Type of Errors That Arise in Quantum Computing

In last week’s article I talked about the type of qubits that are used in quantum computing. As you may be well aware, one or two of these qubits is not enough to make proper use of quantum mechanics. Scaling is both a priority and a difficult obstacle for scientists involved in QC. However, the more this technology is scaled, the more errors prop up due to things like decoherence, quantum noise, and cross-talk. Due to the miniscule nature of quantum mechanics, the qubits can succumb to tiny changes in the environment they inhabit, which quantum computers have to account for to accurately encode qubit information that is scientifically reliable. This issue is mitigated in a variety of ways, including but not limited to quantum error correction codes or QEC for short, quantum error mitigation (QEM), and architectural changes to the logical qubits to mitigate crosstalk.

Decoherence 

This type of “error” may happen as the molecule or qubit in this case is measured away from its formal environment, which may or may not constitute the collapse of the wave function. With qubits this can be done after or during it has successfully encoded the information to closeby detectors or other informational qubits. Coherence can play a large role in whether or not the logical system as a whole is kept in precise superposition during the algorithmic “transaction” to relay the correct information to those external qubits/detectors. Useful decoherence in the quantum system may also be dependent on the degree to which the system is able to understand the outcomes for the phase relations of particular qubits and then relay that information to the appropriate output.

Quantum noise

This type of error arises from the latent environment in which the qubits are located and is caused mainly by tiny unwanted fluctuations in the measurement of the encoded qubit information. Due to the quantized nature of qubits, they are negatively susceptible to miniscule mechanical and thermal changes that deviate from those desired in the experiment themselves. For example, when information is moving from the superposition qubit to the memory bus that is encoding the information, there is space for mechanical error that may deteriorate the viability of the quantum information being relayed. Furthermore, systems acting as detectors for this information are bound by fundamental constraints due to Heisenberg's Uncertainty principle, which comes with inherent noise that needs to be corrected.

Crosstalk

Is one of the most important errors to adequately fix in modern superconducting qubit microchips, due to the nature of their source. Crosstalk often comes from the very sources which are used to manipulate and encode the qubits themselves. That is, photons and microwave signals that are intended for one source are had by another, which can cause unintended information to be read by the qubit itself. This can also cause unintended further consequences like decoherence due to feedback loops within the architecture itself.

Overall, there are a plethora of sources for errors in quantum computing, these are just some of the most talked about in the literature that I read. This is also a very basic overview of the classification of each error that can occur. If you are at all interested in learning more about quantum computing, I encourage you to dive deeper into the literature on any one of these topics, as I am sure you can find plenty of information. Below are some sources that I have utilized and outlined as useful for this discussion. Thank you.

Quantum Decoherence Forthcoming in Routledge Companion to Philosophy of Physics - Elise M. Crull (April 14, 2018)

Quantum Noise and Quantum Measurement - Aashish A. Clerk

Eagle’s quantum performance progress | IBM Research Blog Oliver Dial (March 23, 2022)

Efficient Noise Mitigation Technique for Quantum Computing - Ali Shaib (March 08, 2023)

Surface Codes: Towards Practical Large-Scale Quantum Computation - Austin G Fowler (September 18, 2012)

Good Quantum Error-Correcting Codes Exist - A.R. Calderbank & Peter W. Shor (September 12, 1995)