Why Good SPAM is Essential for Quantum Computing

By Team QCI

June 1st, 2023

Why is SPAM so important?

One of the fundamental requirements of quantum computing is the ability to prepare and measure quantum states with high precision and accuracy.  A big challenge associated with State Preparation (the SP of SPAM) in quantum computing is the difficulty of preparing a qubit in a known, desired state.

This can be due to environmental factors such as finite temperature or imperfect control of one’s qubits, which have the propensity to disturb a quantum system and introduce errors.  Thus, getting good state preparation is essential to getting reliable algorithm outputs.

And Measurement (the AM in SPAM) is another key ingredient to successful quantum computing. The outcome of a computation is determined by the measurement of the quantum state of the qubits.  A measurement returns users a classical bit (either 0 or 1) as the result.

The measurement process is typically subject to error, which can arise due to a variety of factors.  One of the most significant sources is the interaction between the qubit and the measurement device itself.  The latter can disrupt the qubit and corrupt the algorithm, for example.

The measurement can also be “noisy,” meaning that a user may mistake what is a 0 for what is a 1 and vice versa, which also leads to algorithm errors.  Having low measurement errors, therefore, is a must for a quantum program to be successful.

In the case of error correction, where state preparation and measurement outcomes provide information about whether or not an error occurred, low SPAM errors reduce the likelihood you’ll get confused between what’s a quantum error in your system vs. what’s a measurement error.  This will improve the performance of your error correction and your quantum computer.

SPAM isn’t often emphasized in discussions about fault tolerance or scalability, where the attention gravitates toward single and two-qubit gate fidelities.  However, low SPAM is absolutely a requirement in getting error correction working through scalable means.  And this is what QCI is all about, and why we’re so excited about how dual-rail technology unlocks incredibly high-performing SPAM performance.

As every component of the hardware, as all the gates, and as each remaining aspect of the system improves, no doubt SPAM will have to follow suit.

For a more detailed look on SPAM and how it ties into the greater dual-rail picture with superconducting systems, please see the following publication by Teoh et. al. from the Schoelkopf lab at Yale University: https://arxiv.org/pdf/2212.12077.pdf