems created and enhanced by successive groups of physicists that were costly to maintain.  Now many are seeing the benefits of a new generation of commercially available solutions that allow them to spend more time pursuing their core research goals.

Thus far the focus of the work has been on small scale experiments to control just a handful of qubits. To run the large scale error-corrected circuits that will be be needed to perform useful calculations, the numbers of qubits will need to run into thousands, maybe millions.  To meet the growing needs of both quantum computers and the scientists using them, the next generation of qubit control systems must meet three key requirements:

1. Keep complexity under control. We expect to see a Quantum Moore’s law in operation where the power of quantum computers increases exponentially with time. If complexity follows suit, research goals will not be met. Current control systems are often comprised of a lot of loosely connected components. From a hardware standpoint, scaling such systems exponentially is a near-impossible task. This is where a modular, scalable hardware is required and, as a result, backplane solutions will become a more favourable option. Automating circuit execution, system calibration (both offline and in real time) and system management will also become a priority. While scientists can manage these tasks manually within a 5-10 qubit range, this situation will rapidly change with growing qubit numbers. We are reaching a tipping point where automation will be the only practical option.

2. Provide high availability across the quantum stack. Most of today’s quantum computers are experimental systems that are only used for a matter of minutes each day, lying idle most of the time. As quantum computers evolve to run the large error-corrected circuits, these machines will need to operate reliably for longer time periods. Simply put, quantum computers must transition to utility systems with minimal downtime. This places stringent availability requirements on every component of the quantum computing stack.

3. Be tightly integrated with quantum error decoders and other elements of the quantum computing stack. Quantum error correction must be performed at the clock cycle of the computer, requiring terabytes of data to be processed every second. If errors are not detected and corrected quickly they propagate through the quantum computer and render further calculations useless. This presents a significant engineering and systems integration challenge that requires close collaboration between the teams building the control systems and those building the decoders. Tackling the problem from both sides and building a highly optimised, highly integrated solution is the only way the problem of error correction will be solved.

These are not the only considerations as we scale qubit control. Quantum computers rely on RF signals to control the qubits, and an extremely high RF signal purity of at least 99.99% is required. Operating at 99% is relatively simple but operating at 99.99% means every source of thermal, mechanical and electrical noise must be suppressed. The current focus for control system engineers is building the best-performing RF system possible, because in today’s landscape where only a handful of qubits need to be controlled, it’s the most pressing need. While this is already a challenge at small scale, it will become even more difficult as the scale and complexity of control systems increases to support larger quantum computers. Maintaining signal purity under these conditions will require a clock maker’s approach to precision.

These requirements must also be met regardless of qubit type. The good news is that while the bare-metal interface between the control system and qubits may differ in terms of frequency and pulse shape, with careful design, the bulk of the control system can remain largely the same. When coupled with superior RF systems performance and the three core requirements highlighted in this post, every quantum computer maker will benefit from improvements in the core technology, keeping the heart of today’s quantum computers beating in the years to come.

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