Lack of fine granularity of phase and probability amplitude may render large complicated quantum algorithms hopeless.Fine granularity of phase is likely problematic.Post-NISQ quantum computing - NPISQ and NPSSQ using near-perfect qubits.Without full QEC, fault-tolerant quantum computing will consist of near-perfect qubits, greater coherence time, shorter gate execution time, error mitigation, error suppression, and circuit repetition.Fault-tolerant quantum computing = quantum error correction.The nails in the coffin for quantum error correction.Rapidly losing faith? Lost all hope is more like it.My thoughts on fault-tolerant quantum computing, quantum error correction, and logical qubits.And quantum error correction eliminates the limit of coherence time, enabling much more complex quantum algorithms. It’s about logical qubits rather than physical qubits. Quantum error correction is automatic and transparent. Note that quantum error correction (QEC) should not be confused with manual error mitigation or quantum error suppression, such as libraries which tweak and tune pulse sequences to minimize gate errors. That said, we are where we are, which is what I need to focus on at the moment. After all, they do say that hope does spring eternal. Generally, fault-tolerant quantum computing is essentially a synonym for implementing quantum error correction, subject to the caveat listed above, that quantum error correction is unlikely to deliver all that it promises, or at least all that its promoters promise.Īll of that said, I concede that at any moment somebody might have a research breakthrough and come up with a startling new approach to quantum error correction which addresses some, many, most, or even all of my concerns. In short, quantum error correction (QEC) as it is currently envisioned won’t lead to full, perfect fault-tolerant quantum computing (FTQC) - as it is currently envisioned and promoted. To be clear, I’m not suggesting that there are logic errors in any of the quantum error correction algorithms, but primarily, even if the algorithms do “work”, that they won’t do the full job and correct for all possible types of errors. Why I’ve Completely Given Up Hope in the Prospects for Quantum Error Correction.In truth, I soft-pedaled the title of this informal paper - it more accurately should have been: Now, it’s time for me to come clean and admit the ugly truth about quantum error correction. In fact, only in the past year has my confidence in quantum error correction fully crumbled. Instead, I fairly quickly concluded that near-perfect physical qubits (four to five nines of qubit fidelity, maybe in the three nines range for some applications) was a much more ideal goal for both the near term and the medium term, if not the long term as well.Įven two years ago I still considered full quantum error correction as a more ideal solution for the longer term, but even then I was aware of most of the issues I summarize in this informal paper. But two years ago as I was writing up my thoughts on fault-tolerant quantum computing, quantum error correction, and logical qubits my confidence in quantum error correction cracked and started a slow and very gradual crumble as I dug into the details and contemplated the consequences, ramifications, and issues involved. Not today, not next year, not in two years, and not in five years - or likely even ever.įive years ago and even just two years ago I was quite enthusiastic and supportive of the eventual prospects for quantum error correction even if it had no near-term prospects. I feel that it would be truly foolhardy to bet heavily that quantum error correction is going to magically lead us to fault-tolerant quantum computing and perfect logical qubits. Why I’m Rapidly Losing Faith in the Prospects for Quantum Error Correctionįull, automatic, and transparent quantum error correction (QEC) is no longer a slam-dunk panacea from my perspective.
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